ABSTRACT: Knowledge of the virgin stress state is essential in rock mechanics design, but stress measurements at large depths present a challenge. This paper describes the experiences of using the overcoring technique in deep surface boreholes. Using the Borre probe, three-dimensional overcoring measurements can be taken to depths of 650 m, with a recent large body of measurements being conducted for the SKB site investigation program for final disposal of spent nuclear fuel. Practical experiences have shown that quality control is a critical factor in obtaining reliable and consistent results. A particular problem is temperature effects during overcoring, and a laboratory study showed a complex interaction of glue and rock behavior under elevated temperatures. Some suggestions on how to control temperature effects are outlined. Furthermore, other quality aspects are discussed and practi-cal recommendations given on how to improve the use and reliability of three-dimensional overcoring for stress measurements.
1. INTRODUCTION
The in situ state of stress is one of the more important rock mechanics parameters. At the same time, it is one of the parameter that is most difficult to determine accurately. Since stress is a fictitious quantity, it can only be inferred by measuring another physical quantity, such as strain (in overcoring measurements) or pressure (in hydraulic measurement methods). Stress measurements at large depths pose a major challenge in rock mechanics. This is particularly true for new sites with no underground access, where all pre-investigative work must be performed through deep boreholes drilled from the ground surface. Hydraulic methods are commonly employed for these types of measurements, but have the drawback that the three-dimensional stress field can only be assessed by combining several methods and/or measurements. Overcoring stress measurements are commonly used in short bore-holes, and have the advantage that the full three-dimensional stress tensor is determined from a single measurement. Perhaps less well-known are the applications of overcoring stress measurements in deep, vertical boreholes. The Borre probe [1] is presently the only three-dimensional overcoring cell that allows measurements in deep (> 500 m) water-filled, vertical boreholes. This paper describes the recent experiences of using this technique, with particular reference to the site investigation program of the Swedish Nuclear Fuel and Waste Management Co. (SKB), aiming at characterizing the suitability of final disposal of spent nuclear fuel at two sites. At the two sites on the Swedish east coast, extensive overcoring stress measurements were conducted in slim boreholes down to 640 m depth for a planned repository sited within 400-700 m depth.
2. OVERCORING USING THE BORRE PROBE
2.1. The Overcoring Method
Stress measurements using overcoring relies on the principle of measuring the strain experienced by a rock sample as it is being relieved from the surrounding rock and, thus, the stresses acting on it. Soft cell methods are based on measuring strains at borehole walls, while assuming that the stiffness of the strain gauges is negligible in relation to the rock, and thus not affecting the results. The stresses may then be calculated using the measured strains and the elastic constants of the rock, assuming continuous, homogenous, isotropic, and linear-elastic material behavior (stress calculation is also possible for anisotropic materials).