Site topographic survey:
It is recommended that a suitably qualified person be engaged to undertake a detailed site survey. This should be based on an appropriate grid with an additional survey in areas of significant variation.
Typical vertical accuracy should be ±0.01 m to enable the preparation of materials budgets and engineering drawings.
Site geology:
Geological conditions form an important part of a site investigation as the presence of shallow bedrock can interfere with construction.
Rock outcroppings are also a major source of leaks in containment structures, as water can travel through cracks and fissures in the rock, rapidly lowering water levels and potentially causing rises in local groundwater levels.
The following geological information should be provided as part of site investigations:
- regional geology and structure
- geological conditions at the site, highlighting any geological structure, hazards or potential problem areas
- geological sections that allow an interpretation of the stratigraphy
- geological mapping where rock structure or geological features are evident.
Site soil survey:
Purpose:
If aquaculturalists do not have detailed information on the engineering properties of the site soils, a suitably qualified engineer or soil scientist should be engaged to undertake a detailed survey of the site soils as part of the risk assessment process. The investigations should establish whether:
- the soils on site are suitable for the construction of containment structures
- soils will need to be imported
- artificial materials will be required to control or manage the quality of leachate, groundwater levels, seepage, erosion, settlement or stability.
It is often cost-effective to simultaneously undertake an acid sulfate soil assessment in accordance with the Queensland guidelines. For more information, contact your local Department of Natural Resources and Water office.
Minimum soil testing requirements:
The investigation program should identify the soil profile and strata, the particle grading relevant to each of the strata, the classification of the soils in the strata plus other relevant physio-chemical parameters. Except as otherwise indicated in AS 1726 Geotechnical site investigations, the minimum investigation requirements should include:
- the properties of the foundation materials in containment areas.
(Each major soil type and distinctive soil layer should be sampled. Depth of sampling should be a minimum of 1.0 m below the deepest intended excavation in the project area to assess for changes in lithology.
Soil data should identify soil stratification throughout the containment area, beneath the embankments, along the routes of any channels, storage areas and at the site of any other proposed containment structures)
- the properties of the materials to be used in the embankment fill
- the permeability of the fill materials to be used in containment structures
- the settlement properties of the foundation materials if there is a risk of settlement during or after construction
- the strength properties of the foundation and fill materials.
The survey should be undertaken based on an appropriate grid with additional survey holes in areas of significant variation. The minimum recommended sampling frequency is included in Appendix 2.
Appropriate soil sampling, collection and preservation techniques are important in ensuring the soil analysis results are representative. Factors that should be considered are:
- sampling locations, pattern, density and depths
- tests on any soils imported to the site
- sampling devices and equipment (e.g. auger type)
- method of sample preservation.
All soil testing should be undertaken in accordance with AS 1289 Methods of testing soils for engineering purposes. Acid sulfate soil testing should be in accordance with the Acid sulfate soils laboratory methods guidelines . Soil testing requirements are outlined in Appendix 2.
Soil classification:
Each soil type listed in AS 1726 Geotechnical site investigations is associated with an engineering property range. These soil types may be correlated roughly with engineering properties for the soils.
These correlations are approximate only and should be used with extreme care. Soil engineering properties should be assessed by a suitably qualified engineer.
The Queensland small embankment dams guidelines (Draft) may be used to guide the assessment of the potential engineering behaviours of the various soil types and their beneficial use (see extract in Appendix 3).
Groundwater survey:
The objective of the groundwater survey is to develop an understanding of the existing groundwater characteristics including the depth of groundwater aquifers, quality of the groundwater and direction of flow.
Regional groundwater is often complex and, in a large site, may include perched and confined aquifers and other complex variations. It is strongly recommended that expert advice is sought on the groundwater investigations and the design and installation of monitoring bores.
Where practical, undertake baseline groundwater monitoring to establish the natural variation in groundwater characteristics including:
- daily variation in groundwater level (e.g. due to gravitational or tidal influence)
- seasonal variation in groundwater level or quality
- response of groundwater to local or regional rainfall (recharge).
The limited knowledge and variability of sub-surface materials means a lengthy period of observation is often needed for an understanding of groundwater behaviour. Therefore, groundwater monitoring should start as early as possible in the life of the project.
This will ensure that, if allegations are made about the unsatisfactory performance of containment structures, the information about groundwater behaviour is as comprehensive as possible.
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