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6.7.2 Excavation Dewatering. At the time. of this 'investigation; groundwater .was encountered as .shallow as' 9.5 feet below existing ground surface. As a result, dewatering or groundwater control is expected to be necessary for the installation of the underground storage tanks. It is recommended that the UST installation be conducted as close to the end of the dry season as possible to take advantage of any seasonal fluctuation in the groundwater level. The existence of and magnitude of any such fluctuation in the groundwater level at the site was beyond the scope of this investigation and thus not evaluated. It is recommended that just prior to excavating for the underground storage tanks, the depth to groundwater be determined and evaluation and design of the dewatering system be made based upon the depth to groundwater existing at that time. Dewatering of the excavation should draw down the groundwater surface to a level adequate for the prevention of bottom heave or "quick" conditions due to hydrostatic pressures. The proposed dewatering procedures should be reviewed by the Project Geotechnical Engineer prior to the start of the installation of any dewatering system. 6.7.3 Excavation Stability Information obtained from the field exploration indicates that the near -surface soils have sufficient strength to stand vertically for the depths expected to be required for the installation of the underground storage tanks. However, due to the high groundwater instability of vertical excavation, walls should be expected. As a result, it is recommended that the excavation walls be either shored or sloped at 0.5:1 (horizontal to vertical) or flatter. The stability of sloped excavation walls and need for flatter wall inclination will depend somewhat on the dewatering procedures and scheduling employed by the contractor. At no time should anyone be allowed to enter excavations that are not shored or sloped in accordance with CAUOSHA regulations. Surcharges from soil stockpiles, structures, vehicles, etc., should not be positioned within ten feet of the excavation. If solid sheet shoring is used in lieu of sloping the excavation side walls as recommended above, the shoring should be based upon the following lateral earth pressures. Active (cantilever): 30 psf per foot of depth. At Rest (braced): 50 psf per foot of depth Passive: 500 psf per foot of depth JN 4=7-095-41 U%WsWW7-06jm.dm 64