When a long wall face is newly opened between two headings, or gate roads, say, 100 m apart, and the face advances from a solid coal pillar or coal rib, the higher strata in the roof form a sort of bridge (or arch) across the excavated area from solid coal of the face to solid coal of the coal pillar or coal rib behind. Timber or steel supports will support the immediate roof. As the face advances and the span of the arch increases the load due to upper strata increases over the abutments of the arch until the bridge breaks down and the roof comes down in the goaf.
The redistribution of forces is shown in fig. 9.4. The dotted arch represents the "Pressure arch". The weight of the higher beds is carried partly by the solid coal at some distance ahead of the face, (between B and C) and partly by the packs at some distance behind the face (between M and E). The pressure arch, thus, has its front abutment in the solid coal and shifts forward as the face advances. It has its back abutment on the packs in the goaf. This back abutment also moves forward as the face advances.
In the figure, the line XY represents the static load due to depth of the seam (called the dead weight of the strata). The front and back abutment pressures are much in excess of this static load. !n the working area, CD, the load is greatly reduced and in the goaf the load increases till it reaches the maximum at the back abutment between M and E.
Just as there is a longitudinal pressure arch at right angles to the face, as described above, there is also a transverse pressure arch which spans the whole face length and has its abutments on the solid coal beyond the gate roads.
Experience shows that the pressure arch in longwall workings has its apex or crest at a minimum height above the seam at nearly 2 times the length of the face. If the depth of the coal seam exceeds twice the length the pressure on account of weight of the rocks outside the pressure arch is transmitted to abutments. If the depth is less, the static pressure of the rocks above the goaf area right up to the surface is to be supported by the props, chocks and the packs or stowing in the goaf. Perhaps for this reason longwall faces with caving have caused difficulties in roof control at depths of less than nearly 100 m.
It will be clear that failures of underground coal pillars and roadways are the combined effects of the induced stresses in the surrounding rocks and coal and the inability of the rocks and coal to withstand them. For example, a pillar fails when the applied axial stress exceeds its compressive strength. Similarly, when the shear stress in the roof rocks at the edges of a rectangular opening exceeds the shear strength of the immediate roof, it fails in shear. How a rock responds to possible state of stress as well as the reasons for its failure should be understood by engineers planning underground supports and workings.
Underground rocks are generally subjected to a load equal to the weight of the overlying rock. Machines for tests of rock samples brought out in the cores of boreholes are generally designed to determine the compressive strength and the most common method of rock testing is the uniaxial compressive strength test. It involves setting a cylindrical (or cubic or prismatic) specimen between the upper and lower platens of the testing machine. The load generated by the testing machine is applied to both ends of the specimen through the platens. The physical properties of the rock determined in the laboratory generally do not represent those of the rock mass in situ, because in-situ rock mass generally contains planes of weakness, which often break apart during preparation of specimens for laboratory testing. The specimens used in the laboratory tests represent generally the intact portion of the rock mass.
The compressive strength of most of the rocks is greater than the tensile strength. The compressive strength increases with lateral stress; so most test systems are also equipped with a pressurisation system and chamber designed to apply a confining pressure to the lateral surface of the specimen during compression test.
The underground conditions to which a rock is subjected in situ can be simulated to a great extent if the rock specimen undergoes a compression test when it is laterally confined by hydraulic pressure on all sides. This is achieved by triaxial load tests. The triaxial tests are usually performed for cylindrical specimens in a triaxial cell which is a cylindrical steel chamber (Fig.9.3), large enough to accommodate a specimen with a length/diameter ratio of nearly two. The confining pressure is applied by using hydraulic oil while the vertical (axial) load is applied by the testing machine. To prevent oil from penetrating into the specimen, it is usually inserted into tightly fitted tubing, which can be rubber, plastic or copper.
The modern machines used in the laboratories employ closed loop servo-controlled testing system.
Plate shows a triaxial test cell for specimens up to 54 mm in diameter, it has an internal heater capable of bringing the specimen to temperatures as high as 200"C. (MTS model 656.01)
This triaxial cell can test specimens upto 150 mm long, and is available with optional platens for testing specimens with diameters smaller than NX size. The pressure rating of this triaxial cell is 20,000 psi (138 MPa) maximum confining pressure, and 20,000 psi "maximum pore pressure. The unit's internal heater will heat specimens' to 200°C typically in less than four hours.
The pressure vessel's internal diameter of 140 mm gives ample room for simultaneous mounting of optional MTS circumferential and axial rock mechanics extensometers. Inside, a Teflon insulating' shroud and heat baffles are incorporated to reduce the effect of temperature on the load cell and instrumentation connector's. The vessel is raised and lowered by hydraulic lifts for easy access to the test specimen area.
One method of determining the shear strength of a rock is the direct shear test, A cylindrical specimen is tight-fitted into a, shear box that consists of an upper and a lower piece. During testing, the lower piece is fixed while a horizontal force is applied to the upper piece in the direction parallel to the contact plane between the upper and lower pieces and gradually increased until failure occurs along the contact plane. The horizontal force at failure divided by the cross-sectional area of the specimen is the shear strength.
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