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COBALT GEOTECHNICAL INVESTIGATION GEOSCIENCES EVERETT,WASHINGTON December 24, 2019 8.1.6 Slab-on-Grade We recommend that the upper 18 inches of the existing fill and/or native soils within slab areas be re- compacted to at least 95 percent of the modified proctor(ASTM Di557 Test Method). Often,a vapor barrier is considered below concrete slab areas.However,the usage of a vapor barrier could result in curling of the concrete slab at joints. Floor covers sensitive to moisture typically requires the usage of a vapor barrier. A materials or structural engineer should be consulted regarding the detailing of the vapor barrier below concrete slabs. Exterior slabs typically do not utilize vapor barriers. The American Concrete Institutes ACI 36oR-o6 Design of Slabs on Grade and ACI 302.1R-o4 Guide for Concrete Floor and Slab Construction are recommended references for vapor barrier selection and floor slab detailing. A minimum 4 inch thick capillary break should be placed over the prepared subgrade. This may consist of pea gravel or 5/8 inch clean angular rock. Slabs on grade may be designed using a coefficient of subgrade reaction of i8o pounds per cubic inch(pci) assuming the slab-on-grade base course is underlain by structural fill placed and compacted as outlined in Section 8.i. A 4 inch thick capillary break material should be placed over the prepared subgrade. This could include pea gravel or 5/8 inch clean angular rock. A perimeter drainage system is recommended unless interior slab areas are elevated a minimum of 12 inches above adjacent exterior grades. If installed a perimeter drainage system should consist of a 4 inch diameter perforated drain pipe surrounded by a minimum 6 inches of drain rock wrapped in a non-woven geosynthetic filter fabric to reduce migration of soil particles into the drainage system. The perimeter drainage system should discharge by gravity flow to a suitable stormwater system. Exterior grades surrounding buildings should be sloped at a minimum of one percent to facilitate surface water flow away from the building and preferably with a relatively impermeable surface cover immediately adjacent to the building. 8.1.7 Groundwater Influence on Construction Groundwater was not encountered in any of the explorations. There is a chance that light groundwater could be encountered above the unweathered glacial till, where encountered. The depth to groundwater would likely be 5 to 8 feet below grade,if present. If groundwater is encountered, we anticipate that sump excavations and small diameter pumps systems will adequately de-water short-term excavations, if required. Any system should be designed by the contractor. We can provide additional recommendations upon request. 8.1.8 Utilities Utility trenches should be excavated according to accepted engineering practices following OSHA (Occupational Safety and Health Administration) standards, by a contractor experienced in such work. The contractor is responsible for the safety of open trenches. Traffic and vibration adjacent to trench walls should be reduced; cyclic wetting and drying of excavation side slopes should be avoided. Depending upon the location and depth of some utility trenches,groundwater flow into open excavations could be experienced,especially during or shortly following periods of precipitation. 7 PO Box 82243 Kenmore,WA 98028 cobaltgeoagmail.conl 2o6-331-1o97