PC-2026-15 - Laserfiche WebLink

330 Land Company, LLC Arroyo Lago 9785.004.001 Design-Level Geotechnical Report Page | 3 April 16, 2024 Latest Revision October 31, 2025 1.3 PROJECT DESCRIPTION We understand the development of the 26.6-acre site will comprise 194 market-rate single-family homes. Forty nine of the homes will be designed with affordable junior Accessory Dwelling Units. The development will also include paved drive-lanes and parking areas spanning the community footprint, an approximately 0.7-acre park located in the community center, 0.5 mile of public walking trails, concrete flatwork, underground utilities, retaining walls, and landscaping features. The vesting tentative map prepared by CBG dated July 2025, shows anticipated earthwork of up to 7 feet of fill and 4 feet of cut to achieve proposed pad elevations, with an average of 1 to 3 feet of fill across the site. We anticipate structures will be of wood-frame construction and one to two stories in height. Structural loads are yet to be determined; however, we assume that structural loads will be representative for this type of construction. 2.0 FINDINGS 2.1 SITE HISTORY Based on review of historical aerials, topographic maps, and provided geotechnical reports, we understand the site was historically part of a mining quarry. Open pit excavation operations within site limits began between 1979 and 1982, in which the site was divided into a larger north ern pit and a smaller southern pit (also known as the Busch Pit) at the southwestern end of the site. The remaining area to the east of Busch Pit was quarried and sloped to accommodate vehicular dirt roads along the boundary of the northern pit and Busch Pit. The approximate limits of Busch Pit are shown in Figure 2A; a historical aerial from 1984 depicting the approximate extents of the northern pit and Busch Pit are shown in Figure 2B. The northern pit appears to have been quarried to at least 160 feet below ground surface (bgs). We understand backfill operations of the northern pit were completed by 1993. The work was reportedly performed in accordance with the County of Alameda Surface Mining Permit Reclamation Plan (SMP-31), which indicated that the backfill was to be placed in the northern pit in a compacted manner; however, we are unaware of documentation detailing the fill specifications or operations. It is unknown whether the fill was placed in a manner that would be considered to be in conformance with the standard of practice at that time. Therefore, we consider the fill placed in the northern pit to be undocumented and potentially non-engineered. Busch Pit appears to have been quarried to at least 50 to 70 feet bgs at the same time as the northern pit. Busch Pit was utilized as a stormwater retention pond after the northern pit was filled under SMP-31. According to the testing and observation report provided by Haley & Aldrich dated December 18, 2013, Busch Pit was backfilled to match adjacent site grades in 2013. The fill was placed at a minimum of 90 percent relative compaction in accordance with ASTM D1557. A historical on-site stockpile was used as backfill material for Busch Pit, and generally consisted of sandy clay to clayey sand. The historical fill stockpile appears to vary from 5 to 15 feet in height. The stockpile was placed sometime between 1993 and 2002. The October 2003 photo shows a cut in the southwestern corner of the stockpile, presumably used as material to grade the residential lots to the northwest of the site. The stockpile resided in its October 2003 configuration