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r <br /> ' May 28, 2007 <br /> Proposed Office Building <br /> L&A Job No. 7A053 <br /> ' Page 11 <br /> such as Miradrain 6000 by Mirafi Inc. or equivalent, may be placed against the perimeter vault <br /> walls as the vertical drainage blanket. The vertical drainage blanket or drain mat should be <br /> ' hydraulically connected to the drain line at the base of the perimeter walls. Sufficient number of <br /> cleanouts at strategic locations should be installed for periodical cleaning of the vault wall drain <br /> line to prevent clogging. <br /> The perimeter walls of the detention vault would also serve as retaining walls to support cut <br /> banks and backfill. The perimeter walls of the vault capped with a lid would be restrained at <br /> Itheir top from horizontal movement and should be designed for at-rest lateral soil pressure. For <br /> the condition that groundwater behind the perimeter vault walls can be fully drained by the drain <br /> line provided at the base of the walls, we recommend an at-rest soil pressure of 50 pcf equivalent <br /> fluid density(EFD) be used for the design of vault perimeter walls. To counter the at-rest soil <br /> pressure, a passive lateral soil pressure of 375 pcf EFD may be used, except that the passive <br /> pressure within the top 12 inches of the finish subgrade should be ignored. The above passive <br /> pressure assumes the backslope of the walls is level or ascending away from the walls. The at- <br /> rest soil pressure may also be resisted by the friction force between the footings and the subgrade <br /> soils based on a coefficient of friction of 0.60. <br /> If the site grades are such that it is not feasible to completely drain groundwater behind the vault <br /> walls with a gravity drain line system, the hydrostatic pressure on the perimeter vault walls <br /> should also be taken into consideration for the design of the vault perimeter walls. For the <br /> condition that a perimeter drain line has to be placed higher than the footing level, the perimeter <br /> vault walls should be designed for a lateral soils pressure of 50 pcf EFD above the drain line <br /> level and a combined lateral soil and hydrostatic pressure of 80 pcf EFD below the drain line <br /> level. The above lateral pressures on the walls may be countered by a passive soil pressure of <br /> 375 pcf EFD above the drain line and 210 pcf EFD below. <br /> LIU & AS <br /> SOCIATES, INC. <br /> P <br />