330 HEATHER RD 2020-06-17 - Laserfiche WebLink

Brian and Connie Hallgarth JN November 15, 2018 Page 2 At the crest of the steep slope, along the west edge of the rear yard, are several large evergreen (fir and cedar) trees. These have trunk diameters of 2 to 3 feet. They do not show any curvature indicative of recent or historic slope movement. The steep slope itself is overgrown, and we could not see indications of recent instability on the upper portion of the slope. Our review of the Geologic Map of the Everett Quadrangle (USGS) indicates that landslides have occurred along the base of the steep slope in the past. This localized shallow soil movement, particularly at the base of the slope along the creek, is consistent with our knowledge of the area. There are no indications of larger, deep-seated movement. According to the Geologic Map, the eastern portion of the property is underlain by glacial till, a glacially-compressed mixture of gravel, silt, and fine-grained sand. During our visit, we assessed the near-surface conditions by two test holes conducted in the proposed western addition. Both of these test holes exposed dense glacial till at a depth of approximately 2 feet below the existing grade. The Geologic Map indicates that the middle portion of the steep slope is comprised of advance outwash, a glacially-compressed, well-drained sand and gravel. The base of the slope is comprised of transitional beds, which are often fine-grained soils that, while they are also glacially- compressed, are more prone to slope movement. No seepage was observed in the shallow test holes. Perched groundwater is often found at least seasonally overlying the glacial till, and at the interface between the advance outwash and the underlying transitional soils. CONCLUSIONS AND RECOMMENDATIONS GENERAL THIS SECTION CONTAINS A SUMMARY OF OUR STUDY AND FINDINGS FOR THE PURPOSES OF A GENERAL OVERVIEW ONLY. MORE SPECIFIC RECOMMENDATIONS AND CONCLUSIONS ARE CONTAINED IN THE REMAINDER OF THIS REPORT. ANY PARTY RELYING ON THIS REPORT SHOULD READ THE ENTIRE DOCUMENT. Based on our observations, and available information, the planned addition is underlain at a shallow depth by competent, glacially-compressed soil. The footings for the new construction should be excavated to bear directly on dense soil. Any loosened or softened soil must be removed from footing areas before pouring concrete. This can require hand excavation. The planned addition will be set back over 30 feet from the crest of the steep western slope. Considering the geologic information, and our site observations, it is our professional opinion that the potential for deep-seated instability affecting the western yard area is negligible. Localized slides at the toe of the steep slope, or on the face of the slope may occur in the future, but this should have no adverse impact on the proposed addition. By maintaining the planned 30-foot minimum setback, and supporting the new footings directly on competent glacial till, 1)the planned development should be protected against foreseeable future slope movement, and 2) should not adversely affect slope stability on the site or the adjacent lots. No filling or excavation should occur within the 30-foot setback, and the existing vegetation on the steep slope should not be disturbed. Runoff from the small amount of new impervious area should be discharged to the storm system, consistent with the current house. While the steep slope meets the criteria for an Erosion Hazard Area, the planned project will not disturb the slope or the existing vegetation on the slope. As a result, the project will not increase GEOTECH CONSULTANTS,INC.