Why is it necessary to carry out geotechnical investigations?

  • To obtain sufficient site information to decide on the technical feasibility and economics of a proposed project.
  • To identify geotechnical factors which need to be addressed in the planning and design of the proposed project.

    • Why is it necessary to carry out an environmental investigation?
    To identify, assess and minimise potential risks to human safety or nature from a proposed development.

    What should a site investigation report provide?
    As a general rule, a site investigation report should:

  • present field and laboratory test data
  • provide interpretive comments on the data as appropriate, eg. Groundwater is at 3m, but is probably a \'perched\' water table.
  • review results against other relevant data and, for environmental work, statutory or other stipulated guidelines
  • review proposed construction or development and the influence of the subsurface conditions
  • suggest appropriate parameters for design, and/or suggest and assess remediation proposals for the site (environmental only)
  • predict performance
  • discuss potential design and construction solutions and risks on the basis of \'if you do this, this will happen\'

    Most methods of investigation generally only indicate subsurface conditions at the specific locations where the tests or samples were taken. They cannot necessarily be relied upon to accurately reflect strata variations which may exist between test or sample locations.

    • How do I decide the extent of investigation required?
    Deciding the extent of an investigation is a balance between uncertainties associated with anticipated conditions and the economic effects of these uncertainties on overall project costs. There are diminishing returns but generally the more paid for the investigation, the less risk of economic surprises later on.

    The extent and cost of the geotechnical investigation should reflect the coverage required to reasonably define or confirm overall site geology, and the accuracy with which natural variations in ground properties are to be estimated. An investigation is generally for design and costing purposes, and samples only a small proportion of the whole site. Differences inevitably occur during construction between conditions expected and those encountered. This can lead to costly overruns and disputes regarding latent conditions, variations in design, and even accusations of negligence if the investigation can be perceived as in any way inadequate. Unexpected conditions include natural geological features, such as dykes, sink holes, faults and adverse jointing; as well as man-made problems, such as highly variable filling, service trenches, buried former structures, features of archaeological significance, and site contamination.

    In deciding on the number of boreholes or sampling points on a site of given area, environmental consultants regularly make use of tables which give, say, the minimum number of sampling points to detect a \"hot spot\", that is, contaminated area of minimum diameter, with 95% confidence.

    For geotechnical investigations many of the possible site anomalies which are more likely to be encountered are subvertical lineaments, such as dykes, trenches or faults. A grid of the order of the width of the lineament would generally be required for reliable location. The chances of locating these geological features can be increased by inclined drilling. Austroads Bridge Design Code (1992) recommends for GC2 investigations, bores or test pits at no more than 30 m spacings for culverts, retaining walls etc, or, for bridge piers, at least one bore or test pit per pier and abutment.

    The depth of test bores and probes should be sufficient such that all significant soil and rock strata likely to affect behaviour of the structure have been profiled, sampled or tested so that they can be adequately characterised in the ground model. For piled foundations, this usually means probing or drilling the bores to several metres below founding levels of the piles. For friction piles or piles founding on an intermediate hard or dense layer, at least two bores should generally be carried to unweathered bedrock level to provide data for assessment of group effects, such as deep seated settlements due to loads from the whole structure.

    What type of investigations are there?

    Investigation is generally an iterative procedure in which field work is carried out to fill gaps in existing knowledge of a site. The types of investigation are broadly:

    Desk study
    Site Assessment
    Preliminary site investigation
    Comprehensive site investigation
    Supplementary site investigation
    Field trial
    Monitoring of construction

    How are soils generally classified?
    Geotechnical consultants generally classify soil types according to the predominating particle size in accordance with AS 1726 Geotechnical Site Investigations Code. If you wish to view a table, indicating approximate field and in situ test classifications of relative density for granular soils and strength for silts and clays, click here