Michael Hood

Observations of crack growth in hard rock loaded by an indenter

Abstract A description is given of the processes of rock fragmentation induced by circular flat-bottomed punches from 5 to 20 mm dia loading orthogonal to the flat surface of cylindrical specimens of Sierra granite of 89 mm dia and confined by a steel belt, and 100 mm cube specimens confined in a biaxial frame. It is found that the rock deforms elastically until the applied punch load exceeds 45% of the maximum load that the rock cna sustain. At loads greater than 45% of the maximum, a crack is initiated around the perimeter of the punch and this crack propagates in the well-known conical Herizian manner. When the crack begins to propagate, the location of its front is reasonably well defined. However, as it extends with continued application of the load, an intense zone of microcracks surrounds and conceals the crack tip. Fractures initiating within this microcracked region propagate to the surface and form rock chips. The effects of punch size and confining stress, orthogonal to the punch, on the rock indentation strength are examined. Some decrease in punching strength with increasing punch size is noted; there is an increase in strength with increasing confining stress. At low confining stress, specimens split by cleavage through the axis of the punch. The theory of elasticity is used to show that this could occur with a punch loading against a semi-infinite rock surface and may therefore be an important phenomenon, not just an artifact of finite laboratory specimens.

Wear and failure mechanisms of polycrystalline diamond compact bits

Abstract Four mechanisms of polycrystalline diamond compact cutter failure are identified in a detailed study of cutters damaged in both laboratory rock-cutting tests and field operation. One mechanism is smooth wear. This occurs when individual diamond grains are polished away by a combination of high mechanical and thermal loads. A second mechanism is microchipping of the polycrystalline diamond table. This is caused mainly by the action of the bit cutting forces. A third mechanism is gross fracturing. This is caused by the application of excessive normal force to the bit. A fourth mechanism is delamination at the interface between the diamond table and the cemented tungsten carbide substrate. This results both from impact loading in the bit normal-force direction and thermal stress build-up at the interface between the diamond layer and carbide substrate due to the mismatch in the thermal expansion coefficients of these materials. All these processes lead to the formation of a wear flat beneath the cutter. Once this flat forms, repeated frictional heating and cooling cause thermal fatigue which results in the commonly observed heat checking on this surface.

Waterjet-assisted rock cutting systems — the present state of the art

SummaryA review of the benefits of assisting mechanical tools, notably drag bits, with moderate pressure waterjets suitably directed with respect to the bit is given. These benefits include reduced bit forces, especially the bit normal force, reduced bit wear, reduced dust make, and reduced incidence of frictional sparking. The research work that has been conducted to date to investigate this phenomenon has been empirical in nature. Experiments are described that extend the data bank of this empirical knowledge. In addition, experiments aimed at gaining a better understanding of the fundamentals of the rock fragmentation process with this hybrid cutting method are outlined.Results from the first of these experimental series are used to make recommendations as to the position of the jet with respect to the bit, the stand-off distance between the nozzle exit and the bit/rock interface, and the jet energy. In addition, preliminary findings are reported regarding the increase necessary in the jet energy when the bit velocity is increased. Results from the second test series are discussed in the context of rock fracture behaviour induced by mechanical bits acting alone. The likely influence of waterjets on these fracture processes is analysed. It is concluded that, in terms of the bit force reductions, a dominant influence of the jets when used in conjunction with sharp drag bits, is continuous removal of the rock debris that forms ahead of the advancing bit. The observed reductions in bit wear and incidence of frictional sparking are attributed to reduced heat loading of the bit during the cutting operation. Reductions in the dust make are attributed to effective wetting of the fine rock particles before they become entrained in the airstream.

A STUDY OF ROCK EROSION USING HIGH PRESSURE WATER JETS

Abstract This study is directed towards developing a better understanding of the erosion mechanisms involved in the use of high-pressure water jets for cutting in rock materials. Theories to explain the cutting action of high-pressure water jets have been proposed by Crow [1,2] (Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. 10, 567–584, 1973 and 12, 203–212, 1975) and Rehbinder [3] (Rock Mech. 12, 247–257, 1980). These theories predict the depth of kerf cut by the jet in terms of the rock and fluid properties and the water jet system parameters. While both theories produce reasonable results for certain cases, neither is completely satisfying. For pedagogical purposes this paper is organized in sections. Following the Introduction, a review is given of published empirical findings from studies of rock erosion using high-pressure water jets. Then the theories of Crow and Rehbinder are compared with published experimental results to determine how well these two models predict the kerf depth in rock cut by a water jet. Next, results are given of new water jet rock kerfing experiments conducted using a factorial approach in the experimental design. Empirical models fitted to these results identify the relative importance of both the rock properties and the water jet parameters in the erosion process. There follows a description of a new laboratory technique employed to examine micro-structural rock damage caused by the water jet in eroding the rock. The objective of all of this work is to gain new insight into the mechanisms of rocl erosion. The concluding section then outlines a proposed new physical model for rock erosion by a high-pressure water jet.

Surface strain over longwall coal mines: its relation to the subsidence trough curvature and to surface topography

The paper presents an analysis carried out on a US longwall mine located in rugged terrain with the object of determining the effects of topography on surface strains and of defining a characteristic pattern for those effects.

Empirical methods of subsidence prediction—a case study from Illinois

Abstract Subsidence profiles above two adjacent panels in Illinois are compared with profiles predicting subsidence behaviour obtained using the (i) National Coal Board (NCB) [1] method, (ii) the profile function method and (iii) the influence function method. The NCB method predicts the maximum subsidence values at the centre of the troughs accurately but the overall shapes of these predicted profiles do not match the profiles from the measured data well. Consistent values for the angles of draw were measured by these angles were different in the transverse (average values 43°) and in the longitudinal (average value 17.5°) directions. Time dependent subsidence effects are shown to be small but measurable. These displacements continue at a linear rate for at least a 12 month period. A comparison of measured horizontal distances, interpreted as horizontal strain, and the NCB predictions for strain shows that the peak measured strains are greater by a factor of about four than the predicted strains. The relationship between surface curvature and strain is investigated. Problems associated with calculation of surface curvatures from vertical displacement data are highlighted and a recommendation is made for future studies to consider direct measurement of this parameter. Surface curvatures above a moving face are found to be about three times less than the curvatures at the stationary end of the panel. On the other hand, a hyperbolic tangent profile function is shown to serve as an accurate predictive tool for subsidence behaviour in two adjacent longwall panels at Old Ben Number 24 mine in Illinois. This function predicts not only the vertical displacements but also the surface curvatures above both panels. Influence functions are shown to be more problematic, although potentially more flexible, in their application than the profile functions.

Fatigue test on polycrystalline diamond compacts

Abstract We have carried out compressional fatigue tests on notched polycrystalline diamond compacts (PDC). Fatigue cracks were seen to grow in both the sintered tungsten carbide and the polycrystalline diamond parts of the compact. The cracks in the cemented carbide were very fine and sharp, with an unstressed opening of about 1 μm. That the cracks propagated under the far-field compression was mainly a result of the induced residual tensile stresses that arose upon unloading from the compressive load. These cracks grew at a decreasing rate away from the starting notch, no doubt because compressive load was transferred directly across the crack faces by crack closure during the (compressive) loading part of the cycle. The fact that the fatigue crack was propagating into a region of greatly reduced stress may also have contributed to the deceleration in crack propagation. The cracks in the polycrystalline diamond, however, were relatively wide. At the start of crack growth, we believe that failure of the diamond occurred under mainly uniaxial loading, as happens in the case of borehole breakouts. This is characterized by the formation of spalled flakes whose long axes are nearly parallel with the direction of the applied load. As the crack deepened, the size of the fracture zone decreased to an approximately constant width of about 30 μm. Failure at the root of the crack was then more probably in multiaxial compression, in which the diamond grains were crushed and disintegrated over a certain volume, resulting in the detachment of fragments of diamond, and the maintenance of a wide crack.

Optimum bunker size and location in underground coal mine conveyor systems

SummaryThe transportation of coal out of a mine is usually done by a serially linked system of conveyors. It is apparent that in a serially dependent system of this type the failure of any one component (conveyor) causes the complete system to be shut down. The provision of bunkers at strategically located points in the conveyor system can result in the production of coal even in the event of failure of some outbye conveyors. Although the use of bunkers can increase mine system availability, the maximum possible system availability may not be attained if the bunker capacity is inadequate and/or the location of these bunkers in the system is inappropriate. This paper presents a stochastic model by which the optimum capacities and locations of bunkers can be determined for a system consisting of a single production section and a system of serially linked conveyors.