Experimental and theoretical study of the fault slip events of rock masses around underground tunnels induced by external disturbances

Fault slip events of rock masses around underground tunnels, such as irreversible displacements and slip-type rock bursts, can easily occur after external disturbances. To investigate the mechanism and threshold conditions of fault slip events triggered by external disturbances, a series of slip experiments using red sandstone blocks were performed with various horizontal pulling forces and vertical dynamic disturbances. The experimental results show that the relative vertical displacements of the adjacent blocks were the main reasons for anomalously low friction phenomenon; the initial stress state strongly affected the final residual horizontal displacements and the threshold conditions of fault slip events; as the shear force ratio β increased, the stabilities of the rock blocks became increasingly sensitive to disturbance energies, and the threshold disturbance energies for causing fault slip events decreased; and when β was close to 1, even a considerably small disturbance triggered a sustained slip failure. Based on energy analyses, a theoretical model was developed, and a dimensionless energy parameter I was introduced to quantifiably characterize the threshold conditions of different types of fault slip events. The threshold condition for rock blocks to start slipping was that the I-value should reach a threshold value, while the threshold conditions of a sustained slip failure were that the I-value should reach a larger threshold value and β should simultaneously exceed a certain value. The expression for the I-value was given and, can reflect the dependency relationships between the disturbance energies, shear force ratios and rock parameters, i.e., the yield strain, the length to height ratio and the ratio of the elastic coefficients. The experimental results and the threshold conditions derived from the theoretical model are essential when carrying out stability analyses and geohazard predictions in rock engineering.