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Helix Design Group,Inc.-Greenwood Heating&Air Conditioning,Everett,WA April 22,2022
<br /> Geotechnical Engineering Report Z0219
<br /> The following sections of this report present our specific geotechnical cattati- ' 4.4
<br /> recommendations concerning site preparation, spread footings, slab-on-grade floors, drainage
<br /> systems,subgrade and retaining walls,asphalt pavement,and structural fill. The Washington State
<br /> Department of Transportation(WSDOT)Standard.Specifications and Standard Plans cited herein
<br /> refer to WSDOT publications M41-10, Standard Specifications for Road, Bridge, and Municipal
<br /> Construction,and M21-01,Standard Plans for Road, Bridge,and Municipal Construction,respectively.
<br /> 4_1 Site Preparation
<br /> Preparation of the project site should involve erosion control, temporary drainage, clearing,
<br /> stripping, excavations, cutting, subgrade compaction, and filling.
<br /> Erosion Control: Before new construction begins,an appropriate erosion control system should be
<br /> installed. This system should collect and filter all surface water runoff through silt fencing. We
<br /> anticipate a system of berms and drainage ditches around construction areas will provide an
<br /> adequate collection system. Silt fencing fabric should meet the requirements of WSDOT Standard
<br /> Specification 9-33.2 Table 6. In addition,silt fencing should embed a minimum of 6 inches below
<br /> existing grade. An erosion control system requires occasional observation and maintenance.
<br /> Specifically,holes in the filter and areas where the filter has shifted above ground surface should be
<br /> replaced or repaired as soon as they are identified.
<br /> Temporary Drainage: We recommend intercepting and diverting any potential sources of surface
<br /> or near-surface water within the construction zones before stripping begins. Because the selection
<br /> of an appropriate drainage system will depend on the water quantity,season,weather conditions,
<br /> construction sequence, and contractor's methods, final decisions regarding drainage systems are
<br /> best made in the field at the time of construction. Based on our current understanding of the
<br /> construction plans,surface and subsurface conditions,we anticipate that curbs,berms,or ditches
<br /> placed around the work areas will adequately intercept surface water runoff.
<br /> Clearing and Stripping: After surface and near-surface water sources have been controlled, sod,
<br /> topsoil, and root-rich soil should be stripped from the site. Our subsurface explorations indicate
<br /> that the organic horizon can reach thicknesses of up to 8 inches. Stripping is best performed during
<br /> a period of dry weather.
<br /> Site Excavations: Based on our explorations, we expect that project excavations will encounter
<br /> densely consolidated glacial till soils comprised of silty sand with few to some gravel. This soil
<br /> group can be readily excavated using standard excavation equipment, but special teeth, or
<br /> "rippers" may be needed in order to rapidly remove native soils.
<br /> Dewatering: We encountered perched groundwater seepage at a depth of 3 to 31/2 feet towards the
<br /> northeast corner of the project area, in the vicinity of test pit exploration TP-1. This seepage
<br /> occurred along the contact between,weathered and unweathered glacial till deposits.- Perched
<br /> groundwater may be encountered on site during extended periods of wet weather. If groundwater
<br /> is encountered during earthwork activities,we anticipate that an internal system of ditches,sump
<br /> holes, and pumps will be adequate to temporarily dewater excavations.
<br /> Migizi Group,Inc. Page 8 of 16
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