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WHAT LIES BENEATH

Improving cost-effectiveness and reducing risk are essential to a project. It is crucial that buried obstacles are identified before groundworks commence.

Geophysics can be key in achieving this. A geophysical survey can potentially identify buried structures or hazards over an entire site and as it is non-intrusive there is no exposure to buried hazards, and no risk of creating contaminant pathways.

This ability to examine the ground without disturbing it was one of the factors which led a local authority to request a geophysical survey at the site of a former gasworks. Most of the gasworks had been demolished in the 1970s and the site had been levelled. Unfortunately accurate records of the position of the gasometers and other buildings were not kept and the local authority wanted to know the locations of the buried gasometers so that it could safely manage the future use of the site, which had since been turned into a car park.

It was impractical to carry out major excavations to locate buried structures. A limited borehole survey had failed to locate them but it had identified contaminated ground, further emphasising the need for an alternative solution to intrusive work. Specialist firm Phase Site Investigations recommended carrying out a combined magnetic and electromagnetic survey. Each of these techniques can identity certain types of buried structure and combining them increases the range of features than can be found.

The client also requested a utility tracing survey to accurately map and record the position of buried services so future intrusive work could avoid them. A combination of radio frequency location and ground penetrating radar (GPR) was used for this. As well as identifying non-metallic apparatus the GPR data could also be used to provide depth information for anomalies identified by the combined electromagnetic and magnetic surveys.

The non-intrusive survey successfully located three gasometers and several associated buildings and the routes of the utility apparatus were accurately located. The survey was tied-in to topographic features on site and the results of the survey were plotted against a digital base plan. The level of detail and information obtained by the survey could only be matched by intrusive means if the entire site had been stripped, which even if it had been a practical option would have been prohibitively expensive.

Combining several different techniques to meet a variety of project aims is one of the key benefits that a well-planned geophysical survey can offer. Buried structures, utility apparatus, mineshafts, archaeological features and even unexploded ordnance can all be located during the same survey if the correct techniques and survey strategy are adopted.

Each objective or application does not have to be conducted in isolation and carrying out a multi-technique survey not only provides further cost savings but it allows a more reliable interpretation of the data. A geophysical survey may not always be suitable or beneficial at a given site and an investigation strategy should be determined on a site by site basis to ensure that the maximum information is obtained.

Non-intrusive surveys, when implemented by qualified and experienced personnel, have the flexibility to be adapted to a wide variety of sites and conditions. When used correctly their greatest benefit is that they can reduce the risk of unexpected buried hazards, which allows projects to be planned with more confidence and to keep down overall project costs.

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