9530 HOLLY DR 2025-08-05 - Laserfiche WebLink

October 19, 2021 Page 6 of 12 Geotechnical Evaluation Passive Earth Pressure on Low Side of Wall Neglect upper 2 feet, then 275 pcf EFD1 (Allowable, includes F.S. - 1.5) Soil -Footing Coefficient of Sliding Friction (Allowable; 0.40 includes F.S. - 1.5) *H is the height of the wall; Increase based on one in 500 year seismic event (to percent probability of being exceeded in 50 years), 'EFD — Equivalent Fluid Density The stated lateral earth pressures do not include the effects of hydrostatic pressure generated by water accumulation behind the retaining walls. Uniform horizontal lateral active and at -rest pressures on the retaining walls from vertical surcharges behind the wall may be calculated using active and at -rest lateral earth pressure coefficients of 0.3 and 0.5, respectively. A soil unit weight Of 125 pcf may be used to calculate vertical earth surcharges. To reduce the potential for the buildup of water pressure against the walls, continuous footing drains (with cleanouts) should be provided at the bases of the walls. The footing drains should consist of a minimum 4-inch diameter perforated pipe, sloped to drain, with perforations placed down and enveloped by a minimum 6 inches of pea gravel in all directions. The backfill adjacent to and extending a lateral distance behind the walls at least 2 feet should consist of free -draining granular material. All free draining backfill should contain less than 3 percent fines (passing the U.S. Standard No. 20o Sieve) based upon the fraction passing the U.S. Standard No. 4 Sieve with at least 30 percent of the material being retained on the U.S. Standard No. 4 Sieve. The primary purpose of the free -draining material is the reduction of hydrostatic pressure. Some potential for the moisture to contact the back face of the wall may exist, even with treatment, which may require that more extensive waterproofing be specified for walls, which require interior moisture sensitive finishes. We recommend that the backfill be compacted to at least 90 percent of the maximum dry density based on ASTM Test Method D1557. In place density tests should be performed to verify adequate compaction. Soil compactors place transient surcharges on the backfill. Consequently, only light hand operated equipment is recommended within 3 feet of walls so that excessive stress is not imposed on the walls. Sformwafer Management Feasibility The site is underlain by relatively dense glacial till. The unweathered till was partially cemented and acts as a restrictive layer. We performed a small scale pilot infiltration test (PIT) in TP-2. The test was performed in general accordance with the Washington State Department of Ecology stormwater manual. The area was excavated to a testing depth of approximately 2.5 feet below the ground surface. The design infiltration rate was determined by applying correction factors to the measured infiltration rate as prescribed in Volume III, Section 3.3.6 of the DOE. The measured rate must be reduced through appropriate correction factors for site variability (CFv), uncertainty of test method (CFT), and degree of influent control (CFM) to prevent siltation and bio-buildup. It should be noted that construction traffic or other disturbance to the target infiltration area could compact the soil, which may decrease the effective infiltration rates. The correction factors and resulting design infiltration rate are also shown in the table below. www.cobaltgeo.com (2o6) 331-1097