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Construction of mound systems for subsurface disposal fact sheet

Construction of mound systems for subsurface disposal

A mound is capable of producing high quality clarified effluent for subsurface disposal. An area of coarse aggregate is used to distribute the effluent onto the surface of the pea gravel media beneath. This fine gravel filters the effluent and provides the within-mound biological treatment. The basal area of the mound covers the natural soil, which then absorbs the treated effluent.

Mounds can be landscaped either grassed or planted with shallow rooted plants. Shrubs planted around the base of the mound should be tolerant of moisture, as mound perimeter may become moist. Planting on top of the mound should be drought-tolerant, as the upper portion of the mound can become dry. The distribution bed area (top of mound) should be grassed and be accessible for future maintenance.

Mound area shall be regularly checked for excess surface moisture breakout. All vegetation should be routinely trimmed or mowed and removed from surface area. Pre-treatment process such as septic treatment plant or pumps shall be maintained in accordance with manufacturers and local authority requirements.


Mounds are normally used on relatively flat slopes that have site or soil restrictions. Sloping sites require specific installation procedures.

The restrictions may be:

(a) A topsoil layer overlying a limiting layer such as rock, hardpan or high water table; adequate sub surface soil depth is required

(b) Slowly permeable soils

(c) Freely permeable soils, e.g. rapidly or well drained soils or landfill areas, where conventional systems should not be used.

The mound is constructed directly onto the natural ground surface, which is ploughed beforehand. Effluent is dose loaded into the mound with treatment primarily taking place within the sandfill of the mound. The treated effluent discharges directly into the underlying soil.

Mounds shall be sized according to the loading rate for the sand-fill, on the underlying soil basal-area, the soil permeability and on the vertical or horizontal linear loading rate of the soils in the area of the mound.

On slopes, the system shall be extended along the contour to control the linear loading rate to prevent effluent breakout occurring. The mound shall be designed with a toe extension on the downhill slope that will increase the available surface area of the parent soil for soakage.

Slopes up to 1 in 6.7 (15%) can accommodate a mound design. On slopes, the construction of the mound is configured differently resulting in an extended base area to that for flat land to prevent seepage emerging at the toe. These sites require shallow step cuts into the topsoil profile. The dosing cell is offset onto the back topside area of the mound. Specialist advice may be required.

Construction and installation

Before on-site construction work is commenced, the site shall be protected from vehicular traffic (to avoid compaction) and shall be isolated or marked out so that other nearby construction activity does not damage the area.

The ground surface and mound shall be constructed with care.

It is essential that both the ground surface and the mound itself are properly prepared, interfaced and that attention is given to the details of the mound design.

Before construction commences:

a) The site shall be cleared of shrubs and trees. Trees shall be cut at ground surface and the stumps removed and backfilled to natural surrounding soil conditions.

b) The mound perimeter and bed shall be marked out in proper orientation. It is advisable to also install reference stakes set some distance from the mound perimeter in case the corner markers are disturbed.

c) The area within the mound area shall be ploughed. (Ploughing 18-20cm deep). Roughening the surface with backhoe teeth may be satisfactory.

d) Basal area contours are not to slope to outer edges.

e) Perimeter area of mound is to be toed below the natural surface level to approximately 100mm

f) Ploughing shall not be done when the soil is too wet. This leads to smearing and compacting of the soil. For example. if a sample of the soil taken from the bottom of the basal area and it forms a wire when rolled between the palms, the soil is too wet. If it crumbles, ploughing may proceed.

g) On sloping sites step cuts of at least 100 mm depth contoured back into embankment shall be undertaken to enable maximum topsoil retention. Contours are not to slope to outer edges of cell or mound perimeter. Lowest contour cut is to have all outer edges step cut into existing topsoil layer.

Construction of the mound

The mound is built up of a fine gravel fill media 5-7 mm with a distribution bed of coarse aggregate 20-30 mm containing the effluent distribution system covered with a fabric and topsoil. It shall be constructed as follows.

Delivery pipe

Install the lilac colour-coded delivery pipe from the dosing chamber so it may drain after dosing. Backfill and compact the soil around the pipe.

Fine gravel fill media

The sand-fill media shall be:

(a) Washed river gravel free of contaminants such as clay, limestone or organic material. It shall have a grain size of 5mm to 7mm.

(b) Carefully placed onto the ploughed area and moved into place either manually or by using a tractor bucket. It is essential to keep vehicles off the ploughed area and to have a minimum of 150mm of material beneath the tracks of the tractor to minimize compaction of the natural soil.

(c) Built up at 1:5 slope approx 60o angle slope until its height reaches the elevation of the top of the distribution bed.

Note the gravel shall be supported by progressively adding the outer surface soil layer.

Distribution cell

The distribution cell shall be:

(a) Formed in the top of the fill media. It shall have a level base. The base shall be at the design elevation.

(b) Carefully filled with graded aggregate (20mm to 40mm) and levelled at a minimum depth of 150mm. In working the aggregate, do not create ruts in the bottom of the distribution bed. At this level install the effluent distribution pipe work.

Effluent distribution network

The effluent distribution network shall be assembled and connected to the delivery pipes. This manifold shall be placed on the aggregate and should be placed so it will drain between doses, either out of the laterals or back into the pumping main. The laterals should be laid level. Class 12-25mm pipe complying with AS/NZS 1477 pipe diameter 25mm with 6mm discharge holes at 200-500mm centres shall be used.

Finish of distribution cell and mound

To finish the distribution cell:

(a) Additional aggregate of 100mm depth shall be placed on the distribution cell to a total depth of 300 mm.

(b) A suitable backfill barrier shall be installed over the aggregate such as a filter cloth.

(c) A fine textured porous topsoil material shall be placed over the entire mound surface at the required 1:3 slope approx 30o angle slope.

(d) A further 150mm (minimum) layer of good quality topsoil may be placed over the entire mound surface.

(e) The mound surface shall be covered using grasses adapted to the area or planted as per planting detail. Shallow rooted ground cover can be planted around the base and up the side slopes.

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