PC-2026-15 - Laserfiche WebLink

330 Land Company, LLC Arroyo Lago 9785.004.001 Design-Level Geotechnical Report Page | 5 April 16, 2024 Latest Revision October 31, 2025 2.3 FIELD EXPLORATION We observed drilling of four borings at the locations shown in the Site Plan, Figure 2. The locations of the explorations were selected to supplement previous exploration data. We retained the services of a drilling subcontractor who provided a crew operating a truck-mounted drill rig to advance the borings using 6-inch-diameter solid-flight and mud-rotary auger methods. We advanced the borings to depths ranging from approximately 50½ feet to 101½ feet below existing grade. An engineer observed the drilling and logged the subsurface conditions at each location. The borings were logged in the field and soil samples were collected using either a 2½-inch inside diameter (I.D.) California-type split-spoon sampler fitted with 6-inch-long brass liners, a 2-inch outside diameter (O.D.) standard penetration test split-spoon sampler, or a 3-inch O.D. Shelby Tube sampler. The penetration of the samplers was recorded as the number of blows needed to drive the sampler 18 inches in 6-inch increments. The boring logs record blow count results as the actual number of blows required for the last 1 foot of penetration; no conversion factors have been applied. We drove the samplers with a 140-pound hammer falling a distance of 30 inches employing an automatic trip system on the track-mounted rig. The locations of our explorations are approximately located, and we estimated their locations using cellular global positioning system and their proximity to existing site features; therefore, the locations shown should be considered accurate only to the degree implied by the method used. We permitted and backfilled our explorations in accordance with Zone 7 Water Agency requirements. We used the field logs to develop the report logs in Appendix A. The logs depict subsurface conditions at the exploration locations for the date of exploration; however, subsurface conditions may vary with time. 2.4 GEOLOGY AND SEISMICITY 2.4.1 Regional Geology The site is located in the California Coast Ranges geomorphic province, which is dominated by a series of northwest-trending mountain ranges that have been folded and faulted in a tectonic regime that involves both translational and compressional deformations . Bedrock in the Coast Ranges consists of igneous, metamorphic, and sedimentary rocks that range in age from Jurassic to Pleistocene. The site is located in the tri-valley basin located near the intersection of Livermore Valley, Amador Valley, and San Ramon Valley. The tri-valley basin is generally regarded as a trough of sediments within the Diablo Range. The basin is filled with Quaternary-age sediments derived from erosion of the surrounding highlands. The sediments have been divided into the Plio-Pleistocene Livermore Gravels and younger Pleistocene to Holocene alluvium. 2.4.2 Local Geology Geologic mapping prepared by Dibblee (2005) indicates the site is underlain by alluvial gravel, sand, and clay (Qa), while adjacent EPSP areas were mapped as Gravel Pits (GP) (Figure 3). In the site vicinity, Holocene flood deposits are mapped. In addition, general bedrock mapped in the adjacent hills consists of late-Pliocene to early-Pliocene-age Livermore gravel.