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STATS Limited, Porterswood House, Porters Wood, St. Albans AL3 6PQ, UK (george.tuckwell{at}stats.co.uk)
Microgravity is established as a technique for the detection of natural and man-made cavities. However, past published examples have concentrated on substantial natural cavities or mine workings. Two cases are described for which the voids present are relatively small and discontinuous, and where much of the affected ground is characterized by low-density ground rather than large open voids. Here we use the term ‘low-density ground’ to encompass ground that has been disturbed by the collapse or partial collapse of material into a void such that the affected ground shows an anomalously low density in comparison with the surrounding unaffected ground. Case study 1 was undertaken with Mouchel Parkman on behalf of Hertfordshire County Council. A doline had opened up within a school playground. The collapse was expected to be related to natural voids within the chalk at 7–10 m depth. The low-density areas identified in the microgravity survey, in addition to some control locations, were subsequently proved by dynamic probing. The investigation accurately identified voided and poorly consolidated ground, and provided the basis for an assessment of the risks of imminent and potential future collapse. Case study 2 was undertaken with Laing O'Rourke, who were principal contractor for the construction of the A590 bypass, Cumbria. Exploratory boreholes confirmed the presence of loose material in the glacial till overburden associated with suspected voids within the limestone bedrock. The microgravity survey identified areas of low subsurface density, which were subsequently targeted by boreholes. A detailed ground model constructed from borehole data allowed a synthetic gravity map to be calculated for comparison with the measured gravity. Poorly consolidated ground, depth to bedrock and surface topographic effects could be isolated and a clear interpretation of subsurface produced. The microgravity technique is shown to be an effective tool for the investigation of poorly compacted ground and small shallow voids in complex and disturbed ground conditions. When followed by targeted intrusive investigation such surveys can yield a great deal of information that would not otherwise be available to the engineer.
