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COBALT GEOTECHNICAL INVESTIGATION EVERETT, WASHINGTON December 5, 2019 8.1.E 'Groundwater Influence on Construction Groundwater was not encountered in any of the explorations. There is a chance that light groundwater could be encountered above the unweathered glacial till, where encountered. The depth to groundwater would likely be 5 to 8 feet below grade, if present. If groundwater is -encountered, we •anticipate that sump excavations and small diameter pumps systems will adequately de -water short-term excavations, if required. Any system should be designed by the contractor. We can provide additional recommendations upon request. 8.18 Utilities Utility trenches should be excavated according to accepted engineering practices following OSHA (Occupational Safety and Health Administration) standards, by a contractor experienced in such work. The contractor is responsible for the safety of open trenches. Traffic and vibration adjacent to trench walls should be reduced; cyclic wetting and drying of excavation side slopes should be avoided. Depending.upon the location and.depth of some utility trenches, groundwater flow into open excavations could be experienced, especially during or shortly following periods of precipitation. In general, silty soils were encountered at shallow depths in the explorations at this site. These soils have low cohesion and density and. -will have a tendency to cave or slough in excavations. Shoring or sloping back trench sidewalls is required within these soils in excavations greater than 4 feet deep. All utility trench backfill should consist of imported structural fill or suitable on site soils. Utility trench backfill placed in or adjacent to buildings and exterior slabs should be compacted to at least 95 percent of the maximum dry density based on ASTM Test Method D1557. The upper 5 feet of utility trench backfill placed in pavement areas should be compacted to at least 95 percent of the maximum dry density based on ASTM Test Method D1557. Below 5 feet, utility trench backfill in pavement areas should be compacted to at least 90 percent of the maximum dry density based on ASTM Test Method D1557. Pipe bedding should be in accordance with the pipe manufacturer's recommendations. The contractor is responsible for removing all water -sensitive soils from the trenches regardless of the backfill location and compaction requirements. Depending on the depth and location of the proposed utilities, we anticipate the need to re -compact existing fill soils below the utility structures and pipes. The contractor should use appropriate equipment and methods to avoid damage to the utilities and/or structures during fill placement and compaction procedures. 8.1.9 Pavement Recommendations The near surface subgrade soils generally consist of silty sand with gravel. These soils are rated as good for pavement subgrade material (depending on silt content and moisture conditions). We estimate that the subgrade will have. a California Bearing Ratio (CBR) value of 10 and a modulus of subgrade reaction value of k = 200 pei, provided the subgrade is prepared in general accordance with our recommendations. YVe-recommend that •at a -minimum,: 121nches,of.the :existing-subgrade material. be moisture conditioned (as necessary) and re -compacted to prepare for the construction of pavement sections. Deeper levels of recompaction or overexcavation and replacement may be necessary in areas where fill and/or very poor (soft/loose) soils are present. 10 PO Box 82243 Kenmore, WA 98o28 cobaltgeo-Punail.com 2o6-331-1097