Development of the miniature seismic reflection (MSR) system for nondestructive evaluation of concrete shaft and tunnel linings

Nondestructive evaluation of the structural integrity of shaft and tunnel concrete linings is the focus of this thesis. A nondestructive testing system was assembled based on the principle of miniature seismic reflection (MSR). The MSR system consists of spherical tip impactors and a pair of vertical and tangential displacement transducers. Mechanical impact causes generation of stress P- and S-waves in the test object. The elastic waveforms undergo multiple reflections between the top and bottom of the testing layer. The surface displacements are captured by a vertical and tangential displacement transducer. The signals are transformed from time domain waveforms to frequency spectra. The vertical displacement transducer is sensitive to normal surface displacements and the highest amplitude peak on the computed frequency spectrum is related to the resonance of the P-wave between the top and the bottom layer. Similarly, the tangential displacement transducer is sensitive to the horizontal surface displacements, and the maximum peak value in the generated frequency spectrum corresponds to the resonance of the S-wave between the two layers of the testing object. Thus, knowing the thickness of the given layer, as well as the measured frequencies, allows P- and S-wave velocities to be calculated. Alternatively, if the thickness is unknown, the time-distance graph of the primary surface wave arrivals can be used to calculate the P-wave velocities and, subsequently, the thickness of the layer. The MSR response depends on the material properties of the testing object. The elastic wave velocities can be used to calculate directly the dynamic elastic properties of the test object. In this study, simulated fractures and steel reinforcement bars were detected and located using the reflected P-waves. In addition, the changes in elastic properties of various types of concrete mixes were monitored for a 28-day curing period. The MSR elastic constants were then compared with dynamic and static values obtained by standard methods in order to assess their accuracy. For the field trial, various sections of two concrete shaft linings at different elevations were investigated and the dynamic elastic properties of the linings were evaluated. Also, the thickness of a concrete tunnel lining was computed using the MSR system. Finally, the MSR system was used to compute the dynamic elastic properties of different rock types. The system was used to monitor changes in elastic properties of rock cores as a uniaxial load was applied on them. For the field trial, the MSR system was used to detect the position of discontinuities in a rock mass and to evaluate the rock's dynamic elastic properties in an underground mining environment.