1601 FLORIDA DR Geotech Report 2022-10-25

February 13,2018 Page 2 of 3 Stormwater Management Recommendations Soil & Groundwater Conditions As part of our evaluation,we excavated and sampled five hand borings up to 5 feet below grade at or near proposed infiltration system locations. Figure 1 shows the general location of the hand borings. Liu and Associates previously excavated three test pits up to 9 feet below grade. All of the hand borings encountered 4 to 8 inches of grass and topsoil underlain by approximately 2.5 to 3.5 feet of loose to medium dense, silty-fine to medium grained sand with gravel (Weathered Glacial Till). This layer was underlain by dense, silty-fine to medium grained sand with variable amounts of gravel, which continued to the termination depths of the hand borings (6 feet below grade). Perched groundwater was observed in HB-1 at 2 feet below grade,in HB-2 at 2.5 feet below grade, in HB-3 at 2.5 feet below grade, in HB-4 at 3 feet below grade, and in HB-5 at 3.25 feet below grade. The observed groundwater conditions are typical for a site underlain by glacial till. During the late winter to spring months, perched groundwater develops between the weathered and unweathered glacial till,migrating laterally along this contact to downgradient areas. We also reviewed available exploration logs from nearby site locations. The soils described in these boring and test pit logs were consistent with those encountered in the hand borings. Stormwater Management Feasibility The uppermost 3 to 5 feet (or more) of native soils consist of weathered glacial till. The United States Department of Agriculture (USDA) textural classification for the near surface weathered glacial till is loamy sand. The following options or combination (as needed) may be considered for stormwater management at this site: • Permeable asphalt or concrete walkways and driveways(where proposed) • Limited infiltration systems utilizing a 75-feet long trenches located in the southeast portion of Lot i and northwest portion of Lot 5 We recommend that infiltration trenches be located along topography and spaced at least 6 feet on center. Trenches will be 12 to 18 inches below existing grades. Permeable Pavements Typically, pervious pavements are supported by a leveling course and storage reservoir course placed on prepared native soils. These courses typically consist of open graded angular rock,5/8 to 2 inches in diameter,with a total thickness ranging from 6 to 18 inches. We recommend removal of loose topsoil and roots greater than 1 inch in diameter prior to placement of the clean crushed rock. The exposed subgrades should NOT be re-compacted to 95 percent of the modified proctor as is typical for roadway and parking lot subgrade preparation. We should be on site to verify 'firm and unyielding' soil conditions are present prior to rock placement. For this site,this generally equates to a relative soil compaction of 90 to 92 percent of the standard proctor. Local scarification may be necessary to loosen surface materials at the subgrade elevations.