avation spacing = - excavation width natural soil infiltration rate For example, if: trench width = 3 ft mound fill infiltration rate = 1.2 gpd/ft², and natural soil infiltration rate = 0.71 gpd/ft2, then: trench = The trenches should have a minimum of 2 ft 1 in. between them. If you must use two beds, this trench spacing formula can be used to determine how far apart the beds must be. It is important that the bottoms of all trenches in a mound with multiple trenches be at the same elevation if they are fed by the same distribution system. You could, however, design a mound with trenches or beds at different CONVENTIONAL SYSTEM MODIFICATIONS elevations, as long as each one had its own dosing system and they were separated by sufficient distance. Minimum trench and bed depth is about 9 in. There must be at least 6 in. of gravel under the distribution pipes, 1 in. or SO beside the distribution pipes, and 2 in. above them. Overall mound dimensions are easily calculated. The major consider- ation is to assure that there is enough natural soil area under the mound to absorb all the wastewater coming from the trenches or bed within the mound. On flat land or on a crest, effluent reaching the natural ground surface from the absorption area within the mound will tend to spread out in every direction. Where the mound will be located across a slope, the effluent is considered to flow only downhill from under the absorption field in the mound. Therefore, on flat land or on a crest, the total area under the mound is available to absorb the effluent, but on the side of a slope, only the natural soil under the absorption field area and the soil under the mound downhill from the absorption field area can be considered as available to accept the effluent (Figure 6.7). Since the maximum slope for the mound sidewalls is three to one (18°), and since there is a minimum sidewall width of 10 ft required, there will automatically be sufficient area under the mound to absorb all the effluent under most circumstances. However, where the