Appendix E - Geology and Soils Geotechnical Supporting Information COMBINED

Ninyo & Moore | Hidden Canyon Residences and Preserve, Pleasanton, California | 403620002 | February 12, 2025 Cornerstone Earth Group (2022) indicated that detailed mitigation recommendations regarding the on-site colluvial materials will be presented in future design-level geotechnical reports. For the larger suspected landslide mass on the Shriner parcel, Cornerstone Earth Group (2020) drilled two borings, EB-1 and EB-2, and used a dry rock coring technique to observe the underlying bedrock within the limits of the suspected landslide mass. Based on the subsurface information collected from the borings and one test pit, and their review of aerial photographs, they concluded that the landslide was not present. For proposed cut and fill slopes, Cornerstone Earth Group (2020) recommended that all cut slopes in bedrock have a maximum inclination of 2H:1V, and all cut slopes should be observed by an engineering geologist during site grading to confirm that cuts exposed relatively competent, favorably bedded rock. For fill slopes derived from on-site soil or bedrock, they recommended a maximum ratio of 2H:1V, and that fill slopes be overbuilt and trimmed back, exposing engineered fill when complete. If adverse bedding conditions are encountered during site grading, some slopes may need to be over-excavated and re-constructed as a buttress fill slope. Cornerstone Earth Group (2020) recommended that south-facing cut slopes greater than 10 feet high be over-excavated and reconstructed as an engineered fill buttress. Their report also includes preliminary recommendations for keyway sizes, construction parameters, and subdrainage. In general, they recommended that fill placed on existing ground inclined at 6:1 or greater, or new cut slope buttress fills, should be benched into the existing slope and a keyway constructed at the toe of the fill. Keyways are recommended to be at least 5 feet below grade or into competent bedrock, be at least 20 to 25 feet wide, and be angled slightly into the slope. Typical keyway and bench detail is shown on Figure 6 in the Cornerstone Earth Group (2020) report. Building Pad Preparation The Cornerstone Earth Group (2020) report includes recommendations for preparation of the proposed building pads and specific recommendations for building pads where a cut/fill transition may occur. They recommended over-excavation of the cut portion of the pad in order to provide a more uniform thickness of fill beneath the pads where a cut/fill transition occurs. They recommended a minimum of 5 feet of over-excavation in the cut portion for these pads. Typical cut/fill transition- over-excavation detail is shown on Figure 5 in the Cornerstone Earth Group (2020) report. Design-Level Geotechnical Evaluation The Cornerstone Earth Group (2020) report recommended that a design-level geotechnical investigation be performed for the project once detailed site development plans are available. The 4