 increased width requires. The absorption area required is determined as for the conven- tional trench, using the results of a percolation test done at the depth of the trench and using the tables presented earlier. The bot- tom area required for a conventional trench is reduced in consider- ation of the increased sidewall area. Do not count the thickness of any clay or other relatively impermeable soil layers encountered along the side of the percolation test hole as part of the absorptive surface, but add the thickness of these layers in calculating the needed depth of gravel below the distribution pipe. The Manual of Septic Tank Practice provides for a reduction in deep trench bottom area. The bottom area of the deep trench as compared to the conventional trench is given by the formula: deep trench bottom area = w + 2 w + 1 + 2 d conventional trench bottom area, where W = width of trench in feet, and d = depth of gravel and rock below the distribution pipe in feet. For example, if there will be 4 ft of gravel and rock below the distribution pipe, and the trench is to be 3 ft wide, then a require- ment for 300 ft2 of bottom area for a conventional trench would be reduced to ONSITE WASTEWATER DISPOSAL This is a 58% reduction. The trench will have to be only 125 divided by 3, or 42 ft long. As the Manual of Septic Tank Practice calls for a 6-in. layer of gravel under the distribution pipe, this formula for reducing the bottom area for a deep trench assumes that more than 6 in. of gravel exist below the distribution pipe. The Uniform Plumbing Code calls for 12 in. minimum. The Code also seems to limit the depth of gravel for which sidewall credit can be obtained to 3 ft. That's ridiculous. So how much credit you should receive for using deep trenches is another matter you may have to take up with local officials. Large rocks-on the order of 6-in. diameter cobbles-can be used in the lower portions of deep trenches to save a little on the materi- als cost. If large rock is used, there should be 6 in. or SO of interm