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ti <br />- DAVID L. NELSON AND ASSOCIATES, INC. <br />Consulting Engineering Geology <br />July 13, 1990 <br />Project No. 9490 <br />Page ft <br />disturbance and softening of the bearing soils during construction. The <br />thickness of the rock or gravel will be dependent on the material being <br />protected, and its use. <br />Pressures On Subgrade Halls <br />The design lateral earth pressures acting on retaining walls are a function of <br />the type of wall (for example, yielding or non -yielding), the method of <br />construction, and the type of soil used behind the wall. For planning <br />purposes for fixed or non -yielding walls, we recommend that the "at rest" <br />condition should be assumed, and that an equivalent fluid pressure of 50 <br />pounds per cubic foot (pcf) be used for design purposes. For yielding walls, <br />such as cantilever walls, the "active" condition should be assumed and an <br />equivalent fluid pressure of 35 pcf be used. To develop the "active" pressure <br />condition, the wall must be allowed to yield about its base a distance of <br />about 0.001 times its total height. <br />The effect of surcharges, such as traffic or floor loads, should also be <br />considered. For a uniformly distributed load behind the wall, a corresponding <br />uniformly distributed pressure equal to 30 percent or So percent of the <br />surcharge should be added to the lateral soil pressure for yielding and non - <br />yielding walls, respectively. if filling behind the wall is performed, <br />compaction of backfill within one-half of the wall embedment height behind the <br />wall should be performed with light equipment, such that the wall is not <br />adversely stressed. <br />For planning purposes, design for lateral earth pressures acting on foundation <br />walls could be -resisted in the native glacial bearing soils by the use of a <br />combination of foundation base friction and passive pressure. A base friction <br />coefficient of 0.10 is considered appropriate within the glacial deposits and <br />Whidbey Formation. A passive equivalent fluid pressure of 250 pcf should be <br />used for the on site soils. Where walls are located near slopes, the ground <br />surface in front of the wall should be level for a distance of at least 3 <br />times the depth of burial of the footing, to allow the above passive <br />