Experiments and discrete element simulations on the influence of symmetrical forms of joints on the propagation of blasting cracks

Natural joints and cracks in rocks seriously threaten the stability of blasting operations. To investigate how symmetrical forms of joints affect the propagation of blasting cracks, blasting loading tests were performed on red sandstone specimens with three types of prefabricated joints using electronic detonators, and the propagation speed of blasting cracks on the specimen surface was monitored with the aid of crack propagation gauges (CPGs). Based on the mechanical parameters determined from laboratory experiments and the derived blasting stress time history curves, a blasting model of jointed rock mass was established using PFC 2D. With this model, the crack propagation behavior at the microscopic scale and the evolution process of blasting stress waves in symmetrical forms of joints was discussed. The following key conclusions were drawn: (1) Blasting cracks exhibit different propagation morphologies in rock specimens which bear various types of prefabricated joints. Under the arrangement of Type Ⅲ prefabricated joints (the joint spacing gradually narrows), only Crack a (blasting cracks propagating along the main joints) that propagates in the main crack direction appears. Under the arrangement of Type I (parallel joints) and Type II (the joint spacing gradually broadens) prefabricated joints, the propagation of Crack a is significantly suppressed, and the overall distribution of blasting cracks is more complex. (2) The propagation speeds of Crack a were calculated on the basis of the voltage signals of CPGs. The calculation results reveal that the propagation speeds of Crack a in three types of prefabricated joints follow the order: III > I > II. Besides, the propagation length and spacing of Crack a are barely influenced by the joint spacing under Type II, while they are greatly swayed by the joint spacing under Types I and III. (3) The tangential stress under blasting load was extracted in the numerical simulation, and it is found that the tangential stress in Specimens Ⅰ and Ⅱ is small, and it is highly sensitive to the blasting stress wave reflected by the prefabricated joints. A comparison of the stress states of symmetrical joints in Specimens I and II indicates that when the incident wave propagates in the same direction towards the inner side of the joints, the symmetrical joints in Specimens II experience a greater shear force than those in Specimens I, resulting in a lower normal stress and a higher degree of crack development in the joint direction in Specimens II.