1023 100TH ST SW Geotech Report 2022-10-25

• COBALT GEOTECHNICAL INVESTIGATION ,-A. GEOSCIENCES EVERETT, WASHINGTON December 3,2019 Minimal cuts should be proposed in areas where permeable pavements are utilized so that infiltration in the weathered glacial till can occur. If rain gardens are utilized, they should be located near booth Street SW with overflow to City infrastructure. We recommend minimizing cuts to the base of the rain garden to i8 inches below existing elevations to maintain at least 12 inches above the glacial till (restrictive layer). We analyzed a soil sample from a hand boring near the south property line at a depth of 2 feet. The Cation Exchange Capacity was 5.6 meq and the organic content was 2.6 percent by weight. These values may be used in water quality considerations for any shallow rain gardens. We anticipate that at least one detention system will be necessary to manage runoff from new impervious surfaces. We should be provided with final plans for review to determine if the intent of our recommendations has been incorporated or if additional recommendations are needed. 8.1.6 Slab-on-Grade We recommend that the upper i8 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 D1557 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 3o2.1R-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 18o pounds per cubic inch(pci) assuming the slab-on-grade base course is underlain by structural fill placed and compacted as outlined in Section 8.1. 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 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 groundwater could be encountered above the unweathered glacial till,where encountered. The depth to groundwater would likely be 2 to 5 feet below grade. 7 PO Box 82243 Kenmore,WA 98028 cobaltgeofalni ail.cotn 206-331-io97