The term "dominant parting plane" has been used to describe the discontinuities which are parallel to bedding and which influence the strata mechanics of tabular deposits. A novel direct shear machine was used to test simulated dominant parting planes. The results showed that the energy dissipated during the direct shear process was markedly dependent upon the mineralogy of the shear surface. The Rosin- Rammler equation was found to describe the weight distribution of sized shear debris, with the distribution constant, n, being correlated with the dynamic (sliding) coefficient of friction, μs. This may enable μs to be inferred for naturally-sheared discontinuities from an analysis of shear debris.
The concept of the dominant parting plane was proposed by Smart (1981, 1986) following observations made of strata deformation processes around mine excavations. This concept suggests that dominant parting planes exert a major control over the reaction of any succession of strata of sedimentary origin to the creation of a void within it.
A parting plane is a plane of relative weakness aligned parallel to the bedding, which is caused by a change in mineralogy, ego by the concentration of clay minerals, producing a thin layer of weak rock. While the layer may only be a few millimetres in thickness, it may extend for kilometres in the plane of the bedding.
It is proposed that a dominant parting plane acts as a focus for stress relief, permitting relative lateral movement between, and/or the separation of, adjacent strata as they move in both the vertical and horizontal directions toward a void. The relative lateral movement, or shearing action, typically produces a thin layer of rock debris containing striations Which indicate the direction and extent of Slip.
The direct shear machine (see Figure 2.1) was conceived and designed specifically to investigate the dynamic shear properties of parting planes. This machine, which is capable of applying 500kN normal and shear loads, incorporates some unique features, at least in the context of shear testing of rock. The shear table utilises a linear hydrostatic bearing to provide effectively frictionless motion, even when acted on by the 500kN normal reaction. This bearing action is created by feeding hydraulic oil through four orifices on the underside of the shear table. 20MPa pressure is dissipated in a O.05mm gap between the rims of the orifices and a lower bearing plate.
The hydrostatic bearing is fed from a 4kW power pack which supplies the 20MPa feed and scavenges the spent oil. The vertical normal reaction across the shear table is developed by a 500kN actuator with 200mm travel. The cross head on which this actuator is mounted can be adjusted vertically through six fixed positions, enabling a maximum clearance of 630mm to be generated between the actuator and the shear table. The horizontal displacement of the shear table is generated by two opposed 500kN rams with a stroke of 100mm, each ram acting only in push.
(Figure in full paper)
