Constructing effluent ponds

This guide is for producers, contractors, consultants, and project managers involved in the construction of effluent ponds. It provides quantitative standards to assist the industry to meet the accepted maximum permeability of 0.1 mm per day.

Because in-situ and laboratory measurement of soil permeability is difficult and relatively inaccurate, rather than relying on permeability standards, this document provides recognised standards for clay lining materials and methods. By applying these standards, an acceptable degree of impermeability should be achieved consistently.

This guide is based on established engineering principles; however, the recommended methods may be revised from time-to-time, as new methods are developed. Proposals involving alternative materials or methods may be submitted to the administering authority for consideration.

Material

The material used to clay-line the ponds must be well-graded impervious material, classified as either CL, CI, CH, SC or GC in accordance with the soil classification system described in Appendix A (Table A1) of Australian Standard 1726.

Note: The classification symbols represent inorganic clays having low, intermediate and high plasticity, clayey sands and clayey gravel, including gravel-clay-sand mixtures, respectively. Furthermore, the lining material must conform with the particle size distribution and plasticity limits in the table below.

Table 1. Particle size distribution
AS metric sieve size (mm) Percentage passing (by dry weight)
75.000 100
19.000 70-100
2.360 40-100
0.075 25-90
Table 2. Plasticity limits on fines fraction, passing 0.425 mm sieve
Measure Score
Liquid limit WL 30-60%
Plasticity index Ip >10%

If materials complying with the above plasticity limits are not readily available, clays having liquid limits between 60% and 80% may be used as lining material, provided that the clay lining layer is covered with a layer of compacted gravel (or other approved material). The compacted gravel layer should have a minimum thickness of 100 mm to prevent the clay lining from drying out and cracking.

Testing of materials to determine compliance with the above requirements must be carried out in accordance with the appropriate sections of Australian Standard 1289. The administering authority may direct the licensee to provide test results certified by an accredited soils laboratory (accredited by the National Association of Testing Authorities (NATA) or equivalent).

Topsoil, tree roots and organic matter must not be used as clay lining material. Furthermore, any other material, which does not compact properly must not be placed in any of the areas to be clay-lined.

Wherever non-dispersive materials are available, they are to be used in preference to materials shown to be dispersive using the Emerson test, as described in Method 3.8.1 of AS 1289. Note: A Class-8 material is considered to be non-dispersive.

Placement of material

Effluent ponds capable of storing water up to a maximum depth of 2 m, must be lined with complying material to a minimum total thickness of 300 mm. Ponds capable of storing water at depths in excess of 2 m, must be lined with complying material to a minimum total thickness of 450 mm. This can be achieved by placing the material at the correct moisture content in progressive, uniform, horizontal layers, not exceeding 150 mm in thickness, after compaction.

Under no circumstances is the compacted thickness of clay lining material to be less than the required minimum thickness.

Correct moisture content

Prior to compaction, all material used for lining purposes must be conditioned to have a moisture content within the range of 2% wet to 2% dry of the optimum moisture content required to produce the maximum dry density when compacted in accordance with Method 5.1.1 of AS 1289.

As a guide, the required moisture content is as wet as it can be rolled without clogging a sheep's-foot roller. A preliminary assessment of the moisture content can be made by rolling a sample of the material between the hands. If it can be rolled to pencil thickness without breaking, it should be satisfactory.

Compaction

Each layer of material must be compacted to produce either a field dry density of at least 95% of the standard maximum laboratory dry density determined in accordance with Method 5.4.1 of AS 1289, or alternatively, a Hilf density ratio of at least 95% when tested in accordance with Method 5.7.1 of AS 1289.

This degree of compaction may generally be achieved by rolling each layer of material, placed at the correct moisture content, with at least eight passes of a sheep's-foot roller. As a guide, compaction will generally be sufficient when there is a clearance of 100 mm between the drum of the roller and the compacted material.

Sheep's-foot roller

Sheep's-foot roller specifications for fulfilling compaction requirements:

  • The diameter of the drum/s cannot be less than 1 m.
  • The length of the drum/s must be approximately 1.2 times the drum diameter.
  • The feet must extend approximately 175 mm radially from the drum and be of the taper-foot type, with a cross-sectional area close to the outer end of not less than 3200 mm² and not more than 4500 mm².
  • The number of feet shall be such that their total area close to the outer ends shall be between 5% and approximately 8% of the area of the cylinder, which would enclose all the feet, i.e. a cylinder having a diameter equal to the diameter of the drum plus twice the length of each foot.
  • The weight of the roller ballast, shall be such that the bearing pressure thus obtained shall be not less than 1750 kilopascals, in accordance with the following formula:
    bearing pressure (kPa) = mass (kg) x 9.81 x 1000 ÷ area of contact of one row of feet (mm²).

Other types of rollers and configurations may be used provided that the required compaction is achieved.

Test for adequate compaction

The administering authority may request compaction testing. Compaction testing must be performed in accordance with AS 1289 and be certified by an accredited soils laboratory or equivalent. A copy of the certified test results are then forwarded to the administering authority.

If the test results fail to comply with the compaction requirements, remedial measures are to be implemented as directed by the administering authority before the pond can be used.

Synthetic liners

Alternate material and installation specifications relating to the use of synthetic lining materials may be used in lieu of clay lining. In Queensland, synthetic lining proposals are subject to assessment by the administering authority.