For the fundamental study of the behavior of rock joints, and the development of constitutive models for joint response, a hollow cylinder apparatus (HCA) has been developed to overcome the deficiencies associated with the direct shear device. A series of baseline tests have been performed to compare the results obtained with the HCA to those of the direct shear device. The results indicate that the HCA yields slightly lower friction angles than the direct shear device. However, the initial stiffness obtained in the HCA is greater, probably a result of the increased rigidity of the torsional configuration.
The convenience of the direct shear device explains its widespread use to determine the behavior of rock joint strength and deformational response (Brown 1981; Franklin 1985; Sun, et al. 1985; Hutson and Dowding 1990). Along with this convenience come several significant limitations. These limitations include the inability to determine the principal stresses except at failure, non-uniform stresses at the joint, and high stress concentrations at the edges. In addition, the response under large displacements can only be assessed by reversing the direction of shear, and joint water pressure is difficult to control and measure. Although the direct shear device is appropriate for design and analysis, it is not adequate for a fundamental study of the behavior of rock joints or for the development of constitutive models for joint response. The historical use of thin-walled annular specimens of soil subjected to an applied torque is well documented and is gaining popularity (Hvorslev 1939; Bishop, et al. 1971; Lade 1981; Hight, et al. 1983; Saada 1988). Due to the favorable stress state in these devices, similar geometries have been used in the studies of other materials like intact rock (Handin, et al. 1967; Christensen, et al. 1974; Cox and Scholz 1988) and the high temperature testing of concrete and rock (Bazant, et al. 1981; Bazant, et al. 1986). The geometry of the hollow cylinder apparatus (HCA) lends itself to an investigation of the effects of various stress paths and anisotropy associated with many materials (Saada 1988). An additional advantage is the continuous contact of the discontinuity surface, which alleviates the stress concentrations that occur in direct shear at the leading and trailing edges of the joint. The University of Tennessee-Hollow Cylinder Apparatus (UT-HCA) has been developed to perform fundamental investigations needed for studying and modeling rock joint behavior (Drumm 1988) (Figure 2). Unlike earlier HCA's used for joint testing, the UT- HCA will allow the application and control of the confining pressure applied to both the inner and outer cylinder walls, and the joint water pressure at the joint interface. The sample used in the UT-HCA has an inside and outside diameter of 100mm and 150mm, respectively. An MTS biaxial load frame with an electro-hydraulic closed-loop system is used to control the normal stress (axial force) and shear stress (torque). A function generator controls the rate of displacement and cyclic rotation of the UT-HCA specimen.
