# Study on Rock Damage Mechanism for Lateral Blasting under High In Situ Stresses
## 4 Discussion and Conclusions  
This study mainly investigates the blast-induced damage characteristics considering the lateral free surface in highly stressed rock mass by using a 3D numerical model. Firstly, the numerical model is calibrated by comparing the results of single-hole crater blasting model test and the numerical simulation. The results of the test and simulation are in good agreement in the upper part of the crater boundary, but the test result gives a slightly larger contour in the lower part. This may be due to the existence of an uneven weakening part in the lower part during the pouring process of cemented sand, which is more likely to be damaged and not considered in the numerical simulation. On the whole, the numerical model is reasonably accurate to study the blast-induced damage characteristics. And then, the influence of uniaxial static load under different burdens, biaxial static load and span ratio on the damage distribution were evaluated with the calibrated numerical model. When the free surface exists, the damage mechanisms are clearly different from these of the plane problem without considering the free surface and the 3D stress state. The development of the blast-induced damage away from the charge is governed by the static load, especially near the free surface. For uniaxial loading, the damage zone and the crater volume \(V\) increase with the increase of uniaxial static load P1 or P2. The \(V\) is greater when the borehole is placed on the side of the static load. For example, when \(W=4\) cm, compared with P2, the \(V\) is increased by 15-31% for P1 with the same static load value. The variations of crater volume also show that the uniaxial static load can change the optimal burden of charge and increase the critical embedding depth of the charge, especially for P1. Static load significantly affects the blast-induced damage distribution, especially for the radial tensile fractures zone and the reflected tensile fractures zone. For the radial tensile fractures zone, the long axis of the bottom circle of the crater turns parallel to the max principal compressive stress, which has been proposed by many researchers [2; 3; 4; 8; 13; 14; 15]. However, the law only applies to the radial tensile fractures but not to the reflected tensile fractures. For the latter, the opposite law that the long axis of the reflected tensile fracture zone is perpendicular to the max principal compressive stress will be obtained. The variation law of the crater volume is different for the case of the biaxial static load. When P1 = 5 MPa is kept and the borehole is placed on the side of P1, the \(V\) reduces monotonously with the increase of P2 before \(\lambda\) increases to 2.0. When the borehole is placed on the other side, the \(V\) first reduces until \(\lambda\) increases to 0.4 and then increases with the increase of P2 before \(\lambda\) increases to 2.0. The turning point at \(\lambda=0.4\) is mainly caused by the transformation of the dominant factor affecting the crater shape and the damage mechanism from P1 to P2. The crater volume is greater when the borehole is placed on the side of the max static load, where the strain energy density is lower. Meanwhile, the \(V\) is also more sensitive to the variation of static load on the same side than the other side. The crater volume increases with \(k\) increases, which indicates that in the stressed rock mass, the explosion crater can be improved by increasing the free surface span on the side of the borehole. Especially for the case of unequal biaxial loading with the same span (\(\lambda\
eq 1\) and \(k=1\)), the rock on the side of max principle stress should be excavated first, where the rock breaking efficiency is higher than the other side, and then the span on the other side can be increased (\(k\) increases), which is beneficial to improving the blasting efficiency on this side.