Geophysics is the solution for locating buried voids

Phase SI spent several weeks earlier this summer carrying out a multi-phase geophysical survey at a site in North Yorkshire to locate solution features and associated voids.

Solution features are a known hazard in the area: a house adjacent to the site had recently been demolished due to subsidence and an initial borehole survey of the site had identified variations in geology and potential voids. Our client needed to know the location and extent of any solution features that were present within the site and also if there was any associated voiding or if the features had collapsed and been in filled over time. Peat and clay were present near the surface in different parts of the site and the client also wanted to know the extent of these deposits and if there were any other significant geological variations within the site.

The site was relatively large (2.4 ha) and it would have been prohibitively expensive to try and map the geological variations using boreholes, which would also would potentially have given an incomplete picture of the sub-surface conditions. Phase SI therefore recommended carrying out a geophysical survey comprising electromagentics (EM) to map the near-surface geological variations and microgravity to locate voids or areas of ‘loose’ infill material. These were followed by targeted resistivity imaging to provide depth information on features identified by the EM and microgravity surveys. Phase SI also carried out a full topographic survey of the site prior to the geophysical survey so that the results could be displayed relative to an accurate map base, which the client could then use as an accurate base for their design and planning.

The survey successfully mapped near-surface geological variations and detected several solution features. The data suggested that some of these features had partially collapsed and been infilled whilst others appeared to contain loose material or voids. It was possible to determine the lateral extent and depth (up to between 10 m and 16 m below ground level) for most of the features that were identified. Although several major geophysical variations located at the edges of the site could not be fully categorised by the geophysical survey. A geophysical survey needs to fully encompass sub-surface features to allow a reliable interpretation of the resultant anomalies to be made and this was not possible in some parts of the site.

The survey was able to highlight which parts of the site would require more detailed investigation (targeted boreholes) and potential remediation prior to any redevelopment and which areas did not have any near-surface hazards or variations. The majority of the solution features had not been identified by the initial borehole investigation. Without a geophysical survey the information available on which to base the proposed redevelopment would have been incomplete and could have resulted in major problems and very significant costs if construction had started in any of the problem areas.