Construction: Elevated Sand Mound

Construction: Elevated Sand Mound. Good construction techniques are essential if the mound is to function properly. 1. Site Preparation: a. Step 1: Rope off the site to prevent damage to the area during other construction activity on the lot. Vehicular traffic over the area should be prohibited to avoid soil compaction. b. Step 2: Stake out the mound perimeter and bed in the proper orientation. Reference stakes set some distance from the mound perimeter are also required in case the corner stakes are disturbed. c. Step 3: Cut and remove any excessive vegetation. Trees should be cut at ground surface and the stumps left in place. d. Step 4: Measure the average ground elevation along the upslope edge of the bed to determine the bottom elevation of the bed. e. Step 5: Install the delivery pipe from the dosing chamber to the center of the mound. Lay the pipe below the frost or slope it uniformly back to the dosing chamber so it may drain after dosing. Back fill and compact the soil around the pipe. f. Step 6: Plow the area within the mound perimeter. Use a two bottom or larger moldboard plow, plowing 7 to 8 inches deep parallel to the contour. Single bottom plows should not be used, as the trace wheel runs in every furrow, compacting the soil. Each furrow should be thrown upslope. A chisel plow may be used in place of a moldboard plow. Roughening the surface with backhoe teeth may be satisfactory, especially in wooded sites with stumps. Rototilling is not recommended because of the damage it does to the soil structure. However, rototilling may be used in granular soils, such as sands. Plowing should not be done when the soil is too wet. Smearing and compaction of the soil will occur. If a sample of the soil taken from the plow depth forms a wire when rolled between the palms, the soil is too wet. If it crumbles, plowing may proceed. 2. Fill Placement: a. Step 1: Place the fill material on the upslope edges of the plowed area. Keep trucks off the plowed area. Minimize traffic on the downslope side. b. Step 2: Move the fill material into place using a small track type tractor with a blade. Always keep a minimum of 6 inches of material beneath the tracks of the tractor to minimize compaction of the natural soil. The fill material should be worked in this manner until the height of the fill reaches the elevation of the top of the absorption bed. c. Step 3: With the blade of the tractor, form the absorption bed. Hand level the bottom of the bed, checking it for the proper elevation. Shape the sides to the desired slope. 3. Distribution Network Placement: a. Step 1: Carefully place the coarse aggregate in the bed. Do not create ruts in the bottom of the bed. Level the aggregate to a minimum depth of 6 inches. b. Step 2: Assemble the distribution network on the aggregate. The manifold should be placed so it will drain between doses, either out of the laterals or back into the pump chamber. The laterals should be laid level. c. Step 3: Place additional aggregate to a depth of at least 2 inches over the crown of the pipe. d. Step 4: Place a suitable backfill barrier over the aggregate. 4. Covering: a. Step 1: Place finer textured soil material such as clay or silt loam over the top of the bed to a minimum depth of 6 inches. b. Step 2: Place 6 inches of good quality topsoil over the entire mound surface. c. Step 3: Plant grass over the entire mound using grasses adapted to the area. Shrubs can be planted around the base and up the sideslopes. Shrubs should be somewhat moisture tolerant since the downslope perimeter may become moist during early spring and late fall. Plantings on top of the mound should be drought tolerant, as the upper portion of the mound can become dry during the summer. 5. Operation and Maintenance: a. Routine Maintenance: A properly designed and constructed mound should operate satisfactorily with virtually no regular maintenance. b. Rehabilitation: Three failure conditions may occur within the mound: (1) severe clogging at the bottom of the absorption area, (2) severe clogging at the fill material and natural soil interface, and (3) plugging of the distribution network. Usually these failures can be easily corrected: i. If severe clogging occurs at the bottom of the absorption bed, its cause should first be determined. If it is due to failure to maintain the pretreatment unit, hydrogen peroxide to oxidize the accumulated organics at the infiltrative surface could be used. The chemical can be applied directly to the bed or through the dosing chamber. Because of the danger in handling this strong oxidant, this treatment should be done by professionals. ii. If the clogging is due to overloading or unusual wastewater characteristics, efforts should be made to reduce the wastewater volume or strength. It may be necessary to enlarge the mound. The mound cap should be removed and the aggregate in the absorption bed stripped out. The area downslope of the mound should be plowed and additional fill added to enlarge the mound to the proper size. The absorption bed can then be reconstructed. iii. Severe clogging at the fill and natural soil interface will cause surface seepage at the base of the mound. This area should be permitted to dry and the downslope area plowed. Additional fill can then be added. If this does not correct the problem, the site may have to be abandoned. iv. Partial plugging of the distribution piping may be detected by extremely long dosing times. The ends of the distribution laterals should be exposed and the pump activated to flush out any solid material. If necessary, the pipe can be rodded. Source: Miss. Code Ann. § 41-67-3. FIGURE 1 – Elevated Sand Mound (Example Sketch Only) Source: Miss. Code Ann. § 41-67-3 FIGURE 2 – Side View of Distribution System and Absorption Area Source: Miss. Code Ann. § 41-67-3. FIGURE 3 – Top View of Absorption Area with Distribution Network and Field Line Pipe Source: Miss. Code Ann. § 41-67-3. FIGURE 19 – Level Site Placement Source: Miss. Code Ann. § 41-67-3. FIGURE 20 – Sloping Site Placement Source: Miss. Code Ann. § 41-67-3. FIGURE 21 – Side Slope Calculation (Examples) Source: Miss. Code Ann. § 41-67-3. TABLE 13 – Sand Mound Setbacks (from edge of Basal Area) Source: Miss. Code Ann. § 41-67-3. TABLE 14 – Soil Loading Rates (sand mounds) Source: Miss. Code Ann. § 41-67-3. TABLE 15 – Slope Correction Factors