1311 W MUKILTEO BLVD 2022-06-29 - Laserfiche WebLink

Geotechnical Engineering Evalual • NGA File No. 1295621 Tsang Slope Restoration October 21,2021 Everett,Washington Page 2 B SCOPE The scope of our services pertaining to this agreement included the following: 1. A review of available soil and geologic maps of the area. 2. Exploring the subsurface soil and groundwater conditions in the vicinity of the existing retaining wall and on the slopes as necessary. 3. Mapping the site slopes and develop a geotechnical cross-section to qualitatively assess slope stability of the site. 4. Providing general recommendations for wall removal and regrading/restoration of the site. 5. Providing general recommendations for site drainage and erosion control. 6. Documenting the results of our findings, conclusions,and recommendations in a written geotechnical letter. Surface Conditions The property is rectangular in shape and covers 0.25 acres in area. It is currently occupied by a single- family residence, with a large driveway and parking area in the northeastern portion of the site. The property is bordered by West Mukilteo Boulevard to the southeast,and residential properties on all other sides. Topographically,the site consists entirely of a west to northwest-facing steep slope,terraced by a series of rockeries. The block wall ranges between 6- and 10 feet in exposed height and retains the concrete driveway and is composed of ecology blocks approximately 2.0 feet in width. The wall is embedded 2.0 feet below the ground surface. Two terraced rockeries underlying the block wall are approximately 9.0 feet in height and are separated by about 8 feet of terraced space. The lower portion of the site includes scattered boulders which do not retain significant material. Slope morphology and surface conditions are shown on the Cross-Section A-A' in Figure 3. Subsurface Conditions Geology: The geologic units for this site are shown on Geologic Map of the Everett 7.5-Minute Quadrangle, Snohomish County. Washington, by Minard, J.P. (USGS, 1985). The site is mapped-as 'Transitional Beds'(Qtb),which are an obsolete classification for a grouping of several geologic units both glacial and non-glacial in origin and lying stratigraphically beneath sands of the Vashon advance outwash. 'Transitional Beds' customarily consist of thick beds of gray clay, silt and fine to very fine sand. Advance outwash is generally described as relatively clean sand or pebbly sand. Our explorations within the disturbed portion of the slope generally encountered undocumented fill underlain by fine to coarse sand with varying amounts of gravel and silt,which we interpreted as glacial advance outwash deposits. NELSON GEOTECHNICAL ASSOCIATES, INC.