Site Requirements for Elevated Sand Mounds

Site Requirements for Elevated Sand Mounds. It is not possible to outline every conceivable soil, site or design situation which may occur. The following section addresses basic criteria that every elevated sand-mound will need to follow: 1. Site conditions where elevated sand mounds are applicable: a. Permeable soils with high water tables: The elevated sand mound is useful in many difficult soils and can be effective in overcoming high water tables. In fact, the use of an elevated sand mound on permeable soils with high water tables may be the most practical use of this system. Whether the water table is seasonal or permanent, these soils have inadequate vertical separation to provide satisfactory treatment with conventional systems. The mound system addresses these conditions by elevating the absorption area to achieve the needed vertical separation. Passing the effluent through the filter media will result in a more thoroughly treated effluent, before it reaches the water table. b. Slowly permeable soils: The elevated sand mound has an application on these soils, although may be costly due to the size of the basal area required. The elevated sand mound applies the effluent to the lighter textured top soil over a large area moving laterally until it is absorbed into the less permeable subsoil. On slowly permeable soils with high water tables, 5:1 side slopes are recommended. c. Excessively permeable soils: These sites present the risk of inadequate wastewater treatment before it reaches unprotected aquifers. The elevated sand-mound system treats the wastewater to a higher level before it reaches the excessively permeable sub-soil. 2. Slope limitations with elevated sand mounds: Slope limitations for elevated sand-mounds are more restrictive than for conventional systems, particularly for mounds used on sites with slowly permeable soils. Elevated sand-mounds should not be considered on sites with slowly permeable soils and slopes of 6% or steeper. Elevated sand-mounds should not be considered on sites with permeable soils and slopes of 12% or greater. [See Figures 19 and 20 for placement on flat and sloping sites.] 3. Minimum soil depth requirements: This is probably the most important factor determining how well the elevated sand-mound will function. If the soil has a restrictive horizon, the seasonal water table may not be any closer than 6 inches from the surface. If the soil does not have a restrictive horizon, the seasonal water table may not be any closer than 12 inches from the surface. If the restrictive horizon is not well defined, 12 inches of unsaturated soil is required. In all cases, there shall be a minimum of a 24-inch separation between the bottom of the absorption area and the water table. 4. Topography – Slopes: On permeable soils the maximum slope for the elevated sand mound is 12%. On slowly permeable soils (light clay loam or heavier) the maximum allowable slope for the elevated sand-mound is 6%. A crest of a slope is preferred because the elevated sand-mound can be situated to allow flow in both directions away from the filled area. It is certainly preferred that the design allows for the effluent to flow away from the elevated sand mound. 5. Level sites: Design should allow the effluent to flow in every direction away from the elevated sand-mound. On level sites with slowly permeable soils, effluent may have a tendency to stack under the absorption area that may result in surface seepage around the base of the mound. The elevated sand-mounds should be placed in areas that allow the effluent to flow away from the filled area. 6. Setback requirements: The setback requirements in Table 13 are provided as measured from the perimeter of the basal area, although no part of the system shall extend fully to a property line. The edge of the side slope must be at least 3 feet from a property line. 7. Reserve area: An area must be set aside to replace the elevated sand mound in the case of failure. Due to the nature of a mound failure the following criteria must be met: a. The area must be large enough to replace the entire system in a new untouched area. b. The area must meet all the initial requirements of the original mound system, including but not limited to soil conditions, water table restrictions and setback requirements. c. The area must not be used by the property owner in a way which would adversely affect the placement of a new elevated sand mound system.