Effects of Loading Conditions on Fracture Processes near a charge hole in Blasting

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Fracture control is very important in rock excavation and partial demolition of concrete structures with blasting. In this research, effects of loading conditions on fracture processes near a charge hole were examined. The effectiveness of the decoupling of the charge, tamping, notched blast hole for blast crack control were studied experimentally.(1) Visualization system by the schlieren method and dynamic strain measurement system were constructed. A schlieren system using an Ar-ion laser was set up for visualization of stress waves and cracks in the blasting processes. A high-speed video camera system was used to record the dynamic events. In dynamic strain measurement, strain amplifiers with wide-band frequency characteristics (DC-500 kHz) were used. Strain signals were recorded in a digital storage-type oscilloscope with 8 bits vertical resolution.(2) Blasting experiments with PMMA blocks (100x100xl30mm, decoupling index ; 1.1, 2.57, 3.5) and an electric detonator were carried to … More analyze the decoupling effect on the motion of stress waves and fracturing near the charge hole. In this experiment, the fracture patterns formed in the block were also determined by the collection of fragments. High-speed video images showed that the optical clarity of the PMMA block near the free surfaces is lost by the tension of stress waves. The dynamic fracture processes near the charge hole are changed as the decoupling index increases. The strain measurements showed that an air cavity between the charge and the walls of the charge hole causes influencs to the compressive wave propagation in the medium.(3) Blasting experiments with mortar and concrete specimens (200 mm in dia. , 200 mm in length, decoupling index ; 1.1, 1.86, 2.57, 4.85) were carried out to demonstrate the effectiveness of the decoupling of the charge, tamping, notched blast hole for blast crack control. High-speed video images showed that the cracks propagate from the charge hole towards the free surface, and that the decoupling of the charge is effective in controlling crack initiation from the notched charge hole and in producing controlled fracture plane. Less