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A laboratory demonstration of failure mechanics in horizontally bedded, cross-jointed roofs
Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. Vol. 20, No. 6, pp. 297-298, 1983
Printed in Great Britain. All rights reserved
0148-9062/83 $3.00 + 0.00 Copyright © 1983 Pergamon Press Ltd
Technical Note A Laboratory Demonstration of Failure Mechanics in Horizontally Bedded, Cross-jointed Roofs B. STIMPSON*
INTRODUCTION Roof falls are the major source of fatalities and injuries in underground coal mining. Though safe procedures are the major way of reducing these tragic accidents and much research and development is being undertaken to develop improved roof supports and safer methods of mining and support installation, it is also important that engineers, geologists and mine operators increase their understanding of the mechanics of roof failure. As an instructor of university mining students, the writer has had as one of his teaching objectives the development of simple physical modelling techniques which can be used in the laboratory to demonstrate how rock masses behave as opposed to intact rock samples [1-5]. These models are not exact analogues but do, nonetheless, demonstrate general principles and to some degree simulate actual, full-scale behaviour. Of course, the real "laboratory" in the mine will reveal other phenomena that cannot be readily duplicated in the laboratory; however, simple physical models provide a convenient bridge between classical rock mechanics and practical rock engineering. This note describes a simple and inexpensive physical modelling technique for demonstrating the modes of failure of a coal mine roof in horizontally bedded, cross-jointed strata.
and cross-joints, and to simulate various thicknesses of bed and rock strengths. From the results of experiments with various preformed building and framing materials from readily available commerical sources, it was determined that plaster-board or "Gyproc" was the most suitable for mechanical properties, ease of handling, availability and cheapness. In Canada, it can be purchased in two thicknesses, 1.27 and 15.9mm, in sheets measuring 2.44 x 1.22 m. Beds of various thicknesses can be created by gluing the plaster-board with general purpose glues. PROCEDURE
Having established the size of the model and the number of beds to be simulated, the plates are cut to the size required with a jig or band saw from a large sheet of plaster-board. The cross-joints are formed by the following simple procedure. The locations of joints on the surface of each plate are marked with a pencil line and a cut carefully made with a band saw along each of the lines. The cut should penetrate the thin paper covering of the plaster-board and make a shallow groove or notch in the plaster. The positions of the joints on each plate are easily arranged so that when the model is constructed the cross-joints can either be continuous across beds or offset. Next, each plate is inserted in a vice and a groove aligned just above the vice (Fig. 1). A rotating motion of the hands will snap the board and SIMULATING A COAL MINE ROOF create a rough tensile fracture, the sides of which will A simple physical model of a coal mine roof should interlock perfectly (Fig. 2). (Note: only one side of each provide for, at a minimum, the simulation of relatively plate need be grooved.) The two sides of the plate will smooth, horizontal parting planes and rough, discon- still be joined by a thin layer of paper on one side (the tinuous, cross-joints. The latter may continue across ungrooved side) (Fig. 1). This hinge of paper can be parting planes or may be abruptly terminated at parting carefully torn offf and the process repeated for the other planes. Thus, a classical model of this situation may be joints on the plate. Each rectangular block bounded by visualized as a brick or masonry wall. In more compli- two joint surfaces should be numbered to ease reconcated models it would be useful to model lateral loads, struction should the model be accidentally disturbed. The roof layers are now reconstructed on a layer of to vary the frictional properties of both parting planes wood which has been cut into several discrete blocks. * Department of Mineral Engineering, University of Alberta, When the required number of layers has been built, a Edmonton, Alberta, Canada T6G 2G6. lateral load should be applied to the model to ensure that t The thin layer of paper on plaster-board provides a graphic all the joints are closed. The lateral load can then be illustration of the strength provided by a thin layer o f shotcrete as removed unless it is desired to demonstrate the influence evidenced by a comparison o f the results o f a simple three-point beam of lateral forces on roof stability. bending test on a plaster-board beam with its paper covering intact and the same test on a beam with the covering removed. Failure can be slowly induced in the model by care~Ms 2o/~D
297
298
STIMPSON: TECHNICAL NOTE
fully removing, one at a time, the blocks of wood which simulate the coal seam. Figure 3 illustrates the effect of increasing the span of an opening beneath an I l-layer roof containing beds of equal thickness in which the cross-joints between each layer are offset. Many basic rock mechanics principles are demonstrated in this model and can form the basis for introducing students to mathematical models of roof behaviour.
Fig, 1. Formation of model joint by tensile failure.
produced which fractures in tension and compression under quite small loads (uniaxial compressive strengths of the order of 140-300 kPa).
Received
25 March 1983; revised 20 M a y 1983.
Fig. 2. Rough tensile joint in plaster-board.
Fig. 3. Sequence of photographs from top left to bottom right showing development of roof failure in horizontally bedded, cross-jointed model under zero lateral load. OTHER APPLICATIONS
AND REFINEMENTS
The model may be extended, at the cost in some cases of a loss in simplicity, to simulate subsidence, longwall caving, and rock bolting. In the latter case, nylon fishing line (about 150N tensile strength) bonded into holes drilled in the plaster (full-column or point anchor) is a suitable, readily available, inexpensive material for simulating the rock bolt. Lateral loads can be added by construction of a reaction frame and rams, and to a limited extent the strength of the plaster can be changed by either soaking in water or curing in an oven at 90°C for several days. In both cases, a very weak material is
REFERENCES
1. Stimpson B. A new approach to simulating rock joints in physical models. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. 16, 217-219 (1979). 2. Stimpson B. A simple physical modelling technique for the demonstration of interaction between undergound openings. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. 16, 215-216 (1979). 3. Baumgartner P. and Stimpson B. Development of a tiltable base friction frame for kinematic studies of caving at various depths. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. 16, 265-267 (1979). 4. Stimpson B. Laboratory techniques for demonstrating rock mass behaviour. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. 18, 535-537 (1981). 5. Stimpson B. A physical modelling technique for simulating the behaviour of coal pillars. Int. J. Rock Mech. Min. Sei. & Geomech. Abstr. 19, 347-351 (1982).
Printed in Great Britain. All rights reserved
0148-9062/83 $3.00 + 0.00 Copyright © 1983 Pergamon Press Ltd
Technical Note A Laboratory Demonstration of Failure Mechanics in Horizontally Bedded, Cross-jointed Roofs B. STIMPSON*
INTRODUCTION Roof falls are the major source of fatalities and injuries in underground coal mining. Though safe procedures are the major way of reducing these tragic accidents and much research and development is being undertaken to develop improved roof supports and safer methods of mining and support installation, it is also important that engineers, geologists and mine operators increase their understanding of the mechanics of roof failure. As an instructor of university mining students, the writer has had as one of his teaching objectives the development of simple physical modelling techniques which can be used in the laboratory to demonstrate how rock masses behave as opposed to intact rock samples [1-5]. These models are not exact analogues but do, nonetheless, demonstrate general principles and to some degree simulate actual, full-scale behaviour. Of course, the real "laboratory" in the mine will reveal other phenomena that cannot be readily duplicated in the laboratory; however, simple physical models provide a convenient bridge between classical rock mechanics and practical rock engineering. This note describes a simple and inexpensive physical modelling technique for demonstrating the modes of failure of a coal mine roof in horizontally bedded, cross-jointed strata.
and cross-joints, and to simulate various thicknesses of bed and rock strengths. From the results of experiments with various preformed building and framing materials from readily available commerical sources, it was determined that plaster-board or "Gyproc" was the most suitable for mechanical properties, ease of handling, availability and cheapness. In Canada, it can be purchased in two thicknesses, 1.27 and 15.9mm, in sheets measuring 2.44 x 1.22 m. Beds of various thicknesses can be created by gluing the plaster-board with general purpose glues. PROCEDURE
Having established the size of the model and the number of beds to be simulated, the plates are cut to the size required with a jig or band saw from a large sheet of plaster-board. The cross-joints are formed by the following simple procedure. The locations of joints on the surface of each plate are marked with a pencil line and a cut carefully made with a band saw along each of the lines. The cut should penetrate the thin paper covering of the plaster-board and make a shallow groove or notch in the plaster. The positions of the joints on each plate are easily arranged so that when the model is constructed the cross-joints can either be continuous across beds or offset. Next, each plate is inserted in a vice and a groove aligned just above the vice (Fig. 1). A rotating motion of the hands will snap the board and SIMULATING A COAL MINE ROOF create a rough tensile fracture, the sides of which will A simple physical model of a coal mine roof should interlock perfectly (Fig. 2). (Note: only one side of each provide for, at a minimum, the simulation of relatively plate need be grooved.) The two sides of the plate will smooth, horizontal parting planes and rough, discon- still be joined by a thin layer of paper on one side (the tinuous, cross-joints. The latter may continue across ungrooved side) (Fig. 1). This hinge of paper can be parting planes or may be abruptly terminated at parting carefully torn offf and the process repeated for the other planes. Thus, a classical model of this situation may be joints on the plate. Each rectangular block bounded by visualized as a brick or masonry wall. In more compli- two joint surfaces should be numbered to ease reconcated models it would be useful to model lateral loads, struction should the model be accidentally disturbed. The roof layers are now reconstructed on a layer of to vary the frictional properties of both parting planes wood which has been cut into several discrete blocks. * Department of Mineral Engineering, University of Alberta, When the required number of layers has been built, a Edmonton, Alberta, Canada T6G 2G6. lateral load should be applied to the model to ensure that t The thin layer of paper on plaster-board provides a graphic all the joints are closed. The lateral load can then be illustration of the strength provided by a thin layer o f shotcrete as removed unless it is desired to demonstrate the influence evidenced by a comparison o f the results o f a simple three-point beam of lateral forces on roof stability. bending test on a plaster-board beam with its paper covering intact and the same test on a beam with the covering removed. Failure can be slowly induced in the model by care~Ms 2o/~D
297
298
STIMPSON: TECHNICAL NOTE
fully removing, one at a time, the blocks of wood which simulate the coal seam. Figure 3 illustrates the effect of increasing the span of an opening beneath an I l-layer roof containing beds of equal thickness in which the cross-joints between each layer are offset. Many basic rock mechanics principles are demonstrated in this model and can form the basis for introducing students to mathematical models of roof behaviour.
Fig, 1. Formation of model joint by tensile failure.
produced which fractures in tension and compression under quite small loads (uniaxial compressive strengths of the order of 140-300 kPa).
Received
25 March 1983; revised 20 M a y 1983.
Fig. 2. Rough tensile joint in plaster-board.
Fig. 3. Sequence of photographs from top left to bottom right showing development of roof failure in horizontally bedded, cross-jointed model under zero lateral load. OTHER APPLICATIONS
AND REFINEMENTS
The model may be extended, at the cost in some cases of a loss in simplicity, to simulate subsidence, longwall caving, and rock bolting. In the latter case, nylon fishing line (about 150N tensile strength) bonded into holes drilled in the plaster (full-column or point anchor) is a suitable, readily available, inexpensive material for simulating the rock bolt. Lateral loads can be added by construction of a reaction frame and rams, and to a limited extent the strength of the plaster can be changed by either soaking in water or curing in an oven at 90°C for several days. In both cases, a very weak material is
REFERENCES
1. Stimpson B. A new approach to simulating rock joints in physical models. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. 16, 217-219 (1979). 2. Stimpson B. A simple physical modelling technique for the demonstration of interaction between undergound openings. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. 16, 215-216 (1979). 3. Baumgartner P. and Stimpson B. Development of a tiltable base friction frame for kinematic studies of caving at various depths. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. 16, 265-267 (1979). 4. Stimpson B. Laboratory techniques for demonstrating rock mass behaviour. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. 18, 535-537 (1981). 5. Stimpson B. A physical modelling technique for simulating the behaviour of coal pillars. Int. J. Rock Mech. Min. Sei. & Geomech. Abstr. 19, 347-351 (1982).