the trench bottom area tables will work satisfac- torily, for a shorter period of time. Since beds generally do not work as well as trenches, it would be wise to increase the bottom area of beds 30% to 50% over what would be required for trenches, particu- larly in soils with percolation rates in excess of 30 min per in. If the requirement were for 400 ft2 of trench bottom area, the bed should have 520 to 600 ft2. The bed should be 2 to 3 ft deep and about 20 ft wide, with 6 to 12 in. of gravel below the distribution pipes. Extra gravel is wasted in beds. As with trenches, a mound of gravel should cover the distri- bution pipes a minimum of 2 in. The distribution pipe itself should be of looped design with about 6 ft between the laterals (Figure 5.17). The entire surface of the gravel should be protected with untreated building paper, marsh hay, straw, synthetic filter fabric, or other suitable material. Beds are very susceptible to clogging by soil filtering through the gravel, and for this reason, the synthetic fabric would be a good investment. The bed area should be mounded with backfill dirt SO that after settling, there remains a mound to shed rainwater. CONVENTIONAL WASTE DISPOSAL SYSTEMS Figure 5.17 Looped distribution system in a seepage bed. Materials The materials required for bed construction are the same as required for conventional trenches. A 400 ft2 bed 20 ft by 20 ft and 24 in. deep would require about 7.5 yd3 of gravel and about 60 linear feet of distribution pipe. Construction One of the major reasons why beds don't have a good reputation is that heavy equipment often drives over the bed bottom during construction and seals it by compaction. Later, sewage comes to the surface. For this reason, it is highly recommended that beds be no wider than twice the length of the backhoe's reach. This way, the excavation equipment doesn't need to drive in the bed to dig or to place gravel. All such work can be done from the banks. Twenty or 30 ft is a reasonable width for beds. The same methods used to ass