4702 W GLENHAVEN DR 2018-05-16 - Laserfiche WebLink

47U2«'Ulenhati�en Drive CAO,Snohomish County,Vb'ashington Materials Testing& Consulting,Inc. Jule 31_ 20L7 7f�B03(� cemented hardpan to depths in eacess of 35 inches BPG. Stratigraphy varies in this soil unit and the depth to the hardpan can be as little as 20 inches. Permeability is, again, moderate to rapid in the surface and sub surface soils and very slow through the gravelly hardpan, but runoff is rapid and the hazard of water erosion is high in this soil unit. The Department of Ecology (DoE) Coastal Zone Atlas (CZA) online resource rnaps slopes closer to the ocean as unstable, and a eecent slide is mapped approximately 1000 feet the NE ofi the site. This recent slide does not appear to be associated with the slopes on the subject property. The project site and the immediate vicinity are mapped as stable. We caution that these resources are ultimately dated as well as regional in their interpretation of slope conditions. The mapping scale may not be directly suitable for site-specific interpretation of the degr-ee of stability based on mapped boundaries which bisect the site proximity, but should be considered as a Uroad context for the variabrlity and potential risk of the area. Discussion and Recommendations: The findings of MTC's subsurface e�plorations and slope reconnaissance at the project site appear generally consistent with available geol�gic literature and indicate favorable conditions exist for the scale of development. Based on available soil data from MTC's explorations regional groundwater level is interpreted to be below the base elevation of the slope and out of the realm of concern for this study- The existing slope face is generally vegetated with trees and common undergrowth. Signs of surface erosion are minimal and no indications of past or active slope instability were seen in the immediate site vicinity. Perched water may be of concern in the location of the proposed building site that can be mitigated ��vith appropriate drainage controls, exp(ained below. Soil creep due to loose, weathered surface soils which are presumed to cap denser, more consolidated soils at shallow depths along the within the site can be mitigated by long-term slope management, also described below. Further investigation in the rainy season may be necessary to more accurately assess perched water conditions. Based on the information herein, MTC provides the following sit� analysis, and development-specific, recommendations to be followed to minimize the inherent risks of development in this critical area. Building Setbnck The building setback may be measured froln the bottom of the footing to the face of significantly steep slopes comprising the bluffs west of the site, in accordance with the International Building Code (1805.3.1). All foundation elements shall be constructed on undisturbed native material or structural fill material per relevant IBC, ASTM, WSDOT, and Snohomish County standards within the existing setback distance. 5