1033 51ST PL SW Geotech Report 2022-10-28

The near-surface soil typically has a fines content in excess of 15 percent. In areas where moisture-sensitive floor slab and flooring will be installed, the installation of a vapor barrier is warranted to reduce the potential for moisture transmission through and efflorescence growth on the slab and flooring. If the project includes highly moisture-sensitive flooring,we recommend that 10-or 15-mil Stego Wrap be considered for this project. The recommended procedures for installing Stego Wrap are to pour the floor slab concrete directly over the vapor barrier. We recommend that the structural engineer be contacted to determine if the mix design for the concrete should be modified assuming the above-referenced construction sequence. Actual selection and design of an appropriate vapor'barrier, if needed, should be based on discussions among members of the design team. 7.4 RETAINING STRUCTURES We understand that a below-grade detention vault and tuck-under, below-grade walls maybe used for the project. We are providing the following general retaining wall recommendations for design of cast-in-place walls. Our retaining wall design recommendations are based on the following assumptions: (1) the walls consist of conventional, cantilevered retaining walls, (2) the walls are less than 10 feet in height,(3) the backfill is drained and consists of imported granular materials, and (4) the backfill has a slope flatter than 4H:1 V. Re-evaluation of our recommendations will be required if the retaining wall design criteria for the project varies from these assumptions. 7.4.1 Wall Design.Parameters For unrestrained retaining walls, an equivalent fluid density of 35 pcf should be used to estimate the active earth pressure. A superimposed seismic lateral force should be calculated based on a dynamic force-of 6.5H2 pounds perWlineal.foot of wall (where H is the height of the wall in feet) and applied a distance of 0.6H above the base of the wall. Where retaining walls are restrained from rotation prior to being backfiiled, an equivalent fluid density of 55 pcf should be used for design for the at-rest condition. If surcharges(e.g., building foundations, vehicles,etc.) are located within a horizontal distance from the back of a wall equal to twice the height of the wall, additional pressures wiil need to be • accounted for in the wall design. Our office should be contacted for appropriate wall surcharges based on the.actual magnitude and_configuration of the applied loads. The base of the wall footing excavations should extend a minimum of 12 inches below the lowest adjacent grade. The footing excavations should then be underlain by a minimum 6-inch-thick layer of crushed surfacing base course. The wallfootings should be,designed•-in-accordance with the"Shallow Foundations"section. 7.4.2 Wall Backfill Backfill material placed behind retaining walls and extending a horizontal distance of/ZH (where H is the height of the retaining wall) should consist of select granular material that meets the specifications provided in WSS 9-03.1,2(2)-Gravel Backfill for Walls. We recommend the select GEODESIGN= ' 6 NWHame-1-01:101618