Elasto-Plastic Numerical Analyses for Predicting Cave-Ins of Tunnels and Caverns

Abstract

AbstractThis paper deals with elasto-plastic numerical analyses for predicting the collapses, such as cave-ins, of tunnels and underground caverns. Cave-ins often occur during the excavation of tunnels and caverns, particularly those excavated in weak rocks and in highly jointed rock masses. However, cave-ins cannot be predicted by conventional elasto-plastic numerical analyses even though commercially available computer software is used. This is probably due to the fact that the gravitational force may not be considered properly in the analyses. In the conventional numerical analyses of underground structures, such as tunnels and caverns, the initial stresses caused by the gravitational force are firstly determined by linear elastic analyses. This means that the gravitational force is replaced with the Cauchy stress (surface traction vectors), resulting in no more body force existing in the analyses. This computational procedure may be questionable in elasto-plastic analyses because there is no guarantee that the principle of superposition is valid for nonlinear analyses. In other words, stresses in the plastic zone must always satisfy the yielding criterion. Therefore, the stresses in the plastic zone may become smaller if the strength of the rocks is very low. However, it is obvious that the gravitational force must always be preserved, so that the stresses in the plastic zone cannot become smaller than a certain level of stresses caused by the gravitational force. Therefore, if the stresses in the plastic zone tend to become smaller than the certain level of stresses, the underground structures will collapse due to cave-ins.KeywordsTunnels and cavernsCave-inNumerical elasto-plastic analysesGravitational force