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Geotechnical Engineering Evaluation NGA File No. 1033518 Larsen Property Garage May 8,2018 Everett;;Washington Page 12 Materials: Structural fill should consist of a good quality, granular soil, free of organics and other deleterious material,and be well graded to a maximum size of about three inches. All-weather structural fill should contain no more than five-percent fines (soil finer than U.S. No. 200 sieve, based on that fraction passing the U.S. 3/4-inch sieve). Some of the on-site soils could be used as structural fill,but that will highly depend on the moisture content of the material at the time of construction. We should be retained to evaluate proposed structural fill material prior to placement. Fill Placement: Following subgrade preparation, placement of structural fill may proceed. All filling should be accomplished in uniform lifts up to eight inches thick. Each lift should be spread evenly and be thoroughly compacted prior to placement of subsequent lifts. All structuralfill should be compacted to a minimum of 95 percent of its maximum dry. density. Maximum dry density, in this report,refers to that density as determined by the ASTM D-1557 Compaction Test procedure. The moisture content of the soils to be compacted should be within about two percent of optimum so..that a readily compactable condition exists.. It may be necessary to over-excavate and remove wet soils in cases where drying to a compactable condition is not feasible.. All compaction should be accomplished by equipment of a type and size sufficient.to attain the desired degree of compaction. Slab-on-Grade We understand that a slab-on-grade is proposed for the new garage structure. We encountered up to three feet of unsuitable fill soils underlying the proposed slab area. Slabs placed on fill could be at risk for cracking and settlement over time. Depending on tolerance to,future slab settlement,varying degrees of subgrade preparation can be utilized. If no settlement or cracking of the new slab can be tolerated, the entire slab area should be overexcavated to expose competent native soils encountered at depth and the overexcavation backfilled with granular structural fill soils: The slab can then be supported directly on the structural fill soils. However, if some slab settlement, cracking, and future maintenance of the slab can be tolerated, the existing fill should be partially over-excavated and replaced with crushed rock. We recommend that at least 18 inches of the undocumented fill be overexcavated and replaced with 2-inch crushed rock for support of the new slab. The crushed rock should be placed in maximum 8-inch lifts and each lift compacted to a firm unyielding condition. The new slab could be supported directly on the crushed rock. The slab should be additionally reinforced and cold joints should be incorporated into the slab design to further reduce the effects of differential settlement. Some slab settlement, differential settlement, and cracking should still be.expected if the slab is not supported on structural fill extending down to the competent native soils encountered at roughly three feet below current ground surface. NELSON GEOTECHNICAL ASSOCIATES,INC.