375 ft2 of trench bottom area, and you choose a 3-ft trench width, then 375 ft2 ft = 125 ft length Two trenches would each be 62 ft 6 in. long (125 ft divided by two). Because each trench would exceed 50 ft in length, the effluent should enter at the middle of the trenches and be fed into four legs of the distribution system "H," each only 31 ft 3 in. in length. Trench spacing is important to keep effluent from saturating the soil between trenches. All effluent should be absorbed into the nat- ural soil before it mingles with effluent from another trench. In order to figure out how much room to leave between trenches, we must know the natural soil infiltration rate. The natural soil infiltration rate is found by using data from a stan- dard percolation test as described in Chapter 3. The difference is that the percolation test holes should be only 2 ft deep where the water table is high or where the soils are tight (slower than 60 min per in. percolation rate), and are only 12 to 16 in. deep where shal- low soils exist over bedrock. Never should the percolation test hole intercept the water table or bedrock, nor should it penetrate a slowly percolating layer of soil to less dense soil underneath. The natural soil percolation rate is found from the percolation test, and the natural soil infiltration rate is found by dividing five by the square root of the percolation rate. ONSITE WASTEWATER DISPOSAL Table 6.3 Natural Soil Infiltration Rates Soil Texture Infiltration Rate (gpd/ft2 Sand, loamy sand 0.8-1.2 Sandy loam, loam, silt loam 0.6-0.8 Sandy clay loam, clay loam 0.4-0.6 Silty clay loam, sandy loam 0.2-0.4 Clay loam, silty loam, clay 0.1-0.2 Example For example, if the percolation rate is 49 min/in., then the natural soil infiltration rate is 5 V49=57=0.71gpd/ft = Table 6.3 shows typical infiltration rates for various soil textures SO that you can see if the infiltration rate you find is reasonable. Now we are ready to calculate the required trench (or bed) spacing. excavation width X fill infiltration rate Exc