## 3 Test Results and Analyses
### Strain Field Evolution Characteristics of the Free Surface
#### 3.3.2 Uniaxial Static Compressive Load  
Figure 6 shows the magnitude and direction of the maximum principal strain on the free surface. \(\mathbf{t}=0\) corresponds to the distribution of the maximum static principal strain \(\varepsilon_{10}\) immediately after the static load was applied and before the blasting was started. Figure 7 shows the variations in maximum principal strains \(\varepsilon_{1}\) at various points on the \(x\) and \(y\) axes at different moments. In Figure 6a, the maximum static principal strain \(\varepsilon_{10}\) is tensile and distributed relatively evenly. Its magnitude slightly fluctuates around \(\varepsilon_{10}=605\times 10^{-6}\). The principal strains at various points are perpendicular to the direction of the static compression and parallel to the \(x\) axis. The monitoring results agree well with Equation (1). The above results indicate that despite the blashhole, no strain concentration occurs on the free surface and the strain field is uniformly distributed. The blashhole is completely filled and thus it is rational to ignore the effect of the blashhole in the analysis of the static strain. Figure 4: Variation of \(\varepsilon_{1}\) at various points on the \(x\) and \(y\) axes at different moments in case of no static load: (**a**) \(x\) axis direction; (**b**) \(y\) axis direction. Figure 5: Initial pattern of cracks on the free surface in case of no static load. Under the uniaxial compression, the tensile strain concentration zone is elliptical, with the long axis in the direction of the static load and the short axis perpendicular to the direction of the static load. The length of the long axis gradually increases, while no considerable change in the short axis is observed with the increase in static load. A compressive strain concentration zone appears on the extension line of the short axis of the tensile strain concentration zone, symmetrical with respect to the extension line. The compressive strain concentration zone gradually enlarges with the action time of the impact waves. Besides, the direction of the maximum principal strains in the strain concentration zone is perpendicular to the static loading direction and consistent with the direction of the initial static strain. However, the direction of the maximum principal strain in the tensile-compressive strain transition zone changes (Figure 6). Generally, the direction of the maximum principal strain field under the coupled static stress and blasting-induced dynamic stress is controlled by the static strain field. Under the uniaxial compressive stress, the maximum dynamic tensile principal strain rapidly increases from the blasthole center along the loading direction or \(y\) direction (Figure 7b). The strain in the direction perpendicular to the loading direction or \(x\) direction rapidly decreases. The maximum principal strain changes from tensile to compressive (Figure 7a). In reference [20], the evolution of maximum principal strain in the crater blasting process was analyzed. The initial state of specimen was the state after the static stress was applied. In this paper, the evolution of maximum principal strain under the coupling action of static stress and blasting stress was analyzed. The initial state of specimen is the state without static stress. Therefore, although the experimental conditions are the  
Figure 6: Magnitude and direction of maximum principal strain on the free surface in case of uniaxial static load: (**a–f**) images of maximum principal strain after 0 \(\upmu\)s, 30 \(\upmu\)s, 40 \(\upmu\)s, 50 \(\upmu\)s, 60 \(\upmu\)s and 70 \(\upmu\)s of explosion. Figure 7: Variation of \(\varepsilon_{1}\) at various points on the \(x\) and \(y\) axes at different moments in case of uniaxial static load: (**a**) \(x\) axis direction; (**b**) \(y\) axis direction. same, the maximum principal strain field shows some differences. At the same time, it can be seen that the strain caused by blasting and static stress are nonlinear superposition.