Discussion on Session 6: laboratory testing

Professor King of Imperial College spoke about the development of a device for the true triaxial testing of rock specimens, which could also be used for cemented sands and clays. Each of the three principal stresses may be varied independently under servo-control in the range 0 - 115MPa, with pore pressures to 5MPa possible (Figure 1). Stress is tranmsmitted to each of the six faces of the cubic specimen via 5mm thick faceplates matching approximately the elastic properties of the materials being tested. Magnesium has been found a suitable material for use with soft sandstones. Deformation of the specimen is measured by three pairs of LVDTs. Feedback from the LVDTs may be used for servo-control of the specimen deformation. Each of the six transducer holders contains stacks of piezoelectric transducers capable of producing or detecting pulses of compressional (P) or two shear (S) waves polarised at right angles propagating in the three principal directions. The bandwidths of the P and S wave pulses are in the range 350-850kHz. The system permits the measurement of the deformation and elastic-wave velocities and attenuation in each direction. Nine components of velocity are measured: three compressional and six shear. The redundancy in S wave velocity measurements provides the opportunity to confirm that the state of stress within the specimen is indeed homogenous. Permeability and electrical conductivity may be measured in the vertical direction. Tests have been performed on Penrith Sandstone specimens, during which aligned cracks were introduced by increasing o', and o', in unison while maintaining o'3 low. The three deformations, all nine components of velocity and permeability in the vertical direction were measured during this cracking cycle. Finally, the three principal stresses were increased together in order to close the cracks formed. The same set of measurements were repeated during the crack closing cycle. There was an excellent correlation between permeability in the direction of the set of oriented cracks and the S wave velocity in a direction perpendicular to the set. The system will next be used to model the behaviour of soft sandstones adjacent to a borehole during drilling.