# Failure Mechanism of Back-Break in Bench Blasting of Thin Terrane
## 1 Introduction  
Bench blasting technology is widely used in hydropower, civil engineering, mining and other industries of all kinds of rock fragmentation engineering and is an efficient, low-cost method suitable for all kinds of rock excavation. However, complex geological and topographical conditions make it difficult to completely control the blasting excavation effect [1, 2] especially in a layered rock mass with different dip angles and inclinations; the blasting effect is obviously different. In bench blasting, the unevenness of the free surface is often caused by back-break, which increases the difficulty of the blast hole layout. The unreasonable shape of a free surface is easy to make the resistance line uneven and directly affects the crushing effect of the blasted rock mass [3]. It will not only affect the distribution of the excavated rock mass and the shape of blasting pile but also increase the formation probability of the bottom foundation [4]. In addition, the unstable bench increases the risk of hole drilling and endangers the safety of construction machinery and related personnel at the bottom of the bench. Therefore, in bench blasting, we should not only pay attention to the rock breaking effect of the blasted rock but also control the reasonable shape of the free surface after blasting and study the influence mechanism of the rock layer characteristics on the shape of the free surface after blasting, which is of great significance in controlling the bench blasting effect and improving the engineering efficiency. At present, the mainstream view is that the failure of the bench rock mass is the result of the combined action of explosion shock and quasi-static explosion gas [5; 6]. In a layered rock mass, the physical and mechanical properties and failure modes of the rock mass change noticeably. For example, [PERSON] et al. [7] and [PERSON] et al. [8] found that the strength characteristics of the layered rock mass changed with the interlayer azimuth and stress state and established the failure criterion of the layered rock mass based on the stress value. It is thought that the macroscopic failure of a layered rock mass depends entirely on its stress state. [PERSON] et al. [9] also proved through theoretical analysis that layered joints have an obvious influence on the physical and mechanical properties of the rock mass itself and proposed the deformation characteristics and failure characteristics of a layered jointed rock mass under different stress states. In addition, the layered joints also have a significant effect on the explosion stress and explosion-induced gas rock breaking. [PERSON] [10; 11] designed a blasting model test of layered concrete and found that the existence of the structural plane caused the rapid attenuation of the blasting stress wave, which made the detonation gas products escape prematurely. The blasting quality was worse than that of a homogeneous rock mass and the blasting quality was better when the hole arrangement direction was parallel to the structural plane but the blasting effect was worse when the hole arrangement direction was oblique. Using a wave equation, [PERSON] [12] theoretically deduced the shear stress value derived from the tensile stress at the tail of a P-wave at the joint surface and considered that the rock mass near the layered joint surface was more prone to fracture. [PERSON] et al. [13] combined the actual geological conditions according to the geometric relationship between the fault and the joint fissure zone and discussed the attenuation law of the explosion stress wave when passing through a joint fissure. [PERSON] [14; 15] also studied the propagation law of the stress wave under different joint shapes, incidence angles, spacing, stiffness and other parameters and analyzed the explosion effect under different layered joint spacing and filler stiffness. Most of the current research work is based on the material properties and failure characteristics of the layered rock mass.