Abstract: Considering the effect of joint mass on the propagation of stress waves, this study derived formulas for the transmission and reflection coefficients of harmonic waves through the thick viscoelastic joints based on the displacement function of waves. The correlation coefficient of waves was used to describe the waveform changes of the wavelet through viscoelastic joints. Then, we discussed the applicable conditions of a simplified displacement discontinuity model with thick viscoelastic joints. When the correlation coefficient of transmission wave was 0.9, which was calculated by the thick viscoelastic joint model and the displacement discontinuity model, the corresponding thickness of the joint was defined as critical thickness. It was found that there was little impact of the impedance ratio between the rock mass and joint on the critical thickness. The relationship between the critical thickness and the centre frequency followed a negative exponential law. Meanwhile, the larger incident angle was, the lesser impact of centre frequency on critical thickness was. The field tests showed that when the thickness of the joint was 0.03 m, mechanical parameters were extremely close to the results calculated by the thick viscoelastic joint model and the displacement discontinuity model. With the increase of the joint thickness and the centre frequency of the wavelet, the deviation of calculated results using the displacement discontinuity model was larger, which was consistent with the results of theoretical analysis.

Key words: displacement discontinuity model, viscoelastic joint, critical thickness, joint stiffness