Laserfiche WebLink
October 6, 2oi9. <br />Page 2 of 6 <br />Limited Geotechnical Investigation <br />The hand borings encountered approximately 6 inches of topsoil underlain by about 2.5 to 3.5 feet <br />of loose to medium dense, silty -fine to medium grained sand with gravel (Weathered Glacial Till). <br />These materials were underlain by medium `dense to dense, silty -fine to medium grained sand <br />with gravel (Glacial Till), which continued to the termination depth of the hand borings. <br />Groundwater was not encountered in the hand borings during our investigation. We anticipate <br />that perched groundwater could be encountered seasonally between weathered and unweathered <br />glacial till.' <br />..Recommendations <br />The proposed residential development is feasible from a geotechnical standpoint. If fill is <br />encountered within the building footprint, it will be necessary to overexcavate and replace those <br />materials with structural fill prior to foundation placement. <br />Infiltration of runoff from new impervious surfaces is not feasible. The site is underlain by fill <br />and glacial till, neither of which is suitable for infiltration. ' We recommend direction connection <br />to stormwater utility infrastructure. <br />Site Preparation <br />Trees, shrubs and other vegetation should be removed prior to stripping of surficial organic -rich <br />soil and fill. Based on observations from the site investigation program, it is anticipated that the <br />stripping depth will be 6 to 12 inches. Deeper excavations will be necessary below large trees and <br />in'anyareas underlain by undocumented fill materials. <br />The native soils consist of silty -sand with gravel. These soils may be used as structural fill- <br />provided they achieve compaction requirements and are within 3 percent of the optimum <br />moisture.. Some of these soils may only be suitable for use as fill during the summer months, as <br />they will be above the optimum moisture levels in their current state. - These soils are variably,' <br />moisture sensitive and may degrade during,periods of wet weather and under equipment traffic. <br />Imported structural fill should consist of a sand and gravel mixture with a maximum grain size of . <br />3 inches and less than 5 percent fines (material passing the U.S. Standard No. 200' Sieve). <br />Structural fill should be placed in maximum lift thicknesses of 12 inches and should be compacted <br />to a minimum of 95 percent of the modified proctor maximum dry density, as determined by the <br />ASTM D 1557 test method. <br />Temporary Excavations <br />Based'on our understanding of the project, we anticipate that the grading could include local cuts <br />on, the order'bf- Approximately 4feet or'less'•for°foundation and utility placement. Excavations <br />should be sloped no steeper than 1.5H:1V (Horizontal:Vertical) in loose native and fill soils,1HaV <br />in medium dense native soils, and 3/411:1V in dense to very dense native soils. If an excavation is <br />subject to heavy vibration or surcharge loads, we recommend that the excavations be sloped no <br />steeper than 2H:1V, where room permits. <br />Temporary cuts should be in accordance with the Washington Administrative Code (WAC) Part <br />N, Excavation, Trenching, -and -Shoring. Temporary slopes should be visually inspected daily by a <br />qualified person during construction activities and the inspections should be documented in daily <br />reports. The contractor; is responsible for maintaining the stability of the temporary cut slopes <br />and reducing slope erosion during construction. <br />www.cobaltgeo.com (206) 331-1097 <br />