04 ATTACHMENT 6 - Laserfiche WebLink

Project No. E9164-04-0 2 - 5 - April 15, 2020 Seiches are large waves generated in enclosed bodies of water in response to ground shaking. No major water- retaining structures are located immediately up gradient from the project site. Flooding from a seismically- induced seiche is considered unlikely. 5. SOIL AND GROUNDWATER CONDITIONS 5.1 Alluvium Geologic references map Quaternary-age alluvial deposits at the site. In our soil borings, the deposits generally were observed as stiff to very stiff sandy lean clay and medium dense clayey sand and gravel below a depth of approximately 13 feet. Our laboratory testing indicates the alluvium has low expansion potential. 5.2 Orinda Formation Our Borings B2 and B4 through B7 encountered Tertiary age Orinda Formation at depths ranging approximately between 15 and 19 ½ feet. As encountered in our borings, the formational materials were observed as very dense and very weathered sandstone and mudstone/claystone. The formational materials extended to the maximum depths explored -- approximately 24 ½ feet below existing grade in Boring B4. 5.3 Groundwater Groundwater was not encountered in our soil borings to the maximum depth explored—approximately 24 ½ feet below existing grade. Some soil borings in the prior study by others encountered groundwater at depths of 15 to 19 feet. The borings for that study were performed in February of 2015. Since our borings did not encounter groundwater, nor did several of the borings in that previous study, we interpret the groundwater to be a seasonal and/or intermittent perched groundwater condition whereby groundwater accumulates in the coarse-grained alluvial soils atop the underlying dense formational materials. Actual groundwater levels will fluctuate seasonally and with variations in rainfall, temperature and other factors and may be higher or lower than observed during our study or discussed herein.