2211 GIBSON PL Geotech Report 2023-04-06

August 18, 2014 Gibson Bluff L&A Job No. 14-056 Page 15 For walls with ascending backslope, an additional pressure of 0.75 pcf per degree of the backslope angle above horizon should be added to the above design pressures. To counter the above active or at-rest pressure, a passive lateral soil pressure of 300 pcf EFD may be used. This passive pressure value is applicable only to walls with a level or ascending backslope for a horizontal distance at least 1.5 times the wall height. To resist against sliding, the friction force between the footings and the subgrade soils may be calculated based on a coefficient of friction of 0.55. The above soil parameters are ultimate values based on a fully drained condition of the walls, and proper factors of safety should be applied in the design of the basement walls against sliding and overturning failures. The foundation walls should also be designed for seismic loading based on a 100-year seismic event. Based on the soil conditions in the detention vault area, we recommend the vault walls be designed for an invert triangular lateral soil pressure diagram with the pressure at the top of the triangle to be 10H psf for a 100-year seismic event, where H is the height from finish grade at or above the top of the wall to the bottom of footings in feet. A one-third increase in the above recommended allowable soil bearing pressure may be used when considering the seismic loading condition. A vertical drainage blanket, at least 12-inch-thick horizontally, consisting of clean, free- draining, pea gravel or washed gravel should be placed against the back of foundation walls to within 18 inches of the finish grade to prevent accumulation of groundwater behind and buildup of hydrostatic pressure against the walls. This drainage blanket fill should be compacted to a non-yielding state with a vibratory compactor. Structural fill should be used for the remaining wall backfill. The top 18 inches of the backfill may LIU & ASSOCIATES, INC.