Measurement of In-Situ Initial Stress and Excavation-Induced Stress Changes in the Vicinity of Underground Opening

ABSTRACT: A private underground research laboratory has been constructed by Obayashi Corporation to study various aspects related to the design and construction of underground openings. As part of the comprehensive research programme, a representative state of in-situ initial stress of the site has been determined by stress measurements at a number of points by means of the conical-ended borehole technique. Following the measurements of initial stress, a tunnel with a cross section of 4m-high and 5m-wide was excavated in the rock mass. The stress changes have been measured at many locations in the rock mass during the advance of the tunnel. The distribution of stress changes around the opening is calculated and presented here. In order to evaluate the stress response to excavation, to use the monitored results as an aid for future design process and, also, to validate an appreciation of the numerical analysis for the subject, the three-dimensional finite element analysis was performed, taking the excavation sequences into consideration. As a results, although the predictions by the analysis generally agree with the monitored results, the stress redistribution obtained from in-situ measurements is much more various than that of the numerical estimation. The results from this study demonstrate that the stress changes induced by excavation would be highly influenced by the joints in a rock mass. Those, also, suggest both the propriety and the limitation of numerical analyses applied for the predictions of stress redistribution.