1815 13TH ST SEATTLE CHILDRENS NORTH CLINIC Geotech Report 2022-10-25

We recommend that the condition of all subgrade areas be observed by GeoEngineers to evaluate whether the work is completed in accordance with our recommendations and whether the subsurface conditions are as anticipated. Below-Grade Utility Vault Wall Design A storm water vault is planned for the project and is proposed to be located below the parking lot to the east of the CUP building. Lateral earth pressures for design of storm water vault walls should be evaluated using an equivalent fluid density of 55 pcf if the walls will be restrained against rotation by the vault lid when backfill is placed. If the walls will not be restrained from rotation,we recommend using an equivalent fluid density of 35 pcf.Walls are assumed to be restrained if top movement during backfilling is less than H/1000,where H is the wall height. In addition,the walls should be designed for full hydrostatic pressures unless adequate drainage as described below can be provided and the wall drain pipes can be tight-lined to a suitable discharge location.For hydrostatic conditions,the wall should be designed using an equivalent fluid density of 90 pcf. These lateral soil pressures assume that the ground surface behind the wall is horizontal. Drainage for the vault can be provided as outlined in "Below-Grade Walls" below. These lateral soil pressures do not include the effects of surcharges such as traffic loads or other surface loading. Surcharge effects should be included as appropriate. If vehicles can approach the below-grade walls to within one-half the height of the wall, a traffic surcharge should be added to the wall pressure. For cars and small trucks, the traffic surcharge can be approximated using a uniform lateral surcharge pressure of 65 psf. Buoyancy and Uplift Given the low permeability of the native glacially consolidated soils,it is possible that perched water will be present in the backfill material surrounding the vault if no drainage is provided. If no drainage is provided buoyancy and uplift is a consideration. Resistance to uplift can be developed by the dead weight of the structure and friction along the sides of the structure. Frictional resistance can be computed using a coefficient of friction of 0.40 applied to the lateral soil pressures. This coefficient of friction is an allowable value and includes a factor of safety. We recommend that lateral soil pressures for uplift resistance be computed using an equivalent fluid density of 30 pcf. In addition to the above means of resisting uplift,the structure may be constructed with footings that extend beyond the structure walls so that the weight of overlying soil resists a portion of the uplift. For this purpose,the overlying backfill may be assumed to have a submerged unit weight of 60 pcf. Below-Grade Walls Conventional cast-in-place walls may be necessary for small retaining structures located on-site.The lateral soil pressures acting on conventional cast-in-place subsurface walls will depend on the nature,density and configuration of the soil behind the wall and the amount of lateral wall movement that can occur as backfill is placed. For walls that are free to yield at the top at least 0.1 percent of the height of the wall,soil pressures will be less than if movement is limited by such factors as wall stiffness or bracing. Assuming that the walls are backfilled and drainage is provided as outlined in the following paragraphs, we recommend that yielding walls supporting horizontal backfill be designed using an equivalent fluid density of 35 pcf (triangular GEOENGINEERSI April 22,2016 I Page 7 File No.10738-010-01