Accurate magnitude of the biaxial ground pressures existing in a viscoelastic rock mass can be determined by pressure convergence tests with a combination of one cylindrical and two flat hydraulic pressure cells installed in a single hole drilled into that rock mass. Consequently, the triaxial ground pressures can be measured by drilling a pair of orthogonal holes for such instrumentation. Unlike most of the known methods of in situ stress determination that determine the magnitude of the stress indirectly from the elastic stress-strain relation by measuring strain or displacement in place and modulus of deformation in the laboratory, this method directly measures the in situ stress without any knowledge of the value of modulus of deformation. Furthermore, the complete state of stress existing in the viscoelastic rock mass also can be determined by this method if other techniques are supplemented for determining the directions of the principal stresses.
In this paper, the theoretical background, measuring equipment, and measurement procedure are discussed. Validity of the principles and practicality of the equipment are demonstrated by the in situ measurements conducted for a variety of rock types.
In the early 1970's the author initiated a concept of determining the magnitude of the biaxial ground pressures existing in a viscoelastic rock mass by pressure convergence tests with a combination of one cylindrical and two flat hydraulic pressure cells in one hole drilled into that rock mass(Lu, 1973). With this instrumentation, the cylindrical cell gives the sum whereas the two flat cells give the ratio of the biaxial pressures. Consequently, by drilling a pair of orthogonal holes for such instrumentation, the magnitude of the triaxial ground pressures can be obtained provided that the rock mass is homogeneous and the stresses are uniform within the test site.
Unlike most of the known methods of in situ stress determination, which determine the stress by measuring strain or displacement in situ and modulus of deformation in the laboratory, this method directly measures the in situ stress without any knowledge of the value of modulus of deformation. The time dependent in situ modulus of deformation of a viscoelastic rock mass is impossible to determine accurately in the laboratory.
In this paper the underlying theory, measuring equipment, and measurement procedure are described. Several measured results are presented to verify the validity and practicality of the technique.
The state of stress in an in situ intact rock mass, whether it is homogeneous or heterogeneous, isotropic or anisotropic, and elastic or inelastic, is initially in equilibrium or inactive. However, once a hole is drilled into such a rock mass the ground pressure existing in the vicinity of the hole will become active due to stress relief of the portion of the hole.
Usually, most geologic materials are elastic, but some of them may show time effects such as creep under an intermediate (compared with the breaking strength of rock)constant stress(e.g. Griggs, 1939). However, the general problem of stress-strain analysis is the same for elastic and for viscoelastic structures.
