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Interwest Construction Co.-Railway Avenue Improvements,Everett,WA Feb. 14,2019/Revised Feb. 7,2020 <br /> Geotechnical Engineering Report P1486-T18 <br /> would not be suitable for reuse as structural fill due to its high relative organics <br /> content. If clean, fine to medium sands are encountered, free of organics or wood, <br /> they may be well-suited for use as structural fill. <br /> Permanent Slopes: All permanent cut slopes and fill slopes should be adequately inclined to reduce <br /> long-term raveling,sloughing,and erosion. We generally recommend that no permanent slopes be <br /> steeper than 2H:1V. For all soil types, the use of flatter slopes (such as 21/2H:1V) would further <br /> reduce long-term erosion and facilitate revegetation. <br /> Slope Protection: We recommend that a permanent berm, swale, or curb be constructed along the <br /> top edge of all permanent slopes to intercept surface flow. Also, a hardy vegetative groundcover <br /> should be established as soon as feasible, to further protect the slopes from runoff water erosion. <br /> Alternatively,permanent slopes could be armored with quarry spalls or a geosynthetic erosion mat. <br /> 4.2 Subsurface Utilities <br /> Improvement plans for this project include a 12-inch water line and a 54-inch storm line. Listed <br /> below are excavation methods and construction considerations for the placement of the utilities. <br /> 4.2.1 Excavation Methods <br /> In our opinion,conventional trenchingmethods usingtrench boxes and open cuts are feasible at this <br /> P � p <br /> site. Construction dewatering may be required as discussed above. It should be expected that <br /> buried organics or timber could be present along the pipeline alignment which may be difficult to <br /> excavate. <br /> 4.2.2 Open Cut Slopes <br /> If open cuts are used,we expect that temporary slopes of 11/2:1 (horizontal to vertical)can be used as <br /> a guide for cuts in the loose to medium dense deposits encountered in the test borings. These <br /> guidelines assume that surface loads such as equipment loads and storage loads will be kept a <br /> sufficient distance away from the top of the cut so that the stability of the slope is not affected. The <br /> guidelines also assume that the excavations will be dewatered in such a way that significant <br /> seepage is not present on the slope face. Flatter slopes and/or shoring might be necessary for those <br /> portions of the excavations which are subjected to significant seepage in order to maintain the <br /> stability of the cut. All temporary slopes should be constructed in accordance with applicable <br /> WISHA Guidelines. <br /> It should be expected that the excavation face will experience some sloughing and raveling. Heavy <br /> plastic sheeting should be used to protect the exposed face from erosion due to rainfall. Berms, <br /> swales,or drainage ditches should be installed around the perimeter of the excavation to intercept <br /> surface runoff. <br /> At locations where a slope retention system is not required due to space limitations,we believe that <br /> the contractor should have the primary responsibility for the decision as to whether or not to use an <br /> open cut slope rather than a retention system,and for determining the slope angle. The presence of <br /> buried utilities close to the excavation should be considered when evaluating the need for a slope <br /> retention system. <br /> ' Migizi Group, Inc. Page 8 of 13 <br /> �Ijl <br />