3301 SEAWAY BLVD Geotech Report 2022-10-25

January 13,2016 Project No. T-7342 59 Stormwater Detention Pond A stormwater detention pond will be constructed in the eastern portion of the development. The proposed pond floor will be 6 to 24 feet below current site grades and formed by a combination of excavation below current site grades and fill containment berm construction. Our field exploration indicates that the soils in this area consist of medium dense to very dense silty sand overlying dense outwash sand with silt and silty sand. We expect that the cleaner outwash soils will be exposed along the side slopes and floor of the pond. In order to prevent infiltration losses through this soil unit and potential seepage impacts to the adjacent steep slope, we recommend lining the pond with a flexible membrane liner(FML). Soil used to construct the northeastern pond berm can consist of onsite granular soils that have a maximum aggregate size of 3 inches and a minimum of 20 percent soil fines. The results of laboratory testing indicate that soils meeting this gradational requirement exist on-site. Terra Associates,Inc. should examine and test all on-site or imported materials proposed for use as berm fill prior to their use. All fill material placed for formation of the containment berms must be placed and compacted structurally as recommended in Section 5.2 of this report. The berm fill should be keyed into the underlying competent native soil a minimum depth of two feet. The interior slopes of the pond are shown at 3:1 (Horizontal:Vertical) with the exterior slopes at 2:1. Finished slope faces should be thoroughly compacted and vegetated to guard against erosion. 5.10 Drainage Surface Final exterior grades should promote free and positive drainage away from the site at all times. Water must not be allowed to pond or collect adjacent to foundations or within the immediate building areas. We recommend providing a positive drainage gradient away from the building perimeters. If this gradient cannot be provided, surface water should be collected adjacent to the structures and disposed to appropriate storm facilities. Subsurface We recommend installing perimeter foundation drains adjacent to shallow foundations where paved surfaces do not extend to building perimeter and positive drainage away from the structure is not provided. The drains can be laid to grade at an invert elevation equivalent to the bottom of footing grade. The drains can consist of four-inch diameter perforated PVC pipe that is enveloped in washed pea gravel-sized drainage aggregate. The aggregate should extend six inches above and to the sides of the pipe. Roof and foundation drains should be tightlined separately to the storm drains. All drains should be provided with cleanouts at easily accessible locations. Page No. 13