ABSTRACT
Defect of the existing overcoring methods is that their application is limited to the rock in such conditions that the density of fissures is low and observation hole and/or hole bottom could be well finished. Due to this fact, most of the data relating to in-situ stress is from hard and moderately hard rocks. Thus it has been felt very necessary to develop an overcoring system that has wider applicability in order to obtain the in-situ state of stress in soft rocks as well. To solve this problem, new stress measuring system has been developed, including the stress meter, which can measure seven components of radial and axial deformations of pilot borehole during overcoring. From these deformation data, 3-dimensional stress states in rock are determined. The stressmeter can be inserted into the 40 mm pilot borehole, irrespective to its condition. That is, the measurement can be conducted as well in the pilot borehole filled with water. Succeeding to the insertion of the stressmeter into the pilot borehole, the overcoring work using double core tubing can start without any loss of time. After the core with the stress meter inside it is recovered, it can be prepared for the field uniaxial loading test to determine the elastic properties of the rock. After the test, the stress meter is recovered and the data is downloaded to computer for stress calculations. The stress meter then can be used again to measure another point. Advantages of this stressmeter are small in size, convenient to use, reusable, measurement in a borehole at any required depth, continuous data available during the whole cycle of measurement which constitutes from the insertion of the meter to retrieving it from the core, and no loss in time allowing the overcoring process to be short.
We have an experience that two or three data were obtained in one day in a certain field, using downward borehole driven in the soft rock having compressive strength about 10 MPa, and it was cleared that this measuring system is practical to be used in the field.
