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5.ENVIRONMENTAL CHECKLIST MOUNTAIN HOUSE NEIGHBORHOODS K AND L INITIAL STUDY 6. GEOLOGY AND SOILS Gni stress and result in damage to foundations. Expansive soils are commonly identified with an expansion index test that evaluates the percentage of clays and liquid limit. It is generally accepted that soils with an expansion index greater than 50 are susceptible to soil expansion. For Neighborhood K,surface residual soils were characterized as having an expansion index of 25 to 57. For Neighborhood L,the expansion index ranged from 12 to 40(Engeo, 2007b and 2010). Liquefaction Another issue is liquefaction potential. Liquefaction hazards are most common in loose to medium dense, granular and saturated soils;such soils include sands and silts in which the space between individual particles is completely filled with water. Liquefaction occurs when soil with little or no cohesion loses strength when saturated and starts to flow. This is triggered by shaking, such as that caused by an earthquake, or could be due to any significant ground vibration. Liquefaction can occur when the strength and density of a soil is decreased by seismic shaking. Liquefaction is caused by four main factors:depth of - groundwater,soil type, soil density, and the seismicity of the area. Liquefaction can be responsible for widespread structural failure, lateral spreading of liquefied deposits and overlying soils, and localized settlement of the ground surface. The liquefiable potential of site soils was determined by measuring penetration resistance using the Standard Penetration Test(SPT). The results of the analysis indicate that isolated areas of subsurface sands exist in Neighborhoods K and L that have the potential to liquefy during a strong seismic event. Latera!Spreading Lateral spreading is defined as the lateral movement of earth materials and overlying structures during an earthquake as a result of pore pressure build-up or liquefaction. Some of the proposed lake slopes in Neighborhoods K and L are located within potentially liquefiable material that has the potential for lateral spreading to occur during a design-level liquefaction event. However, due to the non-continuous nature of the potentially liquefiable material,the lateral movement in a design-level earthquake is expected to be minimal (Engeo,2007b and 2010) I Groundwater Generally,groundwater is a reflection of the surface topography. In the project area,depth to groundwater is expected to fluctuate in response to both seasonal rainfall and irrigation of surrounding farmland. During subsurface investigations conducted in 2007 and 2010,groundwater was encountered in all soil borings in Neighborhood K at depths of 3 to 12 feet below ground surface(bgs)and in Neighborhood L at 4.5 to 14.5 feet bgs. It was noted that dewatering of groundwater may be required for backfilling of irrigation ditches and installation of underground utilities(Engeo, 2007b and 2010). (gran) 5-76