The Deformation Rate Analysis (ORA) technique can be used to estimate the in situ stress from orientated core obtained from exploration or for example from a block of rock extracted from the side of a drive. In the former case it doesn't require underground access and in both cases the results are free from influence of anisotropy unlike all other methods available; it also costs much less than conventional stress measurement methods (e.g. USBM, HI, HF) as neither special access nor specific drilling are required. However, the nature of this method makes it very sensitive to the test environment and the rock properties can influence the accuracy of prediction. In order to achieve a good quality result, extra attention and back calculation are required.
In this paper we discuss a successful case using ORA to predict the in situ stress. The test was done at a location where the ice sheet was present many thousands years ago. Without the knowledge of the stress condition, the client supplied information on sample depth below surface, regional structure, and nearby openings prior to the testing. The rock core was carefully selected and some trial tests were conducted to check the reliability of result. In the analysis stage, the result was examined using available information to eliminate the induced stress or any artificial stress generated during core extraction. After all procedures been undertaken, the result was compared with the in situ stress obtain by a different method by the client and the difference between two methods was confirmed to be minimal.
