AU2013224760A1 - Support and system for an excavated tunnel - Google Patents

Abstract

A support mesh (10) for an excavated tunnel surface, the mesh comprising a plurality of spaced longitudinal wires (12) and a plurality of spaced cross wires (14) welded together to define a grid, at least one pair of opposed longitudinal wires (20) adapted to receive a rock bolt therebetween so as to secure the mesh to the surface, wherein the wires are coated with a polymeric thermoset coating. )~ ~'rr ii U Ii '' ~itEHtL I IIt fl441 I' ~ it ~i~iiJLf N t 4]4. 'tiI# i~ X'~4 NJ ~4 14~ ii iijjj V 3' 14 Ku ' N k

Description

1 SUPPORT AND SYSTEM FOR AN EXCAVATED TUNNEL FIELD OF THE INVENTION [0001]The present invention relates to a support mesh for supporting a surface of an 5 underground excavation. In particular, the present invention is directed towards supporting a roof surface in a mine tunnel. The present invention also relates to a method for supporting an underground surface and a system for supporting the surface of a mine. 10 BACKGROUND [0002]The present invention will be described with particular reference to applications for supporting a roof in an underground mine and especially a coal mine. However, it will be appreciated that the present invention has wider applications and may be used to support an underground roof surface in any suitable excavation including 15 civil tunnels such as road an train tunnels. [0003] It is known to stabilise roof strata in underground excavations by the use of rock bolts. Rock bolts are long anchor bolts that transfer load from an unstable exterior to the confined more stable interior rock. Whilst rock bolting prevents catastrophic 20 failure of the roof, most rock fall injuries in mines are caused by falls of smaller rocks. In order to address this problem, it is known to use screening in the form of a welded steel mesh. The mesh passively confines the surface so as to prevent fragments of rock and coal from falling from the roof and ribs in the spacing between the rock bolts. 25 [0004] Roof support mesh is typically available in rectangular sections formed from parallel line wires and parallel cross wires welded together in a square or rectangular pattern. The mesh is available in panels of about 750mm to 1500mm and in widths and between 3m to 6m in length. 30 2 [0005]The strength of the mesh depends upon the gauge of the steel. As the strength dictates the amount of rock load that can be tolerated, strength is an important consideration. Typically steel wire having a diameter of between 4mm to 6mm, commonly 5mm is used. It will be appreciated that mesh panels having these 5 dimensions are heavy and generally require two operators to handle a mesh panel. [0006] Roof support mesh in an underground mine is generally installed in a tunnel immediately after a new section of roadway has been excavated. After 10 excavation, the mesh panels are positioned across the roadway with one end overlapping a previously installed panel. The rock bolts are then positioned and installed so as to clamp the mesh to the roof strata. [0007] It is known to provide additional lengths of wire at those positions of the mesh 15 where the rock bolts are installed so as to reinforce those sections. The reinforcing wires may also serve to provide a guide for the positioning of the rock bolts. [0008] Installation of support mesh is expensive in terms of capital and material handling 20 costs. Further costs are incurred indirectly as a result of the delays taken in installing the mesh as this halts the progress of a continuous miner. However, it is considered that the additional safety afforded by the mesh outweighs the costs. [0009]A recognized disadvantage of steel mesh is its susceptibility to corrosion. As the 25 wires are relatively thin as soon as a wire begins to corrode there is a loss of strength. Providing thicker gauge wire that will retain structural integrity for a longer period is not an option as this would increase costs and more importantly make the panels too heavy to be manually handled and installed. 30 3 [0010] Constant monitoring of the mesh to detect corrosion must be carried out. It will be appreciated that a catastrophic failure of a section of mesh cannot be tolerated. Accordingly, as the steel wire corrodes, the mesh must be replaced. The time period between installation and replacement can vary considerably depending 5 upon the conditions within a particular mine. Such conditions include, moisture, pH and temperature. [0011 In order to address the problem of corrosion, the standard approach in the mining industry is to provide galvanized mesh. However, galvanizing simply delays or 10 prolongs corrosion rather than preventing it. Further, the mesh panels are subject to considerable abrasive and other mechanical forces during transport, storage at the mine site and eventual installation. For example, there may be many kilometres of roadway between outdoor storage of the panels to where there are installed. The panels are often towed behind a rock bolting machine to the site of 15 installation. This can scratch or damage the galvanised layer, thereby creating a seed point for corrosion. [0012] Re-installation of mesh involves a significant cost to a mine operator in terms of capital outlay, labour costs and mine down time. In order to reduce the costs of 20 mesh installation, the industry has looked to methods of automating installation of a roof support material. Although self-drilling systems for installing rock bolts have been investigated, mesh installation is necessarily a manual operation in view of the size and rigidity of the mesh. By way of example, current research at the University of Wollongong is aimed at providing an alternative to steel mesh that 25 can be installed automatically with a view to fully automate the roadway support process. The alternative being investigated is a polymer based spray on skin that cures in situ once applied. A suitable material must satisfy numerous criteria including toxicity, strength, temperature, moisture, coal dust and pH sensitivity, anti-static, flexural properties, fire retardancy and longevity. However, to date no 30 practical alternative has been proposed. [0013]Accordingly, there is a recognized need in the mining industry to provide an alternative to the current system.

4 SUMMARY [0014]According to a first broad form of the present disclosure, there is provided a support mesh for an excavated tunnel surface, the mesh comprising a plurality of spaced longitudinal wires and a plurality of spaced cross wires welded together to 5 define a grid, at least one pair of opposed longitudinal wires adapted to receive a rock bolt therebetween so as to secure the mesh to the surface, wherein the wires are coated with a polymeric thermoset coating. [0015]The surface that may be supported by the mesh of the present invention may be 10 any surface of a tunnel that has been excavated. The tunnel may have been dug through any type of geological strata such as shale, sandstone, chalk, hard rock, soft rock, earth, coal or any mixture thereof. The mesh of the present invention is particularly suitable for providing support to an excavated rock surface 15 [001 6]The mesh can provide support for any suitable excavated surface such as a road or train tunnel or other civil tunnel. The excavated surface is most suitably a tunnel in a mine and in particular a coal mine. The mesh is comprised of a plurality of longitudinal steel wires welded to a plurality of cross wires to form a grid. The grid may be square and/or rectangular. 20 [0017]The dimensions of the mesh are suitably adapted to the surface to be supported. For installation to the roof of a coal mine roadway, the mesh may be in the form of a panel. In this case, length of the longitudinal wires generally corresponds to the width of the roadway. Typical coal mine roadways have a width of between about 25 2400mm and 5800mm. The length of the cross wires may be between 800mm to 1600mm, suitably 1200mm. [0018]The physical properties of the wire are selected so that the mesh has the desired strength, depending the intended use. For coal mine applications, a suitable 30 minimum yield strength is about 400MPa, typically about 45OMPa. A suitable minimum ultimate strength is 45OMPa, typically about 50OMPa. A desirable minimum weld strength is 300MPa, preferably 35OMPa. Suitable wire diameters 5 are between about 3mm to about 8mm, typically between 4mm to to 6mm. In one aspect, the wire diameter is about 5mm. [0019]At least one pair of longitudinal wires are rock bolt retaining wires adapted to 5 receive a rock bolt therebetween. Such adaption may be that the opposed pairs of wires have a spacing less than the rest of the remaining wires in the mesh. Alternatively or in addition to, the wires may have a thicker diameter than the remaining wires in the mesh. 10 [0020]The mesh may further include one or more longitudinal rock bolt locating wires, that in use serve to indicate where a rock bolt is to be installed relative to the wire. The locating wires suitably do not support a load and are simply present as locaters. Thus, the locating wires may be of a smaller diameter, such as for example 2mm to 4mm. The smaller diameter also makes them easier to identify. 15 Identification may be alternatively or in addition to by means of a unique spacing between other wires. [0021]The wires are coated with a polymeric thermoset coating. Thermoset coatings are cross linked coatings that once cured cannot be melted as opposed to 20 thermoplastic coatings that can soften upon heating. The coating serves to protect the steel from environmental conditions such as pH, dust, moisture and the like that can initiate and facilitate corrosion. [0022] It is known in the construction industry to coat steel bars for use as concrete 25 reinforcement with an epoxy coating to protect against corrosion. However, the environmental and physical conditions experienced by steel reinforcement within concrete bears little relation to the environmental and physical conditions experienced by a support mesh as used in a coal mine. Reinforcing steel is only subjected to the particular environment within concrete. On the other hand, 30 support mesh in a mine is subject to variations in pH, temperature, volatile chemicals, coal dust, coal dust suppression agents and the like. Further properties necessary for use within a mine such as non-flammability and antistatic properties are irrelevant to concrete reinforcement.

6 [0023]The present inventor has surprisingly and unexpectedly discovered that a thin layer of a thermoset material may protect a mine support mesh from corrosion under the harsh environmental conditions within a coal mine. Still further, the 5 coating has anti-static and non-flammability properties. [0024]An especially suitable coating is a fusion bonded epoxy coating. Such coatings are powder based and are sprayed on as a powder followed by curing at high temperatures. When heated the powder melts and cures to form a coating that is 10 tightly bonded to the steel. [0025] In order to facilitate bonding and adhesion and to protect against delamination, it is the steel wire may be deformed. Deformed wire refers to wire having deformations such as lugs, ribs or other surface irregularities rolled into the 15 surface of the steel. [0026] In some applications, the thickness of the polymeric layer is suitably kept to a minimum so as to minimise the overall increase in weight of a mesh panel. This may be important for manual handling. Mesh panels for supporting coal mine 20 roofs can weigh up to about 30kg. Typically, the thickness of the coating is less than 0.5mm and suitably between about 0.05mm to 0.3mm, typically about 0.15mm to 0.2mm. [0027]The present disclosure also relates to a method of supporting an excavated tunnel 25 surface, the method comprising providing a support mesh panel comprising a plurality of spaced longitudinal wires and a plurality of spaced cross wires welded together to define a grid, the wires being coated with a polymeric thermoset coating placing the panel against the surface to be supported and fixing the panel to the surface to be supported. 30 7 [0028]When the surface is a rock surface, the, the panel is typically fixed to the surface by rock bolts. The anchor plate of the rock bolt bears against the wires and subjects the wires to stress and tension. The mesh therefore may have at least one pair of opposed wires adapted to receive a rock bolt therebetween. As these 5 wires are subject to sheer stress from the anchor bolt plate, they are suitably of a thicker diameter than the remaining wires. Alternatively or in addition to reinforcement may be provided by two wires placed closely together. [0029] Preferably, the contact side of the anchor bolt plate is also coated with the 10 polymer. This can reduce the risk of the anchor bolt plate scratching or abrading the coating on the wires. [0030]According to a further broad aspect of the disclosure, there is provided a system for protecting an excavated tunnel surface, the system comprising at least one a 15 support mesh panel comprising a plurality of spaced longitudinal wires and a plurality of spaced cross wires welded together to define a grid, the wires being coated with a polymeric thermoset coating and at least one rock bolt having an anchor plate for fixing the at least one support mesh panel to the surface, wherein the mesh contact side of the anchor bolt is coated with a polymeric thermoset 20 coating. BRIEF DESCRIPTION OF THE FIGURES [0031]Figure 1 is a plan view of a one embodiment of a mesh panel according to the invention; 25 [0032] Figure 2 is a cross section of the panel of Figure 1; [0033] Figure 3 is a schematic view of a cross section of mine roadway showing two adjoining panels as shown in Figure 1 fixed in place on the roof of a mine roadway; and [0034] Figure 4 is a plan view showing how two panels of 30 8 DETAILED DESCRIPTION [0035] Figure 1 shows a support mesh in the form of a rectangular panel 10 having a plurality of longitudinal wires 12 and a plurality of cross wires 14. The wires 12, 14 are welded together at the intersections to define rectangular grids. The 5 longitudinal edges 16, 18 of the panel are respectively downturned and upturned as may be seen in figure 2. This produces tension on the panel 10 to increase the rigidity of the panel 10 to assist in material handling. [0036]The panel is 4805mm long corresponding to the width of a typical coal mine 10 roadway. The width is 1200mm. [0037] In use, the panel 10 is mounted across a mine roadway with the longitudinal wires 12 transverse to the direction of the roadway. The panel 10 is positioned such that it overlaps a previously installed panel and bolted in place. Typical bolting 15 patterns of coal mine roadways have a line of rock bolts, typically 5, installed across the width of the roadway at about 1 m intervals. The width of the panel 10 is thus dimensioned to accommodate such a bolting pattern as will be described further below. 20 [0038] Both the longitudinal 12 and cross wires 14 are deformed wire of 5mm diameter, except for bolt retaining wires 20 and locating wires 22, discussed below. All of the wires are coated with an epoxy resin, the coating having a thickness of between 0.15 to 0.2mm. 25 [0039] Epoxy coatings for fusion bonding to steel are well known to those in the steel coating arts. The coating is applied as per conventional fusion bonding technology. Epoxy resin powder 80% is mixed with a curing agent at 10%, a pigment filler 8% and addition agent at 2%. The steel is heated to temperatures of between 180L' and 25OLC and the powder is sprayed onto the heated metal 30 whereby it melts and cures.

9 [0040]The longitudinal 12 and cross wires 14 have minimum yield strength of about 45OMPa, a minimum ultimate strength of about 500MPa and a minimum weld strength of about 300MPa. 5 [0041]The panel 10 includes two pairs of opposed longitudinal bolt retaining wires 20 located along an edge of the panel 10 for receiving a rock bolt thread therebetween. These pairs of wires 20 have a diameter of 8mm and are spaced closer together at 60mm apart than the 5mm wires at 100mm apart. This thicker 10 diameter resists the additional stress applied by the rock bolt anchor. [0042]On the opposite side of the panel 10 is a locating wire 22 of narrower diameter of 3mm that is closely spaced to a 5mm wire 24. The 3mm wire acts as a guide or reference wire to assist in aligning the next panel 10 to be installed. This not only 15 ensures that the rock bolts are placed at the predesigned spacing but also avoids excess overlap between panels. It will be appreciated that even a small excess overlap of 10 to 20mm over kilometres of roadway can translate into a considerable number of excess panels and associated cost capital and labour costs. Further, less than optimum rock bolt spacing can translate to excess 20 bolting cots [0043]The panel 10 further includes a second pair of opposed closely spaced wires, about 40mm apart 26 running along the centre of the panel. This allows spot bolts or an intermediate row of bolts to be installed. These wires may be thicker i.e. 25 about 8mm or may be of the same diameter s the remaining wires i.e .5mm. Installation of the central bolts is generally only required where there is poor strata and additional support is required. [0044] Figure 3 shows a schematic view of a cross section through a roadway roof 30 30 showing adjacent panels 1 OA, 1 OB that are overlapping and fixed by rock bolts. There are rock bolts 34 are at the free ends of each panel 1 OA, 1 OB, rock bolts 46 and rock bolts 38 at the overlapping edges. The rock bolts have a bolt plate 40 that contacts and holds the panels in place and a threaded end e32 extending 10 therefrom into the strata. The underside of the bolt plate is also coated with the epoxy resin. This can minimise the potential damage to the wire during the bolting phase. 5 [0045] Figure 4 shows a plan view of the overlapping panels 1 OA, 1 OB. [0046]The present invention provides a mesh for supporting an excavated tunnel surface and in particular the roof of a mine that is resistant to corrosion, without significantly increasing the weight. A still further and unexpected advantage is an 10 improvement in manual handling. The coating allows the heavy panels to be dragged across a surface more easily compared with conventional mesh support panels. [0047] It will be appreciated that various changes and modifications may be made to the 15 present invention as described and claimed herein without departing from the spirit and scope thereof.

Claims (19)

1. A support mesh for an excavated tunnel surface, the mesh comprising a 5 plurality of spaced longitudinal wires and a plurality of spaced cross wires welded together to define a grid, at least one pair of opposed longitudinal wires adapted to receive a rock bolt therebetween so as to secure the mesh to the surface, wherein the wires are coated with a polymeric thermoset coating. 10

2. The mesh of claim 1, wherein the polymeric coating is an epoxy resin.

3. The mesh of claim 1, wherein the wires are deformed wire.

4. The mesh of any one of claims 1 to 3, wherein the polymeric coating is coating is less than about 0.5mm.

5. The mesh of claim 4, wherein the coating is between about 0.15mm to 15 about 0.2mm.

6. The mesh of any one of claims 1 to 5, wherein the wire has a minimum yield strength of about 400MPa a minimum ultimate strength of about 450MPa.

7. The mesh of claim 6, wherein the minimum yield strength is about and the 20 minimum ultimate strength is about 500MPa.

8. The mesh of any one of claims 1 to 7, wherein the minimum weld strength is about of 30OMPa.

9. The mesh of claim 8, wherein the minimum weld strength is about 350MPa.

10. The mesh of any one of claims 1 to 9, wherein the at least one pair of 25 longitudinal wires adapted to receive a rock bolt therebetween have a spacing less than the longitudinal wires forming the remainder of the mesh.

11. The mesh of claim 710, wherein the at least one pair of longitudinal wires adapted to receive a rock bolt therebetween have a thicker diameter than the wires forming the remainder of the mesh. 12

12. The mesh of any one of claims 1 to 11, wherein the mesh further includes one or more longitudinal rock bolt locating wires, that in use serve to indicate where a rock bolt is to be installed relative to the wire.

13. The mesh of claim 12, wherein the at least one locating wire has a diameter 5 smaller than the diameter of the wires forming the remainder of the mesh.

14. A method of supporting an excavated tunnel surface, the method comprising providing a support mesh panel comprising a plurality of spaced longitudinal wires and a plurality of spaced cross wires welded together to define a grid, the wires being coated with a polymeric thermoset coating 10 placing the panel against the surface to be supported and fixing the panel to the surface to be supported.

15. The method of claim 14, wherein the support mesh panel is a mesh of any one of claims 1 to 13.

16. The method of claim 14 or claim 15, wherein the excavated tunnel surface 15 is a rock surface.

17. A system for protecting an excavated tunnel surface, the system comprising at least one a support mesh panel comprising a plurality of spaced longitudinal wires and a plurality of spaced cross wires welded together to define a grid, the wires being coated with a polymeric thermoset 20 coating and at least one rock bolt having an anchor plate for fixing the at least one support mesh panel to the surface, wherein the mesh contact side of the anchor bolt is coated with a polymeric thermoset coating.

18. The system of claim 17, wherein the support mesh panel is a mesh of any one of claims 1 to 13. 25

19. The system of claim 17 or claim 18, wherein the excavated tunnel surface is a rock surface.