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10.2.3 Sl e Stabili

Landslides: Six small landslides and one large landslide were mapped within the landfill

footprint. Other landslides of varying sizes were mapped within the project boundary, but
outside of proposed impact zones (Janes, 1991). The landslides are predominantly located
in the southwestern and western areas of the project within sedimentary rock units of the
Towsley and Pico Formations and Eocene rocks. One landslide was identified within the
igneous and metamorphic basement rocks of the San Gabriel Formation. Many of the

larger landslides identified on the property are located on large dip slopes and appear to
have occurred as failures along bedding planes that dip out-of-slope at less than the slope
angle (EMCON, 1992).

The primary contributing factors to the landslides at the project property appear to be the
removal of lateral support, surcharge, earthquakes, and composition (Janes, 1991). As
533.5570»;
shown on Plate 1, the majority of the landslides at the project property occur in the vicinity
of mapped faults. Numerous slides were mapped along the slopes adjacent to Whitney
Canyon fault and Elsmere field faults A and B (Plate 1). The topographic expression of the
fault traces at the project property are generally steep, northwest-southeast to north-south
oriented incised drainages, which do not provide lateral support to west and southwest

dipping beds. As drainages are incised, lateral support is removed, and slope failure occurs
in the weak sediments. This geometric relationship is most pronounced in the southern
portion of the project property. The siltstone members of the Upper Towsley Formation
and Pico Formation are the most susceptible to failure. Pervasive fracturing permits
percolation of precipitation which promotes deep weathering and shrink and swell of clay
minerals; this effect results in a decrease of shear strength, and promotes slope failure (Saul

and Wootton, 1983). Exploratory test pits in landslide material indicated slide deposits are
predominantly siltstone with some sand and silty sand. The siltstone material was generally
described as soft, weathered, and dry to moist, with no indication of sliding. The
coarser-grained material was generally described as medium dense, moist to very moist, and
highly weathered (EMCON, 1992a).

The uppermost siltstone member of the Towsley Formation appears to be the most
susceptible to slope instability due to unfavorable bedding structure and relatively weak
bedrock strength (EMCON, 1992b), and existing landslides tend to be larger in aerial extent.
Landslides involving Pico Formation are typically smaller and are located along steep slopes

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