Determination of ground pressure existing in a viscoelastic rock mass by use of hydraulic borehole pressure cells

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.