200 W MARINE VIEW DR BAYWOOD Geotech Report 2025-08-11

October 16, 2020 Project No. T-8221 The preceding discussion is intended to be a brief review of the soil conditions observed at the site. More detailed descriptions of the subsurface conditions are summarized on the CPT Logs attached in Appendix A. Logs of test borings and test pits advanced at the site by others are provided in Appendix C. The approximate locations of the CPTs and test borings/test pits by others are shown on Figure 2. 3.3 Groundwater Dissipation tests at CPT-2 through CPT-4 indicate hydrostatic groundwater levels ranging from five to ten feet below existing grades at those locations. Groundwater levels would be expected to be higher during late winter to early spring(January through April) and highly influenced by tidal variations. 1 4.0 SEISMIC Liquefaction is a phenomenon where there is a reduction or complete loss of soil strength due to an increase in water pressure induced by vibrations. Liquefaction mainly affects geologically recent deposits of fine-grained sands ' underlying the groundwater table. Soils of this nature derive their strength from intergranular friction. The generated water pressure or pore pressure essentially separates the soil grains and eliminates this intergranular friction;thus, eliminating the soil's strength. We completed a liquefaction analysis using the computer program LiquefyPro published by CivilTech Corporation. The analysis was completed using a PGAM of.46 and a magnitude 7 earthquake. This PGAM represents acceleration ' for the maximum considered event (MCE)which is an earthquake having a 2 percent probability of exceedance in 50 years. The value was obtained from the Structural Engineers Association of California(SEAOC)U.S. Seismic Design Maps web site (https://seismicmaps.org/) accessed on September 20, 2019. The results of the liquefaction analysis are attached in Appendix B. The results of our analysis indicate soil liquefaction could occur during the design earthquake event. Analysis indicates that liquefaction could result in total settlements approaching four to seven inches, one-half of which would likely be differential in nature. In our opinion, this amount of settlement would not structurally impair the ' building. However,cosmetic damage to the structure in the form of misaligned doors and windows,cracking, and floor settlement could occur. If the owner is not willing to accept the risk of building damage requiring repair should liquefaction induced settlements occur, foundations should be supported on ground improved using stone columns designed to mitigate soil liquefaction settlements below the building foundations or on piles. The potential for lateral spreading to occur during a design level seismic event was evaluated by performing a Newmark analysis using the RocScience SLIDE program. Post liquefaction residual strengths were assigned to the lower sand layer below the groundwater table that are shown to liquefy during the design earthquake. The residual strengths were assigned using the average correlated standard penetration test "N" values determined at CPT-02 with Seed and Harder 1990. Ground motion used in the analysis was the seismic record for the 1995 Kobe Japan earthquake. This record was selected because it had a PGA of.5 which nearly matches the PGAM for the site. A I summary printout of the SLIDE Newmark analysis is also included in Appendix B. ' Page No. 3