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No. 57, as shown on Figure 3. A subgrade modulus of 150 pounds per cubic inch (pci) can be used in <br /> design. <br /> A vapor barrier should be utilized where control of moisture in the slab is critical (e.g., where an adhesive <br /> is used for tiled or carpeted floors).We recommend a vapor barrier consisting of polyethylene sheeting with <br /> bonded seams. <br /> 4.4. Stormwater Vault <br /> In our opinion,the proposed stormwater vault can be satisfactorily supported on the very dense glacial till <br /> or on a minimum 2-foot thickness of structural fill compacted as recommended in this report.An allowable <br /> bearing pressure of 3,000 psf is appropriate for design of foundations overlying structural fill. This value <br /> can be increased to 8,000 psf for footings supported on dense to very dense glacial till.To provide a level <br /> foundation pad and prevent disturbance, we recommend placing a minimum 6-inch-thick layer of crushed <br /> rock beneath the vault slab. We recommend a coefficient of subgrade reaction of 200 pci for a mat <br /> foundation supported on crushed rock overlying the hard/very dense native soils. This value should be <br /> reduced to 100 pci where the supporting soils consist of structural fill. <br /> We recommend that slab subgrade areas be evaluated by a geotechnical engineer immediately prior to <br /> placing the crushed rock to confirm that subsurface conditions are as expected and that the bearing surface <br /> has been prepared adequately. <br /> The effects of buoyancy under empty vault conditions should be considered in design. Buoyancy effects <br /> can be resisted by the weight of the concrete structure,the weight of zones of soil which are located above <br /> the slab floor which protrude beyond the walls, and soil friction along the sides of the walls. For design <br /> purposes, we recommend that hydrostatic uplift pressures be considered to within 5 feet of the ground <br /> surface. Frictional resistance can be computed using a coefficient of friction of 0.35 applied to the lateral <br /> soil pressures.The lateral soil pressure can be estimated using a fluid density of 50 pcf to a depth of 5 feet <br /> and 25 pcf below a depth of 5 feet.This coefficient of friction value includes a factor of safety of about 1.5. <br /> We estimate that the post-construction settlement of the mat foundation may be on the order of 1/2 inch or' <br /> less. Differential settlements measured over a distance of approximately 25 feet may be on the order of <br /> 1/4 inch. We expect that settlements for these conditions will tend to occur rapidly after the loads are <br /> applied. <br /> 4.5. Site Preparation <br /> The surficial soils possess high fines (silt) content such that repeated construction traffic will result in <br /> considerable disturbance and generation of mud during wet weather construction. If wet weather <br /> construction is required, we recommend the existing pavement surface be utilized for construction traffic <br /> to the extent practical. <br /> 4.6. Earthwork <br /> We recommend that all new fill brought to the site be placed and compacted as structural fill. A minimum <br /> 6-inch-thick capillary break layer should underlie all slabs-on-grade. We recommend a minimum 6-inch- <br /> thick base course layer be incorporated in new pavement areas.The base course should contain less than <br /> 5 percent fines to perform as a drainage layer as discussed in the Pavement Section of this report. <br /> GEOENGINEERS November 14,2019 Page 7 <br /> File No.2095-019-07 <br />