Application of the microgravity technique to cavity location in the investigations for major civil engineering works

Abstract

AbstractThe detection of subsurface cavities, including mine workings, mine shafts and solution features, is an essential component of any site investigation for major civil engineering works and often relies on drilling investigations to identify the presence of any cavities. However, there is no standard, cost-effective site investigation technique which can be readily used for the physical investigation of such features.Whilst a desk study may yield documentary information on the presence of recorded mine workings and shafts, the location of solution features is generally even more problematical. Two complementary approaches have been developed for the location of subsurface cavities. Firstly, closely spaced boreholes are drilled in a specific pattern to locate cavities. This method can prove prohibitively expensive, with no guarantee of intersecting all voids or cavities. Secondly, remote sensing geophysical techniques have been used. Such techniques rely on the existence of contrasts in physical properties between the rock mass and the cavities, which can be detected using suitable geophysical methods.This paper describes the application of the microgravity technique to the detection of solution cavities and mine workings with reference to three case histories. In the first and second examples the microgravity technique was used as a reconnaissance method for defining targets for subsequent physical investigation; in the third, the technique was used to define the extent of solution features, having been initially and unexpectedly encountered by a drilling programme. These examples demonstrate the applicability of the microgravity method in detecting and delineating both solution cavities and mine workings within differing geological settings.