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 Footing Settlements: Assuming that structural fill soils are compacted to a medium dense or denser state,we estimate that total post-construction settlements of properly designed footings bearing on properly prepared subgrades will not exceed 1 inch. Differential settlements for comparably loaded elements may approach one-half of the actual total settlement over horizontal distances of approximately 50 feet. Footing Backfill: To provide erosion protection and lateral load resistance,we recommend that all footing excavations be backfilled on both sides of the footings and stemwalls after the concrete has cured. Either imported structural fill or non-organic onsite soils can be used for this purpose, contingent on suitable moisture content at the time of placement. Regardless of soil type,all footing backfill soil should be compacted to a density of at least 90 percent (based on ASTM:D-1557). Lateral Resistance: Footings that have been properly backfilled as recommended above will resist lateral movements by means of passive earth pressure and base friction. We recommend using an allowable passive earth pressure of 225 psf and an allowable base friction coefficient of 0.35 for site soils. 4.3 Slab-On-Grade Floors In our opinion, soil-supported slab-on-grade floors can be used if the subgrades are properly prepared. We offer the following comments and recommendations concerning slab-on-grade floors. Floor Subbase: Structural fill subbases do not appear to be needed under soil-supported slab-on- grade floors 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 subgrades should be vigorously surface compacted to a medium dense or denser condition before slab construction begins. Capillary Break and Vapor Barrier: To retard the upward wicking of moisture beneath the floor slab, we recommend that a capillary break be placed over the subgrade. Ideally, this capillary break would consist of a 4-inch-thick layer of pea gravel or other clean, uniform, well-rounded gravel,such as"Gravel Backfill for Drains"per WSDOT Standard Specification 9-03.12(4),but clean angular gravel can be used if it adequately prevents capillary wicking. In addition, a layer of plastic sheeting(such as Crosstuff,Visqueen,or Moistop)should be placed over the capillary break to serve as a vapor barrier. During subsequent casting of the concrete slab,the contractor should exercise care to avoid puncturing this vapor barrier. Migizi Group,Inc. Page 11 of 16