Auscultation of a chalky rock mass by microgravity Survey and cone penetrometer tests

As part of the construction of a building in the area of Mons (Belgium) upon mainly chalky bedrock, a microgravity survey and a set of Dutch cone penetrometer tests have been used to detect deconsolidated areas and eventually mining cavities whose existence was already presumed on basis of geotechnical maps. Significant relationships have been established between residual gravity anomalies and in situ cone resistance (strength) measurements. Depending on the depth value chosen to integrate the strengths, the correlation coefficient increases, reaches a maximum and then decreases. The detection of small galleries is made easier in the case of deconsolidation around the gallery but is more difficult in case of a sound gallery topped by a strong layer. In the case of karstic phenomena, the deconsolidation existing around the natural fractured areas allows the detection of relatively small holes when they are surrounded by loose formations.