Dynamic Stresses Induced within Rock in Case of Blasting with One Free Face

Abstract

In this study, the behaviour of the dynamic principal stresses induced within rock by an explosion under confined condition was analyzed in case of blasting with one free face by applying the values of measured radial displacement, particle velocity and so on to equations derived from the theory of elasticity, and at the same time, the dynamic stresses on the free face were also calculated from the strain measured by the wire strain gauges affixed on the free face. The results obtained are briefly shown below. 1) The maximum stress on the free face appeared at the intersection between the free face and the burden, and its magnitude was about twice as large as the magnitude of the tangential tensile stress imparted by the incident stress wave within the range related to the breakage by blasting. 2) The maximum value of the compressive stress near the free face was decreased due to the existence of the free face, while that of the tensile stress was increased. 3) Let us consider the spherical coordinate of which origin is the center of a spherical charge. Then, at any point near the free face, the directions of the principal stresses in the γθ plane changed with time except in the direction of the burden, but at another point near an explosion point where the principal stresses were mainly composed of the stresses caused by the initial longitudinal wave, the directions of the principal stresses coincided with those of the coordinate used. 4) The maximum value of the tensile stress in φ direction was larger than those in the γθ plane on and near the free face. 5) The tensile stress caused by the Hopkinson effect appeared only near the burden. 6) It is presumed from the results of stress analysis that radial cracks will be produced in the rock mass near an explosion point, but with the approach of the free face the directions of the cracks will be curved towards the direction parallel to the free face. 7) The appearance of the breakage deduced from this investigation agrees fairly well with the practical observations.