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HomeMy WebLinkAbout04_06_Geology and SoilsCity of South San Francisco Environmental Setting, Impacts, and Mitigation Geology and Soils 751 Gateway Boulevard Project 4.6-1 September 2020 ICF 0662.19 4.6 Geology and Soils 4.6.1 Introduction This section describes the environmental and regulatory setting for geology and soils. It also describes impacts associated with geology and soils that would result from implementation of the proposed project and mitigation for significant impacts where feasible and appropriate. 4.6.2 Environmental Setting 4.6.2.1 Physiography South San Francisco comprises three distinct topographic zones: a lowland zone, primarily east of U.S. 101, underlain by deposits of bay mud up to 80 feet; an upland zone, mostly urbanized with cut and fill in some areas superimposed over alluvial soils of the Colma Creek floodplain; and a hillside zone with some slopes of more than 30 percent, with soils characterized as sandy and gravelly loams having generally high to very high erosion potential. The project site is in the lowland zone at approximately 34 to 21 feet above mean sea level. It gently slopes from west to east, toward Gateway Boulevard. 4.6.2.2 Subsurface Conditions The project site is underlain by medium-dense to very dense sands, with some very stiff to hard clays overlying residual soil. Bedrock was encountered at depths between 40.5 feet below ground surface (bgs) and 80 feet bgs. Rock was not encountered in some borings, including boring LB-8, which extended to 101.5 feet bgs. Within the building footprint, bedrock is expected to be present approximately 40 to 75 feet bgs. 4.6.2.3 Seismicity and Seismic Hazards Primary Seismic Hazards Surface Fault Rupture The project site is not located within an Alquist-Priolo earthquake fault zone and no known fault or potentially active fault exists on the project site.1 The nearest fault is the Hillside fault, located approximately 0.3 mile south of the project site. The Hillside fault is pre-Quaternary (i.e., older than 1.6 million years or without recognized Quaternary displacement), and a review of the Quaternary Fault and Fold Database as well as the Fault Activity Map of California concluded that the Hillside fault was inactive, with the latest activity occurring at least 1.6 million years ago. In a seismically active area such as the San Francisco Bay Area, the possibility of future faulting occurring in areas where faults have not been mapped is small but the possibility exists. 1 California Geologic Survey, 2000. San Francisco South Quadrangle Earthquake Fault Zones and Seismic Hazard Zones Map, released November 17, 2000. Available: http://gmw.conservation.ca.gov/SHP/EZRIM/Maps/ SAN_FRANCISCO_SOUTH_EZRIM.pdf. Accessed May 10, 2018. City of South San Francisco Environmental Setting, Impacts, and Mitigation Geology and Soils 751 Gateway Boulevard Project 4.6-2 September 2020 ICF 0662.19 Seismic Ground Shaking Ground shaking is the most widespread hazardous phenomenon associated with seismic activity. The project site is within a seismically active area that will most likely experience periodic minor earthquakes and a major earthquake (i.e., moment magnitude greater than 6) on one of the nearby faults during the service life of the project. Table 4.6-1 identifies the major faults in the project area and their distance from the project site. The San Andreas, Hayward, and Calaveras faults are the most active and have the highest probability of experiencing a magnitude 6.7 or greater earthquake in the next 30 years. Table 4.6-1. Regional Faults and Seismicity Fault Segment Approximate Distance from Project Site (mile) Direction from Project Site Mean Moment Magnitude N. San Andreas – Peninsula 3.1 West 7.2 N. San Andreas (1906 event) 3.1 West 8.05 San Gregorio Connected 8.7 West 7.5 N. San Andreas – North Coast 13.0 Northwest 7.5 Total Hayward 14.9 Northeast 7.0 Total Hayward-Rodgers Creek 14.9 Northeast 7.3 Monte Vista-Shannon 17.4 Southeast 6.5 Total Calaveras 23.6 East 7.0 Mount Diablo Thrust 34.9 Northeast 6.7 Green Valley Connected 28 Northeast 6.8 Rodgers Creek 29.8 North 7.1 Point Reyes 31.1 Northwest 6.9 Source: Langan Engineering and Environmental Services, Inc. 2019. Geotechnical Investigation, 751 Gateway Boulevard, South San Francisco, CA 75065-1501. November. Oakland, CA. The San Andreas fault is the nearest active fault to the project site. Since 1800, four major earthquakes have been recorded on the San Andreas fault. The Hayward fault experienced a major earthquake in 1868, and the Calaveras experienced significant earthquakes in 1861 and 1984. The 2014 Working Group on California Earthquake Probabilities forecast that there is a 72 percent chance that an earthquake with a magnitude 6.7 or greater in the San Francisco Bay Area over the next 30 years.2 The intensity of earthquake ground motion at the project site would depend on the characteristics of the generating fault, the distance to the earthquake epicenter, the magnitude, and the duration of the earthquake. 2 The 2014 Working Group on California Earthquake Probabilities (2015). “UCERF3: A new earthquake forecast for California’s complex fault system”, U.S. Geological Survey 2015–3009, 6 p., http://dx.doi.org/10.3133/fs20153009. City of South San Francisco Environmental Setting, Impacts, and Mitigation Geology and Soils 751 Gateway Boulevard Project 4.6-3 September 2020 ICF 0662.19 Secondary Seismic Hazards Liquefaction Liquefaction occurs when saturated soils lose cohesion, strength, and stiffness with applied shaking, such as that from an earthquake. The lack of cohesion causes solid soil to behave like a liquid, resulting in ground failure. When a load such as a structure is placed on ground that is subject to liquefaction, ground failure can result in the structure sinking and soil being displaced. Ground failure can take on many forms, including flow failures, lateral spreading, lowering of the ground surface, ground settlement, loss of bearing strength, ground fissures, and sand boils. Liquefaction within subsurface layers, which can occur during ground shaking associated with an earthquake, can also result in ground settlement. The project site is within an area that has not been evaluated for liquefaction or seismic landslides by the California Geological Survey. The Health and Safety Element of the General Plan notes that a large portion of the City, primarily east of U.S. 101, is underlain by deposits of bay mud, up to 80 feet deep in some places, that could be subject to liquefaction. The geotechnical investigation prepared for the project concluded that some of the subsurface soil layers could liquefy during an earthquake, resulting in settlement on the order of 1 inch. The liquefiable layers do not appear to be continuous and would not create bearing issues for the foundation. However, liquefaction could lead to differential settlement. Lateral Spreading Lateral spreading is a phenomenon in which a surficial soil displaces along a shear zone that formed within an underlying liquefied layer. The surficial blocks are transported downslope or in the direction of a free face, such as a bay, by earthquake and gravitational forces. Lateral spreading is generally the most pervasive and damaging type of liquefaction-induced ground failure generated by earthquakes. San Mateo County, where the project site is located, has not been evaluated for seismic hazard zones for liquefaction or seismic landslides.3 The geotechnical investigation prepared for the project indicated that soils would need to consist of saturated, cohesionless sandy sediments for significant lateral spreading to occur. In general, the potentially liquefiable soils underlying the project site consist of clayey and silty sands that are not likely to be continuous beneath the site. Therefore, the potential for lateral spreading at the project site is low. 4.6.2.4 Expansive Soils and Weak Soils Seismic densification can occur when strong ground shaking in loose, clean granular deposits above the water table results in ground surface settlement. The geotechnical investigation prepared for the project encountered approximately 13 feet of medium-dense to dense sand above the water table and estimated that up to 0.5 inch of settlement could occur because of seismic densification. However, the maximum predicted amount does not necessarily occur at the same locations. Laboratory testing performed on near-surface samples of clay indicates that the site has low expansion potential,4 with plasticity indices of 7 to 15. The geotechnical investigation prepared for the project indicated that the project site has a low expansion potential. 3 California Geological Survey. 2020. Earthquake Zones of Required Investigation. Available: https://maps.conservation.ca.gov/cgs/EQZApp/app/. Accessed: June 4, 2020. 4 Expansive soil undergoes volume changes with changes in moisture content. City of South San Francisco Environmental Setting, Impacts, and Mitigation Geology and Soils 751 Gateway Boulevard Project 4.6-4 September 2020 ICF 0662.19 4.6.2.5 Landslides Landslides occur when the stability of a slope changes from a stable to an unstable condition. The stability of a slope is affected by the following primary factors: inclination, material type, moisture content, orientation of layering, and vegetative cover. In general, steeper slopes are less stable than more gently inclined ones. San Mateo County, where the project site is located, has not been evaluated for seismic hazard zones for seismic landslides. In South San Francisco, the highest landslide risk is near the south flank of San Bruno Mountain, which is approximately 1.5 miles north of the project site. The project site, which is approximately 34 to 21 feet above mean sea level, slopes gently from west to east, toward Gateway Boulevard. Therefore, due to the distance between the project site and potential landslide areas, the likelihood of a landslide at the project site is low. 4.6.2.6 Paleontological Resources Geologic units present at the project site are older Holocene- and Pleistocene-aged continental and marine deposits (Qc) at ground surface and the Franciscan Formation, specifically sandstone, shale, and conglomerate (KJfss), at depth.5 The Holocene- and Pleistocene-aged continental and marine deposits consist of sand, silt, clay, and gravel and include the Colma Formation, as at the project site.6 The Colma Formation is a gravelly, sandy clay.7 The Franciscan Formation consists of chaotic mixtures of rock masses in a sheared matrix. The older Holocene- and Pleistocene-aged continental and marine deposits include the Colma Formation, which is known to have yielded vertebrate fossils.8 At a site on Pacific Avenue in San Francisco, Mammuthus (an extinct genus that belongs to the order of trunked mammals, including mammoth) and Bison (bison) fossils were recovered. Furthermore, vertebrate paleontological resources have been recovered from sites in South San Francisco from sediments of a similar age. The University of California Museum of Paleontology identified remains of Alces (moose and elk) and Equus (horse, donkey, and zebra) in this area.9 The geotechnical investigation identified the Franciscan Formation at depths exceeding the maximum depth of excavation; however, because the project site is adjacent to a surface exposure of the Franciscan Formation, it is possible that this unit could underlie areas of proposed excavation. Paleontological resources records have identified significant fossils in the Franciscan Formation.10 Vertebrate paleontological resources recovered from this unit include Ichthyosaurus (San Joaquin County) and Plesiosaurus (San Luis Obispo County). Although vertebrate fossils are uncommon in this geologic unit, fossils have been important in understanding formation of the Franciscan Formation.11 5 Wagner, D.L., E.J. Bortugno, and R.D. McJunkin. 1991. Geologic Map Explanation of the San Francisco-San Jose Quadrangle, California, 1991. Available: ftp://ftp.consrv.ca.gov/pub/dmg/pubs/rgm/RGM_005A/RGM_005A_ SanFrancisco-SanJose_1991_Sheet2of5.pdf. Accessed: March 12, 2020. 6 Ibid. 7 Rodda, P.U., and N. Baghai. 1993. Late Pleistocene Vertebrates from Downtown San Francisco, California. Journal of Paleontology 67(5):1058–1063. 8 Ibid. 9 University of California Museum of Paleontology. 2020. Advanced Specimen Search, San Mateo County. Available: https://ucmpdb.berkeley.edu/advanced.html. Accessed: March 12, 2020. 10 University of California Museum of Paleontology. 2020. Advanced Specimen Search, Franciscan Formation. Available: https://ucmpdb.berkeley.edu/advanced.html. Accessed: March 12, 2020. 11 Wakabayashi, J. 1992. Nappes, Tectonics of Oblique Plate Convergence, and Metamorphic Evolution Related to 140 Million Years of Continuous Subduction, Franciscan Complex, California. The Journal of Geology 100:1(19- 40). Chicago, IL: University of Chicago Press. City of South San Francisco Environmental Setting, Impacts, and Mitigation Geology and Soils 751 Gateway Boulevard Project 4.6-5 September 2020 ICF 0662.19 4.6.3 Regulatory Framework 4.6.3.1 Federal Earthquake Hazard Reduction Act of 1977 Federal laws codified in United States Code Title 42, Chapter 86, were enacted to reduce risks to life and property from earthquakes in the United States through the establishment and maintenance of an effective earthquake hazards reduction program. Implementation of the requirements are regulated, monitored, and enforced at the state and local levels. 4.6.3.2 State The Alquist-Priolo Earthquake Fault Zoning Act of 1972 The Alquist-Priolo Earthquake Fault Zoning Act of 1972 (Alquist-Priolo Act) (PRC Section 2621 et seq.) is intended to reduce the risk to life and property from surface fault rupture during earthquakes. The Alquist-Priolo Act prohibits the location and construction of most types of structures intended for human occupancy12 over active fault traces and strictly regulates construction in corridors along active faults. The California state geologist has established regulatory zones along active faults,13 called “earthquake fault zones,” and published maps that identify areas where surface traces of active faults are present.14 Seismic Hazards Mapping Act The Seismic Hazards Mapping Act of 1990 (PRC Sections 2690–2699.6) directs the California Geological Survey to identify and map areas that are prone to liquefaction and landslides resulting from seismic evens. The act mandates project sponsors to have a site-specific geotechnical investigation performed to identify potential seismic hazards and formulate mitigation measures prior to permitting most developments within specific zoned areas. California Building Standards Code The California Building Standards Code, or state building code, is codified in CCR Title 24. The state building code provides standards that must be met to safeguard life and limb, health, property, and public welfare by regulating and controlling the design, construction, quality of materials, use and occupancy, location, and maintenance of all buildings and structures within the state. The state building code generally applies to all occupancies in California, with modifications adopted in some instances by state agencies or local governing bodies. The current state building code incorporates, by adoption, the 2018 edition of the International Building Code of the International Code Council, with California amendments. These amendments include building design and construction criteria that have been tailored for California earthquake conditions. 12 With reference to the Alquist-Priolo Act, a structure for human occupancy is defined as one “used or intended for supporting or sheltering any use or occupancy that is expected to have a human occupancy rate of more than 2,000 person-hours per year” (CCR, Title 14, Division 2, Section 3601[e]). 13 An active fault, for the purposes of the Alquist-Priolo Act, is one that has ruptured in the past 11,000 years. 14 California Geological Survey. 2020. The Alquist-Priolo Earthquake Fault Zoning Act. Available: http://www.conservation.ca.gov/cgs/rghm/ap. Accessed: March 17, 2020. City of South San Francisco Environmental Setting, Impacts, and Mitigation Geology and Soils 751 Gateway Boulevard Project 4.6-6 September 2020 ICF 0662.19 Chapter 16 of the state building code deals with structural design requirements governing seismically resistant construction (Section 1604), including, but not limited to, factors and coefficients used to establish a seismic site class and seismic occupancy category appropriate for the soil/rock at the building location and the proposed building design (Sections 1613.5 through 1613.7). Chapter 18 includes, but is not limited to, the requirements for foundation and soil investigations (Section 1803); excavation, grading, and fill (Section 1804); allowable load-bearing values of soils (Section 1806); foundations and retaining walls (Section 1807); and foundation support systems (Sections 1808 through 1810). Chapter 33 includes, but is not limited to, requirements for safeguards at work sites to ensure stable excavations and cut-and-fill slopes (Section 3304) as well as the protection of adjacent properties, including requirements for noticing (Section 3307). Appendix J of the state building code includes, but is not limited to, grading requirements for the design of excavation and fill (Sections J106 and J107), specifying maximum limits on the slope of cut-and-fill surfaces and other criteria, required setbacks and slope protection for cut-and-fill slopes (J108), and erosion control through the provision of drainage facilities and terracing (Sections J109 and J110). California Division of Occupational Safety and Health Regulations Construction activities are subject to occupational safety standards pertaining to excavation, shoring, and trenching, as specified in California Division of Occupational Safety and Health regulations (Title 8). State Historic Significance Criteria As discussed in Section 4.7.5.2, Significance Criteria, Appendix G of the California Environmental Quality Act (CEQA) Guidelines includes the following question: “Would the project directly or indirectly destroy a unique paleontological resource or site?” Although CEQA does not define what constitutes “a unique paleontological resource or site,” Section 21083.2 defines unique archaeological resources as “an archaeological artifact, object, or site about which it can be clearly demonstrated that, without merely adding to the current body of knowledge, there is a high probability that it meets any of the following criteria: l Contains information needed to answer important scientific research questions and that there is a demonstrable public interest in that information. l Has a special and particular quality, such as being the oldest of its type or the best available example of its type. l Is directly associated with a scientifically recognized important prehistoric or historic event or person.” This definition is equally applicable to recognizing a unique paleontological resource or site. CEQA Section 15064.5(a)(3)(D) provides additional guidance, indicating that, generally, a resource shall be considered historically significant if it has yielded, or may be likely to yield, information important in prehistory or history. The CEQA lead agency having jurisdiction over a project is responsible for ensuring that paleontological resources are protected in compliance with CEQA and other applicable statutes. PRC Section 21081.6, Mitigation Monitoring Compliance and Reporting, requires the CEQA lead agency to demonstrate project compliance with the mitigation measures developed during the environmental impact review process. City of South San Francisco Environmental Setting, Impacts, and Mitigation Geology and Soils 751 Gateway Boulevard Project 4.6-7 September 2020 ICF 0662.19 4.6.3.3 Local South San Francisco General Plan The 1999 General Plan provides a vision for long-range physical and economic development of the City, provides strategies and specific implementing actions, and establishes a basis for judging whether specific development proposals and public projects are consistent with the City’s plans and policy standards. The 1999 General Plan provides a vision for long-range physical and economic development of the City, provides strategies and specific implementing actions, and establishes a basis for judging whether specific development proposals and public projects are consistent with the City’s plans and policy standards. The General Plan contains a Health and Safety Element, which acknowledges and mitigates the risks posed by hazards (e.g., fire). The General Plan includes the following policy applicable to seismic activity and geologic hazards: l Policy 8.1-G-1: Minimize the risk to life and property from seismic activity and geologic hazards in South San Francisco. City of South San Francisco Building Code The City Building Division enforces the minimum standards found in the various codes adopted by the state through the Building Standards Commission and as adopted and amended by the City Council. In particular, the City adopted by reference the California Building Standards Code, volumes 1 and 2 (2019 edition), as the building code for the City of South San Francisco.15 East of 101 Area Plan16 The East of 101 Area Plan, which was adopted in 1994 and most recently amended in 2016, sets forth specific land use policies for the East of 101 Area. The City interprets the East of 101 Area Plan as a design-level document. Per Policy IM-5, the Gateway Specific Plan is not affected by the land use regulations of the East of 101 Area Plan. Therefore, the policies in the General Plan Health and Safety Element are the guiding policies and supersede all Geotechnical Safety Element policies set forth in Chapter 10 of the East of 101 Area Plan. Nonetheless, applicable policies from the East of 101 Area Plan Geotechnical Safety Element are as follows: l Policy GEO-1: The City shall assess the need for geotechnical investigations on a project-by- project basis on site in areas of fill shown on Figure 17, and shall require such investigations where needed. l Policy GEO-2: Where fill remains under a proposed structure, project developers shall design and construct appropriate foundations. l Policy GEO-7: New slopes greater than 5 feet in height, either cut in native soils or rock, or created by placing fill material, shall be designed by a geotechnical engineer and should have an appropriate factor of safety under seismic loading. If additional load is to be placed at the top of the slope, or if extending a level area at the toe of the slope requires removal of part of the slope, the proposed configuration shall be checked for an adequate factor of safety by a geotechnical engineer. 15 South San Francisco Municipal Code Section 15.08.010. 16 City of South San Francisco. 1994. East of 101 Area Plan. Prepared by Brady and Associates. Available: https://www.ssf.net/home/showdocument?id=508. Accessed: May 8, 2020. City of South San Francisco Environmental Setting, Impacts, and Mitigation Geology and Soils 751 Gateway Boulevard Project 4.6-8 September 2020 ICF 0662.19 l Policy GEO-8: The surface of fill slopes shall be compacted during construction to reduce the likelihood of surficial sloughing. The surface of cut or fill slopes shall also be protected from erosion due to precipitation or runoff by introducing a vegetative cover on the slope or by other means. Runoff from paved and other levels areas at the top of the slope shall be directed away from the slope. l Policy GEO-10: In fill areas mapped on Figure 17, a geotechnical investigation to determine the true nature of the subsurface materials and the possible effects of liquefaction shall be conducted by the project developer before development. l Policy GEO-11: Development shall be required to mitigate the risk associated with liquefaction. l Policy GEO-12: Structural design of buildings and infrastructure shall be conducted according to the Uniform Building Code and appropriate local codes of practice which specify procedures and details to reduce the effects of ground shaking on structures. 4.6.4 Impacts and Mitigation Measures 4.6.4.1 Significance Criteria Based on Appendix G of the CEQA Guidelines, the proposed project would have a significant geology and soils impact if it would: l Directly or indirectly cause potential substantial adverse effects, including the risk of loss, injury, or death involving: ¡ Rupture of a known earthquake fault, as delineated on the most recent Alquist-Priolo Earthquake Fault Zoning Map issued by the State Geologist for the area or based on other substantial evidence of a known fault. (Refer to Division of Mines and Geology Special Publication 42.); ¡ Strong seismic ground shaking; ¡ Seismically related ground failure, including liquefaction; ¡ Landslides; l Result in substantial soil erosion or the loss of topsoil; l Be located on a geologic unit or soil that is unstable, or that would become unstable as a result of the project, and potentially result in onsite or offsite landslide, lateral spreading, subsidence, liquefaction, or collapse; l Be located on expansive soil, as defined in Table 18-1-B of the Uniform Building Code (1994), creating substantial direct or indirect risks to life or property; l Have soils incapable of adequately supporting the use of septic tanks or alternative waste water disposal systems where sewers are not available for the disposal of wastewater; or l Directly or indirectly destroy a unique paleontological resource or site or unique geologic feature. City of South San Francisco Environmental Setting, Impacts, and Mitigation Geology and Soils 751 Gateway Boulevard Project 4.6-9 September 2020 ICF 0662.19 4.6.4.2 Approach to Analysis Evaluation of the proposed project is based on the geotechnical investigation prepared for the project, unless otherwise noted.17 The geotechnical investigation concluded that the proposed project is feasible from a geotechnical standpoint, provided the recommendations included in the investigation are incorporated into project plans and specifications. In the California Building Industry Association v. Bay Area Air Quality Management District case, decided in 2015,18 the California Supreme Court held that CEQA does not generally require lead agencies to consider how existing environmental conditions might affect a project, except where the project would significantly exacerbate an existing environmental condition. Accordingly, placing new development in an existing or future seismic hazard area or an area with unstable soils is not considered an impact under CEQA unless the project would significantly exacerbate the seismic hazard or unstable soil conditions. Therefore, the analysis below evaluates whether the proposed project would exacerbate existing or future seismic hazards or unstable soils at the project site and result in a substantial risk of loss, injury, or death. Paleontological Resources The Standard Procedures for the Assessment and Mitigation of Adverse Impacts to Paleontological Resources ( Procedures)19 of the Impact Mitigation Guidelines Revision Committee of the Society of Vertebrate Paleontology include procedures for the investigation, collection, preservation, and cataloging of fossil-bearing sites. This includes the designation of paleontological sensitivity. The Procedures are widely accepted among paleontologists and followed by most investigators. The Procedures identify two key phases of paleontological resource protection, (1) assessment and (2) implementation. Assessment involves identifying the potential for a project site or area to contain significant, nonrenewable paleontological resources that could be damaged or destroyed by project excavation or construction. Implementation involves formulating and applying measures to reduce such adverse effects. For the assessment phase, the Society of Vertebrate Paleontology uses one of four sensitivity categories for sedimentary rocks (i.e., high, undetermined, low, no potential) to define the level of potential.20 l High Potential. Assigned to geologic units from which vertebrate or significant invertebrate, plant, or trace fossils have been recovered as well as sedimentary rock units suitable for the preservation of fossils (middle Holocene and older fine-grained fluvial sandstones, fine-grained marine sandstones, etc.). Paleontological potential refers to the potential for yielding abundant fossils, a few significant fossils, or recovered evidence for new and significant taxonomic, phylogenetic, paleoecologic, taphonomic, biochronologic, or stratigraphic data. 17 Langan Engineering and Environmental Services, Inc. 2019. Geotechnical Investigation, 751 Gateway Boulevard, South San Francisco, CA 75065-1501. November. Oakland, CA. 18 California Building Industry Association v. Bay Area Air Quality Management District, 62 Cal.4th 369. Opinion filed December 17, 2015. Available: https://caselaw.findlaw.com/ca-supreme-court/1721100.html. Accessed: March 13, 2020. 19 Society of Vertebrate Paleontology. 2010. Standard Procedures for the Assessment and Mitigation of Adverse Impacts to Paleontological Resources. Available: http://vertpaleo.org/Membership/Member- Ethics/SVP_Impact_ Mitigation_Guidelines.aspx. Accessed: March 12, 2020. 20 Ibid. City of South San Francisco Environmental Setting, Impacts, and Mitigation Geology and Soils 751 Gateway Boulevard Project 4.6-10 September 2020 ICF 0662.19 l Undetermined Potential. Assigned to geologic units for which little information is available concerning their paleontological content, geologic age, and depositional environment. In cases where no subsurface data already exist, paleontological potential can sometimes be assessed by subsurface site investigations. l Low Potential. Field surveys or paleontological research may determine that a geologic unit has low potential for yielding significant fossils (e.g., basalt flows). l No Potential. Some geologic units have no potential to contain significant paleontological resources (e.g., high-grade metamorphic rocks [gneisses and schists] and plutonic igneous rocks [granites and diorites]). The methods used to analyze potential impacts on paleontological resources and develop mitigation for the identified impacts followed the Society of Vertebrate Paleontology’s Procedures. l Assessment ¡ Identify the geologic units that would be affected by the project, based on the project’s depth of excavation—either at the ground surface or below the ground surface, defined as at least 5 feet below the ground surface. ¡ Evaluate the potential of the identified geologic units to contain significant fossils (paleontological sensitivity). ¡ Identify impacts on paleontologically sensitive geologic units as a result of near-term and longer-term construction and operation that involve ground disturbance. ¡ Evaluate impact significance. l Implementation ¡ According to the identified degree of sensitivity, formulate and implement measures to mitigate potential impacts. The potential of the project to affect paleontological resources is related to ground disturbance. Geologic units at the project site were identified through California Geological Survey regional maps.21 A determination regarding the presence of paleontological resources in the units was based on the fossil record, as documented by the University of California Museum of Paleontology.22,23 After the records search, the paleontological sensitivity of the units was assessed according to the Procedures.24 21 Wagner, D.L., E.J. Bortugno, and R.D. McJunkin. 1991. Geologic Map of the San Francisco-San Jose Quadrangle, California, 1:250,000. Available: ftp://ftp.consrv.ca.gov/pub/dmg/pubs/rgm/RGM_005A/ RGM_005A_SanFrancisco-SanJose_1991_Sheet1of5.pdf. Accessed: March 12, 2020. 22 University of California Museum of Paleontology. 2020. Advanced Specimen Search, San Mateo County. Available: https://ucmpdb.berkeley.edu/advanced.html. Accessed: March 12, 2020. 23 University of California Museum of Paleontology. 2020. Advanced Specimen Search, Franciscan Formation. Available: https://ucmpdb.berkeley.edu/advanced.html. Accessed: March 12, 2020. 24 Society of Vertebrate Paleontology. 2010. Standard Procedures for the Assessment and Mitigation of Adverse Impacts to Paleontological Resources. Available: http://vertpaleo.org/Membership/Member- Ethics/SVP_Impact_Mitigation_Guidelines.aspx. Accessed: March 12, 2020. City of South San Francisco Environmental Setting, Impacts, and Mitigation Geology and Soils 751 Gateway Boulevard Project 4.6-11 September 2020 ICF 0662.19 For the purposes of this analysis, an impact on paleontological resources was considered significant, thereby requiring mitigation, if it would result in any of the following: l Damage to, or destruction of, vertebrate paleontological resources. l Damage to, or destruction of, any paleontological resource that: ¡ Provides important information about evolutionary trends, including the development of biological communities; ¡ Demonstrates unusual circumstances in the history of life; ¡ Represents a rare taxon or a rare or unique occurrence; ¡ Is in short supply and in danger of being destroyed or depleted; ¡ Has a special and particular quality, such as being the oldest of its type or the best available example of its type; or l Provides information used to correlate strata for which it may be difficult to obtain other types of age information. 4.6.4.3 Impact Evaluation Impact GEO-1: The proposed project would not directly or indirectly cause potential substantial adverse effects, including the risk of loss, injury, or death involving rupture of a known earthquake fault, strong seismic ground shaking, seismically related ground failure, including liquefaction, or landslides. (Less than Significant) Fault Rupture As discussed in Section 4.6.2.3, Seismicity and Seismic Hazards, the project site is not within an Alquist- Priolo earthquake fault zone, and no known potentially active fault exists in the vicinity of the project site. In addition, the geotechnical investigation found no evidence of active faulting on the project site and concluded that the risk of surface faulting and consequent secondary failure from previous unknown faults is very low. Therefore, the project would not exacerbate the risk of surface fault rupture and this impact would be less than significant. No mitigation is required. Ground Shaking As discussed in Section 4.6.2.3, Seismicity and Seismic Hazards, the project site is in a seismically active area. The project site is expected to experience strong to violent ground shaking during a major earthquake.25 However, the proposed project would comply with the California Building Standards Code’s seismic requirements, which were established to reduce risks to life from damage to newly constructed buildings due to seismic hazards. Therefore, the project would not exacerbate the risk of ground shaking resulting from a seismic and this impact would be less than significant. No mitigation is required. 25 A “strong” earthquake is defined on the Modified Mercalli Intensity scale as an VI. It would be felt by all and cause damage to weak plaster, adobe buildings, and some masonry buildings. A “violent” earthquake is defined on the Modified Mercalli Intensity scale as a IX. It could cause some masonry buildings to collapse and other buildings shift off their foundations (see http://resilience.abag.ca.gov/shaking/mmi/). City of South San Francisco Environmental Setting, Impacts, and Mitigation Geology and Soils 751 Gateway Boulevard Project 4.6-12 September 2020 ICF 0662.19 Soil Liquefaction As discussed in Section 4.6.2.3, Seismicity and Seismic Hazards, the project site is within an area that is underlain by deposits of bay mud. The geotechnical investigation concluded that some underlying layers could liquefy during an earthquake. Therefore, the geotechnical investigation recommends that the building foundation be designed to accommodate localized settlement under the building footprint (i.e., up to 1 inch of differential liquefaction settlement between column locations). The proposed project would comply with the recommendations in the geotechnical investigation and standard regulatory requirements—including completion of a detailed geotechnical investigation required by the California Building Code, which are adopted by reference under the South San Francisco Building Code—and, therefore, would result in a less- than-significant impact related to seismically related ground failure, including liquefaction. No mitigation is required. Seismic Densification As discussed in Section 4.6.2.4, Expansive Soils and Weak Soils, the project site is underlain by approximately 13 feet of medium-dense to dense sand above the water table. This could result in seismically induced settlement of up to 2 inches within the proposed building footprint and 1 inch outside the proposed building footprint. Therefore, the geotechnical investigation recommends that the building foundation be designed to accommodate localized settlement under the building footprint and entrances be designed to accommodate settlement. The proposed project would comply with the recommendations in the geotechnical investigation and standard regulations required by the California Building Code, which are adopted by reference under the South San Francisco Building Code—and, therefore, would result in a less-than-significant impact related to densification-induced settlement. No mitigation is required. Lateral Spreading As discussed in Section 4.6.2.3, Seismicity and Seismic Hazards, the clayey and silty sands underlying the project site are not likely to be continuous; therefore, the potential for lateral spreading at the project site is low. The proposed project would comply with standard regulatory requirements—including completion of a detailed geotechnical investigation required by the California Building Code, which are adopted by reference under the South San Francisco Building Code—and, therefore, would result in a less-than-significant impact related to lateral spreading. No mitigation is required. Landslides As discussed in Section 4.6.2.5, Landslides, the project site has a gentle slope. It is not located in a landslide risk area; therefore, the potential for a landslide occurring at or near the project site is low. The proposed project would comply with standard regulatory requirements—including completion of a detailed geotechnical investigation required by the California Building Code, which is adopted by reference under the South San Francisco Building Code—and, therefore, would result in a less-than-significant impact related to landslides. No mitigation is required. City of South San Francisco Environmental Setting, Impacts, and Mitigation Geology and Soils 751 Gateway Boulevard Project 4.6-13 September 2020 ICF 0662.19 Impact GEO-2: The proposed project would not result in substantial soil erosion or the loss of topsoil. (Less than Significant) The project site, which is approximately 34 to 21 feet above mean sea level, slopes gently from west to east, toward Gateway Boulevard. The proposed project would require grading or disturbing an area of approximately 149,000 square feet during construction and excavating approximately 1,850 cubic yards of soil that would be reused as fill on the site. The proposed project would not involve substantial changes to the existing grade, and no unprotected, exposed soils at risk of substantial erosion would remain on the project site. As discussed in Section 4.10.4, Hydrology, construction activities associated with the proposed project must comply with the National Pollutant Discharge Elimination System (NPDES) Construction General Permit, the Municipal Regional Permit (MRP), and City’s General Plan and Municipal Code. These requirements include preparation and implementation of a stormwater pollution prevention plan (SWPPP) that incorporates best management practices (BMPs), such as the installation of erosion control measures (e.g., silt fences, staked straw bales/wattles, silt/sediment basins or traps), geofabric, sandbag dikes, covers for stockpiles, or storage precautions for outdoor material storage areas. Furthermore, the proposed project would comply with the City’s standard conditions of approval, which requires a grading permit prior to any onsite grading. The City’s grading permit requires applicants to have erosion control measures in place, such as de- silting basins, silt fences, asphaltic emulsions, hay bales, fabric and sand filters, swales, and/or sumps. Therefore, with adherence to the BMPs included in the SWPPP, compliance with the City’s standard conditions of approval regarding grading, and compliance with the California Building Standards Code, impacts related to soil erosion would be less than significant. No mitigation is required. Impact GEO-3: The proposed project would not be located on a geologic unit or soil that is unstable, or that would become unstable as a result of the project. (Less than Significant) As discussed under Impact GEO-1, some of the layers below the water table could be susceptible to liquefaction, resulting in settlement on the order of 1 inch after a seismic event. In addition, seismic densification could occur in the 13 feet of medium-dense to dense sand above the water table, resulting in about 0.5 inch of settlement. The geotechnical investigation estimated up to 2 inches of seismically induced settlement could occur within the proposed building footprint and 1 inch could occur outside the proposed building footprint. Sand boils and liquefaction-related ground fissures can occur when surface layers above the liquefiable soils are thin. Although liquefiable layers have been identified in borings, they are not continuous and are located 20, 30, 45, and/or 60 feet below ground surface. Therefore, the potential of sand boils or fissures during a seismic event is low. Lateral spreading is a phenomenon in which a surficial soil displaces along a shear zone that formed within an underlying liquefied layer. As discussed under Impact GEO-1, the geotechnical investigation determined that the potential for lateral spreading at the project site is low and instability would not occur as a result of the project. Weak soils can compress or subside under the weight of buildings and fill, causing settlement relative to the thickness of the weak soil. Usually the thickness of weak soil will vary, and differential settlement will occur. Weak soils also tend to amplify shaking during an earthquake and can be susceptible to liquefaction. The geotechnical investigation determined that the native soil at the foundation level of the project site has moderate to high strength and relatively low compressiblity. Therefore, the potential for settlement resulting from soil compression at the project site is low. City of South San Francisco Environmental Setting, Impacts, and Mitigation Geology and Soils 751 Gateway Boulevard Project 4.6-14 September 2020 ICF 0662.19 Dewatering, if it is extensive, can result in subsidence. To account for seasonal fluctuations in the groundwater level, the geotechnical investigation considered groundwater levels to be approximately 7.5 to 18.5 feet below ground surface.26 To accommodate utility trenches, the project would require a maximum depth of excavation reaching approximately 9 feet below ground surface. Given the range of groundwater elevation (7.5 feet to 18.5 feet below ground surface), the proposed depth of excavation (9 feet), the specific areas of excavation, and the limited duration of trenching activities, it is unlikely that groundwater would be encountered during project construction. Therefore, construction dewatering is not anticipated. Nonetheless, if excavation is performed during the wet season, the contractor would be prepared for dewatering. Because any dewatering would be limited in geographic extent, in the unlikely event that dewatering is needed, the amount of groundwater removed would be so small as not to pose a risk of subsidence. The proposed project would comply with the recommendations in the geotechnical investigation regarding the design of foundations, floor slabs, and other geotechnical aspects of this project. In addition, the proposed project would comply with regulations required by the California Building Code, which are adopted by reference under the South San Francisco Building Code. Therefore, impacts related to potential liquefaction, lateral spreading, soil compression, and settlement and subsidence due to dewatering in soil that is unstable, or could become unstable as a result of such construction, would be less than significant. No mitigation is required. Impact GEO-4: The proposed project would not be located on expansive soil, as defined in Table 18-1-B of the Uniform Building Code (1994), creating substantial direct or indirect risks to life or property. (Less than Significant) As discussed in Section 4.6.2.4, Expansive Soils and Weak Soils, near-surface samples of clay from the project site indicated that the site has low expansion potential, with plasticity indices of 7 to 15. The geotechnical investigation prepared for the project indicated that the project site has low expansion potential. The proposed project would comply with standard regulatory requirements—including completion of a detailed geotechnical investigation required by the California Building Code, which are adopted by reference under the South San Francisco Building Code—and, therefore, would result in a less-than-significant impact related to expansive soils. No mitigation is required. Impact GEO-5: The proposed project would not have soils that would be incapable of adequately supporting the use of septic tanks or alternative wastewater disposal systems where sewers are not available for the disposal of wastewater. (No Impact) The proposed project would connect to South San Francisco’s sewer and stormwater collection and treatment system. Therefore, the proposed project would not use a septic or alternative water disposal system and would have no impact. No mitigation is required. 26 According to Langan Engineering and Environmental Services, the preparer of the geotechnical investigation for the proposed project, the shallowest groundwater expected during the life of the project would be 7.5 feet below ground surface. This estimate does not account for seal level rise. Ultimately, groundwater levels will depend on season and precipitation. City of South San Francisco Environmental Setting, Impacts, and Mitigation Geology and Soils 751 Gateway Boulevard Project 4.6-15 September 2020 ICF 0662.19 Impact GEO-6: The proposed project could directly or indirectly destroy a unique paleontological resource or site or unique geologic feature. (Less than Significant with Mitigation) As discussed in Section 4.6.2.6, Paleontological Resources, both geologic units underlying the project site are known to have yielded significant fossils. The Colma Formation has yielded vertebrate fossils, and the Franciscan Formation has yielded fossils that are important in understanding this geologic unit. Therefore, the paleontological sensitivity of these geologic units is high, and both have the potential to contain significant fossils. Because paleontological resources are located below the ground surface, ground disturbances such as excavating, grading, and resurfacing can affect any paleontological resources that may be present. The proposed project would require grading or disturbing an area of approximately 149,000 square feet during construction. The proposed project would excavate approximately 1,850 cubic yards of soil that would be reused as fill on-site and would import an additional 750 cubic yards of soil to be used as fill on-site. To accommodate utility trenches, the project would require a maximum depth of excavation reaching approximately 9 feet below ground surface. Therefore, project construction would disturb geologic units with high paleontological sensitivity. Destruction of any paleontological resources present at the project site would constitute a significant impact. Implementation of Mitigation Measure GEO-1, Halt Construction Activity, Evaluate Find, and Implement Mitigation for Paleontological Resources, would reduce this significant impact on paleontological resources to less than significant with mitigation by providing training for construction personnel related to the possibility of encountering fossils. Construction personnel would learn the required actions to take in response to fossil discoveries, such as ceasing all earthmoving activities within 25 feet of any potential fossil find and providing for the recovery of fossils at the project site. Mitigation Measure GEO-1: Halt Construction Activity, Evaluate Find, and Implement Mitigation for Paleontological Resources In the event that previously unidentified paleontological resources are uncovered during site preparation, excavation, or other construction activity, the project sponsor shall cease or ensure that all such activity within 25 feet of the discovery cease until the resources have been evaluated by a qualified professional, and specific measures can be implemented to protect these resources in accordance with sections 21083.2 and 21084.1 of the California Public Resources Code. If the find is significant, a qualified paleontologist shall excavate the find in compliance with state law, keeping project delays to a minimum. If the qualified paleontologist determines the find is not significant then proper recordation and identification shall ensue and the project will continue without delay. 4.6.4.4 Cumulative Impacts Impact C-GEO-1: The project, in combination with past, present, and reasonably foreseeable future projects, would not result in a significant cumulative impact on geology and soils. (Less than Significant) In general, a project’s potential impacts related to geology and soils are individual and localized, depending on the project site and underlying soils. Each structure will have different levels of excavation, cut-and-fill work, and grading, which would affect local geologic conditions in different ways. Therefore, the geographic context for geology and soils is site-specific. The cumulative projects located within approximately 0.5 mile of the project site are described in Section 4.1.5, Approach to Cumulative Impact Analysis, in this draft EIR and shown in Figure 4.1-1. City of South San Francisco Environmental Setting, Impacts, and Mitigation Geology and Soils 751 Gateway Boulevard Project 4.6-16 September 2020 ICF 0662.19 The cumulative projects would be required to go through environmental and regulatory review and comply with the California Building Code. Each project would also be required to have a site-specific geotechnical investigation performed, which would provide design recommendations to reduce each project’s impacts. Similar seismic safety standards and conditions of approval would apply to the reasonably foreseeable future projects. For these reasons, the proposed project, in combination with other past, present, and reasonably foreseeable future projects, would not result in a significant cumulative geology and soils impact. The cumulative impact would be less than significant. No mitigation is required. Impact C-GEO-2: The proposed project would not result in a cumulatively considerable contribution to significant cumulative impacts on paleontological resources. (Less than Significant with Mitigation) The geographic context for paleontology is specific to the geologic unit(s) affected. The cumulative projects located within approximately 0.5 mile of the project site are described in Section 4.1.5, Approach to Cumulative Impact Analysis, in this draft EIR and shown in Figure 4.1-1. The cumulative projects could encounter paleontological resources. Depending on mitigation adopted for the cumulative projects, the cumulative impact could be significant. If paleontological resources are discovered during project construction, implementation of Mitigation Measure GEO-1 would ensure that the proposed project’s contribution to cumulative impacts paleontological resources would be less than cumulatively considerable with mitigation because it would require compliance with state law, which would ensure that any information that can be recovered from any recovered paleontological resources would be recorded and the find itself properly curated.