-ft trench 4 ft wide. This means the mound can be narrower than if two parallel trenches were used with space between them. The narrower the mound on sloping ground, the less fill material is required on the downhill side of the mound. There will be 2 ft of fill below the uphill edge of the trench and the fill below the downhill edge will be deeper by 4 ft X 0.12 = 0.48 ft = 6 in. Add 1 ft of trench and 9 in. of cover to the fill below the trench and we see from Figure 6.9 that at the uphill edge of the trench, the mound will be 3 ft 9 in. (3.75 ft) high and at the downhill edge, the mound will be 4 ft 3 in. (4.25 ft) high. Also as shown in Figure 6.9, the uphill side of the mound will extend 10 ft from the uphill edge of the trench, and the sides will extend 3.75 ft X 3/1 = 11 ft 3 in. The distance from the downhill edge of the trench to the toe of the mound should be 4.25 ft X 3/1 X 1.57 = 20 ft The area under the mound which can be counted as absorption area is the trench length times the area downhill from the uphill edge of the trench. absorption area length = 100 ft absorption area width = 4 ft + 20 ft = 24 ft total absorption area = 24 ft X 100 ft = 2400 ft2 The minimum absorption area necessary is 300 gpd 0.77 gpd/ft2 = 429 ft2, SO the area available is quite sufficient to accept the effluent. Don't feel like the mound is incredibly overdesigned because of the differ- ence in absorption area available as compared to what is minimally required. The extra area is necessary because of the tendency of ONSITE WASTEWATER DISPOSAL 1 1/4 diameter 1/4" hole end cap Figure 6.10 Example distribution system design. water to run downhill instead of straight down through the soil. Wastewater from our dream cabin will sheet out over the bedrock under the shallow soil. Since the person who owns that property downhill is very large and also the Governor's son, we can't have sewage coming to the surface down there. Now let's design the distribution system (refer to Figure 6.10). The single trench is only 4 ft wide, SO a si