WO2009134139A1

WO2009134139A1 - Surface retaining face plate for rock bolts

Info

Publication number: WO2009134139A1
Application number: PCT/NO2009/000161
Authority: WO
Inventors: Charlie Chunlin Li
Original assignee: Dynamic Rock Support As
Priority date: 2008-04-28
Filing date: 2009-04-28
Publication date: 2009-11-05
Also published as: GB2487005B; GB201204559D0; GB201204555D0; NO20082012L; GB2487006B; GB2487006A; GB2487005A

Abstract

The invention relates to a face plate (1) for forming a connection between a rock bolt (1) fixed by grout or otherwise anchored in a borehole and a surface support element such as a wire mesh, concrete or thin sprayed liners. The device according to the invention serves to establish a compatible connection between rock bolts and surface support elements, mesh and/or sprayed concrete, in a ground support system. The main areas of application of the invention are mining, tunneling and other underground excavations where ground support measures are needed. The face plate (1) is made for forming a connection between a rock bolt (2) and a surface support element, the face plate (1) comprising a main plate (10) with a central plate portion (11) with a hole (9) for receiving the rock bolt (2), the central plate portion (11) arranged for being retained by a nut (4). The central plate portion (11) is provided with two or more circumferentially distributed radially extending arms (12) having an inner end (121) connected to the central plate portion (11) and an outer end (122) connected to a peripheral ring (13),the radially extending arms (12) having a first bending stiffness near the radially inner end (121) greater than a second bending stiffness near the radially outer end (122).

Description

SURFACE RETAINING FACE PLATE FOR ROCK BOLTS

Introduction

The invention relates to a face plate for forming a connection between a rock bolt fixed by grout or otherwise anchored in a borehole and a surface support element such as a wire mesh, concrete or thin sprayed liners. The device according to the invention serves to establish a compatible connection between rock bolts and surface support elements mesh and/or sprayed concrete in a ground support system. The main areas of application of the invention are mining, tunneling and other underground excavations where ground support measures are needed.

Background of the invention

Ground support systems for underground excavations may be considered as composed of internal and external elements. Internal elements refer to bolts and cables which are installed in bore holes and fixed to the rock using grout, epoxy resin or other types of anchoring mechanisms. Internally, rock bolts have two roles in ground support systems. They reinforce the rock by holding fractured rock blocks together on one hand and they carry at least part of the surface load on the other hand. External elements refer to steel meshes, straps, so-called "shotcrete", i.e. sprayed concrete, and all other types of elements which provide retaining and containing functions on the surface of an underground opening. The load exerted on the surface support element is transferred to rock bolts. The link between the surface support elements and the rock bolt is a face plate as will be shown in the present application. The plate has to have a compatible link to both the bolt and the surface support element in order to guarantee an effective load transfer from the surface element to the bolt.

In deep mining, either shotcrete or steel meshes or a combination of both are used to provide retaining support to the heavily deformed rock surface. The surface support element has to be connected to rock bolts in order to transfer the surface load to the stable rock strata behind the surface. The connection between the bolt and the surface support element is achieved through the face plate. The most widely used type of face plate either has a circular or a square shape. However these two types of plates are too stiff in accommodating the surface deformation. Figs. 10 and 11 show two examples of failure of the surface support elements. In the situation depicted in figure 10 the mesh wires were cut off by the edges of the plate. In the situation depicted in figure 11 the shotcrete was subjected to punch failure when subjected to a heavy surface load on the shotcrete. In both the cases, the failures of the surface elements were due to the too high stiffness at the plate edges.

The number of bolts and plates used in the mining industry is very large, one may mention that in the Svea coal mine on Spitsbergen, one consumes about 150.000 rock bolts and plates per year. In one single mine in Sweden more than 60.000 bolts and plates have been used. Given the number of mines worldwide it is evident that a minor improvement in the functioning of the rock bolts and retainer plates will be of considerable economic importance. If one may reduce the production costs of the plates, there are large benefits to be accrued, hence the importance of the present invention.

Background art

WO2002 79070 Cassidy et al describes a face plate with a central plate portion with a hole for a rock bolt and radially extending thin arms of even width, said arms holding a square frame. The Cassidy face plate's radial arms will yield in an uncontrolled manner, and will generally yield near the central plate portion due to the bending moment distribution of the peripherally arranged square frame.

USD470041 illustrates a hybrid face plate for retaining a mesh, comprising a lower, larger plate arranged for retaining the mesh, the upper, smaller plate arranged centrally on the larger plate, and the bolt threaded through the centre of both plates, through one cross of the mesh, thus weakening the mesh. USD470041 has radially extending ridges resembling the radial arms of the present invention, but lacks the apertures between the ridges, thus the plate consumes more material during production and provides a smaller areal coverage per steel unit. The large plate is able to be easily deformed together with the surface support element, while the small and strong plate transfers the surface load to the bolt. Obviously, it is costly to manufacture this type of plate as this would necessitate the manufacture and conjunction of two separate elements. Short description of the drawings

Embodiments of the invention are illustrated in the attached drawings.

Fig. 1 illustrates an embodiment of the invention in which the face plate is provided with three arms from a central plate portion, in which the arms' rock facing side are in the same plane as the rock facing side of a ring attached to the outer ends of the arms.

Fig. 2 illustrates an embodiment of the invention corresponding to Fig. 1 , but in which there are four arms holding the ring.

Fig. 3 is similar to Fig. 1 with the difference that the arms are straight and not kinked at the ends.

Fig. 4 is similar to Fig. 2 with the difference that the arms are straight and not kinked at the ends.

Fig. 5 is similar to Fig.3 with the novel feature that the bases of the arms are made more slender by deviating the triangle sides of the overall face plate main body slightly inwards near the middle portions of the triangle sides.

Fig. 6 is an illustration of a combination of the features of Figs. 1 and 5 in that the triangle sides are deviated towards the central hole along their middle portions and that the rock facing sides of the ring and the arms are flush in the same plane.

Fig. 7 is an illustration of an embodiment of the invention in which the central plate portion, the three arms and the ring are made in one integral piece of material.

Fig. 8 is an illustration of an embodiment of the invention in which the central plate portion, the four arms and the ring are made in one integral piece of material. In addition to the sections of the plates along the arms, as illustrated in Figs. 1-8, Fig 8 further contains a section aside from the arms, to illustrate the integrally formed ring. Fig. 9 is a comparative Cartesian diagram showing relative stiffness of material equal areas of a circular face plate of the background art, a square face plate of the background art, and a triangular main body of a face plate according to the invention.

Figs. 10 and 11 are photographic illustrations of square face plate showing mesh wires being cut thus having caused punch failure of shotcrete, respectively, when their bolt heads and face plates are set under tension due to rock deformation.

Fig. 12 shows a partial side view, partial section view, of a rock bolt grouted in a borehole, and the rock surface covered with a shotcrete support element, the rock bolt provided with a face plate according to the invention.

Short summary of the invention

Some of the problems related to background art face plates are remedied by the invention which is a face plate for forming a connection between a rock bolt and a surface support element, said face plate comprising a main plate with a central plate portion with a hole for receiving said rock bolt, said central plate portion arranged for being retained by a nut, wherein said central plate portion being provided with two or more circumferentially distributed radially extending arms having an inner end connected to said central plate portion and an outer end connected to a peripheral ring, said radially extending arms having a first bending stiffness near said radially inner end greater than a second bending stiffness near said radially outer end.

In an advantageous embodiment of the invention, one or more of said radially extending arms has a first cross-section near said radially inner end greater than a second cross-section near said radially outer end.

The plate according to an embodiment of the invention has an advantage in that in addition to the plate being strong enough to carry the surface load, it also has the advantage that it is able to deform compatibly with the surface support element, in that the bending stiffness of the integrated arms of the plate should be smaller at the edges than in the central area. An advantage of the embodiment of the invention is that the face plate will distribute forces from the bolt over the entire area of the central plate and the peripheral ring.

Another advantage of an embodiment of the tri-armed face plate of the invention is the fact that the effective area is more than double the area of a square plate or circular plate of the background art using the same plate mass.

In an advantageous embodiment of the invention a generally circular cross- section of the peripheral ring may further prevent a clipping effect on a surface mesh or other surface support element.

In an advantageous embodiment of the invention production of the face plate may be manufactured by methods such as stamping out the plate comprising the arms from a metal sheet in a single operation, and welding a ring to near the outer ends of the arms in a subsequent operation.

In an advantageous embodiment of the invention production of the face plate may be manufactured by methods such as stamping out the plate comprising the arms from a metal sheet in a single operation, including the ring near the outer ends of the arms, from the same plate material portion.

Embodiments of the invention The invention will in the hereinafter be described with reference to the figures.

The invention comprises a face plate (1) for forming a connection between a rock bolt (2) and a surface support element. The surface support element may be a mesh of wires of diameter 3 to 12 mm, or shotcrete on the surface of the rock wall as shown in Fig. 12. Said face plate (1) comprises a main plate (10) the main plate (10) comprising a central plate portion (11) with a hole (9) for receiving said rock bolt (2) with a threaded portion extending out of the borehole for being retained by a threaded nut (4), please see Figs. 1 to 8. Other mechanisms than a threaded bolt head for threaded nuts may be used for securing the plate to the bolt, such as those used in split set bolts. The central plate portion (11) of the plate (10) according to the invention is provided with three, four, or more circumferentially distributed radially extending arms (12). Each arm (12) has a radially inner end (121) attached to the central plate portion (1 1 ). Each of the arms (12) has a radially outer end (122) connected to a peripheral ring (13) near the outer ends (122). An essential feature of the invention is that the arms (12) have a first bending stiffness near the radially inner end (121) greater than a second bending stiffness near the radially outer end (122). The second bending stiffness near the radially outer end (122) may in an embodiment of the invention be very low. The result of this radially decreasing bending stiffness is a better distributed load transfer from the rock to a wide surface portion of the rock around the borehole, reducing the risk of cutting the surface support element it being a wire mesh or being shotcrete.

In an embodiment of the invention, the radially inner end (121 ) forming the transition to the central plate portion (11 ) may form a part of a circle or be circular, as the bending moment is identical along the circle rather than the transition between the radially inner end (121) and the central plate portion (11) being a straight line. Both embodiments are however within the scope of the invention.

In an embodiment of the invention the central plate portion (11 ) and the radially extending arms (12) are formed in one piece of material. Generally the central plate portion (1 1) and the arms (12) would be made from a steel plate or a steel band and having a plate thickness of 10 to 12 mm in order to provide an adequate plate stiffness such that it is useful for mining purposes. Figs. 1 - 8 show this feature in their depicture of embodiments of the invention. It is of course possible to manufacture the central plate portion (11 ) and the arms (12) in separate portions which are connected e.g. by welding during the production, but it might be less rational to do so. The invention pertains to the physical characteristics of the face plate itself, and thus comprises the necessary means of manufacture of the same. All variants pertaining to constructive details should thus be considered as being comprised by the invention. The arms (12) should however be considered as being an integral part of the plate (1 ) and it is easy to envisage production methods such as stamping out the plate (1 ) comprising the arms (12) from a metal sheet in a single operation, or by welding the arms (12) to a manufactured plate (1 ) in a second operation. Fig. 9 is a comparative Cartesian diagram showing the relative stiffness as a function of radial distance from the centre of three different geometrical shapes of steel plate, namely a circular plate, a square plate and a triangular plate, all having the same areas of 27067 mm². The three different steel plates in the below comparison have equal material thickness and equal areas. First, a horizontal line representing the relative stiffness of a mainly circular face plate of the background art, with radius 93 mm. Secondly, a line representing the relative stiffness of a square face plate of the background art, with side length S=165 mm. A maximum "radius" R' is thus 1 17 mm, counted as the half diameter of the square plate. Finally is shown the relative stiffness of a triangular main body of a face plate (1) according to the invention, having an edge length of 250 mm, with a maximum "radius" of 144.5 mm as counted from the centre of the triangle to an apex. Thus this will also be the radius of the peripheral ring (13) of the face plate (1 ) of the invention. The effective area of a face plate (1 ) according to the invention may be represented by the area of that ring (13), in this case 65600 mm². Thus the effective area of the face plate (1) according to the invention is more than double the area of a corresponding square plate or circular plate using the same mass for the solid central plate portion.

Table 1 below has four columns representing values calculated for different sizes of triangular plates. The upper row lists the edge length of a triangular plate of the different sizes. The second row lists the corresponding radial distance R" from the centre of the face plate (1 ) of the invention to the apex, a radial distance which is also the radius of the ring (13). The third row lists the corresponding radius r" of the central part (11 ) of the plate. The fourth row lists the corresponding areas of the different triangular plates. The fifth row lists the "footprint", i.e. effective areas of the face plates (1) according to the invention, calculated as the area corresponding to a circle having the triangle radii R" as defined in the second row. In the sixth row is listed the radii R of circular plates of the same areas as for the triangle plates, and in the seventh row the radial distances R¹ of area-equivalent square plates having corresponding areas to the triangular plates. The values in the column corresponding to and comparing to a triangular plate of side length 250 mm are indicated in boldface. Table 1 : Comparison of various plates.

As seen, the invention results in an increased effective radius of the "footprint" of the face plate (1) according to the invention compared to prior art circular plates or square plates representing approximately the same plate area. The bending stiffness of the arms (12) decrease with increasing radius, and the increased effective radius R" of the invention will provide a reduced clipping effect of the edge of the plate compared to the background art as illustrated in Fig. 9. The reduced consumption of material is also a considerable advantage when considering the fact that several hundred thousands of face plates may be consumed annually for one single deep mine.

An advantage of the unevenly distributed bending stiffness shown in Fig. 9 is that the central plate portion (11 ) is strong enough to transfer the force from the bolt and the nut (4) and transfer and radially distribute this force over the area of the entire plate according to the invention, while the outer portions of the face plate (1 ) according to the invention will yield and deform with a reduced risk of cutting wires or punch-penetrating concrete. The peripheral (13) ring contributes to transferring and distributing the force from the arms (12) over a larger area of the surface support mesh or shotcrete.

In order to obtain the feature of the arms (12) having a first bending stiffness near the radially inner end (121 ) being greater than the second bending stiffness near the radially outer end (122), the bending stiffness may be varied through the variation for the cross-section of the arms. In Figs. 1 - 8 it is shown embodiments in which the arms (12) have a first cross-section near the radially inner end (121) greater than a second cross-section near the radially outer end (122). In these embodiments the width of the arms (12) taper off with the radial distance from the central hole (9). The central portion of the plate may in an embodiment have a full circular peripheral whereas the cross section of the radially outer end (122) may taper off to a very small cross-section. Alternatively the arms (12) may taper off having even width but decreasing thickness with increasing radial distance from the central hole (9). It may prove easier to manufacture the above embodiment having decreasing width with increasing radial distance, particularly if the face plate (1 ) according to an embodiment of the invention shall have the central plate portion (11 ) and the radially extending arms (12) being formed from a sheet-like plate material. According to an embodiment of the invention the face plate (1 ) and the arms (12) may be made from a steel plate of given thickness. Such steel plates may have a thickness of e.g. 10 - 12 mm although other thicknesses may be considered according to specific applications.

The peripheral ring (13) may be formed from steel wires or rods welded to form a ring, whereafter the ring (13) is welded to the arms (12) near their radially outer ends (122) to form said peripheral ring (13).

A feature of embodiments shown in Fig. 1 , Fig. 2, Fig. 6, Fig. 7, Fig. 8 is that a rock facing surface (131 ) of the peripheral ring (13) and a rock facing surface (123) of the radial arms (12) are in a common plane (Ps). In the embodiments shown in Fig. 1 , Fig. 2, and Fig. 6, the arms (12) have a portion (124) near the outer ends (122) which is kinked away from the rock facing common plane (Ps), in which the portion (124) is arranged for riding on the peripheral ring (13). The face plate (1 ) shown in Fig. 7 and Fig. 8 also have the rock facing side of the arms (12) and the ring flush, and additionally the arms (12) and the peripheral ring (13) are formed in one material piece. Such integral face plates (1 ) with integral ring may be stamped out of a steel plate, laser cut from a plate, or even moulded.

The face plate of some of the embodiments of the invention has a perimeter of the main plate (10) comprising the central plate portion (11) with the arms (12) having generally an overall triangular shape. An important feature with relation to mass production of face plates is the fact that this allows a number of triangular main plates (10) may be cut out of a sheet-like or band-like plate with material loss only due to cutting.

In the face plate (1) according to an embodiment of which examples are illustrated in Fig. 5 and Fig. 6, the triangle shape of the main plate (10) has the middle portion of each lateral edge deviated away from the purely triangular shape in the general radial direction towards the bolt hole (9). This feature provides a more even transition of bending strength of the arms (12), resulting in an reduced bending stiffness of the arms (12) when compared to the purely triangular shape combined central portion and arms. In the embodiments shown in Figs. 1 - 8 the arms (12) are planar. This allows yielding of the plate in a controlled manner depending on stiffness and width, starting at the outer ends of the arms and propagating toward the centre of the plate.

According to an embodiment of the invention the face plate (1 ) has it's central plate portion (11) provided with a bulge (15) having a concave face (111 ) being directed towards the rock surface and with a convex face (112) on the opposite, nut facing side of the central plate portion (11) around the hole (9). This enhances the load-bearing capacity of the face plate (1 ). The nut facing face (112) will support the threaded nut (4) in itself, or a washer (41 ) and the nut (4)

The peripheral ring (13) may have either a generally circular cross-section as shown in Figs. 1 - 6, or a generally rectangular cross-section as illustrated in Fig. 7, Fig. 8. A circular cross-section of the peripheral ring (13) may further prevent a clipping effect on a surface mesh.

Claims

1. A face plate (1) for forming a connection between a rock bolt (2) and a surface support element, said face plate (1) comprising a main plate (10) with a central plate portion (11) with a hole (9) for receiving said rock bolt (2), said central plate portion (11) arranged for being retained by a nut (4), wherein said central plate portion (11 ) being provided with two or more circumferentially distributed radially extending arms (12) having an inner end (121) connected to said central plate portion (11) and an outer end (122) connected to a peripheral ring (13), said radially extending arms (12) having a first bending stiffness near said radially inner end (121) greater than a second bending stiffness near said radially outer end (122).

2. The face plate of claim 1 , said radially extending arms (12) having a first cross- section near said radially inner end (121 ) greater than a second cross-section near said radially outer end (122).

3. The face plate of claim 1 , said central plate portion (11 ) and said radially extending arms (12) being formed in one material piece.

4. The face plate of claim 2 or 3, said central plate portion (11 ) and said radially extending arms (12) being formed from a sheet-like plate material.

5. The face plate of claim 1 , said peripheral ring (13) attached to said radially extending arms (12) near their radially outer ends (122).

6. The face plate of claim 1 , a rock facing surface (131) of said peripheral ring (13) and a rock facing surface (123) of said radially extending arms (12) forming a rock facing common plane (Ps).

7. The face plate of claim 6, said radially extending arms (12) being provided with a portion (124) near said outer ends (122) being kinked away from said rock facing common plane (Ps), said portion (124) arranged for riding on said peripheral ring (13).

8. The face plate of claim 1 , said radially extending arms (12) and said peripheral ring (13) being formed in one material piece.

9. The face plate of claim 1 , said radially extending arms (12) and said peripheral ring (13) being formed from a sheet-like plate material.

10. The face plate of claim 1 , the number of radially extending arms (12) being three.

11. The face plate of claim 1 , the number of radially extending arms (12) being four.

12. The face plate of claim 10, the perimeter of said main plate (10) with said central plate portion (11 ) with said radially extending arms (12) being generally triangular.

13. The face plate of claim 12, the triangle shape of said main plate (10) having the middle portion of each lateral edge of the triangle deviated in the direction of the hole (9).

14. The face plate of claim 1 , the central plate portion (11 ) provided with a bulge (15), said bulge (15) having a concave face (1 11) for being directed towards the rock surface.

15. The face plate of claim 14, said central plate portion (11 ) provided with a convex face (112) around said hole (9), said convex face (112) for supporting said nut (4) or a washer (41) and said nut (4), said bulge (15) arranged for enhancing the load- bearing capacity of the face plate (1 ).