The in situ stresses are important design parameters in rock engineering. Unfortunately, conventional stress measurement techniques are expensive, difficult to execute, and often inconclusive due to spatial variations in stress or scale effects. An innovative and for many applications economical approach has been developed at the Geomechanics Research Centre to determine the stress field at a scale relevant to underground excavations. The stress changes induced by the excavation are measured and used to back-analyze the best fit far-field in situ stresses. The principles and the data analysis procedure are briefly reviewed. This method has been verified by comparison with an analytical model and evaluated by field data from the advance of a drift and the sinking of a shaft at the Underground Research Laboratory (AECL), Pinawa, Manitoba.
In situ stresses need to be considered for the design of underground openings, for stability assessment, for mine design, for the selection of the excavation sequence, for support design, etc. Since in situ stresses cannot be predicted with confidence, they must be determined from field observations. Conventional techniques of stress determination include such techniques as overcoring, hydraulic fracturing, flat jack testing, and many more. Theses methods have several practical limitations and economic draw-backs. Therefore, the search for alternatives continues and currently focuses on two aspects: (a) simplification of the stress relief techniques to avoid overcoring during drilling, and (b) expansion of zone influenced by stress measurement to avoid local stress variations. A borehole slotting method was developed by Bock and Foruria (1984) to reduce the costs.
By this technique, the testing time has been significantly shortened, allowing repetitive measurements within a reasonable time frame. However, it requires a special instrument that is still under development and, at present, can only determine the stresses in the plane perpendicular to the borehole.
Theses methods have a common problem in that the stresses are determined in a relatively small volume of rock of about lm3 per measurement. The stresses may differ significantly within a small distance because of discontinuities, geological structures and variation of rock properties in a natural rock mass. Consequently, these methods may provide inconsistent results if few measurements are taken or tests are conducted at different sites.
A reliable and practically useful stress measurement technique should provide a state of stress that is representative for a domain of comparable size as the planned underground opening. It should also be relatively inexpensive or at least cost less than the conventional overcoring technique.
