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1 <br /> Harding Lawson Associates <br /> C. Extraction Trench Design � <br /> The extraction trench will be located downgradient from the <br /> former UST complex, directly in front of the store (Plate 6) . <br /> The trench will extend to a depth of eight feet at the eastern <br /> ' edge and slope downwards to a depth of 10 feet at the western <br /> edge. A four-inch PVC extraction well will be installed at the <br /> ' western end as a sump to collect groundwater if dewatering is <br /> ' necessary in the trench. <br /> Construction details of the trench are presented on Plate 8. <br /> The trench will be backfilled with Class II permeable material* <br /> to a depth of three feet to prevent sediments from accumulating <br /> in the pore spaces and obstruct groundwater flow. A four-inch- <br /> diameter PVC 0. 02-inch slotted pipe will be lain approximately <br /> one foot above the bottom of the trench, sloping at the same <br /> P g <br /> grade to the west, to facilitate groundwater flow to the sump <br /> area. <br /> ' Vapor extraction lateral piping will be installed at a depth <br /> of three feet and will be covered with one foot of crushed drain <br /> rock to maximize air permeability. Three two-inch PVC vapor <br /> ' laterals, each terminating with 10 to 20 feet of 0 . 03-inch <br /> slotted screened, will be placed along the trench at three <br /> ' separate areas of the trench (Plate 9) . The applied vacuum will <br /> be distributed throughout the trench so most of the soil can be <br /> * State of California Department of Transportation (CALTRANS) <br /> 1988 Standard Specification Section 68-1 . 025. <br /> 22 <br />