Effect of Pre-Existing Symmetrical Cracks on Propagation Behaviors of a Blast-Induced Crack

Abstract

Defects such as voids, pores, and joints will transform into big scale cracks in the rock of tunnel surrounding under dynamic load like blasting and earthquake. In this paper, three kinds of symmetrical cracks were chosen as an example, and experiments and numerical simulations were conducted to study the effect of symmetric cracks on a blast-induced crack. The relationship of main crack propagation characteristic and distribution of symmetrical cracks was investigated. Some circular specimens using two kinds of material, PMMA and sandstone, including a center hole charged with a detonator and pre-existing cracks were used in the experiments. The test system consisted of an oscilloscope and an ultradynamic strain amplifier and crack propagation gauges (CPGs) were employed in monitoring propagation velocity. AUTODYN code was applied in numerical simulation to investigate the propagation behavior of main crack between symmetrical cracks. Linear equation of state and a modified major principal stress failure criterion was utilized to describe the status of rock material. Based on experimental and numerical results, it can be concluded that (1) the pre-existing symmetrical cracks have arrest effect on main crack propagation, (2) compressive stress in y-direction plays very important roles in crack arrest, and (3) the spacing of parallel cracks has a great influence on crack propagation length and velocity.

The Mechanism of Rock Mass Crack Propagation of Principal Stress Rotation in the Process of Tunnel Excavation

“…Equation (6) shows that the crack will expand toward original fracture surface in the direction of θ 0 angle under the I-II composite load, and the sign of the θ 0 angle depends on the sign of K II . When K II is a positive sign, θ 0 will be a negative value, and vice versa.…”

“…Under uniaxial compression, the corresponding fracture angle θ 0 can be obtained by bringing the given crack angle β into formula (6), as shown in Figure 3. When β � 0, θ 0 � arccos(1) � 0 ∘ , the fracture angle of crack growth increases with the increase of crack angle, and the crack growth direction is approximately consistent with the maximum compressive stress direction.…”

“…Accordingly, fractures are more easily expanded under tensile shear stress. During a mining excavation, the principal stress level and the principal stress direction change; in other words, the changes in stress state of surrounding rock at any time lead to a certain range of damage to the rock mass on the excavation face [6,7], which will also have significant impacts on the expansion of rock mass cracks and deformation and failure of surrounding rock [8,9]. e expansion and the distribution of unloading cracks are the basis for the study of rock mass failure and surrounding rock stability in underground engineering.…”