600 54TH ST SW I Geotech Report 2025-01-29

GEOTECHNICAL INVESTIGATION EVERETT,WASHINGTON March 2,2018 Additional seismic considerations include liquefaction potential and amplification of ground motions by soft/loose soil deposits. The liquefaction potential is highest for loose sand with a high groundwater table. The relatively dense soil deposits that underlie the site have a low potential for liquefaction. Areas of fill and loose sand could have a slight potential for liquefaction;however,following fill and loose soil mitigation below foundation elements will adequate reduce this potential for habited structures. 7.o DISCUSSION 7.1.i General It is our opinion that future single-family residences may be supported on shallow foundation systems bearing on medium dense or firmer native soils,re-compacted native soils,or on structural fill placed on native soils per the recommendations in Section 8.1.1 and 8.1.4 of this report. We encountered up to 4 and 7 feet of loose fill within the central and south-central portions of the property,respectively. In the south-central portion of the site,the fill locally contained garbage and other large debris(asphalt,concrete). These soils are not suitable for use as structural fill. Local overexcavation below footing,slab,and paved areas will likely be necessary depending on planned finish grade elevations and overall site grading. All excavations within the building footprints should be backfilled and compacted according to the recommendations in this report. Any filled areas beneath foundation elements should extend a lateral distance equal or greater than one-half the depth of the overexcavation in all directions from the faces of the footings. Soils below foundation elements should be compacted to at least 95 percent of the modified proctor or consist of medium dense to very dense native soils. Previously graded and oversteepened slope areas within building lots 7-io and the detention tract should be re-graded to more stable configurations. This could include benching into the native soils and creating nearly level yard areas of compacted structural fill. We can provide additional recommendations once grading plans have been developed. 8.o Recommendations 8.1.1. Site Preparation Trees,shrubs and other vegetation should be removed prior to stripping of surficial organic-rich soil. Based on observations from the site investigation program,it is anticipated that the stripping depth will range from 6 to 18 inches. Deeper excavations should be expected where undocumented fill is present and below any large trees where root systems extend to greater depths. The excavated material is not suitable as fill material within the proposed building envelopes but could be used as fill material in non-settlement sensitive areas such as landscaping regions.In these non- settlement sensitive areas,the fill should be placed in maximum 12 inch thick lifts that should be compacted to at least 90 percent of the modified proctor(ASTM D 1557 Test Method)maximum dry density. 6