A Blasting Method for Crack Control by Using Bore Holes with Notches

在岩石开挖和混凝土结构局部爆破拆除中，断裂控制是非常重要的。在这项研究中，提出了一种利用带凹槽的钻孔实现高度裂缝控制的新方法。试验研究了该方法作为导孔技术的有效性。利用PMMA板、砂浆试件和电雷管进行了模型试验，研究了应力波与圆孔的相互作用，并验证了带缺口圆孔的作用效果。利用高速摄像机观察了裂纹沿控制断裂面的动态行为。得到了以下结果：(1)阴影图显示了反射波的行为、圆孔的变形以及通过与圆孔的相互作用在孔内产生收敛的空气冲击波。在这些实验事实的基础上，可以预期利用带凹口的圆孔作为导向孔是有效的。结果表明，在两个缺口的尖端附近产生了阴影光斑。高速摄像系统的图像显示，缺口尖端的两个裂纹以相反的方向扩展，并产生了受控断裂面。在形成受控裂缝后，爆炸气体开始膨胀并渗透到裂缝中。这些结果表明，该方法是控制裂隙面位向和控制裂缝向更远距离推进的一种可行方法。

Fracture control is very important in rock excavation and partial demolition of concrete structures with blasting. In this research, a new method for achieving high degree of fracture control by using bore holes with noches is presented. The effectiveness of the method for blast crack control as a guide hole technique is studied experimentally. Model experiments are carried out to examine the interaction of stress waves with circular hole and to demonstrate the effects of the circular hole with notches using PMMA plates, mortar specimens and electric detonators. The dynamic behavior of cracks along a control fracture plane is observed by means of a high-speed video camera. The following results are obtained.(1) Shadowgraphs show the behavior of reflected waves, deformations of the circular hole and the generation of converging air shock waves in the hole by interaction with the hole. On the basis of these experimental facts, it might be expected that utilization of the circular hole with notches is effective as a guide hole. It is shown that shadow spots are produced in the vicinity of the apexes of two notches. These are caused by the tensile deformation of the material near the notches.(2) The images of the high-speed video camera system show that two cracks originating from the apexes of the notches propagate in the direction opposite to each other and the controlled fracture planes are produced. Explosion gases start to expand and penetrate into the cracks after the formation of controlled cracks. These results show that the presented method is a feasible means in controlling the orientation of the fracture planes and in driving the controlled cracks to greater distances.