ers along the side when you calculate the available absorption area. The CONVENTIONAL WASTE DISPOSAL SYSTEMS deeper the pit, the more important it is to take the different percola- tion rates into consideration. Deep pits may not allow percolation test holes to be dug with a shovel or hand auger. A bucket auger or other piece of heavy equipment that is digging the seepage pit will have to stop periodically for you to run a percolation test and do a few calculations. Be sure that there is no possibility that the hole will cave in on you while you're doing the percolation tests. Make a conservative guess at percolation rates using soil samples and Table 3.3 rather than climb into a hole in which you could be buried. Then see if this guess is satisfactory to the regulatory agency. These different percolation rates are taken into account as follows. A pit is to be 20 ft deep (18 ft below the inlet) and has four different soil layers: 4 ft of soil at 10 min per in. 5 ft of soil at 4 min per in. 6 ft of soil at 15 min per in. 3 ft of soil at 20 min per in. A weighted average of the percolation rates must be calculated. Each percolation rate is multiplied by the thickness of its respective soil layer, and the products are added together. That sum is divided by the total thickness of all the layers to arrive at the weighted average percolation rate. Thickness Product Two hundred ten divided by 18 equals the weighted percolation rate (11.7 min per in.). The weighted percolation rate is used with Table 5.1, 5.2, or 5.3 to find out the needed bottom area if a conven- tional trench was to be installed; this same area is used as the side- wall area in a seepage pit. The safety factor built into the trench bottom area calculation is significantly reduced in sizing seepage pits. Using the bottom area table developed by the EPA (Table 5.3) and ONSITE WASTEWATER DISPOSAL Table 5.4 Absorption Areas for Seepage Pits (ft2) Sidewall Depth Below Inlet (ft) Pit Diameter (ft) assuming a three-bedroom house, a percolation rate of 11.7 mi