2320 RAILWAY AVE Geotech Report 2023-02-01

Interwest Construction Co.-Everett Contractor Yard Site Improvements,Everett,WA March 12,2021 Geotechnical Engineering Report P2184-T20 Slope Protection: We recommend that a permanent berm,swale,or curb be constructed along the top edge of all permanent slopes to intercept surface flow. Also, a hardy vegetative groundcover should be established as soon as feasible, to further protect the slopes from runoff water erosion. Alternatively,permanent slopes could be armored with quarry spalls or a geosynthetic erosion mat. 4.2 Subsurface Utilities Improvement plans for this project include the introduction of extensive storm lines around the perimeter of the site, and various side sewers and water line stubs. Listed below are excavation I methods and construction considerations for the placement of the utilities. I I4.2.1 Excavation Methods In our opinion, conventional trenching methods using trench boxes and open cuts are feasible at this site. Construction dewatering may be required as discussed above. It should be expected that buried organics or timber could bepresent alongthe pipeline alignment which maybe difficult g p p g cu t to excavate. 4.2.2 Open Cut Slopes If open cuts are used,we expect that temporary slopes of 11/2:1(horizontal to vertical)can be used as a guide for cuts in the loose to medium dense deposits encountered in the test pits. These guidelines assume that surface loads such as equipment loads and storage loads will be kept a sufficient distance away from the top of the cut so that the stability of the slope is not affected. The guidelines also assume that the excavations will be dewatered in such a way that significant I I I I seepage is not present on the slope face. Flatter slopes and/or shoring might be necessary for those I portions of the excavations which are subjected to significant seepage in order to maintain the stability of the cut. All temporary slopes should be constructed in accordance with applicable WISHA Guidelines. I It should be expected that the excavation face will experience some sloughing and raveling. Heavy plastic sheeting should be used to protect the exposed face from erosion due to rainfall. Berms, swales,or drainage ditches should be installed around the perimeter of the excavation to intercept surface runoff. At locations where a slope retention system is not required due to space limitations,we believe that the contractor should have the primary responsibility for the decision as to whether or not to use an open cut slope rather than a retention system,and for determining the slope angle. The presence of buried utilities close to the excavation should be considered when evaluating the need for a slope retention system. 4.2.3 Lateral Pressures on Shoring Walls I The lateral soil pressures acting on shoring walls will depend on the ground surface configuration I behind the wall, and the amount of lateral wall movement which can occur as the excavation is made. For drained shoring that is free to yield at the top at least one-thousandth of the height of the Iwall, soil pressures will be less than if movement is limited by such factors as wall stiffness or bracing. The followingrecommendations applyto o s the case of shoring with up to one level of _1 I Migizi Group, Inc. Page 9 of 16 MIGILI lilili 1