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GE4TECHNICAL APPENDIX
SCH Number: 95092027
Prepared for the
CITY OF SOUTH SAN FRANCISCO
by
WAGSTAFF AND ASSOCIATES
Urban and Environmental Planners
in association with
Harlan Tait Associates, Engineering Geologists
January 1996
WPS11S4810SE1RlCGV-GE0.548
REPORT
"AREA D" SLOPE STABILITY
ANALYSIS AND REMEDIATION
PHASE I, TERRABAY PROJECT.
SOUTH SAN FRANCISCO, CALIFORNIA
JULY 1995
PREPARED FOR:
SunChase GA Calif I, Inc.
6001 North 24th Street, Suite A
Phoenix, Arizona 85016
1866-002
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...` Geo/Resource Consultants, Inc.
GEOLOG ISTS~'ENG1 N EERS/ENVIRO NMENTAL SCI ENTISTS
505 BEACH STREET, SAN FRANCISCO. CALIFORNIA 941 t 3
REPORT
AREA "D" SLOPE STABILITY A1~IALYSIS AND REMEDIATION
PHASE I, TERRABAY PROJECT
CITY OF SOUTH SAN FRANCISCO, CALIFORNIA
PREPARED FOR:
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SUN CHASE GA CALIF I, INC.
6001 NORTH 24TH STREET, SUITE A
PHOENIX, ARIZONA 85016
PREPARED BY:
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GEO/RESOURCE CONSULTANTS, IlYC.
505 BEACH STREET
SAN FRANCISCO, CALIFORNIA 94133
JULY 1995
GRC PROJECT NO.1866-002
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GEOLOGISTS /ENGINEERS ! ENVIRONIAENTAL SCtENTt5T5
1
7uly 26, 1995
1866-002
SunChase GA Calif I, Inc.
6001 N. 24th Street, Suite A
~` Phoenix, Arizona 85016
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San Frandxo. Gilonra 94133
(t15)77'S~1T7 fAX (It5) 77rr23S.9
Re9bnat o~ ~«,. GNOrr.a -,~.. w.9..,
RE: AREA "D" SLOPE STABILITY ANALYSIS AND REMEDIATION
`^ PHASE I, TERRABAY PROJECT
' SOUTH SAN FRANCISCO, CALIFORNIA
Ladies and Gentlemen:
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!_. Transmitted herein aze the results of our slope stability analysis of the referenced site. During
the course of our study, we have conferred with you and representatives of the City of South San
Francisco.
We have evaluated several remedial alternatives for Area D. The new remediation plan is based
in part on new test borings and updated readings from the existing piezometers, inclinometers
and hydraugers. The preferred remedial alternative consists of removing the upper 10 to 20 feet
of Area D, constructing a keyway at the base of the slope, and providing subdrainage
improvements. With this alternative, slopes of Area D meet or exceed generally accepted
stability criteria. The repair will provide protection for the proposed homes, storm drains and
debris basin, and adjoining HCP lands.
We refer you to the contents of our report for details.
It has been our pleasure to work with you on this important project. If you have any questions
concerning our findings, please call.
r ~ Sincerely,
~ GEO/RESOURCE CONSULTANTS, INC.
-,
., ~---~-
_ Alan D. Tryhorn,
Senior Vice President
`J Glenn Romig
1 , Geotechnical Engineer
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ADT,'ES~ /GR.csc
cc: City of South San Francisco (3 copies)
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Eric S. Ng, P.E., G~
Principal Engineer
TABLE OF CONTENTS
.1 PACE
- 1.0 INTRODUCTION 1
1.1 BACKGROUND 1
1.2 PURPOSE OF THIS INWESTIGATION 1
1.3 SCOPE OF SERVICES 2
;.
2.0
FIELD INVESTIGATION
4
~ 2.1 FIELD EXPLORATION 4
2.2 EXPLORATORY BORINGS 4
2.3 MONITORING OF PIEZOMETERS, INCLINOMETERS
" - AND HYDRAUGERS 5
3.0 GEOLOGIC CONDITIONS ~
3.1 SURFACE CONDITIONS 7
3.2 SUBSURFACE CONDITIONS 7
-- 4.0 FINDINGS AND CONCLUSIONS 10
~ 4.1 EXPLORATORY BORINGS 11
4.2 PIEZOMETER AND INCLINOMETER RESULTS 11
F' 4.3 EXTENT AND DEPTH OF LANDSLIDE I2
~..
5.0
RECOMIvIENDATIONS
14
5.1 PREVIOUS WORK 14
5.2 STABILITY EVALUATION 15
5.3 RECObEvvIEENDED ALTERNATIVE 18
5.4 SITE MONITORING 19
~ ~ 6.0 LIMITATIONS 21
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FIGURES
_ FIGURE 1 SITE VICIIVITY MAP
' FIGURE 2
AND FIGURE 3 TEST BORING LOGS
FIGURE 4 UNIFIED SOIL CLASSIFICATION SYSTEM
FIGURE 5 HYDRAUGER LOCATIONS
FIGURE 6 GEOLOGIC CROSS SECTION A-A'
FIGURE 7
~--~
GEOLOGIC CROSS SECTION B-B'
~ FIGURE 8 RAINFALL AND GROUNDWATER LEVELS
PLATE 1 GEOTECFiNICAL MAP (IN POCKED
TABLES
TABLE 1 WATER LEVEL READINGS
1 TABLE 2 HYDRAUGER READINGS
C1 TABLE 3 SUMMARY OF STABILITY RESULTS
APPENDICES
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APPENDIX A INCLINOMETER DATA
U APPENDIX B SLOPE STABILITY RESULTS
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1.0 INTRODUCTION
This report summarizes the results of our supplementary geologic and slope stability evaluation
- for an area previously known as "Landslide D" in the Terrabay project, South San Francisco,
California. This area is located towards the north side of the Phase 1 development, as shown in
Figure 1.
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~ 1.1 BACKCTROtJND
Terrabay is a residential development that will consist of two phases of single-family houses and
townhouse units. Site grading for the Phase 1 azea was completed in 1989. Area "D" was
(~ recognized as an upper area of relatively shallow, active landslides; however, the overall slide
_,
r mass was believed to be inactive. An initial remedial measure for Area D was developed by
1 building atie-back wall to overcome translational slide movement and localized slumping.
• .. However, during the construction of the tie-back wall, lower sheared and weaker clay surfaces
were discovered. The presence of the lower shear planes below the wall presented a bigger and
• ~ deeper slide potential in the area and made the constructed wall a less effective remedial
measure, than originally planned.
-.
~ In 1992, Leighton and Associates, Inc. (L.&A) together with the Geotechnical Engineer for the
(~ project, PSC Associates (PSC), provided supplementary investigation of Landslide D. The
U evaluation in their report focused on the potential for movement of this slide, its effect on the
development, and possible remedial measures. Several alternative measures were prepared and
they will be discussed later in this report
1.2 pi TitPOSE OF THIS INVESTIGATION
The purpose of this study was to re-evaluate proposed remedial measures for Area D in view of
L safety and constructability, particularly taking into account the significant rainfall that occurred
in the winter of 1994/1995. The study was to maintain the safety standards that were established
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by the City and are recognized as typical in the industry. The study was initiated by SunChase
pursuant to its obligations as developer. Our conclusions are based on reviewing previous data
accumulated by others, as well as our own field investigations which supplemented those data
with two new test borings and recent readings from the on-site piezometers, hydraugers and
inclinometers.
1.3 ~('OPE OF SERVICES
The scope of our services, as stated in our proposals to ..you dated October 10, 1994, and modified
in subsequent meetings, consisted of the following:
(" A. Review of reports by PSC and L&A;
B. Discussion and meeting with City Engineers and Consultants regarding the history and
concerns of the site development;
1.
C. Field reconnaissance and advancing two test borings to supplement and update previous
~ data within the toe of the landslide area;
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[ j D. Collecting readings between November 1994, and July 1995, on the piezometers,
hydraugers and inclinometers that were previously installed on the site;
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E. Evaluation of the remedial alternatives as proposed by the previous consultants and _
recommendation of alternatives that may be most appropriate for the current
development;
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(' F.
Summary our findings in this report.
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During the course of our investigation, we have discussed this project with Mr. Gary Parikh of
Parikh Associates who had substantial participation of the investigation in the Terrabay
development. We also consulted with Mr. John Gibbs, City Project Manager, and with Dr. Eric
McHuron, City consultant, who both had performed substantial review on previous project work
for the City.
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2.0 FIELD INVESTIGATION
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2.1 FIELD EXPL.OR_ATION
' During the period from October 1994 to February 1995, GRC geologists and engineers
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periodically visited the Terrabay site to observe and map geologic conditions. Our
reconnaissance focused on surficial features such as scarps, ground cracks, hummocky ground
bulges, seepage, and rocky outcrops in the vicinity of Area D. We used the previous mapping
~t contained in the L&A report of January 28, 1992, as a base, and modified those maps as
appropriate. Features were mapped at a scale of 1 inch equals 40 feet, and were taped or paced
from topographic and cultural features shown on the map.
` Additional groundwater levels and flow data were collected during spring and summer, 1995, by
;`- our geologists and Dr. McHuron.
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2.2 F7~LORATORY BORINGS
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~ Two borings (BH-1 and BH-2) were drilled at the toe area of the mapped landslide as shown on
Geotechnical Map, Plate 1. 'The boring locations were selected to supplement existing
subsurface data and to help in defining the lower extent and geometry of the slide mass.
~~
C Frequent rainstorms delayed the exploratory drilling until mid-December. on December 20,
{ ~ 1994, attempts were made to access lots 104, 105, 106, and 107 with afour-wheel drive truck
t ~ mounted auger, but were unsuccessful. On December 29 and 30, 1994, we drilled two borings,
using a rotary wash, track-mounted drill rig. Boring BH-1 was drilled on lot 105 to a depth of
`- 64.2 feet, and BH-2 was drilled on lot 106, to a depth of 49.5 feet.
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Both borings were continuously logged by our engineering geologist, and were sampled with
drive samplers and, in selected intervals, continuously with a 36-inch-long Pitcher Barrel
sampler. Samples were extruded in the field, examined, and placed in core boxes for transport to
our laboratory. Additionally, Dr. McHuron examined the core samples in the GRC laboratory.
~ The logs of these borings are presented as Figures 2 and 3. The soils are described in accordance
with the Unified Soil Classification System, as summarized in Figure 4.
2.3 MONITOIZTNG OF PIEZOMETERS. INCLINOMETERS AND HYDRAUGERS
Previous geotechnical investigations by PSC and L&A resulted in the establishment of a number
,,
of instrumentation stations across Area D. These stations are in the form of groundwater level
observation wells (piezometers) and slope movement detection casings (inclinometers). Near-
horizontal drains (hydraugers) were installed at various locations as an aid in lowering
groundwater levels, and they also serve as groundwater monitoring points_ The locations of all
piezometers and inclinometers are shown on Plate 1. Locations of the hydraugers are shown on
Figure 5.
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We collected groundwater level readings from 17 piezometers over an eight-month period (from
November 1994 to July 1995), using a Solinst electric sounder. The result of these readings,
t presented as depth below the ground surface, are shown in Table 1.
~ Measurements from seven inclinometers were made on two separate oc~sions, December 1 and
~-, 15, 1994 by Parikh Consultants. These readings were performed by the same individuals using
similar techniques and equipment as the previous measurements dating back to 1989 and 1991.
This was done to provide comparable data sets, and to avoid wide variations possibly induced by
`~ methods, equipment type and different operators. Two readings were performed to provide
j ' confidence in the interpretation of inclinometer results. Readings were made on two axes within
L each inclinometer casing. One axis was along the longitudinal direction of the slide and the other
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along the transverse direction. A graphical display of previous and current measurements is
presented along with a reference historical reading in Appendix A.
_ The flow rate from 16 hydraugers was measured, from a total of 22 located in the upper slide,
~ lower slide, and building pad levels. The gravity flow from these hydraugers was measured
using a stop watch and graduated glass beaker, over a short duration. To be consistent with
previous reports, the flow rates were calculated on the basis of number of gallons per day. Thus,
the calculated flow rates presented in Table 2 should be considered approximate.
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3.0 GEOLOGIC CONDI'T'IONS
-~ 3.1 Si 1RFACE CONDITIONS
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~.r Area D occupies a portion of the Terrabay Phase I development that includes moderate to steep,
_ southeast-facing natural slopes, steep, south-facing slopes, and level building pads. The slopes
drain to a series of lined ditches and a natural drainage which empties into a concrete debris
basin. Most of the ground surface is covered with grasses, and some shrubs. Clusters of water-
loving plants mark a number of locations, particulazly at seepages, drainage courses, and
- hydrauger dischazge points.
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' ~ The most prominent geomorphic feature of the area is the subdued, hummocky topography of
shallow and deep-seated landslide deposits. The slide mass is flanked on both sides by graded
bedrock slopes. Grading has exposed Franciscan Complex sandstone and melange (mainly
sheazed shale), as well as alluvium and slope debris.
3,2 Si TRST TRFACE CONDITIONS
+ Descriptions of subsurface geologic units are based largely on the summary given in the I.&A
j report, 1992, and modified by our site observations.
Area D is characterized by landslide terrane underlain by a variable thickness of Quaternary-age
~-. unconsolidated surfiicial materials, which are in turn underlain by older bedrock of the Franciscan
Complex. The Franciscan Complex consists of either predominantly (graywacke) sandstone or
predominantly melange (a sheared, deeply weathered mixture of mostly shale, siltstone, and
~' chert). In the study area, these two bedrock units are typically separated by the inactive Hillside
(' fault.
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The unconsolidated deposits mantling Franciscan bedrock are geologically young deposits
derived from older geologic materials on or neaz the site. Topsoil, fill, colluvium, alluvium,
slope debris, debris flow deposits, sand lenses, and landslide deposits are all surficial materials
which cover the bedrock. The areal distribution of the unconsolidated units is shown on the
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Deposits of artificial fill (Map symbol A.f) were mapped neaz the toe of Area D. These fill
materials were placed during grading for the sheaz key of a buttress fill at the toe of Area D,
upslope of the pads of Lots 105 and 106. Fill was derived from on-site soils and placed under
- the observation and testing of PSC.
Most of the upper 40 feet of materials within Area D are composed of debris which has been
transported downslope primarily as slopewash or colluvium and debris flows, and possibly,
'-', wind-blown sands. These materials are composed of mixtures of gravel, sand, silt, and clay.
~ ~ U r debris flow de sits bol aze redominantl li ht brown sand cla with
PPe Po (MaP ~ Qd) P Y g Y Y
gravel. The upper debris flow deposits are generally moist and stiff.
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~ The upper debris flow deposits include discontinuous stringers of light brown silty fine sand.
The sand deposits are thickest neaz the upper reaches of the landslide. These sand stringers may
j be portions of, or derived from, the Colma sand. .
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In the central portion of the slope, the upper debris flow deposits are underlain by a generally
~ continuous layer of silty clay ranging in thickness from approximately 2 to 15 feet. The clay is
~ characteristically light brown with olive to light gray clay stringers with minor amounts of
gravel. Discrete sheaz surfaces were observed within this clayey unit. Additionally, the clay
L' layer was mapped in a portion of the key bottom. This sheared clay typically marks the basal
G rupture surface of the landslide. In the toe azeas, the clay layer (as well as the upper debris flow
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deposits) interfingers with the undifferentiated alluvium/debris flow deposits. This clay layer, as
reported in the I.&A study, does not exist in the lateral mazgins of the landslide deposits.
The lower debris flow and colluvial deposits .(Map symbol Qd2) which underlie the above
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1,~ described clay layer are predominantly reddish-brown clayey gravel with minor amounts of sand
_ and boulders. These materials are interpreted as debris flow deposits which accumulated in the
'.,; deeper portions of the ancient valley. The unit includes material previously called the valley
infill deposits. These deposits do not appeaz to be involved in the potential landslide mass.
Undifferentiated alluvium and debris flow deposits (Map symbol Qal and Qd), located in the tce
~ ~ area of the slope, is a mixture of stiff gravelly clays to dense clayey sands. These materials are
interpreted as alternating, overlapping sequences of debris flow deposits from the surrounding
' hills and water-carried alluvium de sited in the stream channel.
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Underlying the surficial materials are bedrock units of Jurassic to Cretaceous-age Franciscan
Complex, which consists in this area of predominantly sandstone (graywacke) or predominantly
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melange. Melange is typically a weak, highly fractured to sheared mixture of rock types, mainly
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shale and siltstone. The sandstone graywacke is generally fractured and moderately weathered
and varies from hard and strong to weak.
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4.0 FINDINGS AND CONCLUSIONS
Based on the additional test borings and the latest readings from the piezometers, hydraugers and
• inclinometers, we summarize. our conclusions below. These conclusions form the basis of our
recommendations in the subsequent section.
o The extent and the depth of the slide has been further refined. The two borings provided
an indication that the slide zone is less extensive and shallower towazds the toe area than
. estimated in the LBtA (1992) report.
~~ o No significant slope movement was detected by the inclinometers between the previous
- reading (10/23/91) and the recent readings (12/1/94 and 12/15/94). This provides some
confidence that Area D has remained stable within this period at its current configuration
`' following the site grading.
,.
'~ o Piezometer readings during a portion of the very wet '94= 95 winter season indicate higher
water levels than indicated in previous reports. Shallow piezometers typically have
responded directly to infiltration of rainfall, rising dramatically after prolonged rain, and
falling during periods when rainfall ceased. This is especially evident in the July 14,
+' 1995, readings where most of the shallow piezometers have fallen to previous lower
Y
{ levels. Deeper piezometers show very slow changes relative to rainfall, as would be
! expected We expect that some portion of the slope area, such as the upper area behind
~ the existing retaining wall, may be saturated and more prone to localized slope
1 movement.
~ o Water flow from hydraugers was similaz to shallow piezometers in that the flow rates
1 varied with rainfall patterns. Hydraugers appear to be effective in dewatering portions of
the slide mass, and thus confirm assumptions in earlier reports.
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o The existing retaining wall, while deemed to be not effective against deep slope
movement, probably provides some resistance against localized near-surface movement.
The wall probably increases the general stability of the area towazds the twQ ends of the
retaining wall. In addition, it provides a catchment fence to prevent occasional loose
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boulders from reaching lower levels. .
4.1 FXPT.(~RATORY BORINGS
! - The two exploratory borings, BH-1 and BH-2, were located to provide information on the extent
~~ and depth of slide debris in the toe azea. BH-1 provided positive information on a shallower
~--~ slide depth than previously shown, while BH-2 confirmed the general type of soil and rock
i ~ materials.
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i ~ As illustrated in Figure 2, Log of Boring BH-1, we found a thin, gray silty clay layer at a depth
' of 16.8 feet. This clay, which resembled clay described from the central portion of the slide, was
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in abrupt contact with black-stained sandy and gravelly clay. The dip of the contact was about
10 degrees from horizontal. Although the core sample was not oriented, the slide plane geometry
as determined from previous work suggests that the slide plane does not continue to plunge down
at that location, but rises back toward the ground surface.
4.2 "~~^~~TER ND INCLINOMETER RESULTS
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y Based on our review of the piezometric (water level) data we collected during this investigation
and on previous data, we believe that at least two distinct perched groundwater zones exist within
~ the slopes of Area D. An upper, and apparently discontinuous, zone of groundwater in the
(; colluvium and debris flow deposits above the basal rupture surface, is generally quite responsive
~ to large amounts of rainfall. The response of selected piezometric levels to the '94-'95 winter
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rains is displayed in Table 1 and in Figure 8. After nearly twenty consecutive days of rain, some
of the upper zone water levels had risen to an average of 5 to 20 feet below the ground surface.
In a three-day period between two readings (February 2 and s, 199s), no rain was recorded on
site, and most of the piezometers recorded a slight drop in water levels. This pattern is consistent
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~,~ with previous observations and may reflect more permeable sandy soils close to the surface. A
_ _ lower zone of groundwater is at or near the bedrock surface and is separated from the upper zone,
at least in part, by the clay unit at the slide plane. During the high precipitation period, water
levels in the lower zone did not appear to rise dramatically. In contrast to the upper piezometers,
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two piezometers screened below the slide plane level (LSDP- 6B and 7B) continued to show a
- slight rise even after rainfall ceased. This is typical of deeper water bearing units.
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• The results from inclinometers are consistent with previous readings, typically slowing
movement in the upper several feet, probably attributable to surficial soil creep or irregularities
'~ in the inclinometer casing. The magnitude of deflections observed in the graphical plots, which
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are presented in Appendix A, generally fall within the measurement tolerance of the instruments,
and in our opinion, can be considered "noise". Based on inclinometer results, we believe there
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has been no movement of the inactive Area D landslide since the previous instrumentation period
• in 1991.
~•, j 4.3 FA'T' AND DEPTH OF I,ANDSL.IDF.
1 Area D is occupied by a lazge, inactive landslide feature that has developed through translational
r, movement of older debris flow deposits and colluvium. Shallow debris flows and rotational
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landslides of more recent age have occurred within the lazger mass and at its margins. According
to the extensive supplemental geotechnical investigation performed by L,&A (1992), the depth of
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the large inactive landslide is defined by a basal rupture surface approximately 4s to s0 feet deep
f ~ in the central portion of the slide. The rupture surface is at the base of a 2 to 15-foot-thick clay
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layer. The rupture surface is not well defined at the lateral mazgins of the slide, or at the tce area.,
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where the slide mass was removed through ancient alluvial processes.
r The lateral limits of the large slide are indicated on the Geotechnical Map, Plate 1, and are -
~ modified from the L&A report in the tce area. R/e interpret the thin (0.2 feet thick) layer of clay
in BH-1 to represent a basal rupture surface. This relationship is illustrated on Figure 6, Cross-
Section A-A, and Figure 7, Cross-Sectioa B-B'. Based on additional readings and test results,
the depth of this feature suggests a shallower slide plane, and more limited toe extent than
estimated in the I,&A report.
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5.0 gECOM1I~NDATIONS
5.1 PRRVIOt1S WORK
~~ In the previous report by L&A, several remedial measures were examined. The options
presented included construction of a large toe buttress, placement of toe fill, removal of upper
slide materials, and a laterally loaded drilled pier system. All these options were designed to
increase the factor of safety against potential slope movement. The static factor of safety used
for design was 1.5, which was consistent with the design basis for other slopes in the Terrabay
-- project
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'' Alternative 1
(^ The proposed remedial measure in the LBtA report, referred to in this report as Alternative 1,
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consisted of construction of two large sheaz keys, one above and one below the existing tieback
retaining wall, together with some removal of overburden above the wall, and subsurface
1
drainage improvements.
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We have evaluated this proposed remedial measure from both technical and construction
feasibility aspects. From a technical standpoint, the proposed repair scheme would minimize
future slope instability, Provided that the shear keys are extended below the failure planes, the
L i keys are constructed with adequate subsurface drainage, and that adequate compactive effort is
-. used in fill placement. However, from a construction standpoint, we have several concerns
regazding the proposed system.
First of all, the shear keys involve extensive earthwork which requires deep excavation on the
' ~ order of 50 to 60 feet in some areas. Temporary excavation requires open cut slopes on the order
of 1:1 to 1.5:1 (Horizontal to Vertical). Such a deep cut (50 to b0 feet) in a marginally stable
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landslide slope generates a concern about the overall hillside stability during construction. In the
past, we understand that a similaz slope problem was developed during the repair of "Landslide
R" (immediately southwest of "Area D") and major failures occurred on the slide plane.
,f ; Second, the location and geometry of the failure plane within the slide zone are based on data
points from various borings. The actual slide plane configuration within the slope can only be
verified during excavation. It is possible that deeper excavation would be required in some areas.
If this were to occur, a steeper cut would be necessary to achieve the sheaz key construction,
adding to the difficulty of constructing the shear key.
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Third, extensive cuts in the slide toe area would require either partial or total removal of the
' ~ existing debris basin. Such a change would also require a modified design of the debris
catchment system.
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5.2 STA>~IL.ITY EVALUATION
~ -.
We evaluated several remedial alternatives during the course of our work. In order to assess the
feasibility of each alternative, we performed stability analyses of several of them. The computer
. program "TSLOPE", furnished and applied by Dr. Robert Pyke, was used for the stability
j calculations. A summary of the results is presented in Table 3, and brief description of
1, alternatives 2, 3 and 4 is described below. Copies of the computer printouts from the
1 ~ recommended alternative (Alternative 4) are presented in Appendix B.
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' The soil strength parameters used in our stability analysis are identical to those used by L&A in
their January 28, 1992 report. We also assumed simiiaz groundwater conditions.
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We understand from the City of South Saa Francisco that a factor of safety of 1.5 is required for
' static stability for landslide evaluation. In our opinion, a factor of safety of 1.1 is generally used
in practice for seismic stability analysis.
r• .
~ The alternatives evaluated in addition to the I,&A repair scheme, developing a setback from the
existing unrepaired slope (Alternative 2), included construction of a soil buttress at the toe of
i : slope (Alternative 3), and a combination of significant removal of slide mass and construction of
a shear key at the tce of slope (Alternative 4).
-~ ~ternative 2
Our stability analysis indicates that the existing slope does not meet the required factor of safety
of 1.5. A setback could be established to protect the development below the slopes of Area D by
elimination of 7 to 8 lots within a reasonable setback zone. This would protect residents;
however, it would not remove the need for future maintenance due to a potential landslide, or
~ mitigate the impact on underground infrastructure.
Alternative 3
j Constructing an earth buttress at the base of Area D would improve the static and seismic factors
r, of safety to approximately 1.3 and 1.0, respectively. The static factor of safety for this
~ -~ alternative is in the range often accepted for a development such as this, in our opinion, with no
- structures actually being built on it A small amount of movement of the slope during seismic
-' ground shaking would be possible with this alternative. However, the movements would likely
be small. This alternative would also result in eliminating 7 to 8 lots, and would require relaxing
V the requirement for a static factor of safety to under 1 S.
it
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~ ,r Geo/Resource Consultants, Inc.
July 26;1995
1866-002
Page 17
Alternative 4
r(
Removing a thickness of approximately 10 to 20 feet of the slide mass and constructing a shear
key at the bottom of the slide would meet the static and seismic factors of safety above those
~-i commonly required for this type of development. This alternative would entail cutting to a
relatively uniform slope, beginning below the upper concrete drainage ditch, and cutting only
!~ within the Area D limit. Twenty feet of soil would be removed at the lower portion of the slope,
decreasing generally to 10 feet in the upper portion, with less at the very top. Where competent
bedrock is encountered within the cut, such as within the west edge of the slope, the depth of the
- - cut can be reduced. )n addition, a sheaz key similaz to that proposed in the L&A repair scheme
~ -' (Alternative 1) would be constructed at the base of the slope. However, this shear key would
require an unsupported cutslope about 20 feet lower in height because of the proposed removal of
- 20 feet of the slide mass, significantly reducing the potential for safety hazards to develop during
' construction. This proposed remedial scheme is presented on Figure 6, Geologic Cross Section
A-A'. The static and seismic factors of safety for this alternative aze approximately 1.6 and 1.2,
respectively. We also briefly evaluated the sensitivity of the stability analysis to variations in
groundwater conditions. When raising the assumed groundwater level for this alternative by 7
~~
feet, to the final graded ground elevation, the static and seismic factors of safety were reduced to
1.5 and 1.1, respectively. In our opinion, a groundwater level this high would be highly unusual.
However, the analysis shows that even with such high groundwater, the stability remains within
(~ the range required for the development. The details of the stability analysis results aze presented
l! in Appendix B of this report.
i•
~ - ' In our opinion, the fourth alternative is the most desirable alternative, from a safety and
constructability viewpoint. In addition, this alternative is the only one that actually reduces the
~ potential landslide driving forces. The computed factor of safety of this alternative is equivalent
~'1 to the L&A Plan, which has previously been accepted by the City.
u
~ ~~~ ,r Geo/Resource Consultants, Inc.
July 26, 1995
18b6-002
Page 18
5.3 RFrO1ViMENDED AT.TEIZNATNE
The repair should be designed and constructed in accordance with the following criteria.
~;~ 1. The area and depth of the slide mass removal and shear key aze shown on the Geotechnical
Map, Plate 1., and the cross section, Figure 6. The final repair plan should be developed by a
` ' Civil Engineer, working closely with our office; more detailed earthwork criteria will be
provided by our office. The surface drainage features in the area that will be removed,
should be replenished. In addition, to the extent possible, the final ground surface should be
sloped in an easterly direction to divert debris/boulders to the swale and debris basin at the
~ eastern edge of the slope, away from the developed lots. Alternatively, a debris catchment
fence could be constructed at the base of repair.
r' 2. The keyway should extend to Elevation 270 feet, as shown on Figure 6. We anticipate that at
this depth the key will be below the slide planes. Exploration pits should be dug in advance
` of the keyway excavation to confirm that this depth is adequate.
3. A subdrain should be placed in the base of the keyway, and extend up to Elevation 310 feet.
The subdrain should consist of a 12-inch perforated pipe, perforations placed down,
t~ embedded in a 3 foot width of Caltrans specification Class 2 permeable material.
(~ Alternatively a composite geofabric/crushed rock scheme can be used. The 3 foot width of
t• j Class 2 rock should be extended up as fill is placed in the keyway. A solid pipe should be
~, used to direct drained water to a suitable discharge point.
4. On-site soil may be used to backfill the keyway. The fill should be compacted neaz the
~ optimum water moisture and at a relative density of at least 92 percent, as determined by
ASTM Test D-1557, latest edition.
L
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~ ~ie~ ,,r Geo/Resource Consultants, tac.
July 26, 1995
1866-002
Page 19
5. The keyway will extend partially under lots 105 and 106. This will result in up to 30 feet of
fill underlying portions of these lots, and cut surfaces beneath other portions. Because of the
~ :~
variability in soil conditions across these lots, we recommend that the Geotechnical Engineer
of record for the lots prepare specific recommendations for them. The eazlier
,~
~ recommendations will likely not be adequate. In addition, to reduce the potential for
~-- settlement of the keyway fill, structural fill placed on tbe lots should be compacted at a
' relative compaction of 95 percent relative compaction (ASTM 155.
6. Drawings should be developed, showing the existing piezometers, slope indicator casings,
hydraugers, subsurface drains and other underground pipes within the repair area. Those
~~l features which are desired to be kept in operation should be identified in the contract
documents. A contract mechanism to protect these items during construction should be
developed as appropriate.
7. The topsoil from the graded azea should be stripped, to the extent possible, and stockpiled.
~ This topsoil can then be spread over the final graded surface, to promote growth of vegetation
on the repair surface.
C
8. We should be retained to work with the Civil Engineer and your other consultants in
~-
~~ developing plans for the repair, to review the final plans and specifications developed, and to
(~ observe and test the earthwork portion of the repair during construction.
Li
(~ 5.4 SITE MO>`TITOIZiNG
U
We strongly recommend that periodic monitoring be performed at Area D currently and
u throughout the construction period. This includes further readings of hydraugers, inclinometers
and piezometers. We also recommend that new instrumentation be installed upon the completion
of the repair so that future monitoring of the slope is feasible. We would work closely with your
3J
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~ ,r Geo/Resource Consultants, inc.
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July 26, 1995
1866-002
Page 20
Civil Engineer regarding the locations of the new monitoring device and the frequency of future
measurement.
GD99:tE66-R
;r GeoJResource Consultants, Inc.
t
July 26, 1995
1866-002
Page 21
6.0 LIMITATIONS
This report is based upon the services we provided in conducting the study for the specific
purposes of geological evaluation, as described herein. The scope of services associated with
this report was developed specifically for the client in light of their risk management preferences.
This study was performed with the skill and care ordinarily exercised by members of the
`; profession practicing under similaz conditions at the same time, and in the same or similaz
locality. No other warranty, either expressed or implied, is made or intended.
This report is subject to certain limitations that may or may not be noted in the report itself. In
addition, recognize that the passage of time affects the information provided in the report Our
opinions relating to site conditions aze based upon information that existed at the time our
conclusions were formulated. As we are sure you can appreciate, site conditions can change
rapidly, such as seasonally, or in some cases, overnight.
Please understand that the services provided for in this project were limited to the specific
requirements of the client; the limited scope of service allows our firm to form no more than an
opinion of the actual conditions at the site. This letter confers upon no third party the right to
rely upon the information contained in the report. No other pazty is entitled to rely on the report
unless our express written consent is first obtained. Please contact us if you have any questions
r~ or concerns regazding the information contained in the report or these limitations.
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LOG OF BORIN6 BH-1
EQtripment Solid Flight Auger/Rotary Yash
Elevation 297±ft. Date 12/29/94
SANDY CLAY (CL)
brown; soft, wet; rock fragments @ 2'
augered to 10' depth
SILTY SAND (SM)
red brown; medium stilt; wet;
some scattered gravel; rocky @ 5'
CLAYEY GRAVEL (GC)
red brown; low plastic fines; medium stiff;
gravel is fine to 2", angular
switched to rotary dulling @ 10'
hard dulling @ 13'
color change to gray @ 14'
SB~H
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PB-4
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30
PB-7
PB-6
35
PB-9
PB-10
l PB 1 I I I 140 ~~
GeotResource Consultants, lnc.
~~~ Geologists 1 Engineers - Enviroronentsl Scientists
Job No. 1866-002 Appr:~ZDate ~
set casing to 12'
GRAVELLY CLAY (CL-GC)
mottled gray and yellow brown; very stiff--hard;
moist; gravel is fine, some coarse sand;
mostly sandstone and volcanic rock
poss~le slide plane @ 16.8'
angular to sub rounded rock obstruction @ 17'
.15' gray clay seam -stiff, wet, w/sharp
lower contact; dips about 10 degrees
gravel @ 21 ' -moderately hard; very strong
gravel Ca 23.5'
CLAYEY SANDY GRAVEL (GC)
light olive gray, mottled w/orange brown
dulled out to 26 2'
hard dulling @ 27.5'-30'
cuttings are hard silty sand, gray
@ 31.0' chance to red-brown to 32.5', then
mottled w /gray and minor y allow
washed away 32.5 + to 33.4'
CLAYEY SAND (SC)
yellow brown mottled w/olive gray and orange
brown; very fine sand, some medium; some
weathered sandstone gravel
LOG OF BORING BH-1
TERRABAY
AREA D
FIGURE
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LOG OF BORIN6 BH-1
Equipment SohdFlight Auger/Rotary wash
Elevation 297 ± ~- Date 12!29/94
increase in gravel
hard dulling (~ 45'-47.8'
cuttings are sandstone
olive black to olive brown; hard; very strong
some zones of hard dnlting ~ 50'-54.5'
5 5 easy dulling C~ 54.5'
cuttings are green gray and
red-brown, clayey
hard dulling (d 57'-575'
~ 59' -clay, yellow gray; plastic
6O ~s
ssss
s FRANCISCAN MEt.ANGE
ssss
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s s sandstone and stltstone
s
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s olive brown w/clay seams
PB-12 ssss
ss s light olive gray
PB-13 ssss s
65 Boring terminated ~ 642'
70
75
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GeolResource Consu{tams, Inc.
~~ Geologists f Engineers f Environmental Scientists
Job No 1866-002 pppr:~D Date ~~~-
LOG OF BORING BH-1 FIGURE
TERRABAY 2 ~J
AREA D
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6
~ ~ Equipment Rotary Mash
n ~ 297±ft. 12/30/94
Elevation Date
0 GRAVELLY SANDY CLAY (CL)
red-brown; stiff; moist to wet; bw plasticity;
grades to brown, minor red-brown, mottled
rocky dolling Q 4.5'
5 mostly sandstone, gavel is olive black and
red-brown; subangular to subround; deeply to
moderately weathered
coarse black sand; triable, probable Mn02
10 --{ ~ rocky dolling Q 10.5'
1 S rocky dolling Q 15.7'
hard drilling Q 17'-18' fractured ehert Q 18.4'
SANDY CLAY (CL) w/some gravel
orange brown mottled w/light gay; stiff; moist
20 to wet; light gray cby, less sand, more plasti~n
near-veritcal contact from 18.9'-26.5'
25
30
hard dotting Q 26.5' -sandstone boulder,
olive gray; very strong; moderately hard
hard dolling Q 32'-34 ± ,then
easier dolling to 36'
cuttings from 34'-36' in gray clay
35 refusal fi rock (boulder) @ 36'
w/Pitcher sorrel sampler
CLAYEY SAND (SC) AND GRAVEL (GC)
olive gay w /yellow brown; mottled; dense
40 _~L
GeolResource Consultants, Inc.
~~ Geoloflists f Enflineers ! Environanentd Scientists
Job No. 1866-002 pppr:~~Date 7 !s'
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LOG OF BORING BH-2
TERRABAY
AREA D
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a N ~ ~ ~ ~
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m E c°v o ... v, ~ o ~ Elevation 297 *_ tt. Date 12/30/94
Pe PB-6 40
hard dulling Q 41 '
•asy dulling Q 42'-43'
:. hard dulling Q 44'
45
Bulk
5 0 FRANCISCAN MELANGE
dark gray shale
Boring terminated Q 49.5'
55
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~~~ Geologists 1 Engineers ! Envirotunentd Scientists
Job No. ~ 866-002 Appr:~Date ~ ~
LOG OF BORING BH-2
TERRABAY
AREA D
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~T~JIFIED SOIL CLASSIFICATION SYSTEM
MAJOR DIVISIONS SOIL DESCRIPT.I.ON
clean gravels GW t~~ Yell Graded Gravels, Gravel -Sand Mixtures
GRAVELS with little or `
~ ~,,
„ over half of nv fines GP Poorly Graded Gravek, Gravel -Sand Mixtures
~
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g
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c No. 4 sieve
over 1296
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Q L
~ ~ clean sands SW r=' Yen Graded Sands Gravelly Sands
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$ SANDS with Mtle or
o ~
~ over halt of
f
ti nO fines SP Poorly Graded Sands, Gravelly Sands
coarse
rac
on
fmr than sands Mrith SM Silty Sands, Poorly Graded Sand - Silt Mixtures
° No. 4 sieve over 1296
fxks SC Clayey Sands, Poorly Graded Sang - Ciay Mixtures
} SILTS AND CLAYS ML Silts, Very Fine Sands, Silty or Clayey Fine Sands
v
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C N liquid limit less than 50
. CE Low Plasticity Clays, Sandy or Slty Clays
N
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Low Plasticity Organic Silts and Clays
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~ ~ SI LTS AND CLAYS MH Micaceous ~ Diatomaceous Silts, Volcanic Ash, Elastic S,
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CH
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Plastic' s -Fat s
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• • •
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High Plasticity OrgSnie Sits and Clays
° HIGHLY ORGANIC SOILS Pt Peat an0 Other Fibrous Organio Soils
KEY TO SAMPLES
Modified California
Indicates depth of sampling
with no recovery
Hydraulically pushed
Shelby Tube
Indicates depth of Standard
Penetration Test and 2" sample
KEY TO TEST DATA
Shear Strength, psf
Confining Pressure or Normal Load, psf
TxW 750 (2600) Unconsolidated Undrained Triaxial
TxCU 540 (2600) Consolidated Undrained Triaxial
TxCD 800 (2600) Consolidated Drained Triaxial
DS 500 (2000) Direct Shear
~u ~2 400 Shear Strength obtained from
Unconfsxd Compression Test
P 1 =Plasticity Index
C =Consolidation Test
~~~~ Geo/Resource Consultants, Inc ~~~E
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HYDRAUGER LOCATION MAP
AREA D - TERRABAY PROJECT -
SOUTH SAN FRANCISCO, CALIFORNIA
~ - ' '-"" "' 'T' ELEVATION IN FEET
' '-"" "' 'T' ELEVATION IN FEET
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TABLE 2
HYDRAUGER READINGS
AREA D - TERRABAY
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Measured Flan Rats
_, Hydrauger Number {estimated ia;$all°zts k~'~~~..... ... <''~'>`">>`..:.....
1,(20(9Sr~ .
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$f?l~S , ?151
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Upper Slide
# 11 Steady flow, leaking SS
# 18 ~ 1040
# 19 180
# 20 335
# 21 Steady thin flow 30
# 22 125 175 160
Lower Slide
# 5 Moist No drip
# 6 Moist No drip
# 7 Dry Dry
# 8 Steady trickle 50 50
# 9 20 40 30
# 10 30 50 SO
# 25 Drip 25 No flow
# 26 90 175 150
# 27 90 125 8S
# 28 135 230 190
# 29 Flow 145 140
# 30 Drip Slow drip Slow drip
# 31 l?ulsing drip Slow drip No drip
Pad Level
# 15 20
# A (lot 109) 25 30
# B (lot 109) Dry
Notes: Recent slope work @ 14/15 has covered hydraugers; #14 was not located. Calculated flows were
rounded to the nearest Sgpd.
~ cns~:ts~rz .r Geo/Resource Consultants, Inc.
TABLE 3
SifMMARY OF STABILITY RESULTS
AREA D - TERRABAY
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Remedial Sfitic Factror- Seismic Factor Scope a€, ' Potential for:.
Constructability
Alternative of Safety of Safety Constnxxion ProblEms
1) Recommended 1.6 Not presented Extensive -Rebuild High
Leighton and Associates debris basin, deepening
Plan keyway at base of slope,
and providing second
key at midslope area
2) Setback from 1.1 to 12 *w/o - No appr~eciabte Low
Existing Slope groundwater construction.
3) Construct Tce 1.3 to 1.4 0.99 to 1.03 Approx. 30,000 cubic Low
Buttress yards of material
required
4) Preferred Alternative 1.6 1.2 Approx. 45,000 cubic Moderate
Remove 10 to 20 feet of yards removed from
Slide Mass and slope, approx. 20,000
Construct Shear Key** cubic yards reworked for
key at base of slope,
install drain in base of
keyway, replace surface
drains, repair
hydroaugers.
' All cases, other than this one, included a groundwater level similar to that assumed by L&A.
* * See attached stability section showing location of keyway and depth of soil to be removed
y 1 ~ ,r Geo/Resource Consultants, Inc.
APPENDIX A
i~~
Sterling Pacific
Job No. 95125.10
July 24, 1995
Page 3
General Site GeoloQy
General site geology consisted of various types of surficial soils (slopewash, slope debris,
topsoil, colluvium, etc.) overlying formational materials consisting of melange, sandstone, shale
and metamorphic and igneous rocks in a variety of weathering conditions. The presence of
several landslides, in-filled buried valleys, and geologic contacts both depositional and fault
related in nature have added to the complexity of the geology and grading conditions.
The geologic conditions encountered are considered to be substantially as anticipated in the
"Geotechnical Engineering Investigation Report for the proposed Terrabay Village and Terrabay
Park, Neighborhoods A and B" . The areas of significant difference were in landslide "C" where
a buried valley was encountered, and in area of the cut slope along Pazkridge/Skypazk (Goatfarm
Cut Slope). These areas aze discussed in~ further detail in the following text and in the
referenced reports.
Slope Stability
During grading cut slopes were observed by representatives of both PSC and RFA. Based on
our slope stability analysis and field observation, it is our opinion that the cut and fill slopes aze
grossly stable against deep seated failures.
Natural slopes prone to surficial instability have had debris basins, debris walls or debris fences
installed or grading performed to reduce future instability related problems. Surficial slope
stability of properly vegetated and irrigated graded slopes should be adequate. Slopes which are
not planted such as slopes for split level lots are more susceptible to erosion. Maintenance of
drainage devices on slopes, vegetation and proper watering techniques are imperative to future
slope performance.
Landslides/Buried Valleys
Three areas with landslides were addressed during the grading of the Village. Landslide "A"
was located on the west side of the Village at the end of Baycrest Way. Remediation in this azea
consisted of the excavation of a deep keyway and construction of a high density buttress fill with
a minimum relative compaction of 95 percent. As designed, a drain system was installed in the
11L
Sterling Pacific
Job No. 95125.10
July 24, 1995
Page 4
keyway and within the backcut for the buttress fill. This system of drains discharges into the
adjacent controlled drainage systems.
Landslide "B" is located to the north of Highcrest Lane. To aid in the stabilization of the soils
associated with this area, a tie back retaining wall designated Landslide "B" Retaining Wall was
constructed. Memos issued on October 21, 1989 by Jacobs Associates and on October 25, 1989
by PSC Associates recommended a reduction in pier depth to a minimum embedment of 8 feet
into "hard" bedrock and 18 feet below the landslide plane. These recommendations were
approved by Roger Foott Associates in a memo dated October 27, 1989. The area below the
wall received additional fill soils to attain the proposed finished grades and to aid in stabilizing
the slide. The area above the wall was graded to help seal the surface against infiltration of
surficial water.
Landslide "C" involved extensive grading to remove and replace the majority of the slide
materials. A deep keyway and a detailed drainage system were included in the construction as
part of the buttress design. A separate memorandum presenting the construction details and the
stability calculations was prepared separately entitled "Landslide "C" Repair Program date
October 11, 1989" . In addition, a separate memo from Roger Foott Associates was issued
which accepted the repair work for Landslide "C" .
A soil nailing wall was installed in the area of Lots 162 through 168. This wall was adesign-
built-system by Schnabel Construction. The wall was constructed to provide sufficient space for
the proposed buildings and to maintain the street width. (A difference between field survey and
the proposed grading resulted in loss of some horizontal distance). The wall consisted of drilled
shafts with 10 foot long bars grouted into place. The bars were tied into a shotcreted wall face
providing support for the vertical cut. Drainage behind the wall face was provided by vertical
strip drains which were connected to a horizontal collector drain at the base of the wall. This
system is discharged to a controlled drainage device. Maximum wall height is on the order of
8 feet with approximately 7 feet or less exposed.
Retaining wall "E" was constructed along the eastern end of the site along the north side of
Hillside Boulevard from approximately Stations 139+40 to 149+59. The wall was extended
from the original plans to adjust for some topographic and survey differences which were
recognized during construction. The wall varies in height to a maximum of approximately 4'9".
The wall is topped by a V-ditch which is discharged through vertical drains into controlled
drainage devices.
I ~~
Sterling Pacific
Job No. 95125.10
July 24, 1995
Page 5
Rock Fill/Boulder Fill
A deep canyon located at the west end of the Village was designated for a rock fill area due to
its depth relative to the proposed finished grade of the fill. The original plan was a controlled
boulder fill program using oversize rocks (3 foot minus).
Grading began with the removal of loose surficial soils, vegetations and other deleterious
materials. A canyon subdrain was installed and a toe key drain was installed. As fill was
placed in this area, loose material and vegetation were removed from the sites of the canyon and
benches were cut. Prior to placing any fill the bottom of the canyon was covered by a layer of
filter fabric. When the fill operation was started, it was observed that the available oversize
rocks were more weathered than anticipated and were breaking down during compaction
resulting in a significant quantity of fines. Therefore, a change in the design and construction
was implemented subsequent to a program of Field Plate Bearing Tests. The Plate Bearing Test
program is discussed in a subsequent section of this report.
The change included removal of the original fill followed by placing a lift of oversize rock
mixed with fill from the 'PG & E cut'(18 inch minus material). Abundant water was added to
aid in filling the voids and in breaking down the oversize rock. Placement, blending and
compaction was performed by heavy equipment such as Caterpillar D-8 bulldozers, 825
compactors and loaded 631 and 637 scrapers. It was then graded to a crown, directing flow into
perimeter drains and was then covered by a blanket drain which consisted of a 6-inch layer of
gravel covered by a layer of filter fabric. These blanket drains were placed at regular vertical
increments and locations as the fill progressed. Various minimum compaction standards were
utilized for different zones of fill depending on overburden. Fills of more than 40 feet in
thickness were compacted to at least 98 percent relative compaction, fills of 20 to 40 feet were
compacted to at least 95 percent relative compaction and fills of 20 feet or less were compacted
to at least 90 percent relative compaction.
Plate Load Bearing Tests
As discussed above, a plate load bearing testing program was performed on the rock fill to aid
in determining the effectiveness of the compaction procedures and required densities. The
testing procedure included the preparation of a test fill at a variety of moisture contents and
relative compactions. The test were performed on a test pad approximately 60 feet in length,
I ~~
Sterling Pacific
Job No. 95125.10
July 24, 1995
Page 6
25 feet in width and 4 feet in thickness. The plate load tests were conducted using a series of
plates to distribute the load to an 18-inch diameter bearing plate. The reaction load was
provided by a Caterpillar D-8 bulldozer. The jacking system consisted of a 25-ton ram with a
dial gauge read out system. Prior to starting the testing procedure the gauge and ram were
calibrated by Consolidated Engineering Laboratories. Deflection measurements were provided
by a dial gauge and a 10-foot long reference beam.
Tests were performed on materials placed at three different conditions of: 98 percent relative
compaction at 7 percent moisture content; 94 percent relative compaction at 9.4 percent moisture
content; and 98 percent relative compaction at 13 percent moisture content (saturated for over
3 days). Simulated overburden pressures of 9,000 pounds were applied during each test.
Settlements were observed to be permanent with virtually no rebound observed on relaxing of
loads. The data is summarized below:
PERCENT MOISTURE PERCENT RELATIVE SETZ'L.EMENT AT
COMPACTION 9,000 psf
7%
9%
13%
98 % 0.06 INCH
94 % 0.14 INCH
98% 0.18 INCH
These settlements aze well within the tolerable limits for typical wood-framed construction. The
plate load testing program was approved by RFA.
Debris Basin
Debris basin No. 1 was constructed in substantial conformance with the plans and specifications
in the northwest corner of Terrabay Village. The debris basin was designed and constructed to
aid in controlling runoff and potential debris from the upslope drainage. Additional information
utilized for the design of the debris basins was presented in the report entitled "Debris Flow,
Potential Debris Flow Areas, Debris Flow Paths, Potential Debris Paths and Estimated Volume
of Debris Materials in Storm Drainage Basins", File No. A82103-04-I prepazed by PSC
Associates and dated February 15, 1983.
i~~
Sterling Pacific
Job No. 95125.10
July 24, 1995
Page 7
Current Site Conditions
PSC geotechnical engineers visited [he subject site on December 22 and 29, 1994 and July 24,
1995, to perform a reconnaissance of current conditions. The following is a summary of our
observations:
1. Minor erosional gullies have formed in several areas, particularly on cut slopes
and along some of the temporary drain pipes installed for winterization. Ponded
water was observed on some building pads.
2. Severe erosion has occurred on Lot 70, one of the proposed model home
locations. A 1-to 2-foot wide gully, about 5 to 6 feet deep has developed in the
slope between the upper and lower pads.
3. A small landslide has developed on the cut slope along the north side of Highcrest
Lane, just west of the intersection with Northcrest Drive. The slide does not
appear to be impacting any proposed building lots or pavements. The nearest lots
to this slide are about 50 feet away to the east and across the street to the south.
4. A small slump on the slope to the west of the Debris Basin No. 1 access road was
noticed. This slump has been depositing soil into the concrete ditch, which had
recently been cleaned out.
5. At the east end of Highcrest Lane, the street pavement has settled around a
manhole at the south curb above Lots 163 and 164 and the soil nailing wall which
supports this end of the street. Some cracking of the curb and sidewalk was also
noticed. The pattern of the cracking and settlement seems to indicate settlement
of the manhole backfill upon saturation from infiltrated rainwater. No evidence
of movement or cracking of the adjacent soil nailing wall was observed.
Conclusions and Recommendations
1. Based on our observations and test results during the mass grading work, it is our
professional opinion that the mass grading work was performed in substantial
conformance with the geotechnical recommendations presented in our reports. Necessary
~ 1~
Sterling Pacific
]ob No. 95125.10
July 24, 1995
Page 8
modifications which were made during site grading were approved by the City and their
consultant prior to performing these modifications.
2. Foundations should be constructed in accordance with the conclusions and
recommendations presented in the report entitled "Foundation Recommendations
Terrabay Village" reported by PSC Associates, Inc., dated February 13, 1991. Our
recent review of site conditions indicate that these recommendations are still valid for the
proposed development as currently planned and designed.
3. The small landslides, the distressed pavement on Highcrest Lane, and large gullies noted
in this report, plus any more that may develop prior to development should be repaired
in accordance with the grading recommendations presented in our previous reports.
4. Even though most of the loose rocks above the cut slope have been removed, yearly
reconnaissance of large rocks above these slopes should be performed. Any unstable
rocks should be removed or stabilized.
5. A program of annual monitoring and maintenance of slopes, subdrains, debris basins, and
erosion control measures should be implemented until development is completed.
6. Some of the building pads are covered with grass and shrubs. Near surface soils have
undergone several cycles of wetting and drying since the rough grading. Final site
grading consisting of clearing of vegetation, scarification and recompaction of the upper
several inches of the pad soils will be required prior to the start of foundation
construction.
7. All final site grading and foundation construction must be observed and tested by a
representative of PSC. The conclusions and recommendations contained in this report
are contingent on this provision.
8. Any changes to the final grading or foundation plans should be reviewed by our office.
9. Good surface drainage is imperative to the future performance of the site. Positive
measures should be taken to properly finish grade the building pads after the structures
and other improvements are in place to reduce the potential for differential soil
movement, erosion and subsurface seepage. Drainage water from the lot and adjacent
I ~~
Sterling Pacific
Job No. 95125.10
July 24, 1995
Page 9
properties should be directed off the lot and to the street away from the foundations and
the top of the slopes. Experience has shown that even with these provisions, a shallow
groundwater or subsurface water condition can develop in areas where no such water
condition existed prior to site development. Proposed subdrainage systems around the
structures should help alleviate such conditions.
10. It is recommended that homeowners be provided with a copy of the attached Appendix
A "Suggested Guidelines for Maintenance of Hillside Homesites for Slope Stability and
Erosion" or a similar document. This helps the individual homeowner to understand the
importance of hillside maintenance. '
Limitations
Our professional opinions and recommendations contained herein were made in accordance with
generally accepted geotechnical engineering principles and practices and are based on our
previous work for the project and a site reconnaissance and the assumption that the soil
conditions do not deviate from the observed conditions. All work done is in accordance with
generally accepted geotechnical engineering principles and practices. No warranty, expressed
or implied, of merchantability or fitness, is made or intended in connection with our work by
the furnishing of oral or written reports or findings.
The recommendations and conclusions contained herein shall be considered valid only if PSC
Associates, Inc. is retained to review any changes to the plans and to monitor and test all
geotechnical related construction, including final site grading, repairs of existing erosion and
landslides and foundation construction. If these services are performed by others, the
conclusions and recommendations contained herein will be considered null and void and invalid.
'This report has been prepared for the proposed Terrabay Village to assist in the current
evaluation of the property and to assist the architect and engineer in the design of this project.
Fn the event any changes in the design or location of facilities are planned, or if any variations
or undesirable conditions are encountered during construction, our conclusions and
recommendations shall not be considered valid unless the changes or variations are reviewed and
our recommendations modified or approved by us in writing.
i~~
Sterling Pacific
Job No. 95125.10
July 24, 1995
Page 10
This report is issued with the understanding that it is the owner's responsibility to ensure that
the information and recommendations contained herein are called to the attention of the designer
for the project, and that the necessary steps are taken to see that the recommendations are
carried out in the field. Should ownership of this property change hands, the new owner should
be informed of the existence of this report.
The findings in this report are valid as of the present date. However, changes in the conditions
of the property can occur with the passage of time, whether they result from legislation or from
the broadening of knowledge. Accordingly, the findings in this report might be invalidated
wholly or partially, by changes outside of our control. Therefore, this report is subject to
review by the controlling governmental agencies and is valid for a period of one year.
Respectfully submitted,
PSC ASSOCIATES, INC.
Daniel . O'Conn 11, P.E., G.E.
Principal Engineer
DPO:mc
Enclosures: References
Appendix A
c:\wps l~rpt<om\95 vs IO.n4
-~~
LIST OF REFERENCES
1. "Additional Details for use in the Final Site Grading Plans for Neighborhoods A and B,
Terrabay Development, South San Francisco, California', Job No. 83103.10, prepared
by PSC Associates, Inc., dated September 27, 1984.
2. "Additional Slope Stability Analyses, Terrabay Development, South San Francisco,
California, A Development by W.W. Dean & Associates for Resources Engineering and
Management, Grading Design for Terrabay Village and Terrabay Park (Neighborhoods
A and B)", Job No. A83103-01, prepared by PSC Associates, Inc., dated March 20,
1984.
3. "Final Report -Slope Monitoring Services at the Tie-back Soldier Beam Retaining Walls
for Landslides 'B' and 'D", Job No. 83103.31, prepared by PSC Associates, Inc., dated
November 20, 1987.
4. "Geotechnical Engineering Investigation Report, Grading Design for the Proposed
Terrabay Village and Terrabay Park, Neighborhoods A and B, Terrabay Development,
South San Francisco, California", Job No. A83103-01, prepared by PSC Associates,
Inc., dated November 15, 1983.
5. "Geotechncial Engineering Investigation, Proposed Tieback Retaining Walls at Landslides
"B" and "D", Terrabay Development, South San Francisco, California", Job No.
83103.31, prepared by PSC Associates, Inc., dated July 31, 1985.
6. "Geotechnical Engineering Investigation, Recommendations for Tieback Retaining Walls
at Landslides "B" & "D", Terrabay Development, South San Francisco, California,
Addendum -1", Job No. 83103.31 prepared by PSC Associates, Inc., dated May 15,
1989.
7. "Geotechnical Engineering Investigation Recommendation for Tieback Retaining Walls
at Landslide "B" and "D"", Job No. 83103.31, prepared by PSC Associates, Inc., dated
May 15, 1989.
8. Letter of "Clarification for Seismic Design Considerations Tie-back Walls at Landslides
"B" and "D"", Job No. 83103.31, prepared by PSC Associates, Inc., dated June 26,
1989.
i~~
LIST OF REFERENCES (Cont.)
9. Memorandum Regarding "Landslide "C" Repair Program", prepared by PSC Associates,
Inc., dated October 11, 1989.
10. "Debris Flow, Potential Debris Flow Paths, Potential Debris Flow Paths and Estimated
Volume of Debris Materials in Storm Drainage Basins", Job No. A82103-04-I, prepared
by PSC Associates, Inc., dated February 15, 1983.
11. "Foundation Investigation for Proposed Terrabay Village, South San Francisco,
California", Job No. 89102.11, prepared by PSC Associates, Inc., dated February 13,
1991.
12. "Geotechnical Review, Terrabay Village, Terrabay Park, Recreation Center, Terrabay
Development, South San Francisco, California", Job No. 94127.10, prepared by PSC
Associates, Inc., dated December 30, 1994.
13. "Slope Monitoring Services at the Tie-back Soldier Beam Retaining Walls for Landslides
'B' and 'D'", Job No. 83103.31, prepared by PSC Associates, Inc., dated February 23,
1987.
14. "Supplemental Geotechnical Evaluation of Completed "Goat Farm" Cut Slopes, Terrabay
Development -Phase I, South San Francisco, California", Job No. 89140.20, prepared
by PSC Associates, Inc., dated November 29, 1991.
I ~4
APPENDIX A
INCLINOMETER DATA
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APPENDIX B
SLOPE STABILITY RESULTS
ALTERNATIVE 4
Slope stability analysis was based on SPENCER's method using Computer Code TLLOPE
developed by TAGA Engineering Software Services in Lafayette, California. The cases
evaluated aze listed below.
• - (1) Static Condition -low water table, 7 feet below existing ground.
(2) Seismic Condition -low water table, 7 feet below existing ground.
(;) Static Condition -high water table, at finished ground surface.
(4) Seismic Condition -keyway backslopes high water table, at finished ground surface.
(5) Static Condition -during construction, assuming no water.
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ADDENDUM
"AREA D" SLOPE STABILITY
ANALYSIS AND REMEDIATION
PHASE I, TERRABAY PROJECT
SOUTH SAN FRANCISCO, CALIFORNIA
SEPTEMBER 1995
1866-002
PREPARED FOR:
SunChase GA Calif I, Inc.
6001 North 24th Street, Suite A
Phoenix, Arizona 85016
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GEOLOGISTS/ENGINEERS~ENV)RONMENTALSCIENTISTS
505 BEACH STREET, SAN FRANCISCO, CALIFORNIA 94113
Geo/Resource Consultants, Inc.
GEOLOGISTS ENGINEERS 'ENVIRONMENTAL SCIENTISTS
Corpcra:e lieadgw~rs
sos Beach Street
San Francesco. California 94133
(415) 775-3177 FAX (415) 7752359
°eg~onal OH:CeS An20na Cahfoma Hawau Yryr+a
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September 5, 1995
1866-002
SunChase GA Calif I, Inc.
6001 North 24th Street, Suite A
Phoenix, Arizona 85016
Attention: Mr. Larry Harris
RE: SUPPLEMENTARY REMEDIAL MEASURES
AREA "D" SLOPE STABILITY
PHASE 1 TERRABAY PROJECT
SOUTH SAN FRANCISCO, CALIFORNIA
Ladies and Gentlemen:
This letter report presents additional remedial measures to enhance the stability of Area "D" in
the Terrabay project referenced above. In our report dated July 26, 1995, Geo/Resource
Consultants, Inc. (GRC) evaluated five alternatives for remediation including the alternative
proposed by Leighton and Associates (L&A). Out of these five alternatives, we believe
Alternative 4, which consists of removing a thickness of 10 to 20 feet of slide material across the
entire slide and constructing a shear key at the bottom of the slope, is the most desirable
alternative. This alternative actually removes some of the driving forces from the potential slide
and improves the constructability of the required sheaz keys.. The computed factor safety of this
alternative is equivalent to the previous L&A plan, which has been accepted by the City.
Upon further review by various parties, including L&A, while Alternative 4 satisfies the static
factor of safety requirement of 1.5 for considering potential slide movement below the bottom
sheaz key, it did not meet the requirement in considering shallower slides above the bottom sheaz
key. To achieve the same level of factor of safety for all modes of potential failure, we propose
to lower the groundwater level within the existing inactive slide deposits to below the slide
plane, approximately 20 feet below the finished slope grade. To accomplish this goal, GRC
recommends two additional drainage keyways be established within the existing inactive slide
deposits. These two drainage keyways will be located across the entire width of Area D and
extend to below the slide plane, at the approximate locations shown on the profile attached as
Figure 1.
The depth of the drainage keyway excavations is anticipated to be about 20 feet, with temporary
construction slopes to be determined during grading. Drainage will be provided by placing a
minimum 6-inch-diameter perforated pipe covered by Caltrans Class II permeable drain rock, or
Z GD100:t866-L1
September 5, 1995
1866-002
Page 2
,.
alternatively, wrapped by crushed rock and filter fabric, and sloped to drain to the Swale
immediately north of Area "D". The keyways will be constructed of compacted fill, placed at
90% relative compaction in accordance with our report. The locations of these two cross slide
drainage keyways were selected to effectively intercept subsurface water infiltration in the
remaining potential slide mass. The placement of these two keyways also enhances overall
stability by replacing the slide plane and slide mass with engineered fill.
Based on the above drainage measures added to the Alternative 4, the safety factors were
computed to be 1.8 for static condition and 1.3 for seismic condition for potential slides above
the bottom shear key. The details of the analytical data are shown in the appendix of this letter
report.
In our opinion, placement of these keyway drains will eliminate the need for additional
hydraugers; existing hydraugers, where salvaged during construction, may be rehabilitated and
used. Selected piezometers and inclinometers should be preserved for long term monitoring of
Area "D".
We trust that we have provided the information you need at this time.
Very truly yours,
GEO/RESOURCE CONSULTANTS, INC.
Jam'"
Glenn A. Romig, P.E., G.E.
Geotechnical Engineer
~~ ~ ~ /v
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Eric S. Ng, P.E., .E.
Principal Engineer
Alan D. Tryho .G.
Senior Vice President
ADT/ESN/GAR:csc
Attachment: Figure 1
Appendix
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APPENDIX
SLOPE STABII.ITY RESULTS
ALTERNATIVE 4
Slope stability analysis was based on SPENCER's method using Computer Code TLLOPE
developed by TAGA Engineering Softwaze Services in Lafayette, California. The cases
evaluated aze listed below.
(1) Static Condition -low water table
(2) Seismic Condition -low water table
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GRADING REPORT SHOWING
SUMMARY OF TESTING AND OBSERVATION
SERVICES DURING THE MASS GRADING
OF 'I'ERRABAY VII,LAGE AT TERRABAY
DEVELOPMENT, SOUTH SAN FRANCLSCO, CA
FOR
Sterling Pacif c Management Services, Inc.
6001 N. 24th Street, Suite A
Phoenix, AZ 85016
n1~
ASSOCIATES INC
Geotechnfca/ & Env/ronmental Consultants
Construction Materla/s Test/ng ServJces
CORPORATE HEADQiJARTE1tS
1185 Terra Bella, P.O. Box 699
Mountain View, CA 94042-0699
Ph: (41~ 969-1144 • Fax: (41~ 969-5523
Job No. 95125.10 July 24, 1995
GRADING REPORT SHOWING
SLTMIVIARY OF TESTING AND OBSERVATION
SERVICES DURING THE MASS GRADING
OF TERRABAY VII.,LAGE AT TERRABAY DEVELOPMENT
SOUTH SAN FRANCISCO, CALIFORNIA
FOR
STERLING PACIFIC MANAGEMENT SERVICES, INC.
Introduction
This report presents a summary of the observations and testing services provided by PSC
Associates, Inc. (PSC) during the mass grading operations for the proposed Terrabay Village
at the Terrabay Development in the City of South San Francisco. Our services were primarily
provided between June, 1989, and December, 1990. We also performed recent observations of
site conditions in December, 1994, and July, 1995, as descn~bed in this report.
The grading contractor for the mass grading phase of the project was Piombo Construction
Company. Project plans were prepared by C-REM Engineers (now known as KLH Engineers,
Inc.) and are entitled "Stage I Grading, Rough Grading Plans for Terrabay Development", dated
April 17, 1989. Grading was performed in substantial conformance with the project plans and
Stage I Grading Specifications, also by C-REM. Any modifications were approved by the City
during the construction.
A representative of Roger Foott Associates (RFA), the City's geotechnical consultant, was
present on a full time basis during the mass grading. RFA also reviewed and approved the
various geotechnical engineering designs and construction methods. RFA was recently acquired
by GEI Consultants.
Project Description
Terrabay Village (Village) is one of the two residential units completed during the Phase I
grading of the Terrabay Development. The Village is located at the west end of the Terrabay
site. It is bounded by Hillside Boulevard Extension on the south, the South San Francisco Drive
Entrance on the east, open space to the north and the Hillside School to the west. Grading for
the Village entailed the construction of 169 building pads and several interior streets. Elevations
on the site range from a low of approximately 190 feet (NiSL) near the southeast corner of the
site to a high of approximately 525 feet (MSL) located near the north central portion of the site.
Prior to grading, the natural topography of the site sloped towards the south, with two main
drainage courses located on the east and west ends. Fill slopes constructed during the grading
of the Village were at inclinations of 2:1 (horizontal to vertical), with maximum vertical distance
of about 160 feet between the toe and top of fill slopes, and maximum thickness of about 60
I ~~
Sterling Pacific
Job No. 95125.10
July 24, 1995
Page 2
feet. Cut slopes constructed had maximum heights on the order of 80 and 40 feet with
inclinations of 2:1 and 1 3/4:1 (horizontal to vertical), respectively. Additional site grading
partially filled the major eastern drainage course and a desilting basin was installed at the upper
grading extent in the western drainage course.
Geotechnical conditions within the Village were presented prior to grading in the "Geotechnical
Engineering Investigation Report, Grading Design for the Proposed Terrabay Village and
Terrabay Park, Neighborhoods A and B, Terrabay Development, South San Francisco,
California, Job No. A83103-01", prepared by PSC Associates (Reference 1). Actual conditions
were reviewed during the grading and supplemental geotechnical investigations and analyses
were performed as the conditions encountered warranted.
Several areas with special geotechnical or design conditions were encountered during grading.
These areas included landslide stabilization or removal, rock cuts, retaining walls, high density
fills, toe keys and subsurface drains. In general, the extent of these areas was indicated in the
previously referenced report or aze discussed in this report. These areas are shown on the
Grading Operation Guidelines Index Map, which is part of the C-REM rough grading plans.
Grading Summary
Grading generally began with the removal of brush and vegetation from the areas to be graded.
Loose surficial soils were removed to firm natural ground in areas to receive fill. Prior to
placing fill, the exposed ground surface was scarified, moisture conditioned and compacted. Fill
soils derived from onsite cutting operations were then placed and compacted in layers until
design elevations were obtained.
During the grading operations, compaction procedures were observed and in-place density tests
(ASTM D 2922-81) were performed to evaluate the relative compaction of the fill. Field
observations and the results of the in-place density tests indicated that the fill was generally
compacted to design specifications of at least 90 percent or 95 percent relative compaction
depending on the specific requirements. Laboratory tests were performed on samples of the
materials used for fill to evaluate moisture-density relationships, optimum moisture content and
maximum dry density (ASTM D-1557-78). Results of all field and laboratory tests aze tabulated.
in our project files and are available for review.
I ~~
GRADING REPORT SHOWING
SUMMARY OF TESTING AND OBSERVATION
SERVICES DURING THE MASS GRADING
OF 'I'ERRABAY PARS AT TERRABAY
DEVELOPMENT, SOUTH SAN FRANCISCO, CA
FOR
Sterling Pacific Management Services, Inc.
6001 N. 24th Street, Suite A
Phoenix, AZ 85016
n1~
Geotechn/ca/ & Envlionmenta/ Consultants
Construction Mate~la/s Testing Services
CORPORATE HEADQUARTERS
1185 Terra Bella, P.O. Box 699
Mountain View, CA 94042-0699
Ph: (415) 969-1144 • Fax: (415) 969-5523
Job No. 95125.10 July 27, 1995
GRADING REPORT SHOWING
SitMMARY OF TESTING AND OBSERVATION
SERVICES DURING THE MASS GRADING
OF 'I'ERRABAY PARK AT TERRABAY DEVELOPMENT
SOUTH SAN FRANCISCO, CALIFORNIA
FOR
STERLING PACIFIC MANAGEMENT SERVICES, INC.
Introduction
This report presents a summary of the observations and testing services provided by PSC
Associates, Inc. (PSC) during the mass grading operations for the proposed Terrabay Park at
the Terrabay Development in the City of South San Francisco. Our services were primarily
provided between June, 1989, and December, 1990. We also performed recent observations of
site conditions in December, 1994, and May, 1995, as described in this report.
The grading contractor for the mass grading phase of the project was Piombo Construction
Company. Project plans were prepared by C-REM Engineers (now known as KLH Engineers,
Inc.) and are entitled Stage I Grading, Rough Grading Plans for Terrabay Development", dated
April 17, 1989. Grading was performed in substantial conformance with the project plans and
Stage I Grading Specifications, also by C-REM. Any modifications were approved by the City
during the construction.
A representative of Roger Foott Associates (RFA), the City's geotechnical consultant, was
present on a full time basis during the mass grading. RFA also reviewed and approved the
various geotechnical engineering designs and construction methods. RFA was recently acquired
by GEI Consultants.
Terrabay Park encompasses an area which includes Landslide "D", a major landslide that has
not yet been repaired. A supplemental investigation report by Leighton and Associates dated
January 28, 1992, presented a scheme to repair the slide. Another firm, GeoResource
Consultants, performed a recent investigation of the slide. These reports should be consulted
for conclusions and recommendations regarding the stability of the slide area and development
of the adjacent lots.
Proiect Descriution
Terrabay Park is one of the two residential units that was mass graded as part of the Phase I
grading. Grading was performed to provide building pads for 125 single-family residential units
and the interior streets. Terrabay Park is located in the northwestern portion of the site. It is
bounded on the north by open space, on the south by the alignment of South San Francisco
-~~
Sterling Pacific
Job No. 95125.10
July 27, 1995
Page 2
Drive, on the west by Terrabay Village and on the east by the alignment of Gree~azk Terrace
and undeveloped land. General site topography slopes towards the south. Elevations within the
site range from a high of approximately 500 feet (MSL) near the east side of Terrabay Park to
a low of approximately 210 feet (MSL) located on the south side of the site adjacent to South
San Francisco Drive. Prior to grading, the site was dissected by three main drainage courses
which all trended southeast. Grading resulted in the construction of fill slopes with maximum
vertical distance of approximately 210 feet or less at inclinations of 2.0 to 1.0 (horizontal to
vertical) and cut slopes of 60 feet or less in height with inclinations of 1.5 to 1.0 (horizontal to
vertical) or less. The thickness of fill was generally less than 30 feet.
Geotechnical conditions within the Park were presented prior to grading in the "Geotechnical
Engineering Investigation Report, Grading Design for the Proposed Terrabay Village and
Terrabay Park, Neighborhoods A and B, Terrabay Development, South San Francisco,
California, Job No. A83103-01", prepared by PSC Associates. Actual conditions were reviewed
during the grading and supplemental geotechnical investigations and analyses were performed
as the conditions encountered warranted.
Several areas with special geotechnical or design conditions were encountered during grading.
These azeas included landslide stabilization or removal, rock cuts, retaining walls, high density
fills, toe keys and subsurface drains. In general, the extent of these areas was indicated in the
previously referenced report or are discussed in this report. These areas are shown on the
Grading Operation Guidelines Index Map, which is part of the C-REM rough grading plans.
Grading Summary
Grading generally began with the removal of brush and vegetation from the areas to be graded.
Loose surficial soils were removed to firm natural ground in areas to receive fill. Prior to
placing fill, the exposed ground surface was scarified, moisture conditioned and compacted. Fill
soils derived from onsite cutting operations were then placed and compacted in layers until
design elevations were obtained.
During the grading operations, compaction procedures were observed and in-place density tests
(ASTM D 2922-81) were performed to evaluate the relative compaction of the fill. Field
observations and the results of the in-place density tests indicated that the fill was generally
compacted to design specifications of at least 90 percent or 95 percent relative compaction
i~~
Sterling Pacific
Job No. 95125.10
July 27, 1995
Page 3
depending on the specific requirements. Laboratory tests were performed on samples of the
materials used for fill to evaluate moisture-density relationships, optimum moisture content and
maximum dry density (ASTM D-1557-78). Results of all field and laboratory tests are tabulated
in our project files and are available for review.
General Site Geology
General site geology consisted of various types of su~cial soils (slopewash, slope debris,
topsoil, colluvium, etc.) overlying formational materials consisting of melange, sandstone, shale
and metamorphic and igneous rocks in a variety of weathering conditions. The presence of
several landslides, in-filled buried valleys, and geologic contacts both depositional and fault
related in nature have added to the complexity of the geology and grading conditions.
The geologic conditions encountered are considered to be substantially as anticipated in the
"Geotechnical Engineering Investigation Report for the proposed Terrabay Village and Terrabay
Park, Neighborhoods A and B" . The areas of significant difference were in landslide "D" and
^R" where buried valleys were encountered. These areas are discussed in further detail in the
following text and in the referenced reports.
The presence of a shear zone was determined during investigation of the site in the vicinity of
Parkridge Circle and Sky Park Circle. This shear zone is believed to be associated with the
Hillside Fault and may represent the northern most splay of the fault zone. However, the named
trace of the Hillside Fault was previously mapped by M.G. Bonilla (1971, USGS MF-311) 500
to 900 feet to the south of this shear zone. No evidence to suggest that this shear zone should
be classified as an active fault was observed during the grading of the site. However, this site
(as well as any other site in this area of California) should be considered to be subject to strong
ground motion as a result of future seismic events originating on any of the nearby major faults.
im act of this shear zone on the project is its potential to impede subsurface water
The primary p
flow. This has been recognized and whenever possible appropriate drainage measures have been
implemented during the grading operation. No other faults or indications of faults were
observed during grading.
I ~~
Sterling Pacific
Job No. 95125.10
July 27, 1995
Page 4
Sloe Stability
During grading cut slopes were observed by representatives of both PSC and RFA. Based on
our slope stability analysis and field observation, it is our opinion that the cut and fill slopes
other than those in the vicinity of Landslide "D" and the adjacent lots, are grossly stable against
deep seated failures.
Natural slopes prone to surficial instability have had debris basins, debris walls or debris fences
installed or grading performed to reduce future instability related problems. Surficial slope
stability of properly vegetated and irrigated graded slopes should be adequate. Slopes which are
not planted such as slopes for split level lots are more susceptible to erosion. Maintenance of
drainage devices on slopes, vegetation and proper watering techniques are imperative to future
slope performance.
Site Monitoring
Slopes which exhibited seasonal seepage were perforated with horizontal drains (hydraugers).
Slope inclinometers were placed in several of the major cut slopes, in Landslide "R" and
Landslide "D" to provide monitoring of slope stability. Piezometers were also located in
Landslide "D" to monitor water levels. Monitoring of Landslide "D" has been performed by
Leighton Associates and GeoResource Consultants as part of their supplemental investigations.
Landslides/Buried Valleys
Two areas of landslides were addressed during the grading of the Terrabay Park.
Landslide "D" is located on the north side of the site near the western terminus of Parkridge
Circle. A tieback retaining wall was proposed to support the upper 6-8 feet of surficial
movements and to help contain potential debris flows. A detailed description of the design
parameters, analysis and construction of the wall are presented in the report entitled
"Geotechnical Engineering Investigation Recommendations for Tieback Retaining Walls at
Landslide "B" and "D", Terrabay Development" prepared by PSC Associates and dated May
15, 1989. This report was superseded by supplemental reports by others, as described below.
i ~~
Sterling Pacific
Job No. 95125.10
July 27, 1995
Page 5
A supplemental geotechnical investigation report of the landslide by Leighton & Associates dated
January 28, 1992, presented a scheme to repair the slide. Another geotechnical consulting firm,
GeoResource Consultants, recently performed a new evaluation of the landslide. PSC has not
been involved in any recent evaluation of the stability of Landslide "D" , and therefore this report
does not include any conclusions or recommendations regarding the potential impact of Landslide
D on the development. These supplemental investigations should be followed for further
development of this portion of the project.
The area designated as Landslide "R" was removed during grading down to the underlying
contact with the weathered rock melange. A keyway and buttress fill were then constructed tq
the proposed design grades. Construction included the installation of a drain at the heel of the
keyway and the placement of a network of horizontal drains at approximately 10 foot vertical
increments. Additional blanket drains were placed in areas where seepage was present. The
drains were connected to the controlled drainage system. The analyses and construction in the
Landslide "R" area is covered in the report entitled "Geotechnical Report on Grading Work to
Repair "Buried Valley"/Landslide "R", Terrabay Development, Phase 1" prepared by PSC
Associates, dated December 10, 1990.
Retaining Walls
Due to field survey differences, the proposed wall at the rear of the Lots 281 and 282 R-as
eliminated. Instead, a retaining wall above lots 281 and 282 was constructed. The wall consists
of 21 inch diameter drilled piers with steel H-beams in concrete. The maximum depth of the
piers is approximately 16 feet below finished grade and the height of the wall is 7.5 feet or less.
A layer of geotextile fabric was placed against the back of the wall and Class II permeable drain
rock was placed to within approx•T~hee allfacing consisted of concrete lagging placed within
constructed at the top of the wall
the flanges of the H-beams.
Canvon Underdrains
A series of subsurface canyon drains were installed in the three main drainages which traversed
the site with a southerly trend prior to grading. The canyons were cleaned out to firm native
material and a fabric wrapped gravel drain was installed. The drains consist of approximately
i ~~
Sterling Pacific
Job No. 95125.10
July 27, 1995
Page 6
9 cubic feet of 1-1/2 inch minus gravel per lineal foot with Mirafi 140 or an approved
equivalent. Additional subsurface drains in the Park included two curtain drains which extend
from the termini of Pazkridge Circle and Skypazk Circle to the intersection with Pazkgrove Drive
and from the back of lot 228 to the western terminus of Baypazk Circle. A subdrain was
extended from Landslide "R" near the northeast comer of lot 309 to the previously mentioned
curtain drain neaz northwest comer of lot 235.
Debris Basins
Three debris basins were constructed in Terrabay Park. All three are located on the north side
of the site at the mouth of the main drainage courses. Debris basin "2A" is located on the east
side of retaining wall at landslide "D" and to the east of the terminus of Pazkridge Circle.
Debris~basin "2C" is located to the north of Parkridge Circle across from lots 311 and 312. The
third debris basin is designated "3A" and is located north of Skypazk Circle across from lot 323.
The debris basins were designed and constructed to aid in controlling runoff and potential debris
from the upslope drainage. Additional information utilized for the design of the debris basins
was presented in the report entitled "Debris Flow, Potential Debris Flow Areas, Debris Flow
Paths, Potential Debris Paths and Estimated Volume of Debris Materials in Storm Drainage
Basins", File No. A82103-04-I prepared by PSC Associates and dated February 15, 1983.
Present Site Conditions
PSC engineers visited the subject site on May 25, 1995 to perform a reconnaissance of the
present conditions. On the same day, a total of 14 test pits were excavated on the building pads,
including the 4 model lots, and at some of the distressed areas, using a rubber tired backhce.
On the pads, the pits were only excavated to a depth of one foot or less to observe the depth of
vegetation and to allow testing of pad compaction at that depth to compare with the surface
readings. Deeper excavations were made on the slopes below Lot 203 and between Lots 294
and 317 to explore conditions of the shallow landslides.
I ~4
Sterling Pacific
Job No. 95125.10
July 27, 1995
Page 7
The following is a summary of our observation from the May 25, 1995 reconnaissance along
with remedial recommendations:
1. Most of the building pads are covered with native grass and occasional weeds or small
brush. Our excavations indicate the native grass does not have extensive roots and most
of the organic material can be lost from the pads during fine grading or removed from
the pads by scraping the surface with a blade from a bulldozer or grader, and should not
require stripping to remove the roots.
2. Our field density tests show the relative compaction of the near surface soils in the area
tested less than the specified 90 percent. This is likely due to the effects of weather
changes since the rough grading. In most areas tested the moisture content was well
above optimum due to the heavy rains from the past few months. These tests indicate
the pad surfaces will need to be scarified and recompacted prior to construction. Some
drying and conditioning of the soils may be required after scarification, prior to
recompaction.
3. Significant erosion was observed on a few building pads and cut slopes above the
development. Substantial gullies have developed on the cut slopes on the south side of
Green Park Terrace, the east side of the Water Tank Access Road, and west of the end
of Parkgrove Drive. Large gullies, several feet in width and depth, have developed
along the temporary drain pipes placed for winterization at the southwest corner of Lot
264, and on the slope leading between Lots 239 and 242 down to Lots 231 and 232.
Each of these areas were observed last December, with some increase in severity noted
during the present reconnaissance. Also, a new area of severe erosion was noted on the
slope between the upper and lower pad of Lot 267. Erosion of the cut slope above Lot
269 was also observed.
The erosion on the pad areas should be repaired by overexcavating all loose material and
replacement to original grades as a compacted, engineered fill. After the gullies are
filled back in, the surface drainage above should be redirected to avoid a reoccurrence
of the erosion.
Erosion on the cut slopes seems to be occurring primarily in the sandstone portion of the
bedrock despite the use of jute mesh as a preventative measure. The erosion seems to
be caused by the runoff of rainfall directly on the slopes, and not from the slopes above.
i~~
Sterling Pacific
Job No. 95125.10
July 27, 1995
Page 8
Also, no vegetation has been established on these slopes. Therefore, a better surface
treatment is needed to reduce future erosion in these areas.
The slopes should first be trimmed below the depth of the gullies, rather than trying to
fill in the gullies. Then a surface treatment either with erosion control blankets or
sprayed-on stabilization mixtures could be used. Selection of the most appropriate
method should be based on aesthetic considerations and with consultation of a local
erosion control expert.
4. The erosion has resulted w fiches of silt was not d especially o Lots 2692 671 266,
Accumulation of up to a fe
238, 237, 232, 231, 305, and 302. This silt should be removed from the pads pnor to
construction.
5. Small shallow landslides have occs Franc sco DDrive was byes rv~edllast December,sl~
below Lot 203 adjacent to South
test pit showed this slide toad area above The slide s about 30 t Osfeet wide,dand
concentrated runoff on the p
extends about 10 to 15 feet above the toe of the slope.
A small landslide was detected in our latest reconnaissance on the slope between Lots
294 and 317. Our test pit sho es bedrock t i b s sab lut 30 to 35a feet wideeeand extends
weathered and sheared melang
from the toe to the top of the slope.
These two slides can be repaired by removal and replacement as engineered fill with
subdrain systems installed, following the recommendations in our report for grading
design dated November 15, 1983.
6. The shallow landslide previously noted on the cut slope above Parkridge Circle just west.
of Debris Basin 2C has become larger, and a second slide ~ ~~ 8 ed,~ lide is~a
west. Anew landslide has occurred on the cut slope abo
shallow failure at the contact between sandstone and shale bedrock, and is about 10 to
15 wide, extending 30 to 40 feet upslope above the bench at midslope.
1 ~~
Sterling Pacific
Job No. 95125.10
July 27, 1995
Page 9
The slide areas in the cut slopes are not presently impacting the lots. The debris basin
area fence has been damaged by one Tide areaslcan be repaired by removing the slide
unpact the road in this area. These s
debris and replacement with oversize rocks and boulders.
Conclusions and Recommendations
1. Based on our observations and test results during the mass grading work, it is our
professional opinion that the mass grading work, with the exception of Landslide "D°
was performed in substantial conformance with the geotechnical recommendations
presented in our reports. Necessary modifications which were made during site grading
were approved by the City and their consultant prior to performing these modifications.
The recent reports by Leighton & Associates and GeoReources should be consulted for
the current status and proposed repair scheme for Landslide "D" and the adjacent lots.
2. Foundations should be constructed in accordance with the conclusions and
recommendations presented in the report entitled "Foundation Recommendations
Terrabay Park" reported by PSC Associates, Inc., dated May 15, 1990. Our recent
review of site conditions indicate that these recommendations are still valid for the
proposed development as currently planned and designed.
3. The small landslides and large gullies noted in this report, plus any more that may
develop prior to development should be repaired in accordance with the grading
recommendations presented herein and in our previous reports.
4. Even though most of the loose rocks~ab a slo es should be performed. mAny unstable
reconnaissance of large rocks abo Pe
rocks should be removed or stabilized.
5, A program of annual monitoring and maintenance of slopes, ~evb elo mentbiss ompleted
erosion control measures should be implemented until p
Subdrains should be checked for proper functioning and repaired or cleaned out if
necessary.
i~~
Sterling Pacific
Job No. 95125.10
July 27, 1995
Page 10
6. Some of the building pads are covered with grass and shrubs. Near surface soils have
undergone several cycles of wetting and drying since the rough grading. Final site
grading consisting of clearing of vegetation, scarification and recompaction of the upper
several inches of the pad soils will be required prior to the start of foundation
construction.
7. All final site grading and foundation construction must be observed and tested by a
representative of PSC. The conclusions and recommendations contained in this report
are contingent on this provision.
8. Any changes to the final grading or foundation plans should be reviewed by our office.
9. Good surface drainage is imperative to the future performance of the site. Positive
measures should be taken to properly finish grade the building pads after the structures
and other improvements are in place to reduce the potential for differential soil
movement, erosion and subsurface seepage. Drainage water from the lot and adjacent
properties should be directed off the lot and to the street away from the foundations and
the top of the slopes. Experience has shown that even with these provisions, a shallow
groundwater or subsurface water condition can develop in areas where no such water
condition existed prior to site development. Proposed subdrainage systems around the
structures should help alleviate such conditions.
10. It is recommended that homeowners be provided with a copy of the attached Appendix
A "Suggested Guidelines for Maintenance of Hillside Homesites for Slope Stability and
Erosion" or a similar document. This helps the individual homeowner to understand the
importance of hillside maintenance.
Limitations
Our professional opinions and recommendations contained herein were made in accordance with
generally accepted geotechnical engineering principles and practices and are based on our
previous work for the project and a site reconnaissance and the assumption that the soil
conditions do not deviate from the observed conditions. All work done is in accordance with
generally accepted geotechnical engineering principles and practices. No warranty, expressed
i~~
Sterling Pacific
Job No. 95125.10
July 27, 1995
Page 11
or implied, of merchantability or fitness, is lmdadme s r intended in connection with our work by
the furnishing of oral or written reports or f g
The recommendations and conclusions contained herein shall be considered valid only if PSC
Associates, Inc. is retained to review any changes to the plans and to monitor and test all
geotechnical related construction, including final site grading, repairs of existing erosion and
landslides (other than Landslide D) and foundation construction. If these services are
performed by others, the conclusions and recommendations contained herein will be considered
null and void and invalid.
This report does not include an evaluation of the stability of Landslide "D" and the adjacent lots,
or a method of repairing this slide. Our previous reports for Landslide "D" have been
superseded by the supplemental investigation reports by Leighton & Associates and GeoRource
Consultants. These supplemental reports should be consulted for development of this part of the
site. PSC hereby disclaims liability or responsibility for Landslide "D" and the adjacent lots.
This report has been prepared for the proposed Terrabay Park to assist in the current evaluation
of the property and to assist the architect and engineer in the design of this project. In the event
any changes in the design or location of facilities are planned, or if any variations or undesirable
conditions are encountered during co a ortioariations areureviewed and o~urerecommendations
be considered valid unless the Chang
modified or approved by us in writing.
This report is issued with the understanding that it is the owner's responsibility to ensure that
the information and recommendations co stems are rtaken to see thaththe recomrnendarions arer
for the project, and that the necessary p
carried out in the field. Should ownership of this property change hands, the new owner should
be informed of the existence of this report.
The findings in this report are valid as of the present date. However, changes in the conditions
of the property can occur with the passage of time, whether they result from legislation or from
the broadening of knowledge. Accordingly, the findings in this report might be invalidated
i ~~
Sterling Pacific
Job No. 95125.10
July 27, 1995
Page 12
wholly or partially, by changes outside of our control. Therefore, this report is subject to
review by the controlling governmental agencies and is valid for a period of one year.
Respectfully submitted,
PSC ASSOCIATES C.
aniel P. O'Connell, P.E., G.E.
Principal Engineer
DPO:mc
Enclosures: References
Appendix A
c:1wp51\rp«om\9512510.727
~FESSIp~,~,.
r.,~ ~~ ~
11L
LIST OF REFERENCES
1. "Additional Details for use in the Final Site Grading Plans for Neighborhoods A and B,
California", Job No. 83103.10, prepared
Terrabay Development, South San Francisco,
by PSC Associates, Inc., dated September 27, 1984.
2. "Additional Slope Stability Analyses, Terrabay Development, South San Francisco,
California, A Development by W.W • Dean & Associates for Resources Engineering and
1~~lanagement, Grading Design for Terrabay Village and Terrabay Park (Neighborhoods
A and B)", Job No. A83103-01, prepared by PSC Associates, Inc., dated March 20,
1984.
3. "Final Report -Slope Monitoring Services at the Tie-back Soldier Beam Retaining Walls
for Landslides 'B' and 'D", Job No. 83103.31, prepared by PSC Associates, Inc., dated
November 20, 1987.
4. "Geotechnical Engineering Investigation Report, Grading Design for the Proposed
Terrabay Village and Terrabay Park, Neighborhoods A and B Ted b baPy DeAssoca tes,
South San Francisco, California", Job No. A83103-01, p p y
Inc., dated November 15, 1983.
5. "Geotechnical Engineering Investigation, Proposed Tieback Retaining Walls at Landslides
"B" and "D" , Terrabay Developmtest~ Inc u dated July 311S 1985 . ~ifornia" , Job No.
83103.31, prepared by PSC Associa ,
Recommendations for Tieback Retaining Walls
(. "Geotechnical Engineering Investigation, California,
at Landslides "B" & oD r83103~31ayr pared by PSCSAssocS tes~inc lsdated May 15,
Addendum -1", Job N P
1989.
7. "Geotechnical Engineering Investigation Recommendation for Tieback Retaining Walls
at Landslide "B" and "D"", Job No. 83103.31, prepared by PSC Associates, Inc., dated
May 15, 1989.
g. Letter of "Clarification for Seismic Design Considerations Tie-back Walls at Landslides
"B" and "D"", Job No. 83103.31, prepared by PSC Associates, Inc., dated June 26,
1989.
i ~~
LIST OF REFERENCES (Cont.)
9. "Debris Flow, Potential Debris Flow Paths, Potential Debris Flow Paths and Esrtimat~
Volume of Debris Materials in Storm Drainage Basins", Job No. A82103-04-I, p p
by PSC Associates, Inc., dated February 15, 1983.
10. "Geotechnical Review, Terrabay Village, Terrabay Park, Recreation Center, Terrabay
Development, South San Francisco, California", Job No. 94127.10, prepared by PSC
Associates, Inc., dated December 30, 1994.
11. "Slope Monitoring Services at the Tie-back Soldier Beam Retaining Walls for Landslides
'B' and 'D'", Job No. 83103.31, prepazed by PSC Associates, Inc., dated February 23,
1987.
12. "Foundation Investigation for Proposed Terrabay Park, South San Francisco, California"•,
Job No. 89102.11, prepazed by PSC Associates, Inc., dated May 15, 1990.
-~~
APPENDIX A
1 ~~
SiJG~'I~D GUIDE r~
N~I'EI~PiCE OF HIIZ.SIDE HC~E'SI~'E5
FQR SLOPE STABILI'T'Y AND ~I~d
StJOGFS'1'ID ~R
N~I~NCE OF HIZZSII~ F~CMESl'I'~
FL7R SI1~PE STABII~I'Y AND EEtO6ION
During the wet weather season, hca~~eown~-rs such as Y~lf. livirq in houses
placed on fill (man-placed earth) or in the vicinity of excavated (cut) of
~ concerned abort the vcmdition of their building site.
fill slopes, minimize the likelihood
In general, modern design and connstzvvction practices codes of the local
of serious larxlslidirg (slope failure) • ~ grades filled land,
jurisdictions (cities and axinties~~ru~onlare among the most stringent in
excavation, terracing arxi slope have been corLStrvc'ted
In addition, most hillside levels
the cotmtty. rofessional standards. Zherefore, the concern of the
according to critical p sl drainage provisions
hameawner should be directed toward maw ~s-
and facilities so that they will perfozm as designed. the following general'
bons and simple prat-autions are p~~ to help you Properly
Main Your hillside hcenesite. Please refer to the attac3ied diagram for
an i.l.lustration of terms -
The general public often regards the natural terrain as stable at work
firma." This is, of course, an erroneous Nature is always
altering the landscape. Hills and mcxuztains ~ the valleys and lo~wlad
(erosion, landsliding, creeping soil et cetera) ~ ~~ leveling the
collect these products. thus the natural process
terrain. Periodically (war millions of years) major land movements rebuild
mountains and hills arxi these processes begin over again. In scm~e areas
these proses-ses are very slow and in others they occur at a relatively rapid
rate.
'Ihe development of hillsides for residential ~ sit~.erand to min~mize the
possible, to enhance the natural stability
probability of instability resulting from the gr~~J necessary to provide
et cetera. Zhis has been dyne by the developer
hcanesites, streets, yards. ~r~g ;nvestigations.
and designers on the basis of geologic ~ a d drainage p~~i~ and
Hover, in order to be successful. the hce~eowner.
facilities must be maintained by Y'~~
As a he~neowner you are aecustc~-ed to maintainincl Yom' h~% that is, y+ou
expect to paint Your hhese periodically, clean out clogged plumbing, raga-ir
roofs, et cetera. MaintP~ance of a hillside hamesite must be considered on
an even more serious basis because neglect can result in serious
corLSeq~iences. In most cases, lot arxi site maintenance can be provided
along with normal care of the grcnmds anr3 larr3scaping • Any e°sts of
maintenance are far cheaper to you than repair aftP-r neglect.
nl~
DON'
1. DON'T alter lot grading without ~e~~ ~1Oe. The ~~ ~°~'
on yowr lot were designed to carry away water rtu~off to a place where
it can be safely distributed.
2 . DON'T block or alter ditches whidz have been graded arcxu~d your Yvotise
or the lot pad. Ztlese shallow ditches have been put there for the
p~u.~pose of quickly rear~ving water towatrl the driveway, street or others
positive outlet.
3. DON'T block or alter ditr3les or drains. If several banes rely on the
same facilities, it is a good idea to cdieck with your r~ghbors- ~''~~'
bacJ~d up on their property may everYtually reach you. Water bacJ~,ed up
in surface drains will overflow arxi infiltrate slopes which leads to
instability. Maintain the grcxmi surface upslope of lined ditches to
ensure that surface water is collecfied in the ditch arr3 is riot
pPnn;teed to collect behind or flora wader the lining. (See detail
sketch on the attached diagram).
4. DON'T permit water to collect or porxi anywhere on Your' lot. Such water'
will either seep into the ground causing tuiwanted saturation, or will
overflow onto slopes and begin erosion. once erosion is started, it is
difficult to control arxi severe damage may result rather quickly.
5. DON'T direct water over slopes even where this may seem a good way to
prevent porxiing. This ten7s to cause erosion and slope instability.
Dry wells are sometimes used to get rid of excess water when alter
means of disposing of water are not readily available. I~owever, such
facilities should be planned arx3 located by a qualified engineer.
6. DON'T let water pond against fourx~ations, r+etainirx3 walls and basement
walls. 'Ilzese walls are built to withstand the orriinazy moisture in the
grcxuxi and, where necessary - are a~anied by su~idra.; ns to carry of f
excess subsurface water. However, excel-s surface water aaLSt be
directed away from these structures
roof drains, gutters or down spouts to pY; ~;,'~
7. DON'T connect for that purpose.
subsurface drains which may not have been designed
Ir~tead, either collect the water in lined ditdzes or w;~erforated
pipes and conduct it to a storm drain, paved road or suitable area of
natural graurl. mere such d~annel flow is direc.'ted onto natural
grcxax3 it must be converted to sheet flow Mess a suitable natural
dzannel exists.
g, DON'T discharge surface water into septic tanks or leaching fields.
Not only are septic tanks constrvcl~ for a different purpose, but they
wi11 tend, because of their '~~, ~ acuminate additional
water from the gtYxmd during a heavy rain. Overloading them
artificially during the rainy season is bad f'ran a slope stability
starx~oint arri is doubly dangerous since their overflow can pose a
serious health hazard. We generally reco®errl that the use of septic
tanks be disoorrtirrued as soon as sewers are grade available.
9. DON'T place loose soil or debris over the sides of slopes. Ir.~ose soil
soaps ~ ~,,~~ more readily than ca~acteci f i11. It is not ~ac'~d
~ the same strength as the slope itself and will terra to slide when
laden with water arr3 may even affect the soil beneath it. Zhe sliding
may clog terrace drains below or may cause additional damage in
w~ea]cenis~g the slope. If you live below a slope, be sure that loose
fill is not chn~ed above your property.
10. DON'T over-irrigate slopes or leave a hose or sprinkler mm~ing
unattended on or near a slope. G cover and other vegetation will
r,eguire moisture during the hot sumoer months, but during the wet
season irrigation can cause grcxind cover to pull loose, which not only
destroys the cover, but also starts serious erosion.
11. DON'T try to conpact earth in trenches by flooding with water. Not
only is flooding the lease efficient way of compactirxl fine grained
soil, but this could saturate and redtii~ the bearing capacity of•
sLtQporting soils.
12 . DON'T change surface grade behind retaini tx~ walls or against bu.ildincJ
walls because this would incrp-are the lateral loading on the walls,
which could result in damage to such walls.
In conclusion, your neighUor's slope, above or below your Property, is as
important to you as the slope that is within your property lines. For this
reason, it is desirable to develop a ~exative attitude ~~~~ hillside
maintenance and we re~eryd developing a "good neighbor" policy. 5hculd
conditions develop off your property which are urx3esirable frcaa indications
given above, n~ action should be taken by you to ensure that prcanpt
remedial measures be taken.
n1~
'Ihe following several pages P guidelines for the general mainter~x~e
of hillside residential property. ~e frequerxY of attention to st~h
measures depPSr]s upon both the corxliticazs at each site arxi th,e nature of
the iaq~ravements on the property
Recc~er~dations aarztained in the Soil Report apply to future site
improvements, but we advise that you include ~ltation of a qualified
professional in the planning, design and ccnstructicaz of any improvements.
Such improvements including patios, swiaming pools, decks, etc. as well as
~; t direr structures.
Plans ar~d details of the i~rovements to your property sha~ld be kept so
that the maintenance procedures on the following Pages may be readily
followed. A continuing Program of maintenance is necessary for assurance of
minim~l_ difficulties. However, forces of nature acting in the future,
mi afurictioning of improvements and othex asPec-'ts may cause results whidi
~~d ~ ;n~peo•~,ed by qualified professionals. Attentica'i to this ~ Y~
responsibility in order that any necessary corrective or preventative
measures may be applied as soon as possible and to the extent required.
Zize qualities of hillside living are superior, but in order to ~- ~~II
each owner must accept the fact that the dynamic natural forces which formed
the hills will contisrue to alter them. Zhe developer, his design
consultants, arxi the constructors are powerless to c3~ange this. Only by
your pnx3ent future actions and diligent inspec-;tion arxi maintenance can
problems be kept to a minurn~.
n1~
Most luuoi ~~le lot problems are associated with water. Ur~trollea water
from a broken Pipe, septic tank or during wet weather causes most damage.
Wet weather is the principal time of slope problems in California because
the rainfall is quite variable anti may be or prolatxied.
'Ilzerefore, drainage and erosion control are iaportant asPec-'ts of h~~esite
stability and the prwisions built into the developed lot must not be
altered without oa~et.ent Professional advice. a of the
provisions must be carried cut to a-ssui"e their v~tirn~d oPemtion.
Zherefore, we Offer the following list of "Dogs" and "Don'ts" as a guide to
you:
DO
1. (heck roof drains, gaffe-rs arxi dam spouts to be sure they are clear.
Deperraing on your location, if you cio not have roof gutters and down
spouts, you may wish to install them because roofs and their wide space
can shed tr~endaus quantities of water. Without gutters or other
adequate drainage provisions, water falling frcffi the eaves collects
against the fau~dation arxi basemerYt walls which is Lu~desirable.
2. Clear drainage ditches and d~erk the3n frequently during the re~Y'
season. AsJc your neighbors to dp likesrise.
3, ~~k interceptor (brew) ditches at the tap of slopes to be sure that
they are clear and that water will Holt Overflow the slope, causing
erosion.
4. Be sure that all drain outlets arxi weep-holes are open arxi clear of
debris, vegetation and other material which could block the3n in a
storm. If blockage is evident, have it cleared.
5. Check for loose fill above and below Your Property ~ Y~ live on a
slope or terrace.
6. Limit watering during the rainy season when little irrigation is
required. Crver-saturation of the grauxi can cause major subsurface
damage.
7. Watch for water bac3aip inside the house at sump drains and toilets,
since this irxiicates drain or sewer bloc~mge.
8. Watch for wet spots oa7 Your property- ~~ may be natural seeps or an
indication of a broken water or sewer line. In either case, obtain
ccs~~petent advice regarding the proble3n and its correction.
9 . E~tercise ord; ~w precautica~. Your' house and building site were
to meet ~ which should protect against most natural
Provided you do your part in maintaining th~a.
n~4
TERMS USED ON HILLSIDE HOMESITES
~ 13 4
8 4
11
Q n ~ ~ ~ r,r-
3 _~~ ~__~
~..J
f__~~ 9
~ r -~
.~ ~~~ . r
~.-i
-~ s TYPICAL SLOPE SECTION
HOT TO SCALE
NOTA710NS:
O NATURAL GROUND SLOPE
O ORIGINAL GROUND SURFACE
O FILL SLOPE
O CUT SLOPE
O FIIL COYPACTED TO ENGINEERING SPEC IfICAT10NS
AND BENCHED INTO FIRY GROUND.
® ROOF GUTTER
O DOWNSPOUT CO`INECTEO TO AN UNPERFORATED PIPE
OR LINED DITCH WATER COLLECTION STSTEY.
US DRAINAGE SCALE OR DITCH
O9 SUBORAIN (PERfORATED PIPE AND/OR PE RYE ABLE
NATERIAt).
OO SUBDRAIN DISCHARGE (UNPERFORATED PIPE).
11 DRAINAGE TERRACE AND DITCH (SEE DETAIL)
O2 BR011 DITCH
13 LINED DRAINAGE DITCH (SEE DETAIL)
14 RETAINING 1fAll
15 VEEP-HOLES THROUGH RETAINING 1I ALL
16 BERY TO DIRECT EATER OFF SLOPE
it OR T2
~~v
'ter
6p~ pi
---- ~ ./
•. •. . ~ POOR
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