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October 19, 2021 <br />Page 9 of 12 <br />Geotechnical Evaluation <br />Test <br />Test <br />Measured <br />Correction Factors <br />Design <br />Number <br />Depth (ft) <br />Infiltration <br />Infiltration <br />Rate (in/hr) <br />Rate <br />CFv <br />CFT <br />CFM <br />(in/hr) <br />TP-2 <br />2.5 <br />0.75 <br />0.9 <br />0.5 <br />0.9 <br />0.30 <br />Localized shallow infiltration consisting of rain gardens is generally feasible; however, the depth <br />and location of any system must be reviewed by the geotechnical engineer to determine if <br />downgradient issues may be expected. Any system (permeable pavement or rain garden) must <br />have adequate overflow to an approved conveyance. <br />All systems should be lower in elevation than adjacent structures. It may be necessary to place <br />cutoff drains between systems and structures depending on the final grades. We should be on site <br />The dense glacial till acts as a restrictive layer in which infiltration is not feasible. Any system <br />must have adequate required clearance above the groundwater table or restrictive layer, generally <br />3.5 to 4.5 feet below grade. <br />We can provide additional recommendations upon request and once civil plans have been <br />prepared. <br />We should be provided with final plans for review to determine if the intent of our <br />recommendations has been incorporated or if additional modifications are needed. <br />Slab -on -Grade <br />We recommend that the upper 18 inches of the native soils within slab areas be re -compacted to <br />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-o6 Design of Slabs on Grade and ACI 302.1R-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 210 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. A 4- to 6-inch-thick capillary break layer should be placed <br />over the prepared subgrade. This material should consist of pea gravel or 5/8 inch clean angular <br />rock. <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 stormwater system. <br />www.cobaltgeo.com (2o6) 331-1097 <br />