9606 19TH AVE SE PREMIER STORAGE Geotech Report 2022-10-25

Premier Storage Everett, LLC ES-2432 June 26, 2012 Page 10 Revised June 28, 2012 Retaining walls should be backfilled with free draining material that extends along the height of the wall, and a distance of at least 18 inches behind the wall. The upper one foot of the wall backfill can consist of a less permeable soil, if desired. A perforated drain pipe should be placed along the base of the wall, and connected to an approved discharge location. A typical retaining wall drainage detail is provided on Plate 3. Drainacte Perched groundwater should be anticipated in site excavations depending on the time of year grading takes place. 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. In our opinion, foundation drains should be installed along building perimeter footings. A typical foundation drain detail is provided as Plate 4. Infiltration Based on the results of laboratory textural analysis of representative soil samples collected at the boring locations, the native soil would be classified as sandy loam using the USDA textural classification method. In accordance with the 2005 Department of Ecology Volume III — Hydrologic Analysis and Flow Control BMPs Table 3.7, a long term infiltration rate of 0.25 inches per hour can be used for design. In our opinion, sufficient overflow provisions should be 9 p incorporated into designs. Excavations and Slopes The Federal Occupation Safety and Health Administration (OSHA) and the Washington Industrial Safety and Health Act (WISHA) provide soil classification in terms of temporary slope inclinations. Based on the soil conditions encountered at the boring locations, where fill and/or groundwater seepage is exposed and the weathered native soils encountered in the upper approximately four to six feet are classified as Type C by OSHA/WISHA. Temporary slopes over four feet in height in Type C soils must be sloped no steeper than 1.5H:1V (Horizontal:Vertical). Medium dense to dense native soils encountered below about four feet where no groundwater seepage is exposed would be classified as Type A by OSHA/WISHA. Temporary slopes over four feet in height in Type A soils must be sloped no steeper than 0.75H:1 V. The presence of perched groundwater may cause caving of the temporary slopes due to hydrostatic pressure. ESNW should observe site excavations to confirm the soil type and allowable slope inclination. If the recommended temporary slope inclination cannot be achieved, temporary shoring may be necessary to support excavations. Permanent slopes should maintain a gradient of 2H:1 V, or flatter, and should be planted with vegetation to enhance stability and to minimize erosion. A representative of ESNW should observe temporary and permanent slopes to confirm the slope inclinations are suitable for the exposed soil conditions, and to provide additional excavation and slope recommendations, as necessary. Earth Solutions NW, LLC