Crosshole seismic CT data field experiments and interpretation for karst caves in deep foundations

Abstract

Foundations at karstic terrains often cause instability problems for the supporting piles of upper buildings, resulting in uneven settlement and/or rock failure. Borehole investigation of the study site showed that the limestone cavities existed from 77.0m to 100.1m below the surface with the mean dimension of 1.52m and maximum of 8.0m. However, it is difficult to characterize these deeply buried karst caves accurately and completely. In this study, we used crosshole seismic CT techniques to image the P-wave velocities of the limestone stratum. Field experiments were designed to acquire seismic data from 44 boreholes and 113 survey lines at depths of >75m. The inverted P-wave velocities were iterated by combining the shortest path raytracing method and the damped least-square QR decomposition (LSQR) method on the basis of the initial model. Issues including the inverted velocities, uncertainty, stability and resolution of the results, and ray coverage were evaluated from the synthetic model. As bases for the interpretation of cavities, fractures and fillings, the quantitative relationship between P-wave velocities and geological anomalies was consequently obtained through correlating the results of field tomography and ultrasonic tests to the borehole data and limestone samples from karst foundations, respectively. Representative P-wave velocity maps were shown and the interpreted cavities were projected onto horizontal planes. Two sampling boreholes and in-situ water injection/pumping tests verified the interpretations. Regarding the number and scale of cavities, each area was divided into three engineering geological zones providing references for the grouting proposals of different zones.