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COBALT GEOSCIENCES GEOTECHNICAL INVESTIGATION EVERETT, WASHINGTON December 5, 2019 8.1.5 Stormwater Management The site is underlain by glacial till. Infiltration is typically not feasible in the areas underlain by glacial till. We performed asmall-scale pilot infiltration test in TP-3 at a depth of 4 feet obelo g and ade. F lcowi ng ntl testing and application of correction factors for site variability (0.33)� g ( 4)� (0.9), the infiltration rate was determined to"be 0.24 inches per hour. This rate is lower than what is considered to be feasible for widespread infiltration. permeable pavemen for new Depending on the final site layout and grading, it maybe plots the existing driveway area that driveways. We understand that there is a detention vaultnssiplace within will be utilized for stormwater. We can provide additional options for stormwater management upon request. We should be provided with final plans for review to determine if the intent of our recommendations has been incorporated or if additional modifications are needed. Sa.6 Slab -on -Grade We recommend that the upper 18 inches of the existing fill anal/or native soils within slab areas be re - compacted to at least 95 percent of the modified proctor (AS TM D1557 Test Method). Often, a vapor barrier is considered below concrete slab areas. However,the. a vaporbarrier could moisture result in curling of the concrete slab at joints. Floor covers sensitive to typically s 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 e Grade a or barrier selection and floor Concrete Floor and Slab Construction are recommended references P 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. sing a coefficient of subgrade reaction laced and compacted asl of 180 pounds per inch (pei) Slabs on grade may be designed u outlined in assuming the slab -on -grade base course is underlain by structural fill p Section 8.1. 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 roc A perimeter drainage system is recommended unless interior slab areas s stem should cominimum ist of 4 inch inches above adjacent. exterior grades. If installed, a perimeter drainagey s a non -woven diameter perforated drain pipe surrounded by a minimum 6 inches of drain rock wrapped 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. m 4. oeprcent tosurface tate Exterior grades surrounding'buildings should be"sloped at a minim nimeermeablea surface over water flow away from the building and preferably with a relatively P immediately adjacent to the building. 9 PO Box 82243 Kenmore; VWA 98028 ..,cobaltseona small com 2o6-331-1097