Dynamic stress adjustment and rock damage during blasting excavation in a deep-buried circular tunnel

Blasting excavation of deep-buried tunnels is a typical dynamic process subjected to the combined effects of in-situ stress redistribution and blast loading. However, many theoretical and numerical studies associated with excavation-induced rock damage tend to focus on the final static stress distribution after excavation. Whereas the instantaneous stress change during the excavation receives much less attention. In this study, a theoretical model for a two-dimensional (2D) circular excavation is developed to investigate the stress evolution and resultant rock damage arising from millisecond-delay blasting. The results show that the rapid stress release occurring on blast-created excavation boundaries generates additional stress fluctuations, giving rise to higher deviatoric stress and creating a wider compression-shear damage zone than the final static stress. The magnitude of the additional stress depends on the in-situ stress level, unloading rate, excavation dimension and rock properties. Under high in-situ stress levels (e.g., greater than 30 MPa), when smooth blasting techniques are used, the dynamic stress redistribution is mainly responsible for the formation of rock damage surrounding the tunnel contour. While in this case, blast-produced stress fluctuations from the outermost stoping holes still have a considerable contribution to the damage growth and aggravation. If aggressive blasting methods are employed in the final contour holes or at lower stress levels, explosion-induced stress waves will contribute more and even become the main cause of the surrounding rock damage. Under this scenario, blast-created tensile failure will persist into the vicinity of the tunnel profile in addition to a wider range of compression-shear damage. It is also found that the preceding blast loading and subsequent in-situ stress unloading cause directions of the maximum and minimum principal stresses to be switched during the process of stress adjustment.