Rule 15-18-77-5.6.17 - System Design1. A soil and site evaluation must be performed on the lot. See also Section VII. The loading rate of the natural soil must be determined from Table 2: a.Table 2: Soil loading rates2. Determine the average daily flow from the residence: Number of bedrooms X 130 gallons per day 3. Determining the size of the absorption area, basal area, side slopes, and maximizing length requirements: a. Sizing the absorption area - The absorption area size shall be determined by the loading rate of the fill material. The fill material shall be coarse sand, 0.5-1.0 mm (USDA designation), and is the same as concrete sand (Section S-703, MS Standard Specification for State Aid road and bridge construction). The loading rate of this material is 1.2 gallons per day per square foot. Note: A fill material as heavy as a light loam may be used, but this will change the size of the absorption area size. Use the appropriate loading rate of the fill to calculate the absorption area. i. Example: Three (3) bedroom home @ 390 gallons per dayii. 390 gallons per day / 1.2 gallons per day per square foot = 325 square feetiii. Absorption area = 325 square feet b. Sizing the basal area: Using the information gathered from the soil and site evaluation, determine the loading rate of the natural soil within two (2) feet of the surface. Use the heaviest textured soil loading rate to size the basal area. Divide the average daily flow from the residence by the loading rate of the natural soil. i. Example: Three (3) bedroom home @ 390 gallons per day a natural soil of a heavy loamii. 390 gallons per day /0.45 gallons per day per square foot = 867 square feet basal areac. Maximizing length of the elevated sand mound: To the greatest extent possible, the elevated sand mound should be as long as possible. The length of the basal area and absorption area must always be at least 4 times the width. However, the width of the absorption area shall never be less than 2 feet.d. Filter media depth: There shall be a vertical separation between the seasonal water table and the bottom of the absorption area of at least 2 feet in every situation. This separation may include up to 12 inches of unsaturated natural soil.e. Calculation of side slopes: Side slope requirements will be different on level sites than on sloping sites. The side slope on the downhill side must be longer than the side slope going up hill. The following chart gives the correction factor on various slope conditions: (Table 3: Correction factors)
i. Example: Given: 3' high mound with 9' side slopes placed on a 6% slope.ii. Table 3: Correction factorsiii. Upslope side slope: 9' X .85 = 7.65' side slopeiv. Downslope side slope: 9' X 1.22 = 10.98' side slope15 Miss. Code. R. 18-77-5.6.17