ing to the Manual of Septic Tank Practice and 12 in. according to the Uniform Plumbing Code. Again, lacking any requirements, you should decide whether to go with more gravel for a longer trench life or less gravel for a less expensive system. In general, the extra gravel is cost-effective. The distribution pipe should be covered with gravel to protect the holes or joints in the pipe from becoming clogged with soil. With 12 in. of gravel below the distribution pipe, 4 in. beside it, and 2 in. above it, the total gravel depth would be 18 in. This is the depth of gravel commonly used. About 12 in. of earth fill should be placed over the gravel, but a 6- to 12-in. layer of straw or a layer of untreated building paper or other water-permeable barrier should be placed over the gravel before fill is added in order to prevent dirt from filtering down into the gravel. By the time the straw or building paper has decomposed, the fill should have stabilized. Or SO the theory goes. In at least some cases, failed trenches which have been excavated had original spaces completely filled with dirt, although building paper or straw had been used (Figure 5.3). A synthetic filter fabric (geotechnical fabric) is sometimes used to line the trench sides before the gravel is pumped in, and then the fabric is folded over the top of the gravel (Figure 5.4). The fabric doesn't decompose and prevents dirt from the soil cover and trench sidewalls from mixing with the gravel, but it is porous and allows air to come into the trench and moisture to go out. Ask a paving contractor about it. If you want a really first-class sewage disposal system, use it. Perhaps you noticed that with a 2-ft deep trench and 18 in. of gravel, there isn't room for 12 in. of soil cover. So mound 6 in. over the top of the trench. This provides for any settling that may occur, and it forms a dome over the trench which promotes rainfall runoff. ONSITE WASTEWATER DISPOSAL Figure 5.3 Gravel voids filled with dirt. With several trenches next to each other, you may wish t