Elliptical Blast Cratering in Low Porosity Soft Rock Due to Emitted and Reflected Pressure Waves Interaction

Two fundamental fracturing mechanisms act as the primary drivers for burden removal in rock blasting: the propagation of radial cracks from the borehole due to the internal detonation pressure and the tensile failure relatively close to a free surface associated with spalling. A third mechanism may also influence cratering when the emitted compressive pulse is compounded by the reflected tensile waves to generate localized zones of high deviatoric stresses. This paper describes the conditions under which this additional mechanism can play an important role in burden removal by blasting, based on recent work performed on low porosity soft rock (i.e. rock salt) with relatively small wave impedance and weak mechanical strength. The process is described conceptually and expressed analytically and then illustrated using numerical modelling results that simulate the development fracture stages. The effect of key parameters, including pulse width, tangential stress magnitude, and damping, are also assessed with simulations performed with the FEM model LS DYNA. The numerical results obtained for rock salt are compared with those of hard rock to evaluate the impact of this third mechanism in low porosity soft rock. The analysis outcomes are also compared with actual observations from a single blast hole test in the field.