HomeMy WebLinkAboutDraft Supplemental EIR Terrabay Specific Plan 01-01-1996
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GEOTECHNICAL APPENDIX
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SCH Number: 95092027
Prepared for the
Ii CITY OF SOUTH SAN FRANCISCO
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Urban and Environmental Planners
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in association with
Harlan Tait Associates. Engineering Geologists
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REPORT
1866-002
"AREA Oil SLOPE STABILITY
ANALYSIS AND REMEDIATION
PHASE I, TERRABA Y PROJECT.
SOUTH SAN FRANCISCO, CALIFORNIA
JULY 1995
PREPARED FOR:
SunChase GA Calif I, Inc.
6001 North 24th Street, Suite A
Phoenix, Arizona 85016
~~~ Geo/Resource Consultants, Inc.
GEOLOGISTSlENGINEERSiENVIAONMENT AL SCIENTlSTS
50S_BEACH STREET, SAN FRANCISCO. CALIFORNIA 94113
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REPORT
AREA "D" SLOPE ST ABILITY Ai~AL YSIS AND REMEDIA nON
PHASE I, TERRABA Y PROJECT
CITY OF SOUTH SAN FRANCISCO, CALIFORNIA
PREPARED FOR:
SUN CHASE GA CALIF I, INC.
6001 NORTH 24TH STREET, SUITE A
PHOENIX, ARIZONA 85016
PREPARED BY:
GEOIRESOURCE CONSULTANTS, INe.
505 BEACH STREET
SAN FRANCISCO, CALIFORNIA 94133
JULY 1995
GRC PROJECT NO. 1866-002
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1866-002
SunChase GA CalifI, Inc.
6001 N. 24th Street, Suite A
Phoenix, Arizona 85016
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RE: AREA "D" SLOPE STABILITY ANALYSIS AND REMEDIATION
PHASE I, TERRABA Y PROJECT
SOUTH SAN FRANCISCO, CALIFORNIA
Ladies and Gentlemen:
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Transmitted herein are 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.
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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.
. ) Sincerely,
I GEOIRESOURCE CONSULTANTS, INC.
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Senior Vice President
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Principal Engineer
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Glenn Romig
Geotechnical Engineer
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City of South San Francisco (3 copies)
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TABLE OF CONTENTS
PAGE
1,0 INTRODUCTION
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1.1 BACKGROUND
1.2 PURPOSE OF THIS INVESTIGATION
1.3 SCOPE OF SERVICES
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2.0 FIELD INVESTIGATION
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2.1 FIELD EXPLORATION
2.2 EXPLORATORY BORlNGS
2.3 MONITORING OF PIEZOMETERS, INCLINOMETERS
AND HYDRAUGERS
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3.0 GEOLOGIC CONDITIONS
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3.1 SURFACE CONDITIONS
3.2 SUBSURFACE CONDITIONS
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4.0 FINDINGS AND CONCLUSIONS
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4.1 EXPLORATORY BORINGS
4.2 PIEZOMETER AND INCLINOMETER RESULTS
4.3 EXTENT AND DEPTH OF LANDSLIDE
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5.0 RECOMMENDATIONS
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5.1 PREVIOUS WORK
5.2 STABILITYEVALUATION
5.3 RECOMMENDED ALTERNATIVE
5.4 SITE MONITORING
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6.0 LIMITATIONS
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FIGURES
FIGURE 1 SITE VICINITY MAP
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FIGURE 2
AND FIGURE 3 TEST BORING LOGS
FIGURE 4 UNIFIED SOIL CLASSIFICA nON SYSTEM
FIGURE 5 HYDRAUGER LOCATIONS
FIGURE 6 GEOLOGIC CROSS SECTION A-A'
FIGURE 7 GEOLOGIC CROSS SECTION B-B'
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\ FIGURE 8 RAINFALL AND GROUNDWATER LEVELS
PLATE 1 GEOTECHNICAL MAP (IN POCKEl)
TABLES
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TABLE 1
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APPENDIX A
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WATER LEVEL READINGS
HYDRAUGER READINGS
SUMMARY OF STABILITY RESULTS
APPENDICES
INCLINOMETER DATA
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.
1.1
BACKGROUND
Terrabay is a residential development that will consist of two phases of single-family houses and
townhouse units. Site grading for the Phase 1 area was completed in 1989. Area "D" was
recognized as an upper area of relatively shallow, active landslides; however, the overall slide
mass was believed to be inactive. An initial remedial measure for Area D was developed by
building a tie-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
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 PURPOSE OF TIllS INVESTIGA nON
The purpose of this study was to re-evaluate proposed remedial measures for Area 0 in view of
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
SCOPE 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;
C.
Field reconnaissance and advancing two test borings to supplement and update previous
data within the toe of the landslide area;
D.
Collecting reatiings between November 1994. and July 1995, on the piezometers,
hydraugers and inclinometers that were previously installed on the site;
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;
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 INVESTIGA nON
2.1 FIELD EXPLORATION
During the period from October 1994 to February 1995, GRC geologists and engineers
. 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
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.
2.2 EXPLORATORY BORINGS
J- Two borings (BH-l 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.
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Frequent rainstorms delayed the exploratory drilling until mid-~ber. On December 20,
1994, attempts were made to access lots 104, lOS, 106, and 107 with a four-wheel drive truck
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-l 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-Iong 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.
.
. i 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.
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2.3 MONITORING 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.
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,
presented as depth below the ground surface, are shown in Table 1.
Measurements from seven inclinometers were ~e on two separate occasions, 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
confidence in the interpretation of inclinometer results. Readings were made on two axes within
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. Th~
the calculated flow rates presented in Table 2 should be considered approximate.
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3.0 GEOLOGIC CONDmONS
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3.1 SURF ACE CONDITIONS
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i--~ Area 0 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-
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hydrauger discharge 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
sheared shale), as well as alluvium and slope debris.
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3.2 SUB SURF ACE CONDmONS
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Descriptions of subsurface geologic units are based largely on the summary given in the L&A
report, 1992, and modified by om site observations.
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Area 0 js characterized by landslide terrane underlain by a variable thickness of Quaternary-age
unconsolidated surficial 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 near 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
Geotechnical Map (plate 1).
Deposits of artificial fill (Map symbol At) were mapped near the toe of Area D. These fill
materials were placed during grading for the shear 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 bas been
transported downslope primarily as slopewash or colluviwn and debris flows, and possibly,
wind-blown sands. These materials are composed of mixtures of gravel, sand, silt, and clay.
Upper debris flow deposits (Map symbol Qd) are predominantly light brown sandy clay with
gravel. The upper debris flow deposits are generally moist and stiff.
The upper debris flow deposits include discontinuous stringers of light brown silty fine sand.
The sand deposits are thickest near the upper reaches of the landslide. These sand stringers may
be portions of, or derived from, the Colma sand.
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 shear surfaces were observed within this clayey unit Additionally, the clay
layer was mapped in a portion of the key bottom. This sheared clay typically marks the basal
rupture surface of the landslide. In the toe areas, 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 L&A study, does not exist in the lateral margins of the landslide deposits.
The lower debris flow and colluvial deposits. (Map symbol Qd2) which underlie the above
described clay layer are predominantly reddish-brown clayey gravel with minor amounts of sand
and boulders. These materials are interpreted ~ 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 appear to be involved in the potential landslide mass.
Undifferentiated alluvium and debris flow deposits (Map symbol Qal and Qd), located in the toe
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 deposited in the stream channel.
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
melange. Melange is typically a weak, highly fractured to sheared mixture of rock types, mainly
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.
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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 towards the toe area than
estimated in the L&A (1992) report.
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No significant slope movement was detected by the inclinometers between the previous
reading (10/23/91) and the recent readings (12/1194 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.
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
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
movement
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Water flow from hydraugers was similar to shallow piezometers in that the flow rates
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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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 towards the two ends of the
retaining wall. In addition, it provides a catchment fence to prevent occasional loose
boulders from reaching lower levels.
4.1 EXPLORATORY BORINGS
The two exploratory borings, BH-l and BH-2, were located to provide information on the extent
and depth of slide debris in the toe area. BH-l provided positive information on a shallower
slide depth than previously shown, while BH-2 confIrmed the general type of soil and rock
materials.
As illustrated in Figure 2, Log of Boring BH-l, 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
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 PIEZOMETER AND INCLINOMETER RESULTS
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 5, 1995), no rain was recorded on
site, and most of the piezometers recorded a slight drop in water levels. This pattern is consistent
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,
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.
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
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
has been no movement of the inactive Area D landslide since the previous instrumentation period
in 1991.
4.3
EXTENT AND DEPTH OF LANDSLIDE
Area D is occupied by a large, inactive landslide feature ~at bas developed through translational
movement of older debris flow deposits and colluvium. Shallow debris flows and rotational
landslides of more recent age have occurred within the larger mass and at its margins. According
to the extensive supplemental geotechnical investigation performed by L&A (1992), the depth of
the large inactive landslide is defined by a basal rupture surface approximately 45 to SO feet deep
in the central portion of the slide. The rupture surface is at the base of a 2 to IS-foot-thick clay
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layer. The rupture surface is not well defined at the lateral margins of the slide, or at the toe area,
where the slide mass was removed through ancient alluvial processes.
The lateral limits of the large slide are indicated on the Geotechnical Map, Plate 1, and are
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in BH-l to represent a basal rupture surface. This relationship is illustrated on Figure 6, Cross-
Section A-A, and Figure 7, Cross-Section 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 L&A report.
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5.0 RECOMMENDATIONS
5.1 PREVIOUS WORK
"-~ In the previous report by L&~ 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
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The proposed remedial measure in the L&A report, referred to in this report as Alternative 1,
consisted of construction of two large shear keys, one above and one below the existing tieback
retaining wall, together with some removal of overburden above the wall, and subsurface
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 minimi7.e
future slope instability, provided that the shear keys are extended below the fililure planes, the
keys are co~ with adequate subsurface drainage, and that adequate compactive effort is
used in till placement. However, from a construction standpoint, we have several concerns
regarding the proposed system.
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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 60 feet) in a marginally stable
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1866-002
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landslide slope generates a concern about the overall hillside stability dming construction. In the
past. we understand that a similar slope problem was developed during the repair of "Landslide
R" (immediately southwest of" Area 0") and major failmes occurred on the slide plane.
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 shear key construction,
adding to the difficulty of constructing the shear key.
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.
5.2 STABILITY EVALUATION
We evaluated several remedial alternatives during the course of om 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
calculations. A snmmary of the results is presented in Table 3, and brief description of
alternatives 2, 3 and 4 is described below. Copies of the computer printouts from the
recommended alternative (Alternative 4) are presented in Appendix B.
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 asswned similar groundwater conditions.
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We understand from the City of South San 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.
I I The alternatives evaluated in addition to the utA repair scheme, developing a setback from the
existing unrepaired slope (Alternative 2), included co~ction of a soil buttress at the toe of
slope (Alternative 3), and a combination of significant removal of slide mass and construction of
a shear key at the toe of slope (Alternative 4).
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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
Constructing an earth buttress at the base of Area D would improve the static and seismic factors
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 elimim~ting 7 to 8 lots, and would require relaxing
the requirement for a static factor of safety to under 1.5.
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1866-002
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Alternative 4
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
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 0 limit. Twenty feet of soil would be removed at the lower portion of the slope,
decreasing generally to 10 feet in the upper portio~ 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. In additio~ a shear key similar 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 are 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 elevatio~ 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 are presented
in Appendix 8 of this report
In our opinio~ the fourth alternative is the most desirable alternative, from a safety and
constructability viewpoint In additio~ this alternative is the only one that actually reduces the
potential landslide driving forces. The computed factor of safety of this alternative is equivalent
to the L&A Plan, which has previously been accepted by the City.
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5.3 RECOMMENDED At TERNA TIVE
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 are shown on the Geotechnical
Map, Plate 1, and the cross section, Figure 6. The fmal 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 debrislboulders 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.
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 feel
The subdrain should consist of a 12-inch perforated pipe, perforations placed down,
embedded in a 3 foot width of Caltrans specification Class 2 permeable material.
Alternatively a composite geofabriclcrushed rock scheme can be used. The 3 foot width of
Class 2 rock should be extended up as fill is placed in the keyway. A solid pipe ~uld 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 near the
optimum water moisture and at a relative density of at least 92 percent, as determined by
ASTM Test D-1557,latest edition.
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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 earlier
recommendations will likely not be adequate. In addition, to reduce the potential for
settlement of the keyway fill, structural fill placed on the lots should be compacted at a
relative compaction of95 percent relative compaction (ASTM 1557).
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6. Drawings should be developed, showing the existing piezometers, slope indicator casings,
hydraugers, subsurface drains and other underground pipes within the repair area. Those
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.
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7. The topsoil from the graded area should be stripped, to the extent possible, and stockpiled.
This topsoil can then be spread over the fmal graded surface, to promote growth of vegetation
on the repair surface.
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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.
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We strongly recommend that periodic monitoring be performed at Area 0 currently and
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 vrith your
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Civil Engineer regarding the locations of the new monitoring device and the frequency of future
measurement.
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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
l ~ profession practicing under similar conditions at the same time, and in the same or similar
locality. No other warranty, either expressed or implied, is made or intended.
This report is subject to certain limitations that mayor 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 are 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 party is entitled to rely on the report
unless our express written consent is first obtained. Please contact us if you have any questions
or concerns regarding the information contained in the report or these limitations.
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LOG OF BORING BH-l
Equipment Solid Flight Au~r/Rot~ 'w'uh
Elevetion 297:!:. ft. Dete 12/29/94
SANOY ClAY (Cl)
brown; soft, Wtt; rock fr~9mtnts @ 2'
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SILTY SAND (SM)
red brown; m~dium stiff; Wtt;
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CLAYEY GRAVEL (Ge)
red bro'Wn; low plastic ('1n~S; I'Mdium stiff;
gr~vel is fint to r, angular
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s~t c~sinq to 12'
hard dnlling @ 13'
color chang~ to gr~" @ 14'
GRAVEllY CLAY (Cl-GC)
mott1~d 9r~y ~nd y.now brown; vtrlJ st;ff-hard;
moist; grav.l is fln~.. som. co~rs. nnd;
most'kj sandstone and volcanic rock
possiblt slide plane @ 16.8'
angular to sub rounded rock obstruction @ 17'
- .15' gr." clay seam - stiff, 'Wet.. w/shwp
lower contact; dips abolrt 10 degrtts
gravel @ 21' - mod.rately hard; very strong
granl@ 23.5'
CLAYEY SANOY GRAVEL (GC)
light olive gr~lJ.. mottl~ w /o,.~ngt brovn
drill.d out to 26.2'
hard drilling @ 27.5'-30'
cuffings art hard silty sand.. gray
@ 31.0' chanqe to nd-brown to 32.S',1htn
mottled "f( /gray and milor ytllow
washed away 32.5 t to 3~.4'
CLAYEY SAND (SC)
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TABLE 2
HYDRAUGER READINGS
AREA D . TERRABAY
, , , , , , , I<,.'.,},..( , , , , , , , , ii.ii~'IiIIJ.,\I~ll,ililllllljllll.lll\llllilli!I\II!\.1Illilllll[~,-.![i.I.~'II:II:~illl:lri.:.il
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. . ...................
: : : , : , : : : : : : . . . . . . . ' . ::i.:;:~.:::;.:::;:::::::..~:.~::::::~,:::....:..R~:.::::;.i.:..:.:::::::::.:::.::....:.::.:.:::t::::l.t:;::::~;::*:::~.:::~~.!;:;:;i:::::i;:::~;j:::::::I:;;;l::r!:~~1~:~:;:~~~1.*;::':::;:;.:;;::::
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Upper Slide
# 1 1 Steady flow, leaking 55
# 1 8 1040
# 1 9 180
# 20 335
# 2 1 5t eady thin flow 30
# 22 125 175 1 60
:
Lower Slide
.
#S Moist No drip I
I
#6 Moist No drip
#7 Dry Dry ;
#8 Steady trickle 50 SO
#9 20 40 30
# 10 30 SO SO
#25 Drip 2S 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 Pulsing drip Slow drip No drip
Pad Level
I
# IS
# A Qot 109)
# B Qot 109)
20
25
Dry
30
Notes: Recent slope work @ 14/1S has covered hydraugers; #14 was not located. Calculated flows were
rounded to the nearest Sgpd
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TABLE 3
SUMMARY OF STABILITY RESULTS
AREA D - TERRABA Y
Remedial . StaticFadDr Seismic Factor Scope of .< potential for
Alternative of Safety of Safety Coostmction ConstnlctabiJity
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 1.2 .w/o No appreciable Low
Existing Slope groundwater construction.
3) Construct Toe 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.
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APPENDIX A
Sterling Pacific
Job No. 95125.10
July 24, 1995
Page 3
General Site Geology
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 Parkridge/Skypark (Goatfarm
Cut Slope). These areas are discussed in' further detail in the following text and in the
referenced reports.
Slo~e Stability
During grading cut slopes were observed by representatives of both PSC and RF A. Based on
our slope stability ailalysis and field observation, it is our opinion that the cut and fIll slopes 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.
Landslides/Buried Vallevs
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 area
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
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 a design-
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.
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
f1lter 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 fmes. 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 Bearin~ 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 f1l1 at a variety of moisture contents and
relative compactions. The test were performed on a test pad approximately 60 feet in length,
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 0-8 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
COMPACTION
SETTLEMENT AT
9,000 psf
7%
9%
13%
98%
94%
98%
0.06 INCH
0.14 INCH
0.18 INCH
These settlements are well within the tolerable limits for typical wood-framed construction. The
plate load testing program was approved by RF A.
Debris Basin
Debris basin No. 1 was constructed in substantial conformance with the plans and specifications
in the northwest comer 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 prepared by PSC
Associates and dated February 15, 1983.
Sterling Pacific
Job No. 95125.10
July 24, 1995
Page 7
Current Site Conditions
PSC geotechnical engineers visited the 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 I-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 15 our
professional opinion that the mass grading work was performed in substantial
conformance with the geotechnical recommendations presented in our reports. Necessary
Sterling Pacific
Job 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 fmal 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 fmal 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
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 fmdings.
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.
In 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.
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 fmdings 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,
DPO:mc
PSC ASSOCIATES, INC.
Enclosures: References
Appendix A
c:\wpSl Upl<om\9S12S 10.724
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, p~epared 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.
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 Ass?Ciates, Inc., dated November 29. 1991.
...J
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APPENDIX B
SLOPE STABILITY RESULTS
ALTERNATIVE 4
Slope stability analysis was based on SPENCER's method using Computer Code TSLOPE
developed by T AGA Engineering Software Services in Lafayette, California. The cases
evaluated are listed below.
(1) Static Condition - low water table, 7 feet below existing ground.
(2) Seismic Condition - low water table, 7 feet below existing ground.
(3) Static Condition - high water table, at finished ground surface.
(4) Seismic Condition - keyway backs lopes high water table, at finished ground surface.
(5)
Static Condition - during construction, assuming no water.
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ADDENDUM
1866-002
"AREA 0" SLOPE STABILITY
ANALYSIS AND REMEDIATION
PHASE I, TERRABA Y PROJECT
SOUTH SAN FRANCISCO, CALIFORNIA
SEPTEMBER 1995
PREPARED FOR:
SunChase GA Calif I, Inc.
6001 North 24th Street, Suite A
"1
~F Geo/Resource Consultants, Inc.
GEOLOGISTSiENGINEERSlENVIRONMENT AL SCIENTISTS
505.BEACH STREET, SAN FRANCISCO, CALiFORNIA 94113
- ~
~ J
~ -J
- -
Geo/Resource Consultants, Inc.
GEOLOGISTS ENGINEERS' ENVIRONMENTAL SCIENTISTS
CQtPc'a:e Hea~
505 Beach SIn!et
San Francisco. California ~133
(415) n5.31n FAX (~15) nS-2359
"eqlOnal Otlices A"zona CahlQtn'a Hawaii v~
September 5, 1995
1866-002
SunChase GA CalifI, Inc.
6001 North 24th Street, Suite A
Phoenix, Arizona 85016
Attention: Mr. Larry Harris
RE: SUPPLEMENTARY REMEDIAL MEASURES
AREA "D" SLOPE STABILITY
PHASEITERRABAYPROJECT
SOUTH SAN FRANCISCO, CALIFORNIA
Ladies and Gentlemen:
.J
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 shear keys. ' The computed factor safety of this
alternative is equivalent to the previous L&A plan, which has been accepted by the City.
Ii
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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
shear key, it did not meet the requirement in considering shallower slides above the bottom shear
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.
r
1-1
,
'-'
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
'1
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GD1OO:Ill66-Ll
~
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.
,f
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,
GEOIRESOURCE CONSULTANTS, INC.
~~"J41r
&4M
Eric S. Ng, P.E., ~~
Principal Engineer
Glenn A. Romig, P.E., G.E.
Geotechnical Engineer
~~
Alan D. Tryho . .G.
Senior Vice President
ADT/ESN/GAR:csc
Attachment: Figure 1
j Appendix
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APPENDIX
SLOPE STABILITY RESULTS
ALTERNATIVE 4
Slope stability analysis was based on SPENCER's method using Computer Code TSLOPE
developed by T AGA Engineering Software Services in Lafayette, California. The cases
evaluated are listed below.
(1)
(2)
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Static Condition - low water table
Seismic Condition - low water table
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TOTI"l.. P.09
GRADING REPORT SHOWING
SUM:MARY OF TESTING AND OBSERVATION
SERVICES DURING THE MASS GRADING
OF TERRABAY VILLAGE AT TERRABAY
DEVELOPMENT, SOUTH SAN FRANCISCO, CA
FOR
Sterling Pacific l\'lanagement Services, Inc.
6001 N. 24th Street, Suite A
Phoenix, AZ 85016
mmIiII1fIJIJ
Geotechnical & EnvIronmental Consultants
Construction MaterIals 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 24, 1995
GRADING REPORT SHOWING
SUl\1MARY OF TESTING AND OBSERVATION
SERVICES DURING THE MASS GRADING
OF TERRABA Y VILLAGE AT TERRABA Y DEVELOPl\1ENT
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 descnl>ed 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. RF A also reviewed and approved the
various geotechnical engineering designs and construction methods. RF A was recently acquired
by GEl Consultants.
Proiect 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 (MSL) near the southeast comer 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
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 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 f1l1. 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 0-1557-78). Results of all field and laboratory tests are tabulated
in our project files and are available for review.
GRADING REPORT SHOWING
SUMMARY OF TESTING AND OBSERVATION
SERVICES DURING THE MASS GRADING
OF TERRABAY PARK AT TERRABAY
DEVELOPMENT, SOUTH SAN FRANCISCO, CA
FOR
Sterling Pacific Management Services, Inc.
6001 N. 24th Street, Suite A
Phoenix, AZ 85016
mmHI1f1JII
Geotechnical & Environmental Consultants
ConstructIon Materials 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 Zl. 1995
GRADING REPORT SHOWING
SUMl\1ARY OF TESTING AND OBSERVATION
SERVICES DURING THE MASS GRADING
OF TERRABAY PARK AT TERRABAY DEVELOPl\1ENT
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. RF A was recently acquired
by GEl 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 finn, 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.
Project Description
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 Greenpark 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 n , 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 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 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.
Gradin~ 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
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 Geolol!V
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 "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-31l) 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.
The primary impact of this shear zone on the project is its potential to impede subsurface water
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.
Sterling Pacific
Job No. 95125.10
July 27, 1995
Page 4
Slope Stability
During grading cut slopes were observed by representatives of both PSC and RF A. 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 Monitorin~
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.
LandslideslBuried 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 superceded by supplemental reports by others, as described below.
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" ILandslide "R", Terrabay Development, Phase 1" prepared by PSC
Associates, dated December 10, 1990.
Retaininf! Walls
Due to field survey differences, the proposed wall at the rear of the Lots 281 and 282 Vias
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 n permeable drain
rock was placed to within approximately 6 inches of the bottom of the v-ditch which was
constructed at the top of the wall. The wall facing consisted of concrete lagging placed within
the flanges of the H-beams.
Canyon Under drains
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 fmn native
material and a fabric wrapped gravel drain was installed. The drains consist of approximately
Sterling Pacific
Job No. 95125.10
July 27, 1995
Page 6
9 cubic feet of 1-112 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 Parkridge Circle and Skypark Circle to the intersection with Parkgrove Drive
and from the back of lot 228 to the western terminus of Baypark Circle. A subdrain was
extended from Landslide "R" near the northeast comer of lot 309 to the previously mentioned
curtain drain near 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 Parkridge 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 Skypark 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-Q4-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 backhoe.
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.
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 fme 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.
J
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 frrst 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 in accumulation of silt on several lots below the slopes.
Accumulation of up to a few inches of silt was noted especially on Lots 269, 267, 266,
238, 237, 232, 231, 305, and 302. This silt should be removed from the pads prior to
construction.
5. Small shallow landslides have occurred in several areas. The small landslide in the slope
below Lot 203 adjacent to South San Francisco Drive was observed last December. Our
test pit showed this slide to be about 4 to 6 feet deep in fill materials weakened by
concentrated runoff on the pad area above. The slide is about 30 to 40 feet wide, and
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 shows the slide to be shallow, less than 5 feet deep, in highly
weathered and sheared melange bedrock. It is about 30 to 35 feet wide, and extends
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 has occurred further to the
west. A new landslide has occurred on the cut slope above Lot 278. This slide is a
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.
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 of the slides, and continuing movements could
impact the road in this area. These slide areas can be repaired by removing the slide
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 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.
Subdrains should be checked for proper functioning and repaired or cleaned out if
necessary.
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 fmal 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
Sterling Pacific
Job No. 95125.10
July 27, 1995
Page 11
or implied, of merchantability or fitness, is made or intended in connection with our work by
the furnishing of oral or written reports or fmdings.
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 fmal site grading, repairs of existing erosion and
landslides (other than Landslide liD ") 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 "DR have been
superceded 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 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.
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 fmdings in this report might be invalidated
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.
vt
~el P. O'Connell, P.E., G.E.
Principal Engineer
DPO:mc
Enclosures: References
Appendix A
c:\wp51 \rpt-com\951251O. 727
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 pst 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. A83103-0l, prepared by PSC Associates,
Inc., dated November 15, 1983.
5. "Geotechnical 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 WaIls
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.
LIST OF REFERENCES (Cont.)
9. "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.
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, prepared by PSC Associates, Inc., dated February 23,
1987.
12. "Foundation Investigation for Proposed Terrabay Park, South San Francisco, California"-.
Job No. 89102.11, prepared by PSC Associates, Inc., dated May 15, 1990.
APPENDIX A
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SUGGESTED GUIDEI..rnES FOR
MAINl'ENANCE OF HIllSIDE lDIESrms
FOR sroPE STABILI'lY AND ER:SICN
'--
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SOOGESTED GUIDELINES FOR
MAINl'ENANCE OF HILlSIDE HCMESITES
FOR SlOPE STABILIT'i AND EROOION
0Jri.rg the wet weather season, homeowners such as ya.1rSel.f, livi.rg in houses
placed on fill (man-placed earth) or in the vicinity of excavated (cut) of
fill slopes, bea:ine concerned aba.It the c:orrlition of their 1:ui.ldi.rq site.
In general, m:rlern design ard construction practices minimize the likelihcxx:l
of serious larrlsliclirq (slope failure). '!he gra~ a:xies of the local
jurisdictions (cities am COLmties) in California conc::erni.n:J filled lam,
excavation, terraci.n:;J am slope const:Iuction are anorq the lOClSt st:ri..n3'ent in
the ca.mtry. In addition, mJSt hillside devel~ have been const::rocted
accordi..rg to critical professional starrlards. '!herefore, the concern of the
h~ should be directed toward mai..nta.i.nirg slopes, drainage provisions
ard facilities so that they will perfonn as designed. '!he fol1OYli.n;J general'
recammerrlations ard simple precautions are presented to help you properly
maintain your hillside horresite. Please refer to the attached diagram for
an illustration of teJ:ms.
'!he general p.1blic often regards the natural terrain as stable - "terra
finna." 'Ih:is is, of course, an erroneous concept. Nature is always at worK
al terin:J the larrlscape. Hills am lOOlIDtains are worn dOY1ll by mass wasti..rq
(erosion, larrlslidi.rq, creepin:J soil et cetera) am the valleys ard lCMlam
collect these products. '!hus the natural process is towal:d levelin:J the
terrain. Pericxtically (over millions of years) major lard lIDVE!l1'eI1ts rebuild
rramtains am hills am these processes begin over again. In same areas
these processes are very SlCM ard in others they c:x:x::ur at a relatively rapid
rate .
'!he develcpnent of hillsides for residential use is carried cut, insofar as
possible, to enhance the natural stability of the site am to minimize the
probability of instability result.in:J iran the gradirg necessary to provide
hanesites, streets, yards, et cetera. 'lhi.s has been done by the develq?er
am designers on the basis of geologic am soil en;Jineeri.n; investigations.
~er, in order to be sucx::essful, the slope am drainage provisions am
facilities m.JSt be maintained by yen, the ha'neoWner.
As a hareowner you are accustamed to mai.nt:ai.nin:l yoor haIe; that is, you
expect to paint yoor house periodically, clean cut clcgged plumbi.n:;J, repair
roofs, et cetera. Maintenance of a hillside homesite l1U.1St be considered an
an even m:>re serious basis because ne::Jlect can result in serious
consequences. In lOClSt cases, lot am site maintenance can be provided
along with nonnal care of the grouOOs am laOOscaPi.n;. Arrj costs of
maintenance are far cheaper to you than repair after neglect.
\....
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[X)N'T
1. IX.N'T alter lot gradiIg withaIt c::x::tq?etent advice. '!he man-made sl~
on ycur lot were designed to carry CJ!.IIay water rorx>ff to a place wilere
it can be safely distr:ib.rt:ed.
2. [X)N'T block or alter ditdles which have been graded ara.m:l yoor boose
or the lot pad. 'Ihese shallow ditches have been prt: there for the
~ of quickly rerrovi.n:J water towanl the driveway, street or other
~itive outlet.
3. [X)N'T block or alter ditches or drains. If several banes rely on the
same facilities, it is a good idea to check with ya;.r neighbors. Water
backed up on their property rray eventually reach yen. Water backed up
in surface drains will overflow am infiltrate slopes wch leads to
instability. Maintain the groun::1 surface upslope of lin=d ditches to
ensure that surface water is collected in the ditch an:i is rot
pennitted to collect behiIrl or flow un:ler the li.ni.n;J. (See detail
sketch on the attached diagram) .
4. [X)N'T permit water to collect or parrl anywhere on ya;.r lot. SUch water.
will either seep into the grourrl causin:J unwanted saturation, or will
overflow onto slopes an:i begin erosion. on:::e erosion is started, it is
diffiallt to control an:i severe damage rray result rather cpickly.
5. IX.N IT direct water OI/er slopes even TNhere this rray seem a good way to
prevent POn.:li.n1. '!his terns to cause erosion ani slope instability.
Dry wells are sareti:mes used to get rid of excess water when other
means of disposin:J of water are rot readily available. However, sud1
facilities should be planned an:i located by a qualified ergi.neer.
6. [X)N' T let water porrl against fourrlations, ~ walls an:i ba-c::Pl'nPnt
walls. 'nlese walls are J::oilt to wit:hstarrl the ordinaIy ooisture in the
grourrl ani, where necessary, are accorrpmied by sulxirai.ns to carry off
excess subsurface water. However, excess surface water must be
directed away iran these structures.
7. [X)N I T cormect roof drains, gutters or dc1.rm spcuts to exi.sti..n:J
subsurface drains which may not have been designed for that pn:pose.
Instead, either collect the water in lined ditches or unperforated
pipes ani con:luct it to a stonn drain, paved road or suitable area of
natural grourrl. Where such charmel flow is directed onto natural
grourrl it must be converted to sheet flow unless a suitable natural
dlannel exists.
8. [X)N I T c:li.scharge surface water into septic tanks or leadl.in:J fields.
Not only are septic tanks constrocted for a different pn:p:se, hIt they
will ten:i, because of their construction, to accIlTTll1ate additional
water fran the groun:i durin:J a heavy rain. OVerload:in;J them
artificially durin:J the rainy SE';:tc:.t"l11 is bad iran a slcp! stability
~ ~
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starrlpoint ani is dalbly dan;leroos si.n=e their overflow can pose a
serialS health hazard. We generally ~n:i that the use of septic
tanks be di.sconti.nued as soon as sewers are :made available.
9. rx:N' T place loose soil or debris over the sides of slq:es. Loose soil
soaks up water lOOre readily than c:x::q:xlcted fill. It is not cx:rrp3.cted
to the same stren:;Jth as the slope itself ani will terrl to slide when
laden with water ani nay even affect the soil beneath it. 'n1e slici.in:J
may clog terrace drains belCM or may cause acHiticnal. damage in
~ the slope. If yoo. live belCM a slope, be sure that loose
fill is not ~ above your property.
10. rx:N'T over-irrigate slopes or leave a hose or sprinkler l:\II'lI1irq
unatt.erXled on or near a slope. Gra.n'rl COller ani other vegetation will
require lOOisture duriI'g the hot Sl.nIIDer lOOl1ths, but duriI'g the wet
season irrigation can cause grourrl CXJ'I/er to p..1l.l loose, which rot only
destroys the CXJ'I/er, but also starts serialS erosion.
11. OON'T tzy to <::oIt'pact earth in trenches by floodi.n:;J with water. Not
only is floodi.n:;J the lease efficient way of cx:rrp3.cti.n;J fine grained
soil, but this could saturate ani reduce the beari..rl;J capacity of'
supportirq soils.
12 . OON' T cha.n;e surface grade behirx:l retai.nirY:J walls or against b..1i1ci.in:J
walls because this would increase the lateral loadi.n:J on the walls,
which could result in damage to such walls.
In conclusion, your neighbor's slope, above or belCM your property, is as
important to you as the slope that is within yoor property lines. For this
reason, it is desirable to develop a a:::x:::p:!rative attitude ~ hillside
naintenance ani we recc:mnen:l developirg a "geed neighbor" policy. Shoold
con::litions develop off yoor property which are urrlesirable fran iniications
given above, necessary action should be taken by you to ensure that prompt
renaiial IOOaSUreS be taken.
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'!he followin} several pages present guidelines for the general naint:enan=e
of hillside residential property. '!be frequerx:y of attention to such
m:asures deperrls upon both the corxli.tians at each site am the nature of
the in'provements on the property.
RecaII11IleOOations contained in the Soil Report awly to :future site
in'provenw=nts, but TNe advise that you inclme consultation of a qualified.
professional in the pl~, design am consb:uction of any iJIprovements.
SUch in'provenw=nts includi..rq patios, sw:i1Im:in1 pools, decks, etc. as TNeli as
buildi.n:J structures.
Plans am details of the improvements to your property should be kept so
that the maintenance procedures on the followin:J pages may be readily
followed. A conti.nui.rg prcgram of maintenance is necessary for assurance of .
mi.niInal difficulties. However, forces of na'bJre acti.n:J in the future,
misfunctioninq of in'proveIrel1ts am other aspects may cause results Wich
should be inspected by qualified. professionals. Attention to this is your
responsibility in order that any necessary corrective or preventative
measures may be applied. as soon as possible am to the extent required.
'!he qualities of hillside livin;J are superior, but in order to maintain them
each owner must accept the fact that the dynamic natural forces which fcn:ne:i
the hills will c:xmtinue to alter them. '!he developer, his design
consultants, am the constructors are pa...>erless to c::han;;e this. Only by
your prudent future actions am diligent inspection am naint:enan=e can
problems be kept to a minimum.
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Most hillside lot prcblems are associ ~ted with water. Uru.altrolled water
!ran a broken pipe, septic tank or duri.n;J wet lllIeather causes m:st damage.
Wet 'Neather is the priIci.pal tilDe of slope prdJlems in Califonrla because
the rainfall is q.rlte variable an:i may be torrential or prolc:n:Jed.
'lherefore, drainage an:i erosion u.JIllL.vl are i:ap:>rtant aspects of bcmesite
stability ani the provisions built into the deve1cp:d lot ltL1St IXJt be
altered wit:hcut ccmprt:.ent professicnal advice. Mai.ntenance of the
provisions nust be carried out to assure their CCI1ti.nued ~tion.
'lherefore, we offer the followil'g list of "OJ's" an:i "Den'ts" as a guide to
yoo:
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1. Check roof drains, gutters arxi down spa.rt:s to be sure they are clear.
Deperrli.rxJ on ycAJr location, if yoo do IXJt have roof gutters ani down
spa.rt:s, yoo nay wish to install them because roofs an:l their wide space
can shed tremerxious quantities of water. wit:hcut gutters or other
adequate drainage provisions, water fallin:J frcm the eaves a:>llects
against the fa.m:lation an:l ~m: walls TNhich is urrlesirable.
2. Clear drainage ditches am check them frequently durin:J the rainy .
sea~. Ask yoor neighbors to do likewise.
3. Check interceptor (brcM) ditches at the tc:p of sl~ to be sure that
they are clear arxi that water will IXJt overflow the slope, causi.n:J
erosion.
4. Be sure that all drain ootlets an:l weep-holes are open arxi clear of
debris, vegetation arxi other material TNhich could block them in a
storm. If blockage is evident, have it cleared.
5. Oleek for loose fill above arxi below yaxr property if yoo live on a
slope or terrace.
6. Limit waterin:J duri.n:J the rainy season when little irrigation is
required. over-saturation of the gro.m:i can cause major subsurface
damage.
7. Watch for water backup inside the boose at SlIItp drains an:i toilets,
sin::e this i.n::licates drain or sewer blockage.
8. Watch for wet spots on your property. 'lhese may be natural seeps or an
iIrlication of a broken water or sewer line. In either case, obtain
cc:xrpetent advice regarcli.n:J the problem ani its a:>rrection.
9. Exercise ordinary precaution. Your house arxi buildi.n; site were
constructed to neet staOOards which sha.11d protect against lOClSt natural
occurrences, provided yoo do ycAJr part in mai.ntai.ni.rq them.
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TERMS USED ON ~lllSIDE HOMESITES
TYPICAL SLOPE SECTION
HOT TO SCALE
NOTATIONS:
CD
o
G)
<9
G)
G)
o
Q)
o
@
@
@
@
@
@
@
NATURAL GROUND SLOPE
ORIGINAL GROUND SURfACE
F III ,SLOPE
CUT SLOPE
fIll COMPACTED TO ENGINEERING SPECIFICATIONS
AND BENCHED INTO FIRM GROUND.
ROOf GUT TER
DOWNSPOUT CONNECTED TO AN U~PERfORATED PIPE
OR lINED OllCH lAlER COllECTION SYSTEM.
DRAIHAGE SWAlE OR DITCH
SUBDRAIH (PERFDRATED PIPE AHD/DR PERMEABLE
IUTERlAl).
SUBDRAIH DISCHARGE (UNPERFDRAlED PIPE).
DRAINAGE lERRACE AND DITCH (SEE OETAll)
BROW DITCH
lINED DRAINAGE DITCH (SEE DETAil)
RETAIHING WAll
WEEP-HOLES THROUGH RETAINING WALL
BERM TO DIRECT WATER OFF SLDPE