Appendix E - Geology and Soils Geotechnical Supporting Information COMBINED

Ninyo & Moore | Hidden Canyon Residences and Preserve, Pleasanton, California | 403620002 | February 12, 2025 underlain by Miocene age marine sedimentary rocks. The underlying sedimentary rock types include sandstone, siltstone, and siliceous shale. These rock units are folded and transected by northwest trending faults. The faults are considered inactive and are not located in an Alquist-Priolo Earthquake Fault Zone (CDMG, 1982). Bedding within the rock units is moderately to steeply dipping and varies in direction due to localized folding and faulting. The sedimentary rocks are overlain by colluvial soils up to 13 feet in thickness (Cornerstone Earth Group, 2020). The landslide hazard map by Majmundar (1996) shows the sloping portions of the Lester parcel within an area marginally susceptible to landsliding, and an area having a relatively high susceptibility to landsliding on the western end of the Shriners parcel. The previous investigation by Lowney Associates (1998) shows four shallow landslides within colluvium filled swales on the south- facing slopes above Devaney Canyon on the Lester parcel. Cornerstone Earth Group (2020) evaluated these landslide features and concluded these features are not the result of slope movements but are the result of seasonal downhill creep of the surficial colluvial soils. The Cornerstone Earth Group (2020) recommended the colluvial material be removed and replaced as an engineered fill buttress. Cornerstone Earth Group (2020) also evaluated the larger suspected landslide mass mapped by Majmunder (1996) on the western end of the Shriner parcel. They drilled two borings and excavated one test pit within the suspected landslide mass and concluded that a landslide is not present. GEOLOGIC ISSUES AND GEOTECHNICAL CONCERNS Our review focused on issues regarding slope stability and recommendations for preparation of the proposed building pads. The following summaries describe how the geotechnical consultant addressed these issues: Slope Stability The Cornerstone Earth Group (2020) report includes a discussion regarding the stability of the natural slopes and proposed cut and fill slopes. They described the presence of colluvial material that will be susceptible to creep and potential slope instability on the slopes above Devaney Canyon and a possible larger landslide mass on the Shriner parcel depicted on a regional landslide hazard map prepared by Majmundar (1996). The colluvial materials are depicted on Figure 2 of their 2020 report and are mapped in the swales that drain into Devaney Canyon. They recommended that the colluvial material be over-excavated and replaced as an engineered fill buttress. In some areas where colluvium over-excavation below keyways is not feasible, they indicated it may be necessary to stabilize the keyways with buried stitch piers, subsurface drains and/or geosynthetic grids. 3