Geotechnical application of borehole GPR - a case history

Borehole GPR measurements were performed to complement the site characterization of a planned expansion of a cement plant. This expansion includes a mill and a reclaim facility adjacent to the present buildings, the whole site being located in a karstic environment. Twenty-one geotechnical exploration borings revealed that the depth to bedrock is very irregular (between 1.5 m and 18 m) and that the rocks likely have vertical alteration channels that extend many meters below the rock surface. The purpose of the GPR survey was to reveal the presence of potential cavities and to better determine the required vertical extent of the caissons of the foundations. In general, the subsurface conditions consist of a top fill layer, which is electrically conductive, a residual clay layer and a limestone bedrock. Very poor EM penetration prevented surface measurements. Hence, 100 MHz borehole antennas were used to perform single-hole reflection and cross-hole transmission measurements. Sixteen geotechnical holes were visited during the survey. All holes were surveyed in reflection mode. Nineteen tomographic panels were scanned. Velocity tomogram were obtained for all the data. Attenuation tomography was performed in fewer occasions, due to higher uncertainty in the quality of the amplitude data. Resistivity probability maps were drawn when both velocity and attenuation data were obtained. The velocity tomography calculations were constrained using velocity profiles along the borings. These profiles were obtained by inversion of the single-hole first arrival data. The velocity tomography results show that globally the area can be separated in two zones, one with an average velocity around 0.1 m/ns and one with a slower 0.09 m/ns average velocity. In addition, low velocity anomalies attributable to weathered zones appear in the tomogram. In conclusion, the GPR results revealed no cavities. Sensitive zones were located, which helped the planning and the budgeting of the caissons construction.