It is recommended that the administering authorities (usually local government, Environmental Protection Agency, Department of Natural Resources and Water and Department of Primary Industries and Fisheries) are notified in writing before construction begins.
Include the date on which construction will begin and the contact details of the earthworks contractor/project manager.
Site preparation:
Before pond construction begins, the site should be cleared of trees, logs, tree roots, and brush. All woody materials should be cleared to avoid leaks in foundations or embankments that could arise as this material decomposes.
Roots also provide easy tracks for some biota to tunnel along, which breaks down the integrity of the bank and should be removed, were possible.
(Note: approvals may be required for the clearing of native vegetation or marine plants. Contact your regional Department of Natural Resources and Water and Department of Primary Industries and Fisheries offices . These latter steps will be a requirement of any development approval issued under the Integrated Planning Act 1997.
All organic material (topsoil), loose or low density fill material, or material that may be compressible, weak or not consistent with the general soils being used to construct containment structures should be removed from foundation areas before the fill is placed.
The material may be stockpiled for later use where topsoil is required for erosion control, landscaping or rehabilitation, or used for other components of the earthworks, such as core fill. In some cases, the material may be moved to a borrow pit for conditioning by blending with other materials to achieve appropriate material suitability.
Material suitability:
The material used for lining the structures should 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 AS 1726 Geotechnical site investigations. Note: The classification symbols represent inorganic clays having low, intermediate and high plasticity; and clayey sands and clayey gravels, including gravel-clay-sand mixtures, respectively.
Placement of material:
Earth material lining:
Where lining material is suitable and of sufficient depth to meet the thickness requirements determined through the risk assessment described above, the lining should be ploughed and ripped to a minimum depth of 200 millimetres (mm) and moisture conditioned and compacted in accordance with the requirements below.
Where in situ material is unsuitable (see Section 5), the material should be either ameliorated in situ or excavated and removed. Where earth lining materials are to be imported, the lining should be constructed in even layers.
The thickness of each layer of soil being compacted should be spread to an even thickness and the compacted thickness of each layer, comprising the lining, should not exceed 200 mm.
The formation of the lining by layering will improve compaction and minimise the potential a weakness in the lining to be created.
In forming the lining, it is strongly recommended that allowance be made in the depth of the compacted layer for the tilling of pond floors between crops and the scouring by water movement, such as aeration. It is recommended that the depth of the compacted layer is sufficient to provide a minimum depth of 200 mm of compacted material that will not be disturbed by future operation and maintenance of the containment structures.
(This does not refer to minor activities such as posts stakes, and pipes.) In some circumstances this can be achieved by covering the compacted layer with material such as sand. It is important that this layer is maintained at all times during the life of the pond.
Embankments:
Pond embankments should be constructed in even layers. The thickness of each layer of soil should be spread to an even thickness and the compacted thickness of each layer should not exceed 200 mm. The formation of the embankments by layering will improve compaction and minimise the potential for weakness in the compacted layer.
In forming embankments it is critical to ensure that a positive cut-off of low permeability material is created between the base of the embankment and the foundation of the containment structure. This is usually achieved by ‘keying’ the embankment into the floor of the containment structure to minimise the risk of structural failure or of water flowing between the join in the foundation and embankment.
Erosion control:
In most cases, local councils have developed specific guidelines for sediment and erosion control with which construction activities will need to comply. In the absence of such guidelines, use Soil erosion and sediment control: Engineering guidelines for Queensland construction sites, a publication of the Institute of Engineers, Australia (Queensland Div.) for guidance on minimising the risk of environmental harm from stormwater runoff during construction.
Pipes, culverts and weirs:
Particular attention should be paid to pipes, culverts and weirs during construction. These structures should be installed to ensure they do not create a weakness in the foundation or embankment. Compaction using small machinery may be required to ensure that appropriate standards of compaction are achieved in the vicinity of the structure.
The installation of baffles or bentonite collars can further minimise the potential for water ?ow (piping) along the outside of the structure. Rock armouring or similar erosion and scour protection should be used to minimise erosion and scour around the inlets and outlets.
Correct moisture content:
Correct moisture content is critical to achieving compaction and low permeability. Prior to compaction, all material used for lining purposes should be conditioned. This is so that its moisture content will fall within two per cent of the optimum moisture content required to produce the maximum dry density when compacted in accordance with AS 1289 Methods of testing soils for engineering purposes (Standard Proctor Compaction). Any deviation from this value will require approval from a certified engineer.
The fill should be placed in continuous operation so that drying out of the surface or wetting of the surface is limited to no more than two per cent variation in moisture content. If a delay in construction occurs and drying or wetting occurs, the layer should be reconditioned to the required moisture content prior to compaction.
Note: as a guide, the required moisture content is as wet as can be rolled without clogging a sheep’s foot roller. Make a preliminary assessment of the required moisture content by rolling a sample of the material between your hands. If it can be rolled to pencil thickness without breaking, it should be satisfactory.
Compaction:
Each layer of material should be compacted to a density greater than 95 per cent of the standard compaction density when tested in accordance with AS 1289 Methods of testing soils for engineering purposes (Standard Proctor Compaction).
Note: 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 an appropriate sheep’s-foot or tamping 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.
Note that it is generally easier to compact and handle material which is a little below the standard optimum moisture content; however, the consequences of doing this are likely to lead to a marked increase in the leakage potential of the finished product.
Documentation:
Construction supervision is an important part of building an aquaculture containment structure.
Supervision ensures that the specification requirements have actually been included in the final product. On the project’s completion, a suitably qualified person should prepare a report confirming that the structure has been built to an appropriate engineering standard. (Generally, this will be consistent with the approved engineering drawings with reasons for variations from the approved drawings documented).
In order to demonstrate compliance with the construction requirements of these guidelines, the placed material will need to be tested, particularly its in situ density. This testing should be carried out in accordance with the appropriate sections of AS1289, Methods of testing soils for engineering purposes.
It is also strongly recommended that all earthworks are audit tested and certified by a suitably qualified person. During construction, all excavations forming part of the permanent works should be geologically mapped. All foundation levels should be recorded, so that the location of any part of the foundations is permanently known. Extensive photography of the earthworks including foundations should be retained permanently.
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