2408 RAVENNA ST 2016-01-01 MF Import

( � ' . Co.�sultation C.etter � Planned Short Plat Road and Detention System Everett,Washington May 16,2000 NCA File No.2594B00 Page 3 However,we enticipate needing to adjust methods and e�ctent of the excavation during conshuction based on conditions observed. High ground water was encountered and should be expected to exist at the ground surface due to the proximiry to the wedand. The detention pipes should be desipied to withstand full hydrostatic uplift. Where properly compacted mineral wil is iu::d for backfill,we recommrnd using a soil unit weight of 60 pounds per cubic foot(pc�W resist uplift forces. This is adjusted fur the buoyent forces. This may be increased to 125 pcf for soil above the existing ground surface that will not be subject to high water levels of the wetlend. Assuming the granular backfill, the weight of overburden should be calculated based on a wedge of soil extending from the spring line of the pipe up at a 1 Vertical to 2 I-Iorizontal slope. An appropriate factor of safety should be used in this calculation. The alternative is to provide drainage around the pipe,assuming there is an appropriate outlet at tl�at elevation. We recommend that when excavating in peat or soft soils, ti:at a minimum width of soil of 4 feet exist between the pipe and the side of the trench at the spring line. This can taper down to the bottom of the trench. The bottom of the trench should be a minimum of the width of the pipe diameter plus 1 foot. All bac�ll should consist of an imported sand and giavel to give lateral support of the pipe. Since d►e subsurface conditions are expected to vary, some increase or dxrease in trnnch width may be appropriate. We should be rctained during construction to observe trench wnditions end provide recommendadons for the trench width. Displacing soft soils at the sides of t6e trench with rock spells may also be a suitable tectu»que. The spalls would be pushed into the soft soils using a hoe-pac. This would require that the sides of the trench sloped such that the spalls could be stacked and compacted into the soR soils. We would expect that this would need to be done prior to placing the pipe. A combinetion of trench width and spalls may be used based on conditions observed in 8ie field. The flexible pipes gaiu significant strength by maintaining the ci�ular shape. It is critical that this shape be maintained during the backfilling process. Intemal bracing of the pipe should be considered prior to placing the fill. Care should be taken to backfill evenly around the pipe to maintain its shape. Bedding of the pipe will also be critical. All peat should be removed from below the pipe. If soR soils such as the silt are observed below the pipe, care should be taken to not disturb these soils. A smoo ��S NELSON-COWRETTE& ASSOCIATES, INC.