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rKLE NFELOER \\� J'Bright People.Righ}So7utFons. Table 5-7 Storm Drainage System Percolation Test Results Test No. Depth (ft.) Percolation Rate (gfd) Soil Type P-1 5 0.20 Lean Clay with Sand P-2 3 0.24 Lean Clay with Sand There are many factors that influence storm water disposal. Clean water was used in our tests, whereas oil residue, silt, leaves, and other deleterious material will likely be included in the actual storm water. Variations in soil conditions within the drainage system will also likely affect percolation characteristics. Based on these factors and our experience with other drainage systems and as typically required by most regulatory agencies, for design purposes, we recommend a Factor of Safety (FOS) of at least 2 be applied to the percolation rates for use in design. For use in design, in our opinion the average percolation rates obtained in tests P-1 and P-2 can be used for soils to a depth of approximately 5 feet below existing grade. After applying a FOS, a design percolation rate of 0.1 gfd should be considered. It should be noted that lean clay was noted in test holes P-1 and P-2 and within the upper approximately 15 feet below existing ground surface in all of our boreholes. In addition, sand lenses were documented throughout the site below depths of about 15 feet below existing ground surface. Percolation rates in these sands may be significantly higher than those presented in the table above. Drainage trenches or dry wells that terminate in sandy soils may be necessary to achieve the desired percolation rates. 5.10.2 Bioswale Construction If bio-infiltration swales/basins or stormwater drainage/treatment construction are considered adjacent to proposed parking lots or exterior flatwork, mitigative measures should be considered in design and construction to reduce potential impacts to flatwork or pavements. Exterior flatwork, concrete curbs, and pavements located directly adjacent to bioswales/stormwater drainage areas may be susceptible to settlement or lateral movement, depending on the configuration of the bioswale/stormwater drainage and the setback between the improvements and edge of the swale/drainage. To reduce the potential for distress to these improvements due to vertical or lateral movement, the following options should be considered in design: • Improvements should be setback from the vertical edge of a bioswale/drainage area such that there is at least 1 foot of horizontal distance between the edge of improvements and 20210067.001 A/ST020R111447 Page 32 of 37 May 11, 2020 ©2020 Kleinfelder www.kleinfelder.com