Use of dynamic photoelasticity to investigate seismic stress fields from blasting

Abstract

At great distances from rock blasting various types of seismic stress waves propagate. Near exposures or holes in the rock a complex pattern of dyamic stress arises owing to reflection of the waves from the free surface and their diffraction and interference. Even in the elastic formulation, theoretical investigations of these wave patterns are very complicated, and at present we have solutions only for infinite regions and simply shaped (e.g., circular) obstacles. A very effective experimental method for investigating problems concerning the propagation and interaction of stress waves in the remote zone of a blast is the method of dynamic photoelasticity, which is based on the use of optical polarization to investigate the stresses with the aid of high-speed kinematography. We have used this method to investigate a wide range of wave problems involving the concentration of dynamic stresses near free and supported holes of various shapes (tunnels, underground cavities, and extraction rooms), the interaction of seismic blast waves with exposed surfaces of ledge-rock excavations, and the propagation of waves in stratified media (screening interlayers of rock crushed by blasting). The dynamic-photoelasticity method is very effective when the stress-wave fields are distributed near the free surface during the blasts. Themore » difficulty of making a quantitative estimate of the influence of various factorsmeans that it is better to make an experimental determination of the dynamic stress fields due to blasting at shallow depths. Since the dynamic photoelasticity method gives a graphic picture of the wave pattern in the model and enables us to estimate the qualitative and quantitative features of the wave fields and to compare cases with different charge depths, solution of the problem, even in the plane formulation, gives much information on the propagation of the stress waves from a shallow blast.« less