905 80TH ST SW MTORRES AMERICA 2018-09-18

February 22,2016 Project No. T-7364 4.7 Slab-on-Grade Floors Slab-on-grade floors may be supported on a subgrade prepared as recommended in Section 4.2 of this report. Immediately below the floor slab, we recommend placing a four-inch thick capillary,break layer composed of clean, coarse sand or fine gravel that has less than three percent passing the No. 200 sieve. This material will reduce the potential for upward capillary movement of water through the underlying soil and subsequent wetting of the floor slab. The capillary break layer will not prevent moisture intrusion through the slab caused by water vapor transmission. Where moisture by vapor transmission is undesirable, such as covered floor,areas, a common practice is to place a durable plastic membrane on the capillary break layer and then cover the membrane with a layer of clean sand or fine gravel to protect it from damage during construction, and aid in uniform curing of the concrete slab. It should be,noted,that if the sand or gravel layer overlying the membrane is saturated prior to pouring the slab, it will be ineffective in assisting uniform curing of the slab and;can actually serve as a water supply for moisture seeping through the slab and affecting floor coverings. Therefore, in our opinion, covering the membrane with a layer of sand or gravel should be avoided if floor slab construction occurs during the wet winter months and the layer cannot be effectively drained. We recommend floor designers and contractors refer to the 2003 American Concrete Institute (ACI) Manual of Concrete Practice, Part 2, 302.1R-96, for further information regarding vapor barrier installation below slab-on-grade floors. 4.8 Infiltration Feasibility In our opinion, on-site infiltration is not a viable option for management of site stormwater. There may be. opportunities to infiltrate limited amounts of stormwater into the upper approximately one to three feet of native soil using natural drainage practices(NDPs). The soils observed above depths of one to three feet would generally be classified as sandy loam using the USDA Textural Triangle. Using Table 3.7 of Volume III, Section 3.3.6 (Design Infiltration Rate Determination Guidelines and Criteria) of the Washington State Department of Ecology Stormwater Management Manual for Western Washington (SMMWW) dated February,2005, an estimated long-term (design) infiltration rate of 025 inches per hour(in/hr)may be used for soils classified as sandy loam. 4.9 Stormwater Detention Pond The grading and drainage plan indicates that the stormwater detention pond in the northern portion of the site will be constructed primarily of excavations below existing ground surface. We expect that perched groundwater seepage will be intercepted by the detention pond excavation, particularly during the wet winter months;however, based on our field observations, we anticipate that the volume of groundwater that might find its way into the pond as seepage would likely be small with respect to the design volume capacity of the pond. Page No. 10