l. Where trenches and a manifold are used, the trench for the manifold will be backfilled with fill soil, not gravel. Tightly tamp this soil as well. Using the backhoe, front end loader, or a wheelbarrow, place 6 in. of gravel on the absorption surface and level the gravel. Be sure that the gravel never extends to the sideslopes. Mounds have failed because gravel was placed so that there was a straight conduit for the effluent to move sideways directly into the top soil. Drill the holes in the underside of the laterals, starting 2 to 3 ft from the manifold. Remove any burrs on the inside of the pipe. Place the distribution system on the gravel, holes down. Check to see that the laterals are level or slope slightly away from the mani- fold. There should be no dips in the lines, SO that the system can drain after each pumping cycle. Digging through frozen soil to thaw or replace frozen pipes will never be a popular hobby. In severe climates, it is recommended that the laterals be connected to the underside of the manifold with tees (Figure 6.11), to ensure com- plete drainage of the manifold after the pumping cycle. Where the ONSITE WASTEWATER DISPOSAL Figure 6.12 Lateral extension for access. mound is higher than the dosing chamber, and where the manifold is fed at its end rather than at the center of the "H" crossbar, the manifold can be sloped toward the supply line to ensure proper draining. Another nice addition to the distribution system is to attach 45° elbows and unperforated extensions to the ends of the distribution laterals (Figure 6.12). These serve as markers to locate the laterals and provide access for unplugging any plugged lines. When the distribution system is properly arranged and cut to the proper lengths, the tees, elbows, and caps are all accounted for and the holes drilled, deburred, and directed downward, glue it all together. Add another 3 in. of gravel to cover the pipes, and place untreated building paper, straw, filter fabric, or other suitable material over the gravel. Then backfill. Th