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GeoTest Services, Inc. <br />2405 Hillside Lane, Everett, WA <br />October 8, 2021 <br />Project No. 21-0845 <br />For structures designed using the seismic design provisions of the International Building Code, <br />GeoTest recommends that retaining walls include a seismic surcharge in addition to the <br />equivalent fluid densities presented above. We recommend that a seismic surcharge of <br />approximately 8H (where H is the height of the wall) be used for design purposes. This surcharge <br />assumes that the wall is allowed to rotate or yield. If the wall is restrained, GeoTest should be <br />contacted so that we can provide a revised seismic surcharge pressure. <br />Passive earth pressures developed against the sides of building foundations, in conjunction with <br />friction developed between the base of the footings and the supporting subgrade, will resist <br />lateral loads transmitted from the structure to its foundation. For design purposes, the passive <br />resistance of well -compacted fill placed against the sides of foundations is equivalent to a fluid <br />with a density of 325 pcf. The recommended value includes a safety factor of about 1.5 and is <br />based on the assumption that the ground surface adjacent to the structure is level in the direction <br />of movement for a distance equal to or greater than twice the embedment depth. The <br />recommended value also assumes drained conditions that will prevent the buildup of hydrostatic <br />pressure in the compacted fill. Retaining walls should include a drain system constructed in <br />general accordance with the recommendations presented in the Foundation and Site Drainage <br />section of this report. In design computations, the upper 12 inches of passive resistance should <br />be neglected if the soil is not covered by floor slabs or pavement. If future plans call for the <br />removal of the soil providing resistance, the passive resistance should not be considered. <br />An allowable coefficient of base friction of 0.35, applied to vertical dead loads only, may be used <br />between the underlying imported granular structural fill and the base of the footing. If <br />foundations will bear directly on the native soil, an allowable coefficient of 0.30 should be used. <br />If passive and frictional resistance are considered together, one half the recommended passive <br />soil resistance value should be used since larger strains are required to mobilize the passive soil <br />resistance as compared to frictional resistance. A safety factor of about 1.5 is included in the base <br />friction design value. GeoTest does not recommend increasing the coefficient of friction to resist <br />seismic or wind loads. <br />Temporary and Permanent Slopes <br />The contractor is responsible for construction slope configurations and maintaining safe working <br />conditions, including temporary excavation stability. All applicable local, state, and federal safety <br />codes should be followed. All open cuts should be monitored during and after excavation for <br />evidence of instability. If instability is detected, the contractor should flatten the side slopes or <br />install temporary shoring. <br />Temporary excavations in excess of 4 feet should be shored or sloped in accordance with Safety <br />Standards for Construction Work Part N, WAC 296-155-66403. <br />15 <br />