Tapercoring is a new method of determining in situ rock stresses anywhere along the length of a borehole. The method is based on a unique way of releasing the three-dimensional in situ rock stresses on the walls of a borehole. The method begins by checking that the borehole walls are suitable for conducting in situ rock property and relief measurements, e.g., continuous, isotropic, and elastic. Next, the Young's Modulus and Poisson's Ratio of the rock mass are determined in situ. Then, a multicomponent deformation or strain sensor is installed and the initial zero readings are recorded, and finally, tapercore drilling in the vicinity of the gage is carried out, i.e. an inclined outwardly tapering circular slot is cut on the borehole walls. Tapercore drilling destresses the material where the sensors are installed. The stress relief readings at various locations and orientations on the borehole wall, together with in situ rock properties determined earlier, provide sufficient data for determining the three dimensional in situ rock stresses.
Overcoring and hydrofracturing are the two most widely used direct methods of determining the in situ state of stress in rock masses. Overcoring has a number of limitations and among them are:
-overcoring can be carried out only at fairly shallow depths;
-the volume of rock affected from which the stresses are calculated are relatively small;
-the elastic properties of the rock mass are needed for transforming the measured
values into rock stresses.
Hydrofracturing, on the other hand, overcomes most of the limitations that are inherent in overcoring. However, hydrofracturing only provides an estimate of the average minimum horizontal principal stress and also assumes the magnitude of a vertical principal stress to be due to the weight of the superincumbent materials which may not necessarily be correct. There are also uncertainties concerning breakdown pressure and instantaneous shut-in pressure (Totme 1983). Notwithstanding, overcoring and hydrofracturing have been the only real and practical methods of determining in situ rock stresses.
Acknowledgment: This paper is submitted in support of the contributions in Rock Mechanics of Professor R. E. Goodman, co-inventor with the author of the Borehole Deepening Method of Stress Measurement. Moreover, this paper suggests the use of the Modified Goodman Jack Method for measuring in situ rock properties in order to ensure more precise in situ stress measurement.