Technical Report NTB 88-39
Grimsel Test SiteRock stress investigations
On the research project "Rock Stress Measurements" the BGR has developed and tested several methods for use in boreholes at a depth of 200 m.
Indirect stress measurements using overcoring methods with BGRprobes and CSIR-triaxial cells as well as direct stress measurements using the hydraulic fracturing method were made. To determine in-situ rock deformation behavior, borehole deformation tests, using a BGR-dilatometer, were performed. Two types of the BGR-probe were applied: a four-component-probe to determine horizontal stresses and a five-component-probe to determine a quasi three-dimensional stress field. For the first time, a computer for data processing was installed in the borehole together with the BGR-probe. Laboratory tests on hollow cylinders were made to study the stress-deformation behavior. To validate and to interpret the measurement results, some test methods were modelled using the finite-element method.
The dilatometer-tests yielded high values of Young's modulus, whereas laboratory tests showed lower values with a distinct deformation anisotropy. Stress measurements with the BGR-probe yielded horizontal stresses higher than the theoretical overburden pressure. These results are comparable to the results of the hydraulic fracturing tests, whereas stresses obtained with CSIRtriaxial cells are lower.
The detailed geological mapping of the borehole indicated relationships between stress and geology. With regard to borehole depth, different zones of rock structure, joint frequency, joint orientation, and orientation of microfissures as well as stress magnitude, stress direction, and degree of deformation anisotropy could be distinguished.