2211 GIBSON PL Geotech Report 2023-04-06

August 18, 2014 Gibson Bluff L&A Job No. 14-056 Page 22 deposits underlying the site at shallow depth are capable of supporting infiltration trenches or galleries to be used for onsite stormwater disposal. Infiltration trenches/galleries should be set back at least 5 feet from property lines, 10 feet from adjacent building footing foundations or utility trenches, and 30 feet from the top of slopes of 40% grade or more. The bottom of infiltration trenches or galleries should be at least one foot lower than adjacent building footing foundations or utility trenches. Infiltration trenches/galleries should be scattered throughout the site to spread water over the entire site and, thus, minimize potential problems from disposed stormwater. Infiltration trenches/galleries installed underneath the access road would a suitable choice. Infiltration Trench/Gallery Construction The trenches should be cut at least 6 inches into the Esperance sand deposits of clean, fine to medium sand. To reach this target soil stratum the trenches would have to be excavated to about 5.0 to 9.0 feet deep or more, depending on the locations of the infiltration trenches/galleries. The condition of the soil unit at bottom of trenches should be verified by a geotechnical engineer. The stability of the trench cut banks should also be verified by a geotechnical engineer during excavation. The trenches should be at least 24 inches wide. The side walls of the trenches should be lined with a layer of non-woven filter fabric, such as MIRAFI 140NS. The trenches are then filled with clean washed 3/4 to 1-1/2 inch gravel or crushed rock to within about 12 inches of the finish grade. The dispersion pipes should be constructed of 4-inch rigid PVC pipes and laid level in the gravel or crushed rock filled trenches at about 24 inches LIU & ASSOCIATES, INC.