Influence of incident angle on spalling damage of underground tunnels subjected to explosion waves

Abstract

Tensile waves formed by the reflection of explosive stress waves at the free surface of a tunnel can lead to the destruction of the surrounding rock in the form of spalling. The incident angle of stress waves is an important factor affecting the characteristics of spalling. Theoretical calculations were conducted on the spalling thickness, spalling range, ejection velocity of rock fragments, and the furthest spalling distance in cases of normal incidence and oblique incidence, respectively. The variations of the spalling with the incident angle along the axial direction of the tunnel are presented. The results show that when the Poisson’s ratio of the medium is less than 0.26, the reflected longitudinal wave transforms from a tensile wave to a compressive wave within a certain range of the incident angle. As the incident angle increases, the thickness of the first spalling layer increases monotonically, while the total spalling thickness decreases monotonically. The critical condition of spalling is reached when the first spalling thickness equals the total spalling thickness. For an explosion 6 m away from the tunnel, the critical spalling distance obtained by oblique incidence is 1.67 m/kg1/3, and the peak particle velocity is 3.11 m/s. The critical spalling distance obtained by normal incidence is 5.64 m/kg1/3, and the peak particle velocity is 0.42 m/s. The theoretical results agree well with the on-site experimental data, indicating that the results of oblique incidence overestimated the critical peak particle velocity for spalling.