417 TAMARACK AVE Geotech Report 2025-08-19

September 23,2022 Page 6 of to Geotechnical Evaluation 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. 200 Sieve)based upon the fraction passing the U.S. Standard No.4 Sieve with at least 3o 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. Stormwater Management Feasibility The site is underlain by weathered and unweathered Vashon Glacial Till. The till becomes cemented at shallow depths and acts as a restrictive layer. The shallow soils were mottled and there is a high likelihood of perched interflow during the wet season. The depth to groundwater could be 3.5 to 8 feet below grade during later winter and spring months. Widespread infiltration is not feasible due to the presence of widespread fill and a restrictive layer at shallow depths. We anticipate that perforated or direct connection to City or County infrastructure will be utilized. We can provide additional recommendations once a civil plan has been prepared. Other shallow systems could be feasible depending on their locations and elevations. Systems other than dispersion devices should not be situated in or above undocumented fill. We should be provided with final plans for review to determine if the intent of our recommendations has been incorporated or if additional modifications are needed. Slab-on-Grade We recommend that the upper 12 inches of the native soils within slab areas be re-compacted to at least 95 percent of the modified proctor (ASTM Di557 Test Method). Any fill should be removed and replaced with imported structural fill or clean angular rock. Often, a vapor barrier is considered below concrete slab areas. However, the usage of a vapor barrier could result in curling of the concrete slab at joints. Floor covers sensitive to moisture typically requires the usage of a vapor barrier. A materials or structural engineer should be consulted regarding the detailing of the vapor barrier below concrete slabs. Exterior slabs typically do not utilize vapor barriers. The American Concrete Institutes ACI 36oR-o6 Design of Slabs on Grade and ACI 3o2.1R-04 Guide for Concrete Floor and Slab Construction are recommended references for vapor barrier selection and floor slab detailing. www.cobaltgeo.com (206)331-1097