917 112TH ST SE Geotech Report 2022-10-25

Habitat for Humanity of Snoh! iIsh County ES-5168 April 26, 2017 Page 9 Drainage Groundwater seepage was observed in a perched condition during the fieldwork (March 2017) at test pit locations TP-1, TP-3, and TP-4 at depths of approximately three to five feet below existing grades. Localized zones of seepage should be expected within site excavations, particularly during winter, spring, and early summer months. Temporary measures to control surface water runoff and groundwater during construction would likely involve interceptor trenches and sumps. ESNW should be consulted during preliminary grading to identify areas of seepage and to provide recommendations to reduce the potential for instability related to seepage effects. Surface grades must be designed to direct water away from buildings. The grade adjacent to buildings should be sloped away from the buildings at a gradient of at least 2 percent for a horizontal distance of at least four feet and up to ten feet (as building and property setbacks allow). In our opinion, perimeter footing drains should be installed at or below the invert of the building footings. A typical footing drain detail is provided on Plate 4 of this report. Infiltration Evaluation The unweathered glacial till soil deposits observed at relatively shallow depths exhibit very poor soil infiltration characteristics. Based on the observed shallow depth to dense unweathered glacial till deposits, we do not recommend utilizing infiltration systems such as trenches, dry wells, infiltration ponds, bioretention facilities, or perforated stub outs for the proposed development. On-site Stormwater Management Pursuant to Everett Municipal Code, implementation of on-site stormwater BMPs are required for proposed developments in,accordance with specified thresholds, standards, and lists. The intent of BMP implementation is to infiltrate, disperse, and retain stormwater runoff on site to the extent feasible. We understand the proposed project intends to add over 5,000 square feet of new hard surface and therefore must comply with Minimum Requirements (MRs) 1 through 9, as outlined on Pages 15 through 22 of Volume I of the 2012 DOE Mannual. MR 5 concerns on- site stormwater management, and the viability of specific BMPs are to be evaluated for each type of proposed surface. The table below summarizes our evaluation of the required BMPs for MR 5, from a geotechnical standpoint. Earth Solutions NW,LLC