Civil Engineering Association

Geophysical Testing of Rock and Its Relationships to Physical Properties

Testing techniques were designed to characterize spatial variability in geotechnical engineering physical parameters of rock formations. Standard methods using seismic waves, which are routinely used for shallow subsurface investigation, have limitations in characterizing challenging profiles at depth that include low-velocity layers and embedded cavities. This research focuses on overcoming these limitations by developing two new methods using both sensitive data and a global inversion scheme. The first method inverts combined surface and borehole travel times for a wave velocity profile. The technique is based on an extremely fast method to compute first-arrival times through the velocity models. The capability of this inversion technique is tested with both synthetic and real experimental data sets. The inversion results show that this technique successfully maps 2-D velocity profiles with high variation. The inverted wave velocities from real data appear to be consistent with cone penetration test (CPR), geotechnical borings, and standard penetration test (SPT) results. The second method inverts full waveforms for a wave velocity profile. The strength of this approach is the ability to generate all possible wave types and, thus, to simulate and accurately model complex seismic wave fields that are then compared with observed data to deduce complex subsurface properties. The capability of this inversion technique is also tested with both synthetic and real experimental data sets. The inversion results from synthetic data show the ability of detecting reverse models that are hardly detected by traditional inversion methods that use only the dispersion property of Rayleigh waves. The inversion results from the real data are generally consistent with crosshole, SPT N-value, and material log results. Employed for site characterization of deep foundation design, the techniques can provide credible information for material at the socket and partially detect anomalies near the socket. Lastly, based upon a laboratory testing program conducted on rock cores, it does appear that relationships between geophysical measurements and geotechnical engineering design parameters are credible, though significant scatter does exist in the data. It could be postulated that geophysical measurements should be capable of identifying large zones of poor quality rock, and the results can provide characterization of spatial variability in geotechnical engineering physical parameters useful in the design of deep foundations.