e. This soil should be more dense than the fill soil. 7. Gravel for bed or trenches. The volume required for a 9-in. gravel depth is calculated as follows: gravel needed (yd3 = 0.278 X bottom area (ft2) The gravel should be 0.5 to 2 in. in diameter and washed. 8. Filter fabric, straw, or untreated building paper to cover the gravel, mortar to seal tank openings, PVC cement, drill with bits, shovel, saw, wiring tools, etc. 9. A backhoe, a plow, and a transit or surveyor's level would be very handy. Construction As with construction of conventional drain fields, the compaction and smearing of the absorption surface of a sand mound will result in system failure. With the mound, we are not talking about the trench sides, we're speaking of the basal area of the mound. The most common symptom of a failed mound is effluent seeping out from the mound toe. If the effluent can't percolate into the natural soil under the mound, it will let you know. So keep heavy equip- ment off the mound site to avoid compaction, and above all, don't work on damp soil. The first construction step is to stake out the perimeter of the mound. Then cut all trees to ground level (leaving the root systems) ONSITE WASTEWATER DISPOSAL and mow down and clear the other vegetation. If the site is vulnera- ble to surface runoff, provide diversion channels or mini-levies. Next, excavate the trench for the supply line. It should extend from the dosing chamber excavation to a point below where the manifold will be. The trench should be either below frost line or should slope SO that the effluent in the pipe will drain back into the dosing chamber or into the mound when the pump switches off. Lay the pipe in the trench, providing a sand bed if the soil is rocky. Leave sufficient pipe at the mound site to reach up to the manifold, and cap the end to prevent dirt from entering. Backfill and compact the soil around the pipe to keep water from flowing along the trench and eroding it. Now comes the step that is the secret of success for the mound. The soil und