A new LVDT-Cell has been developed to measure secondary or induced stresses in the vicinity of an excavated surface. It utilizes the overcoring methodology, measuring the radial convergence of 4 diameters using eight LVDTs, and is similar in concept to the USBM-gauge and IST-tool. A 127-mm diameter pilot-hole is required and the overcore diameter of 200-mm or greater is suggested. The minimum overcoring length is 350mm, and hence a compact drill can be utilized. The in-situ state of stress can be solved by numerical inversion using the results of at least three measurements around the three dimensional tunnel section. The large dimensions of the measurement tool and the ability to utilize multiple measurements at various locations in a tunnel section, provides flexibility in selecting an appropriate rock mass volume. To evaluate the suitability of the LVDT-Cell, tests were carried out in the 4.2m × 4.8 drill and blast TASS-tunnel at Äspö Hard Rock Laboratory. The state of stress established using the LVDT-Cell was in excellent agreement with the state of stress established previously using various traditional borehole methods. The LVDT-Cell results also suggest that the depth of excavation-disturbed zone is relatively mi-nor. In this study, the reliability of the numerical inversion solution for the near tunnel profile measurements is lower than the inversion solution for the deeper measure-ments, which suggests that in case of a drill-and-blast tunnel the minimum measure-ment depth for establishing the state of stress should be approximately 50 cm.
The nuclear waste industry in Finland and Sweden are in the final stages of designing their spent fuel repository. The foot print area of the repository will reach several km2. In addition to the orientation, the stress magnitudes are also required for the design of the underground openings.
