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• July 3,2017 Single-Family Residence LezA Job No. 17-079 Page 16 pressure, a passive lateral soil pressure of 300 pef EFD (ultimate value) may be used. This passive pressure value is applicable only to walls with a level or ascending backsiope away from the walls for a horizontal distance of at least 1_5 times the wall height To resist against sliding, the friction force between footings and subgrade soils may be calculated based on a coefficient of friction of 0.55 (ultimate value). The above soil parameters are ultimate values based on a frilly drained condition of the walls, ancl proper factors of safety should be applied in the design of the building basement walls and retaining walls against sliding and overturning failures. 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 and retaining 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 consist of compacted, clean, sandy soils. Heavy compaction equipment should not be allowed within the walls of a horizontal distance equal to that of the wall heights. A 4-inch perforated PVC footing drain pipe, as recommended in the DRAINAGE CONTROL section of this report, should be provided fo•r the foundation and retaining walls. The drain line should be hydraulically connected to the drainage blanket behind the walls and tightlined to the spreader pipe dispersion system. Retaining walls may be supported on footing foundations constructed on undisturbed advance outwash soil. Preparation of footing bearing soils, allowable soil bearing LIU & ASSOCIATES,INC.