4920 24TH AVE W Geotech Report 2023-06-22

• GeoTest Services, Inc. October 8, 2021 2405 Hillside Lane, Everett,WA Project No.21-0845 simismi footing or 12 inches below the adjacent floor slab grade (whichever is deeper)so that water will be contained. This process prevents water from seeping through walls or floor slabs. The drain system should include cleanouts to allow for periodic maintenance and inspection. GeoTest expects that perched groundwater seepage will be encountered atop the low permeability, previously placed fill soils and native Transitional Beds depending on the time of year. Depending on the final building elevations, an underslab drainage system may be required for the proposed buildings. We can provide further recommendations for underslab drainage upon request. Depending on final site grades and as previously discussed in the Landslide Hazard Areas section of this report, it may be prudent to construct a cut-off or perimeter drain,such as a French drain, to collect upslope water and divert it away from the proposed development. As preliminary grading plans had not been prepared at the time that this report was written, we can provide additional recommendations and consultation regarding site drainage improvements upon request. Please understand that the above recommendations are intended to assist the design engineer and/or architect in development of foundation and site drainage parameters and are based on our experience with similar projects in the area.The final foundation and site drainage plan that will be incorporated into the project plans is to be determined by the design team. Resistance to Lateral Loads The lateral earth pressures that develop against retaining walls will depend on the method of backfill placement, degree of compaction, slope of backfill, type of backfill material, provisions for drainage, magnitude and location of any adjacent surcharge loads, and the degree to which the wall can yield laterally during or after placement of backfill. If the wall is allowed to rotate or yield so the top of the wall moves an amount equal to or greater than about 0.001 to 0.002 times its height (a yielding wall), the soil pressure exerted comprises the active soil pressure. When a wall is restrained against lateral movement or tilting (a nonyielding wall), the soil pressure exerted comprises the at rest soil pressure. Wall restraint may develop if a rigid structural network is constructed prior to backfilling or if the wall is inherently stiff. GeoTest recommends that yielding walls under drained conditions be designed for an equivalent fluid density of 35 pounds per cubic foot (pcf) for structural fill in active soil conditions. Nonyielding walls under drained conditions should be designed for an equivalent fluid density of 55 pcf for structural fill in at-rest conditions. Design of walls should include appropriate lateral pressures caused by surcharge loads located within a horizontal distance equal to or less than the height of the wall. For uniform surcharge pressures, a uniformly distributed lateral pressure equal to 35 percent and 50 percent of the vertical surcharge pressure should be added to the lateral soil pressures for yielding and nonyielding walls, respectively. 14