Analysis of symmetric contact charge action in breaking rock boulders

A new method is proposed for more effective breakage of rock boulders during secondary blasting. It is suggested that by placing two contact charges at symmetrical locations on a boulder's surface, one can take advantage of stress wave interaction within the block and, especially, the superposition of reflected tensile waves from the free boundary of the boulder. Theoretical calculations based on elastodynamics and dynamic photoelastic experiments showed the existence of stress concentration within a finite block under symmetric loading conditions, which could lead to spliting of the block along loading points. Blasting tests on mortar models of various shapes further confirmed the possibility of rock boulder fracture by this method.