 of 47.5 ft. The total basal area would be 37.5 ft X 47.5 ft = 1781 ft2 The minimum basal area required is 450 gpd 0.25 gpd/ft2 = 1800 ft2 Since the basal area of the mound we designed is very slightly less (1% less) than that required, we could either forget about the differ- ence or play it safe and extend one edge a foot. The choice depends on your need for personal security and your perfectionist tenden- cies. The regulatory agency may also have an opinion. Example 2 Now let's design a mound for a sloping site. A schematic diagram of the site is shown in Figure 6.8. The first consideration is that the mound should be laid out along the contour of the slope. If it has some curves in it, that's fine. Just don't put it in a natural depression or channel. 114 ONSITE WASTEWATER DISPOSAL 11'3" 14'7" Figure 6.8 Example mound design. Design assumptions: slope = 6% design flow = 300 gpd fill depth below excavation = 2ft fill infiltration rate = 1.2 gpd/ft2 natural soil percolation rate = 90 min per in. Table 6.1 tells us that with a percolation rate of 90 min per in. and a slope of 6%, we should use trenches in the mound. The required trench bottom area is 300 gpd 1.2 gpd/ft2 = 250 ft2 Because the slope isn't too great, we can use a wide trench - a 5 ft wide trench. So the trench length will be: 250 ft2 5 ft = 50 ft CONVENTIONAL SYSTEM MODIFICATIONS Since we need a minimum clearance of 2 ft from the natural soil to the trench bottom, the uphill edge of the trench bottom will be 2 ft above ground level. Since the trench bottom must be level, the downhill edge of the trench will be higher above ground level because of the slope. The slope is 6% (0.06) so the ground will have dropped 0.3 ft or 4 in. over the 5 ft of trench bottom: 0.06 = 0.3 ft = in. With a greater slope, or if a bed were used, or if parallel trenches were used, the clearance between the excavation bottom and the natural ground surface could be much greater at the downhill edge than at the uphill edge. The mound height at the uphill edge of the trench wi