ANEXO 2: Técnicas instrumentals utilizadas
A.2.2.1. Definición
Less Than Significant Impact. Liquefaction is a seismic phenomenon in which loose, saturated, granular soils behave similarly to a fluid when subject to high‐intensity ground shaking. Specifically, liquefaction occurs when the shock waves from an earthquake of sufficient magnitude and duration compact and decrease the volume of the soil; if drainage cannot occur, this reduction in soil volume will increase the pressure exerted on the water contained in the soil, forcing it upward to the ground surface. This process can transform stable soil material into a fluid‐like state. This fluid‐like state can result in horizontal and vertical movements of soils and building foundations from lateral spreading of liquefied materials and post‐ earthquake settlement of liquefied materials. Liquefaction occurs when three general conditions exist: 1) shallow groundwater; 2) low density non‐cohesive (granular) soils; and 3) high‐intensity ground motion. Studies indicate that saturated, loose to medium dense, near surface cohesionless soils exhibit the highest liquefaction potential, while dry, dense, cohesionless soils and cohesive soils exhibit low to negligible liquefaction potential. The Project Site is located within a City of Los Angeles‐designated Liquefaction Hazard Zone.6 In general, the Project Site is underlain by undocumented fills to a depth of 7 feet below ground surface (“bgs”) that consist of silty sand and some gravel. These undocumented fills were found to be loose to medium dense and dry to slightly moist. Below the undocumented fill material, alluvial soils were encountered. The alluvial soils were stiff to hard silts/clays, medium dense to very dense silty/gravelly sand, and sand. Generally, the soils below the proposed depth of the subterranean garage were found to be dense and very dense silty to gravelly sand. These alluvial soils were dry to slightly moist in the upper most 30 feet and slightly moist to saturated below 30 feet.
The Geotechnical Investigation evaluated whether the fine‐grained soils encountered on the Project Site (silts and clays) are susceptible to liquefaction. Groundwater was encountered in the borings at depths ranging from approximately 30 to 35 feet bgs, and the highest historic groundwater level for the site is approximately 20 feet bgs.
6
City of Los Angeles College Station Project
.
B‐13
The Geotechnical Investigation noted that the soil layers below the upper 20 feet, which are low plasticity silts/clays, have been found to be liquefiable in a seismic event; the fine grained soils encountered on the Project Site are considered to be low to moderately plastic, and were therefore evaluated as potentially liquefiable soils. On‐site soils encountered at depths greater than 20 feet in the borings were gravelly/silty sands to silty/clayey soils, were found to be slightly moist to wet, and based on blow counts and in‐situ densities were found to be medium dense to dense or stiff to hard.
Based on the liquefaction analysis considering a design groundwater elevation of 20 feet and a magnitude weighted peak ground acceleration of 0.4g, the results indicated that the sandy soils encountered in the upper soil layers were not potentially liquefiable; however some fine grained layers of silty/clayey soils were found to be potentially liquefiable between 35–45 feet bgs.
Based on the results of the previous liquefaction analysis, the Geotechnical Investigation concluded that the amount of total seismically induced settlement possible for the design conditions, with the remedial recommendations provided therein, is less than one‐quarter inch. Based on this small amount of settlement during the design seismic event, the Geotechnical Investigation concluded that the potential for dry sand settlement is considered negligible (i.e., less than a tenth of an inch) due the lack of dry, loose granular soils. Nonetheless, the Geotechnical Investigation contains design recommendations to account for one‐quarter inch of post‐construction seismic settlement, including design specifications for conventional and mat foundations.
In addition, as with any development project within the City, the Project would comply with the Uniform Building Code Chapter 18, Division 1, Section 1804.5, Liquefaction Potential and Soil Strength Loss, which requires the preparation of a final geotechnical report that outlines site‐specific design recommendations related to liquefaction and soil‐strength loss. Prior to issuance of the building permit, the Applicant would be required to submit the final geotechnical report to the City’s Department of Building and Safety (“LADBS”), which would review the report and issue an Approval Letter. The Project would be required to comply with the conditions contained within LADBS’s Approval Letter for the Project, which may be subsequently amended or modified. With adherence to the site‐specific design recommendation in the Geotechnical Investigation provided in Appendix B‐1 and the final geotechnical report and any subsequent modifications by LADBS, impacts with regard to liquefaction would be less than significant. No further evaluation of this topic in an EIR is recommended. No mitigation measures are required.
iv. Landslides?
No Impact. The Project Site is not located within a City‐designated Hillside Grading Area, is not subject to the City’s Hillside Ordinance, and is not located in a City‐designated Landslide area.7 Additionally, the ProjectSite is relatively flat, sloping very gently to the southeast. Further, the Project Site is not in immediate proximity to any mountains or steep slopes. As such, the Geotechnical Investigation concluded that there are no indications of landslides close to or within the Project Site. Therefore, the Project would not expose people or structures to potential substantial adverse effects involving landslides and no impact would result. No mitigation measures would be required and no further analysis of this topic in an EIR is recommended.
7
City of Los Angeles College Station Project
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B‐14