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f <br />October 6, 2oi9 <br />Page 4 of 6 <br />Limited GeotecImical Investigation <br />Resistance to lateral footing displacement can be determined using an allowable friction factor of <br />0.40 acting between the base of foundations and the supporting subgrades. Lateral resistance for " <br />footings can.also be developed using an allowable equivalent fluid passive pressure of 225 pounds <br />per cubic foot (pcf) acting against the appropriate vertical footing faces (neglect the upper 12 <br />inches below grade in exterior areas). <br />The allowable friction factor and allowable equivalent fluid passive pressure values include a <br />factor of safety of 1.5. The frictional and passive resistance of the soil may be combined without <br />reduction in determining the total lateral resistance. <br />Care should be taken to prevent wetting or drying of the bearing materials during construction. <br />Any extremely wet or dry materials, or any loose or disturbed materials at the bottom of the <br />footing excavations, should be removed prior to placing concrete. The potential for wetting or <br />drying ,of the bearing materials can Ibe reduced by pouring concrete as soon as possible after <br />completing the footing excavation and evaluating the bearing surface by the geotechnical engineer <br />or his representative. <br />Slab -on -Grade <br />We recommend that the upper 12 inches of the existing fill and/or native soils within slab areas be <br />re -compacted to at least 95 percent of the modified proctor (ASTM D1557 Test Method). <br />often, a vapor barrier is considered below concrete slab areas. However, the usage of a vapor <br />barrier could result in curling of .the concrete slab at joints. Floor covers sensitive to moisture <br />typically requires the usage of a vapor barrier. A materials or structural engineer should be <br />consulted regarding the. detailing of the vapor barrier below concrete slabs. Exterior slabs <br />typically do not utilize vapor.barriers. <br />The American Concrete -Institutes ACI 36oR-06 Design of Slabs on Grade and ACI 302.IR-04 <br />Guide for Concrete Floor and Slab Construction are recommended references for vapor barrier <br />selection and floor slab detailing. <br />Slabs on grade may be designed using a coefficient of subgrade reaction of 18o pounds per cubic <br />inch (pci) assuming the slab -on -grade base course is underlain, by structural fill placed and <br />compacted as outlined in Section 8.1. <br />A perimeter drainage system is recommended unless interior slab areas are elevated a minimum <br />of 12 inches above adjacent exterior grades. If installed, a perimeter drainage system should <br />consist of a 4 inch diameter perforated drain pipe surrounded by a minimum 6 inches of drain <br />rock wrapped in a non -woven geosynthetic filter fabric to reduce migration of soil particles into <br />the drainage system.- The >perimeter �-drainage system should discharge by gravity flow to -: a <br />suitable storinwater system. <br />Exterior grades surrounding buildings should be sloped at a minimum of one percent to facilitate <br />surface water flow, away from the building and preferably with a relatively impermeable surface <br />cover immediately adjacent to the building. <br />www.cobaltgeo.com (206) 331-1097 <br />