Earth pad preparation for deep litter piggeries, solid waste stockpiles and composting areas

This article provides guidance and technical direction to piggery operators, contractors, consultants and project managers involved. It provides quantitative standards for earth pad construction to ensure that consistency is achieved across the industry.

The information is based on established engineering principles; however, it may be revised from time to time as new methods are developed and/or as experience dictates. Proposals involving alternative materials and/or construction methods may be submitted to the administrating authority (currently the Department of Agriculture and Fisheries) for consideration.

1. Clearing and grubbing

The area where the pad is to be established shall be cleared of all trees, scrub and stumps. All tree roots should be grubbed to a minimum depth of 300 mm below natural surface. All trees, scrub, stumps and roots removed from the pad area should be transported to a location clear of the works area and stockpiled or disposed of to the satisfaction of the landowner.

2. Stripping of topsoil

Because of its high organic matter content, topsoil is unsuitable for compaction in the pad foundation. Therefore, unless otherwise determined by the administrating authority, all topsoil shall be stripped from the entire surface of the proposed pad area to a minimum depth of 150 mm. The stripped material shall be stockpiled or disposed of clear of the works area to the satisfaction of the landowner.

3. Pad foundation preparation

Following topsoil stripping and prior to the placement of any fill material, the in-situ foundation should be prepared by the following operations, to produce a satisfactory bonding surface for the placement of subsequent layers of material:

  1. placement and compaction of suitable material into any holes or depressions resulting from the grubbing of tree stumps and roots
  2. scarifying or ripping with a tined implement, to a minimum depth of 150 mm
  3. watering to produce the correct moisture content, as specified in clause 6
  4. compaction in accordance with clause 7.

4. Excavation and placement of pad material

The pad area should be cut and/or filled as required, to produce a smooth, uniform surface in accordance with the design gradients, levels and dimensions shown on the plans, as previously approved by the administering authority.

Provided topsoil stripping exposes a pad foundation material that complies with the suitability requirements specified in clause 5, and further excavation and/or placement of fill are not required to achieve the design pad gradients, levels and dimensions, the pad surface shall be prepared as described above in clauses 3(i) to (iv).

If the pad foundation material exposed following the completion of topsoil stripping does not comply with clause 5, further excavation should be carried out to enable the placement and compaction of a minimum thickness of 300mm (after compaction) of suitable pad material, to produce the design gradients, levels and dimensions.

All fill material placed in the pad shall comply with the suitability requirements specified in clause 5. Following preparation of the pad foundation as described in clause 3 above, all fill material shall be conditioned to the correct moisture content as defined in clause 6, excavated, transported and placed on the pad surface in progressive, approximately horizontal layers, having a uniform thickness of not more than 200 mm prior to compaction.

All unsuitable material excavated from the pad area or external borrow area(s) shall be placed in spoil heaps, clear of the works area, to the satisfaction of the landowner.

5. Materials

5.1 Material specification

Material shall be considered suitable for placement in the pad, subject to compliance with the following requirements:

The material shall be classified as either CL, CI, CH, SC or GC in accordance with the soil classification system described in Appendix A of AS 1726. Furthermore, it should conform with the following particle size distribution and plasticity limits:

Note: The material classification symbols CL, CI, CH, SC and GC represent clays having low, intermediate and high plasticity, clayey sands and clayey gravels respectively.
Table 1: Particle size distribution
AS Metric sieve size (mm)Percentage passing (by dry weight)
75.000100
19.00070-100
2.36040-100
0.07525-90 
Table 2: Plasticity limits on fines fraction, passing 0.425 mm sieve
Liquid limit WL30-60%
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 pad construction material, provided that the pad surface is covered with a layer of compacted gravel (or other approved material), having a minimum thickness of 100 mm, to prevent the clays from drying out and cracking.

Topsoil, tree roots and organic matter must not be used for pad construction. Furthermore, any other material that does not compact properly must not be placed in the pad area.

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 8.1 of AS 1289.

5.2 Material suitability/identification

The visual identification methods described in AS 1726 may be used in the field during construction for determining whether a material complies with the above criteria. However, if there is doubt about the suitability of the material, laboratory testing in accordance with the appropriate sections of AS 1289 should be carried out for confirmation. The administering authority may direct the owner to submit laboratory test results, certified by a soils laboratory accredited by the National Association of Testing Authorities (NATA) or having an equivalent accreditation.

Where the materials available within the general vicinity of the site do not comply with the above criteria, alternative proposals may be submitted to the administering authority for consideration. However, the use of alternative materials and/or construction methods shall be subject to approval by the administering authority.

6. Correct moisture content

All material placed in the pad should 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.

Note: As a guide, the required moisture content for a clay material is as wet as can be rolled without clogging a sheepsfoot roller. A preliminary assessment of the required moisture content of a clay 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.

In the event that water has to be added to achieve the required moisture content, it shall be added to the borrow area in sufficient time to allow even distribution throughout the material before excavation. To achieve effective water distribution, the surface of the material in the borrow area is to be broken up by ripping prior to watering. Part of the required water may be added to the material following placement on the pad area, but only when it is not possible to add all the necessary water in the borrow area.

7. Compaction

Each layer of material placed in accordance with clause 4 above, shall 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.

Note: This degree of compaction may generally be achieved in a clay material by rolling each layer of material, placed at the correct moisture content, with at least eight (8) passes of a sheepsfoot roller of the configuration described in clause 8 below. As a guide, compaction of a clay will generally be sufficient when there is a clearance of 100 mm between the drum of the roller and the compacted material.

8. Sheepsfoot roller

The following specification describes a sheepsfoot roller which would be suitable for fulfilling the compaction requirements described in Clause 7 above, for the materials specified in clause 5.1:

  1. the diameter of the drum(s) should be not less than 1 m
  2. the length of each drum(s) should be approximately 1.2 times the drum diameter
  3. the feet should 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
  4. the number of feet should be such that their total area close to the outer ends shall be between 5% and approximately 8% of the area of the cylinder that 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)
  5. the weight of the roller ballasted, should 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  divided by
                             Area of contact of one row of feet (mm2)

Rollers of other types and configurations may be used provided that the required compaction is achieved in accordance with clause 7.

9. Test for adequate compaction

The administering authority may direct the owner to arrange for compaction testing to be carried out on nominated sections of the pad. Compaction testing is to be performed in accordance with the methods specified in clause 7 of this specification. The test results shall be submitted to the administering authority, following certification by a soils laboratory accredited by the National Association of Testing Authorities (NATA) or a laboratory having equivalent accreditation for the tests performed.

Failure of the test results to comply with the compaction requirements described in clause 7 will result in the applicant being prohibited from using the facility (e.g. stocking the piggery) until appropriate remedial measures are implemented, as directed by the administering authority.

10. Final trimming

Following the completion of compaction, final trimming should be carried out to produce a smooth, uniform pad surface.

11. Pad permeability

The procedures specified in this document are designed to produce a maximum pad permeability of 0.1 mm/day. This criteria may be used by the administering authority to ensure that the appropriate environmental protection standards have been achieved.

The administering authority may direct the owner to arrange for permeability testing to be carried out on a nominated section(s) of the pad. Laboratory permeability testing is to be performed in accordance with the methods specified in either Part 6 of BS 1377 (Triaxial Permeability) or Section F7.1 of AS 1289. The test results shall be submitted to the administering authority, following certification by a National Association of Testing Authorities (NATA) accredited (or equivalent) soils laboratory.

Failure of the test results to comply with the maximum permeability specified above will result in the applicant being prohibited from using the facility (e.g. stocking the piggery) until appropriate remedial measures are implemented, as directed by the administering authority.

12. Alternate methods

The owner may submit to the administering authority proposals to use alternative materials and/or installation methods for the construction of the pad. Possible examples include the use of synthetic lining materials or soil stabilisation with products such as cement, lime, bentonite, etc., in lieu of clay lining. Approval of alternative proposals will be subject to assessment by the administering authority.

13. Indemnity

The administering authority and officers of that authority are not be responsible for any costs incurred by either the piggery owner or his/her contractor, in carrying out any works, as directed by the administrating authority, for the purpose of rectifying design and/or construction faults or omissions.