Cross-hole seismic technique for assessing in situ rock mass conditions around a tunnel

Abstract

The cross-hole seismic technique was used to identify the extent of the Excavation Damage Zone (EDZ) is in a test tunnel and in a tunnel under construction. For the cross-hole test, a new electro-mechanical impact-based transmitter was developed and tested. The transmitter was specially designed to generate appropriate impact energy and fit into a 45-mm rock bolt borehole. In the test tunnel, 79P-wave rays were measured. The interval between the boreholes was 1.5m. From the measured P-wave velocity, 2D tomography was constructed and the average velocity gradient curves at the offsets of 0.21, 0.57, 0.93, and 1.29m were drawn. Given that the wave velocity gradient curves show spatial variation due to the quality of the rock mass, upper and lower bounds that delimit the P-wave velocity were drawn. The extent of the EDZ was determined as the depth at which the average velocity is lower than the lower bound. By applying this approach, the depth of the EDZ at the test tunnel was determined to be about 1.1m from the tunnel surface. The cross-hole seismic measurement was compared with the condition of the extracted rock cores. A gray-scale image of the rock core was compared to the P-wave velocity profile. According to the image, disintegrated rock cores were present up to a depth of 1.1m. From the seismic data and a visual inspection, a similar result was found. The same approach was applied in a tunnel under construction. From the site, the observed depth of the EDZ was about 0.9m. In addition, because of the developed transmitter can generate S-waves, P- and S-wave velocities were measured and compared. The results show that S-waves can be successfully applied to identify an EDZ. The S-wave profile shows less scatter compared to a P-wave measurement.