1707 SCENIC DR SNO CO PUD Geotech Report 2022-10-25

Utility Trenches Trench excavation, pipe bedding, and trench backfilling should be completed using the general procedures described in the 2014 WSDOT Standard Specifications or other suitable procedures specified by the project engineer. The native and fill soils encountered at the site are generally of low corrosivity based on our experience in the Puget Sound area. Utility trench backfill above any pipe zone bedding should consist of structurai fill and should be placed in lifts of 8 inches or less (loose thickness) such that adequate compaction can be achieved throughout the lift. Sand backfill, containing less than 5 percent fines, may be compacted in loose lifts not exceeding 12 inches when placed below 5 feet of the finished ground surface and compacted using a hoe-pack. Each lift must be compacted prior to placing the subsequent lift. Prior to compaction, the backfill should be moisture conditioned in accordance with the criteria discussed above. Figure 3 illustrates recommended trench compaction criteria under hardscape, pavement and non-structural areas. Slope Construction and Regrading Construction of new fill slopes and regarding of existing slopes should be no steeper than 2H:1V, unless structurally reinforced. If existing fill is within the outer 8 feet (horizontal distance) from finished grade of new fill slopes, then existing fill should be removed within 8 feet of the slope face. The outer 8 feet from finished grade of all regraded slopes should also be removed and replaced with structural fill. Structural fill should be placed in approximately 6 inch loose lifts and compacted to at least 90 percent of the MDD per ASTM D 1557. Structural fill should be overbuilt by at least 2 feet and then cut back to the design grade prior to placing landscaping materials. The slopes should be track walked prior to placing landscape materials and hydroseeding. The bottom of the fill zone should be keyed into the existing soiis by at least 3 feet below the lowest adjacent grade. The key should be at least 6 feet wide. The structural fill material should be benched at least 8 feet into the existing slopes as described in Section 2-03.3(14) of the 2014 WSDOT Standard Specifications for embankment construction, and as shown on Figure 4. We recommend that the benches be at least 8 feet wide into the slope, with the vertical height of the benches be limited to no more than 3 feet. The horizontal portion of each bench should be sloped such that surface water runoff is directed downslope. A sand drainage blanket layer should be constructed below structural fill on all slopes, especially slopes located down slope of the infiltration trenches in the 115kV and 230kV yards, as sho�vn on Figure 4. In addition, the sand drainage layer should be constructed where native till soils are encountered in the slope cuts and where groundwater seepage is observed. The sand drainage blanket should be a minimum of 12 inches thick, consist of Sand Drainage Blanket material per WSDOT Standard Specification 9-03.13(1), and be placed on prepared native till soils. The drainage blanket should extend at least 6 feet above the key trench; however, it should extend higher if the native till is encountered in the cut at higher elevations, or if groundwater seepage is observed at higher elevations. The drainage blanket should extend at least 2 feet above observed seepage zones and observed native till contact. The blanket drain should daylight at the toe of the siope. During construction, GeoEngineers should evaluate the cut slope conditions and ground water seepage (if any), and provide additional recommendations for placement of the sand drainage blanket. GEOENGINEER� . October24,2014 Page9 Flle No.0482-027-02