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4. ENVIRONMENTAL ANALYSIS 4.7 GEOLOGY,SOILS.AND SEISMICITY This impact analysis focuses on potential project impacts related to seismic ground shaking, earthquake-induced landslides, and erosion. As discussed previously in this chapter, surface fault rupture and ground failures due to liquefaction are not addressed because these hazards are not considered impacts related to the project. This EIR does not address expansive soils,unstable geologic units,and soils capable of supporting septic systems because these issues are not related to the proposed aggregate extraction project. The evaluation considered project plans,current conditions at the project site, and applicable regulations and guidelines. 4.7.3 IWACTS AND MITIGATION MEASURES Impact 4.7.1: The steepness of temporary excavation slopes proposed during active mining could reach 3/4 :1 (Horizontal to Vertical)and therefore could be subjected to instability under certain static conditions or during an earthquake. Failure of temporary slopes in an active mining environment could cause landslides and subject a limited number of workers to injury and could result in some equipment damage. The proposed permanent 2:1(H:V) slopes could fail during moderate and strong earthquake ground shaking. Failure of the permanent 2:1(H:V)during an earthquake slopes could hamper future,post-mining reclamation efforts by increasing the potential for long term slope instability. This would be a potentially significant impact without mitigation. , At the proposed Vernalis quarry,removal and earthmoving of the sand and gravel deposits would result in excavations approaching 100 feet in depth. As proposed,the temporary excavation slopes exposed during active mining would reach an inclination of 3/4:1 (Horizontal to Vertical [H:V]) or equivalently,a gradient of 53 degrees. These temporary slopes would be unsupported, non-engineered,or otherwise unfinished and could be susceptible to failure during mining operations or during a major earthquake event. If these slopes become saturated during the rainy season, slope conditions could worsen and the potential for slope failure could increase. Permanent slopes under the reclamation plan are proposed at inclination of 2:1 (H:V) without intermediate benches. Slopes at 2:1 (H:V)are typically more stable than steeper slopes but can be subject to failure under earthquake ground motion and localized surface failures from erosion if not adequately engineered and constructed with slope protection measures. Failure of completed 2:1 (H:V) slopes could adversely affect efforts to reclaim the quarry by initiating future slope failure, causing excessive erosion,and limiting reuse of the reclaimed area for agricultural purposes. Landslides and shallow slope failures in non-engineered temporary slopes would be restricted to z localized area. If slope failure occurred in active mining areas, the down slope displacement of soil and rock could cause injury to workers in the immediate vicinity or could cover or compromise the stability of equipment working at the base of the failed slope. Given that mining operations involve select pieces of heavy excavation and earthmoving equipment with a limited number of operators,there would be a limited number of people exposed to slope failure hazards. In non-active mining areas with steep slopes,slope instability and failure would, in most cases, not result in injury or damage because workers or equipment do would not typically occupy these areas. In some areas,failure of steep temporary slopes could cause failure above the slope in non-mined phases. For example, slope failure could adversely affect areas adjacent to Phase II RMC Pacific Vernalis Quavy Mining,od Reclamation Pinim 4.7-16 ESA/203015 ` Administrative Dmrt©R March 2004