2505 57TH ST SW Geotech Report 2022-10-25

August 29, 2019 G-4971 Mr. Jeff Davis Page 2 GEOLOGIC OVERVIEW According to published geologic mapping of the areal, the site soils are identified as Advance Outwash (QQ from the Fraser Glaciation. These deposits typically consist of well -bedded, medium- and fine-grained sands that were deposited in outwash channels created from the terminus of the glacier as it advanced southward into the Puget Sound area approximately 15,000 to 12,000 years ago. These deposits were overridden by the advancing ice and are dense to very dense as a result. SUBSURFACE INVESTIGATION On August 16, 2019, Mr. Bryce Frisher, Staff Geotechnical Engineer from our firm, visited the site to perform a visual reconnaissance of the property and investigate the subsurface soil conditions. We excavated two exploratory soil borings (HA-1 and HA-2) using hand -auger equipment and hand tools during our site visit. Borings HA -I and HA-2 were both located in the east back yard area north of the garage, as shown in Plate 2 — Site Plan. Logs of the conditions encountered in the borings are provided in Attachment I to this report. Soils encountered in boring HA-1 consisted of approximately 1 inch of gray sandy,fill, underlain with damp, loose to medium dense brown to light brown fine-grained sand with trace gravel to a depth of 11 inches. Soils between 11 inches and 18 inches were found to consist of damp, dense brown to light brown sand with silty fines and trace gravel. The boring was terminated at 20 inches due to hand auger refusal in the dense soils. Water seepage was not observed in the boring. Soils encountered in boring HA-2 consisted of approximately 1 inch of light gray sandy fill, underlain with damp, medium dense fine grained brownish gray sand up to a depth of 9 inches. Soils between 9 inches and 15 inches were found to consist of damp, dense grayish brown sand with gray silt and silty sand with trace gravel. The boring was terminated at 15 inches due to hand auger refusal in the dense soils. Water seepage was not observed in the boring. CONCLUSIONS AND RECOMMENDATIONS It is our opinion that the proposed residential addition as described in this report can be accomplished so that it does not increase the risk of soil instability at the site or on adjacent properties if the recommendations provided below are properly implemented. To this end, we recommend that new footings for the project be supported on the medium dense to dense native soils such as those found below a depth of 1.5 feet in borings HA -I and HA-2, located where the footings for the proposed addition will be. Details of these recommendations and other t Distribution and description of geologic units in the Mukilteo quadrangle, Washington, Miscellaneous Field Studies Map MF-1438, by J.P. Minard, 1982. GEO Group Northwest, Inc.