10930 HOLLY DR Geotech Report 2025-08-08

• Helix Design Group,Inc.—Greenwood Heating&Air Conditioning,Everett,WA April 22,2022 Geotechnical Engineering Report Z0219 Backfill Soil: Ideally, all subgrade wall backfill would consist of clean, free-draining, granular material, such as "Gravel Backfill for Walls" per WSDOT Standard Specification 9-03.12(2). A nonwoven geotextile such as Mirafi 140N or equal should be placed between the drainage zone and the backfill soil to prevent drain clogging. Backfill Compaction: Because soil compactors place significant lateral pressures on subgrade walls, we recommend that only small,hand-operated compaction equipment be used within 2 feet of a backfilled wall. Also,all backfill should be compacted to a density as close as possible to 90 percent of the maximum dry density (based on ASTM:D-1557); a greater degree of compaction closely behind the wall would increase the lateral earth pressure,whereas a lesser degree of compaction might lead to excessive post-construction settlements. Grading and Capping: To retard the infiltration of surface water into the backfill soils, we recommend that the backfill surface of exterior walls be adequately sloped to drain away from the wall. Ideally,the backfill surface directly behind the wall would be capped with asphalt,concrete, or 12 inches of low-permeability(silty)soils to minimize or preclude surface water infiltration. Applied Soil Pressure: Walls that are designed to move 0.1 percent of the wall height during and after construction are usually referred to as unrestrained walls. We recommend that unrestrained cantilever walls supporting slopes inclined at 2H:1V or flatter be designed to resist an active pressure (triangular distribution) of 55 pounds per cubic foot (pcf) for drained conditions. The recommended pressure does not include the effects of surcharges from surface loads,hydrostatic pressures,or structural loads. If such surcharges are to apply,they should be added to the above design lateral pressures. Wall Settlements: We estimate that the settlement of the wall footings constructed as recommended will be on the order of 1 inch or less. Most of this settlement is expected to occur as soon as the loads are applied. Differential settlement along the walls is expected to be 1 inch or less over a 50- foot span. 4_6 Asphalt Pavement Since asphaltic pavements will be used for the expanded driveway and parking areas,we offer the following comments and recommendations for pavement design and construction. Subgrade Preparation: Structural fill subbases do not appear to be needed under pavements at the site. However, the final decision regarding the need for subbases should be based on actual subgrade conditions observed at the time of construction. If a subbase is needed, all subbase fill should be compacted to a density of at least 95 percent(based on ASTM:D-1557). All soil subgrades should be thoroughly compacted,then proof-rolled with a loaded dump truck or heavy compactor. Any localized zones of yielding subgrade disclosed during this proof-rolling operation should be over excavated to a maximum depth of 12 inches and replaced with a suitable structural fill material. All structural fill should be compacted according to our recommendations given in Section 4.6. Specifically,the upper 2 feet of soils underlying pavement section should be Migizi Group,Inc. Page 13 of 16 rd,cozi