A circular, servo-controlled, biaxially loaded Shear-Permeability frame, approximately 2 m in diameter, was fabricated and used to investigate the coupled hydraulic-mechanical response of natural and artificial fracture surfaces to normal and shear loading. Loads are provided by two independent actuators, equipped with 0.89 MN load cells. The computer controlledata acquisition system and digital to analog chips provide displacement control while maintaining the specified normal and shear stress boundary conditions. Using the existing actuators, a 200 mm by 300 mm fracture plane can be loaded to about 23 MPa of normal stress or a shear stress of about 10.5 MPa. The normal and shear displacements of each fracture plane are measured using two LVDT's on each corner of the sample. An inflatable packer system provides boundary control for flow and transport experiments within the fracture plane during repeated cycles of normal and shear loading at temperatures that range from 6øC to 80øC.
Direct shear machines have been used extensively to determine the shear strength parameters of soils for many decades and their designs are well-documented in the soil mechanics literature. However, direct shear machines for testing rock and discontinuities in rock have only been used extensively during the last three to four decades. Standardized methods for the design of direct shear machines and the shear test procedures to be followed are given by Gyenge & Herget (1977) and Brown (1981). These authors stated that the six basic components of a conventional shear machine include:
1) A shear box which houses the fractured rock sample and consists of a stationary half and a half that is moveable along the direction of shearing. The sample is encased in the shear box with an encapsulating material such that the plane of discontinuity is coincident with the shearing plane.
2) A mechanism for applying a force normal to the shearing plane, such as a hydraulic jack, and capable of being maintained at a constant value (accuracy of 1-2%).
3) A mechanism for applying a shear force to one half of the shear box along the plane of the discontinuity, and capable of shearing the sample at a uniform rate of displacement (I.2 120 mm/hr). Shear displacement should not be greater than 10% of the sample length (the direction of shearing should be reversible).
4) Equipment, such as load cells with an accuracy of + 2%, for independent measurement of normal and shear forces.
5) Equipment for measuring normal and shear displacements, such as electric transducers fixed on the sample as close to the joint as possible.
6) An automatic data acquisition system to measure,
record and display results during testing.
Jaegar (1971) stated that, in principle, the direct shear system has five degrees of freedom: horizontal and vertical translation in addition to rotation about a vertical axis and two horizontal axes. The above author considers that a sixth degree of freedom, lateral translation, should however be added to the list. Figure I shows the six degrees of freedom (3 translations and 3 rotations) in a direct shear system. The number of degrees of freedom in the design of a direct shear machine should reflect the in-situ conditions the experiment attempts to simulate. In most systems, the horizontal (shear) displacement and vertical (normal) displacement are controlled while lateral displacement and all rotations are prevented. Crawford and Curran (1981) describe a shear machine with three degrees of freedom
