12-12-1987 Planning Packet - Laserfiche WebLink

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r i on paRes B-29 and D-6. To determine the area of rock layer required for a 4-bedroom, type I home, first refer to pace D-7 for the estimated sewage flow rate of 600 gpd. Multiplying 600 gpd times 0.83 uQ ft/gpd results in a rock layer area of 500 sq ft. The rock layer in a mound should be no wider than 10 feet, unless special design considerations are made. Thus, the shape of the rock layer required for a daily sewage flow of 600 gallons is 10 feet wide by 50 feet long. The total width of the mound from dike toe to dike toe is shown on page 5. Dimension d^ is the upslope dike width, and dimension is the dcwnslope dike width. The width of the rock layer is designated as W. The height of the mound above the original soil at the upper edge of the rock layer is designated as h^ and the height at the downslope edge of the rock layer is designated as hi. The slope of the dike is designated with a slope ratio (SR) which is the ratio of the horizontal distance to the vertical distance. For example, an SR of 3 indicates 3 feet horizontal to 1 foot vertical and would be equivalent to a slope of 33 percent. An SR of 4 is a flatter slope and is equivalent to a 25 percent slope. Formulas to determine d^, d^, and h2 are presented on page 5. A table of those values is also presented for convenience. On level ground, d^ equals d^. On sloping ground d^ becomes longer than 1 if the slope ratio is the same for both dikes. The dimension h^ is usually 3.0 feet, consisting of 1.0 foot of clean sand, l.O foot of rock layer, and 1.0 foot of soil cover over the rock. On level ground h_ equals h^, but on sloping ground h. is greater than h^ because tne top of the sand layer and the bottom of the'^rock layer must be level. As an example, if the ground slope is 6 percent, this is equivalent to 6 feet in 100 feet or 0.6 foot in 10 feet. Thus, if h^ equcls 3.0 feet, h- equals 3.0 + 0.6 = 3.6 feet. This value can also be calculated by the formula on page 5 where s is the amount of slope in percent. As can be observed in the table on page 5, the dike widths d. and d^ depend upon the land slope and the dike slope ratio. For example, if the land slope is 6 percent and the dike slope ratio is specified as 4:1, d, equals 9.7 feet and a_ equals 18.9 feet. With a 10-foot wide rock layer, the total dike toe'^to dike toe width of the mound would be 9.7 + 10.0 + 18.9 = 38.6 feet. The length of the mound varies depending upon where it is measured. As can be seen f.~om page 4 in the plan view, the mound shape is trapezoidal. If the length is measured along the center of the rock layer, then the height of the mound at the end of the rock layer is 3.3 feet. If the dike slope ratio is 4:1, the dike will extend out 4.0 x 3.3 or 13.2 feet. The total length of the mound measured at the centerline of the rock layer wi-1 be 13.2 + 50 + 13.2 or 76.4 feet. The mound will be slightly longer near the base of the downslope dike and slightly shorter near the base of the upslope dike. Several mound shapes are shown on pages 6 and 7. The rectangular mound is most commonly used since it is the easiest to construct. While the mound location depends upon soil suitability, every effort should be made to fit the mound into the landscape plan. Hounds can bo used as privacy berms or -iii-