KNOTTED MESH AS SUPPORTING MATERIAL IN MINES

Abstract

A method for lining mining and tunnel constructions with a support material (20) comprises the steps of: •a. providing a plurality of lay wires (24) in the long direction; •b. providing a plurality of stay wires (22) in the cross direction, the stay wires making intersections with said lay wires (24); •c. providing knot wires (28) to attach the lay wires (24) to the stay wires (22) in at least some of the intersections so as to form the support material; •d. installing the support material (20) against a ceiling or a wall of a mining or tunnel construction by means of bolts.

Description

TECHNICAL FIELD

The invention relates to a method for lining a support material in mining and tunnel constructions.

BACKGROUND ART

In the field of mining, a metallic mesh support is required to strengthen the surrounding strata when an opening is created in rock. The function of a support is to activate, conserve and improve the inherent strength (tensile and shear) of the strata, and maintain their load-bearing capacity.

A mining mesh may also comprise a reinforcing element which is considered to modify the internal behavior of the rock mass by the installation of structural elements within it. These elements include tensioned point anchored bolts, friction bolts (split sets) and resin-anchored bolts.

Wire mesh has been used as ground support in mining since the 1950's.

The mesh may comprise welded wire, expanded metal or woven (chain link) mesh.

Wire mesh is used to support small pieces of loose rock and broken rock from falling. Two types of wire mesh currently used in underground operations are chain link mesh and weld mesh. Wire mesh is very economical and quickly installed. It is easy to attach to roof support in hard rock underground mines. Unfortunately the disadvantages of wire mesh are inability to carry excessive load of broken rock without failure. Furthermore wire meshes are easily damaged by flyrock from nearby blasts.

Weldmesh is used traditionally as reinforcement for shotcrete, but is rapidly being replaced by steel fibre reinforced shotcrete. Mesh, like straps, is held in place with additional faceplates or washers and nuts on rock bolts or using separate pins. One big problem with welded mesh is the excessive weight of such meshes which pose an ergonomic issue to the miners wile installation of such meshes in mines. In addition welded meshes have shown failures in tensile wire, welded portions and heat affected zones. Indeed experience has shown that fractures often occur at the level of the welds. In addition welded meshes are often too stiff and do not follow the unevenness of the mining surface.

Chain link meshes, on the other hand, have the drawback of being too flexible. In a standard test, the displacement of a chain link mesh at first break may be higher than 15 inch (381 mm), which is considered as too dangerous as in real life, the mesh will be hanging too low and may lead to minor injury.

SUMMARY OF INVENTION

It is an object of at least certain embodiments of the present invention to devise a wire mesh for lining a support material in mining and tunnel constructions.

It is an object of at least certain embodiments of the present invention to devise a wire mesh that is corrosion resistant and offer longer durability and service life.

It is further object of at least certain embodiments of the present invention to devise a wire mesh of increased panel stiffness comparable to a welded mesh and yet to provide sufficient flexibility for use in mining environments.

It is further object of at least certain embodiments of the present invention to devise a wire mesh of higher tensile strength and lighter weight such that it can be handled by the miner in an ergonomic manner.

It is further object of at least certain embodiments of the present invention to devise a wire mesh that is usable in open pit mining and underground mining.

It is further object of at least certain embodiments of the present invention to devise a wire mesh that can be installed in time efficient manner and is less labour intensive.

Thus, one aspect of the invention is a method for lining a support material in mining and tunnel constructions. The method comprises the steps of:

• • a) providing a plurality of lay wires in the long direction;
  • b) providing a plurality of stay wires in the cross-direction, where these stay wires make intersections with the lay wires;
  • c) providing knot wires to attach the lay wire to the stay wires by means of an intersection knot in at least some the intersections so as to form the support material;
  • d) installing the support material against a ceiling or a wall of a mining or tunnel construction by means of bolts.

The advantage of such knotted mesh is pronounced under the context of roof ceiling and walls in mines.

In comparison with a chain link mesh, using a separate wire to form a knot for fixing the vertical and horizontal wires impart a greater stiffness to the wire mesh thus reduces elongation. The rock that often move and fall are well contained by such a mesh. Another advantage over conventional chain link mesh is that localized damages to specific part of the wire mesh are well contained to that zone. In other words a localized damage on a chain link mesh has a rippling effect that the entire structure collapses thus leading to more dangerous situations inside the mines. This highlights the requirement of the separate wire knot that binds the horizontal and vertical wires.

In comparison with a weld mesh, the separate wire knot allows some sliding of the stay wires over the lay wires, or the other way around, although not leading to the same extent of displacements as chain link meshes. So the knotted mesh provides more flexibility than welded meshes and provides more stiffness than chain link meshes.

In a preferable embodiment of the present invention, the stay wires are perpendicular to the lay wires.

In order to increase the stiffness of the knotted mesh for use in mines and tunnels, the lay wires and the stay wires are preferably patented hard drawn wires of a pearlitic structure.

For lay wires the carbon content may range from 0.30 wt % to 1.0 wt % (wt %=percentage by weight), preferably from 0.40 wt % to 0.90 wt %.

For stay wires the carbon content may range from 0.08 wt % to 1.0 wt %, preferably from 0.30 wt % to 0.50 wt %.

The knot wires may have a carbon content that is much lower, in order to facilitate making the knot. Knot wires may have a carbon content ranging from 0.02 wt % to 0.10 wt %.

The cross-section of the stay wires, the lay wires and the knot wires may have various shapes. This shape may be selected from the group consisting of round, flat, square, rectangular, triangular, trapezoidal, oval, half-round form.

Preferably this cross-section is round. The diameter of the stay wires, lay wires and knot wires may range between 1.0 mm and 6.0 mm, preferably between 1.5 mm and 5.0 mm, e.g. 2.5 mm.

The lay wires, stay wires and knot wires may be coated with a metallic coating. This metallic coating may be selected out of a group consisting of copper, copper alloy, zinc, zinc alloy, nickel, nickel alloy, tin, tin alloy, or combinations thereof.

A zinc aluminum coating has a better overall corrosion resistance than zinc. In contrast with zinc, the zinc aluminum coating is temperature resistant. Still in contrast with zinc, there is no flaking with the zinc aluminum alloy when exposed to high temperatures.

A zinc aluminum coating may have an aluminum content ranging from 2 percent by weight to 12 percent by weight, e.g. ranging from 3 wt % to 11 wt %.

A preferable composition lies around the eutectoid position: Al about 5 wt %.

The zinc alloy coating may further have a wetting agent such as lanthanum or cerium in an amount less than 0.1 wt % of the zinc alloy. The remainder of the coating is zinc and unavoidable impurities.

Another preferable composition contains about 10 wt % aluminum. This increased amount of aluminum provides a better corrosion protection then the eutectoid composition with about 5 wt % of aluminum.

Other elements such as silicon (Si) and magnesium (Mg) may be added to the zinc aluminum coating.

With a view to optimizing the corrosion resistance, a particular good alloy comprises 2 wt % to 10 wt % aluminum and 0.2 wt % to 3.0 wt % magnesium, the remainder being zinc. An example is 5 wt % Al, 0.5 wt % Mg and the rest being Zn.

The knot wire may make various types of knots. The knot may be an S-knot or a fixed knot.

Another aspect of the present invention is the wire mesh further comprises an end knot, wherein said end knot represent the ends of the stay wire twisted at point of intersection to the last lay wire. For this reason, namely for making the knot at the end, stay wires usually have a tensile strength that is less than the tensile strength of the lay wires.

The spacing between two subsequent stay wires and between two subsequent lay wires may range from 2 inches (50.8 mm) and 6 inches (152.8 mm), and is preferably about 3 inches (76.2 mm) to about 4 inches (101.6 mm).

The knotted mesh may be provided in the form of panels or in the form of rolled sheets.

The bolts used to fix the knotted mesh to the ceiling or to the walls of mines or tunnels are rebar bolts, cable bolts or friction bolts.

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

FIG. 1a and FIG. 1b illustrate examples of prior art wire meshes

FIG. 2 illustrates a fixed knot wire mesh for lining a support material in mining.

FIG. 3 illustrates an S-knot wire mesh for lining a support material in mining.

FIG. 4 shows an embodiment of the present invention wherein the wire mesh are installed in form of panels further.

FIG. 5 shows graph depicting the data from static tests conducted on wire mesh of the invention. X-axis represents vertical displacement in inches and y-axis represents vertical load in pounds force.

PRIOR ART

FIG. 1a shows a chain link mesh 10 as known in the prior art. This chain link mesh has the advantage of being very flexible but this flexibility may lead to too large displacements in case of the first rupture. Going to higher carbon contents in order to increase the stiffness is possible but leads to more difficulties in making the chain link fence, having regard to the several bends of all the wires.

FIG. 1b shows a weld mesh 12 as known in the prior art. The weld mesh has the advantage of being stiff but the disadvantages of not following the unevenness of the ceiling or wall of the mines and of being only available in panels.

Invention

FIG. 2 illustrates a first embodiment of a knot fence 20 according to the present invention. The knot fence 20 has stay wires 22 and lay wires 24 which make intersections with one another. The lay wires may have a carbon content ranging from 0.43 wt % to 0.90 wt %, a silicon content ranging from 0.15 wt % to 0.30 wt %, a manganese content ranging from 0.30 wt % to 0.60 wt %, maximum phosphorus content of 0.040 wt %, maximum sulfur content of 0.050 wt %, the remainder being iron and unavoidable impurities. The stay wires may have a carbon content ranging from 0.43 wt % to 0.50 wt %, a silicon content ranging from 0.15 wt % to 0.30 wt %, a manganese content ranging from 0.60 wt % to 0.90 wt %, maximum phosphorus content of 0.040 wt %, maximum sulfur content of 0.050 wt %, the remainder being iron and unavoidable impurities.

The lay wires may be hard conditioned, i.e. hard drawn. The stay wires may be soft annealed with a view to making a knot at the end around the lay wires.

The intersection is shown in more detail at 26. Stay wires 22 and lay wires 24 are attached to each other by means of a fixed knot 28. The term “fixed knot” is shown in ASTM A116.

The knot wire may have a much lower carbon content to facilitate the various bending in the knot. The carbon content of the knot wire may range from 0.02 wt % to 0.10 wt %.

FIG. 3 shows an another embodiment of a knot mesh 30 according to the invention. The stay wires 32 make intersections with the lay wires 34. Such an intersection is shown in more detail at 36. A so-called S-knot 38 attaches a stay wire 32 with a lay wire 34.

The S-knot and fixed knot are examples of the present invention, a person skilled in the art would envisage any type of wire knot provided a separate wire is utilized to tie the vertical and horizontal wire at interesecting points. It is not neccesary that all the point of intersections between vertical and horizontal wires contain a wire knot. wire knot could be placed for instance in an alternating manner.

In one embodiment of the present invention, the spacing between the said stay wires and said lay wires ranges from 2 inches (50.8 mm) to 6 inches (152.4 mm), preferably 3 inches (76.2 mm) to 4 inches (101.6 mm). The stiffness of the wire mesh can be enhanced by using a uniform aperture size such as 3 inches (76.2 mm)×3 inches (76.2 mm) or 4 inches (101.6 mm)×4 inches (101.6 mm).

In one embodiment of the present invention, the wire mesh is in the form of panels. The advantage of such panels is the lighter weight compared to welded mesh thus providing handling of such mesh in an ergonomic manner.

In one embodiment of the present invention, the wire mesh is in the form of rolled sheets. Such a form allows a continous installation in a quicker and efficient manner, further advantge is the overlap between two wire meshes can be minized thus saving the amount of material.

A stay wire or lay wire of the present invention can be made as follows. Starting product is a wire rod (usual diameters 5.50 mm or 6.50 mm) with a plain carbon steel composition along the following lines:

• • a carbon content ranging from 0.30 wt % to 1.0 wt %, e.g. from 0.4 wt % to 0.90 wt %;
  • a silicon content ranging from 0.10 wt % to 2.5 wt %, e.g. from 0.15 wt % to 1.60 wt %;
  • a manganese content ranging from 0.10 wt % to 2.0 wt %, e.g. from 0.50 wt % to 0.90 wt %;
  • a chromium content ranging from 0.0 wt % to 0.5 wt %, e.g. from 0.0 wt % to 0.2 wt %;
  • a vanadium content ranging from 0.0 wt % to 0.5.0 wt %, e.g. from 0.0 wt % to 0.20 wt %;
  • a tungsten content ranging from 0.0 wt % to 0.5 wt %, e.g. from 0.0 wt % to 0.20 wt %.

Tests

Load-displacement tests were conducted on the knot mesh of the present invention. These tests were run on a specially designed frame in a mine roof simulator (MRS). This simulator is designed to simulate loading conditions related to the performance of screen and mesh when used for surface control in underground coal mines.

A test frame is designed to hold the mesh or screen at four corners with bolts and bearing plates. A center load is applied using a one foot square (304.8 mm×304.8 mm) load plate with rounded corners. During loading, the displacement rate is controlled. The load is measured using a load cell with a capacity of 20,000.- lb (9,062.-kgf or 88,898 N). For the mesh tests, both the loads and displacements are recorded over time. The maximum displacement for the system is about 20 inches (508 mm). During the tests, the mesh was deformed at 2 inches (50.8 mm) per minute for a total of 20 inches (508 mm).

Bolts with bearing plates are used to hold the mesh in place on the test frame at four corners. The bolts attaching the mesh to the frame were ¾-inch (19.05 mm) in diameter with the bolts placed on a 4-ft (1219.2 mm) by 4 ft (1219.2 mm) pattern. The load surface beneath the mesh was steel. To hold the mesh in place a torque of 150 ft-lbs (203.2 Nm) was applied to the bolts. The conversion factor of the torque to load is 160.

The reference material used are weld mesh panels which are currently used in most mines, namely 8 gauge wire (4.1 mm diameter) and 10 gauge wire (3.4 mm diameter) with 4 inch (101.6 mm)×4 inch (101.6 mm) apertures. This prior art reference material has a first break at about 10 to 11 inches (254.0 to 279.4 mm) displacement with a load of around 1500 lbs (679.65 kgf or 6,667.37 N). As mentioned, a displacement of 10 inches (254 mm) to 15 inches (381 mm) is considered as dangerous as in real life situation the mesh will be hanging too low and might lead to minor injury.

Various tests carried out on knot meshes have lead to the conclusion that the use of 3 inch (76.2 mm)×3 inch (76.2 mm) apertures and of medium to high carbon content stay and lay wires (0.30 wt % C to 0.90 wt % C) offer a good alternative to weld mesh panels. The first break occurred at a displacement of less than 12 inch (304.8 mm) with a force of around 1600 lbs (724.96 kgf or 7,111.86 N).

Claims

1.-15. (canceled)

16. A method for lining a support material in mining and tunnel constructions said method comprising the steps of:

a. providing a plurality of lay wires in the long direction;

b. providing a plurality of stay wires in the cross direction, said stay wires making intersections with said lay wires;

c. providing knot wires to attach said lay wires to said stay wires in at least some of said intersections so as to form said support material;

d. installing said support material against a ceiling or a wall of a mining or tunnel construction by means of bolts.

17. The method of claim 16, wherein said lay wires and said stay wires are perpendicular to one another.

18. The method of claim 16, wherein said lay wires, said stay wires and/or said knot wires are hard drawn pearlitic steel wires.

19. The method of claim 18, wherein said lay wires are plain carbon steel wires having a carbon content ranging from 0.30 wt % to 1.0 wt %.

20. The method of claim 19, wherein said stay wires are plain carbon steel wires having a carbon content ranging from 0.080 wt % to 1.0 wt %.

21. The method of claim 20, wherein said knot wires are plain carbon steel wires having a carbon content ranging from 0.02 wt % to 0.10 wt %.

22. The method of claim 16, wherein the shape of the cross section of said lay wires, stay wires and knot wires is selected from the group consisting of round, flat, square, rectangular, triangular, trapezoidal, oval, half-round and mixtures thereof.

23. The method of claim 16, wherein said shape of the cross section of said lay wires, stay wires and knot wires is round having a diameter between 1.0 mm and 6.0 mm, preferably between 2.0 mm and 4.0 mm.

24. The method of claim 16, wherein said lay wires, stay wires and/or knot wires are covered with a metallic coating and wherein said metallic coating is selected from a group consisting of copper, copper alloy, zinc, zinc alloy, nickel, nickel alloy, tin or tin alloy or combinations thereof.

25. The method of claim 16, wherein said knot wire is making a knot selected from a group consisting of S-knot and/or a fixed knot.

26. The method of claim 16, wherein said support material further comprises end knots, wherein said end knots are formed by the ends of said stay wire being twisted to the last lay wire.

27. The method of claim 16, wherein spacing between the said stay wires and said lay wires ranges from 2 inches to 6 inches, preferably 3 inches to 4 inches.

28. The method of claim 16, wherein said supporting material is in the form of panels.

29. The method of claim 16, wherein supporting material is in the form of rolled sheets.

30. The method of claim 16, wherein said bolts are selected from a group consisting of rebar bolts, cable bolts or friction bolts.