THE STABILITY OF DEEP CIRCULAR SHAFTS IN HARD ROCK

THE SOLUTION FOR THE STRESSES AROUND A VERTICAL CIRCULAR SHAFT IN AN IDEAL ELASTIC HALF SPACE IS APPLIED TO THE PROBLEM OF FAILURE IN THE WALLS OF THE SHAFT. IT IS SHOWN THAT FAILURE IN SHEAR, ACCORDING TO A HYPOTHESIS SUCH AS THE MOHR-COULOMB CRITERION, IS NOT KINEMATICALLY POSSIBLE, GIVING VALUES OF THE CRITICAL DEPTH OF THE SHAFT WHICH EXCEED THOSE PREDICTED BY EFFECTIVE TENSILE STRESS THEORY. FAILURE ACCORDING TO THIS LATTER THEORY IS SHOWN TO BE KINEMATICALLY POSSIBLE, PARTICULARLY WHEN IT IS RECOGNISED THAT AS A FRACTURE SURFACE DEVELOPS, TRANSIENT FORCES ARE RELEASED WHICH MATERIALLY INFLUENCE THE COLLAPSE MODEL. DESIGN RULES FOR THE EVALUATION OF THE CRITICAL DEPTH OF SHAFTS AND THE DEGREE OF STRUCTURAL SUPPORT REQUIRED WHEN A SHAFT EXCEEDS THE CRITICAL DEPTH ARE GIVEN. /AUTHOR/