rock, the native soil surface must be plowed or tilled before the fill is spread. Use a small tractor to spread the fill in order to minimize compaction. It's best to till the first layer of fill into the native soil SO that there is no abrupt change in soil density as the wastewater moves down. It's best too, if the absorption trenches extend through the fill and at least 6 in. into the native soil. Set the septic tank and dosing chamber. Be sure that the inlet to the dosing chamber will be lower than the outlet of the septic tank. Stake out the distribution laterals on level contours and determine any elevation differences between laterals. Also determine elevation differences between the dosing pump and the lateral lines. A rented level and surveyor's rod will make these measurements easy and accurate. Excavate the trenches for the manifold supply pipe, the manifold itself, and the distribution laterals. Lateral lines should never slope away from the manifold. Any effluent remaining in the distribution laterals when the pump shuts off should drain toward the manifold and then back to the dosing chamber. The laterals may be located about 6 in. above the manifold to help prevent backflow from a higher to a lower lateral where the effluent is being pumped uphill. If the effluent is being pumped downhill, the laterals should be placed below the manifold, as shown in Figure 6.12. In digging the trenches, be sure to consider these differences in trench depths. Assemble each lateral with its 6-in. extensions, 45° elbows, and turn-up ends (Figure 6.12). Drill and deburr the holes in the bottoms of the laterals. The first hole should be drilled 2 to 3 ft from the manifold end of the lateral pipe. Cut the manifold to length and attach the lateral connectors. Build small tamped earth dams between the manifold trench and the lateral trenches. These dams should also be placed every 20 ft or SO in the lateral trenches to the height of the gravel to be placed 152 ONSITE WASTEWATER DISPOSAL Earthen Dams Figure 6.20 Tamped ear