Page 877 - anf1995elsmere_deis

Liooefgotion: Liquefaction is a phenomenon whereby soils lose their supportive capacity
and behave as a liquid during repeated cycles of strong shaking. Prerequisite conditions for
liquefaction are shallow groundwater (typically less than 50 feet) and saturated, loose,
cohesionless granular soils. Liquefaction occurs when the pore-water pressure in the soil

approaches its confining pressure due to seismic loading. Tinsley and others (1985) indicate
sand and silty sand deposits have the greatest susceptibility to liquefaction, while dense
deposits, gravelly and cobbly deposits, and deposits with more than 15 percent clay are less
susceptible. Based on data from project-specific investigation (Janes 1991; EMCON 1992),

including depth to groundwater and characteristics of geologic units, only alluvial deposits
would likely be potentially susceptible to liquefaction effects.

Sgismio sottlemontlDiffgl-gntial Qompgotion; Seismic settlement is the compaction or
consolidation of soils as a result of seisrnically induced ground shaking. Loose, sandy and/or
silty soils are typically most susceptible to seismic settlement. Differential compaction may
occur with variation in soil depth, soil density, and severity of ground shaking across a site.

Seismically-induced settlement can occur in both dry and partially saturated material.

The potential for seismic settlement and differential compaction within bedrock materials
at the project property is considered to be negligible to very low because of the typically
consolidated nature of sedimentary units. Loose and poorly consolidated landslide deposits

and recent alluvium would be more susceptible to these effects.

10.2 GEOTECHNICAL HAZARDS

Existing or potential geotechnical hazards at the project property include moderately
expansive soils, slope erosion, and slope instability. These are discussed below.

10.2.1 Expgngivo Soils

The shrink-swell potential of a soil refers to the anticipated volume change resulting from

changes in the soil’s moisture content. The primary cause of expansion or shrinkage is
volumetric change due to wetting and drying of clay minerals. The type and amount of clay
in the soil controls the amount of soil expansion. The layered sheet structure of certain
types of clay minerals gives them the capacity to absorb water molecules between these
sheets, thus causing expansion of the structure. When the mineral begins to dry, the water

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