GB2131115A

GB2131115A - Reinforcing rock structure and nut elements therefor

Info

Publication number: GB2131115A
Application number: GB08233808A
Authority: GB
Inventors: Carl A Clark; John Rogala
Original assignee: Eastern Co
Current assignee: Eastern Co
Priority date: 1982-11-26
Filing date: 1982-11-26
Publication date: 1984-06-13
Also published as: GB2131115B

Abstract

A bar (16) is fixed and tensioned in a hole (12) drilled in a rock face (14) by means of a nut element (30) screwed onto the outer end of the bar. Initially the nut element is screwed onto the bar until the end of the bar engages an annular dome-shaped abutment portion (46) whereafter rotation of the nut rotates the bar which thus mixes two components (22, 24) of a resin grout from a cartridge (20) that has been ruptured at the inner end of the hole. When the grout has set to rigidify the bar, further torque applied to the nut element causes the end of the bar to displace the portion (46) so that the nut element screws further onto the bar and forces a plate (28), which has been threaded onto the bar, hard against the rock face (14). <IMAGE>

Description

SPECIFICATION Improvements in methods of reinforcing rock structure and in nut elements therefor The present invention relates to rock reinforcement and, more specifically, to novel elements and methods for use, particularly, in resin anchored, tensioned roof support systems.

In mine work, such as-coal mining, or in underground formations such as tunnels or excavations, it is often necessary to reinforce or support the roof and/or walls of the excavation to prevent rock falls or cave-ins. The most common means presently in use for effecting such support include elongate bars or bolts which are inserted in blind drill holes and anchored therein to hold a metal support or bearing plate in tight engagement with the roof or wall surface. Anchoring means within the drill hole normally comprise a mechanical expansion anchor, a hardenable resin or other grouting, or both.

The use of polyester resins in underground formations was disclosed at least as early as 1963 in US-A-3,091,935. It has been found expedient to provide the resin grouting materials in two components each of which remains in a semi-liquid or thixotropic phase until mixed with the other, whereupon curing and hardening progresses to the stage that the steel bolt or bar will fail before the resin bond. The two components, a polyester resin and catalyst, curing or hardening agent, are commonly provided in a single cartridge wherein they are initially separated in individual compartments such as disclosed, for example, in US-A-3,324,663.Upon insertion of the resin cartridge and elongate bar into the drill hole, the end of the bar contacting the cartridge is used to rupture the twocompartment cartridge and mix the components so that the curing and hardening necessary to retain the bar in the hole may take place.

In addition to anchoring one end of the bar in the drill hole it is also desirable the bar be placed in tension in order to reinforce the rock formation surrounding the excavation. To this end, a number of systems have been proposed wherein the bolt or bar is screw threadedly engaged with another member, with some means being provided to limit screw threaded advance as the resin cartridge is broken and the components mixed, and excess torque applied after the resin has hardened serving to break or deform some portion of the structure to allow further threaded movement and tension the bar. The earliest known example of such anchor systems is US-A-3,877,235 wherein a relatively thin plate or diaphragm covering one end of a threaded nut is broken by applying excess torque after hardening of the resin grouting.

In US-A-4,051,683 a shear pin extending across an internally threaded coupling member is broken by application of excess torque.

In US-A-4,122,681 initial screw threaded advance is limited by internally deformed threads, and further screw threaded advance to tension the bar or bolt is effected by application of excess torque after the resin hardens.

Of the considerable number or proposed systems for tensioning resin-anchored bars or bolts by application of excess torque, many involve structural alteration of, or the insertion of frangible elements directly into, screw threaded elements of the system. This presents the possibility of failure due to seizing, fouling or stripping of the threads. Also, many of the previously proposed systems require costly structural additions and/or fabrication operations which diminish the economic effectiveness of the system.

It is an object of the present invention to propose a method and means for providing an improved rock reinforcement system of the type wherein a resin-anchored bar is tensioned by application of excess torque after the resin is mixed and hardened, which is reliable in operation and low in cost.

According to one aspect of the invention, a nut element comprises a body portion having an internally screw threaded opening extending therethrough to accept, in use, into one end an externally screw threaded member of predetermined diameter; an abutment portion fixed to the body portion and extending radially inwardly to obstruct an extension of the externally threaded member extending, in use, through the screw threaded opening at a position spaced from the other end of the threaded opening, whereby screw threaded advance of the externally threaded member, in use, through the body portion is limited by contact of the end thereof with the abutment portion; and the abutment portion being so constructed and arranged that application of a predetermined torque to one of the nut element and the externally screw threaded member while holding the other stationary permits further relative screw threaded advance by forcing the abutment portion out of its obstructing position.

The invention also includes a method of forming a nut element, the method comprising forming an integral, malleable, metal casting including a body portion having an opening extending therethrough between first and second ends; tapping the opening to form internal screw threads extending from the first to the second end; and mechanically deforming a second portion of the casting to extend radially inwardly across a cylindrical plate passing through and of the same diameter as the body portion opening on the second end side of the body portion; whereby the second portion, after being deformed, is capable of limiting screw threaded advance of an exter nally screw threaded member through the internal screw threads from the first to the second end thereof until application of predetermined torque sufficient to move the second portion out of the cylindrical plane and allow the externally screw threaded member to pass therethrough.

The new nut element enables a method of reinforcing a rock formation wherein a blind hole is drilled in the rock face; a two component resin grout cartridge is inserted into the hole; an elongate bar having a screw threaded outer end is inserted into the hole with its outer end projecting from the hole; the bar is manipulated to rupture the cartridge and rotated to mix the the components by means of a tool applied to the new nut element which has been screwed onto the outer end of the bar until the end of the bar engages the abutment portion; and, after the resin grout has set, additional torque is applied to the nut element to cause the outer end of the bar to force its way past the abutment portion and hence the nut element to be screwed further onto the bar and to force a member which is threaded onto the bar into hard engagement with the rock face and thus tension the bar.

There is thus available a rock reinforcement system wherein a length of steel reinforcing bar may be anchored at one end in a blind drill hole by a conventional resin grouting mix and is threaded on the other end, which extends outside the hole. A Member such as a bearing plate is supported on the threaded end of the bar by the nut element which constitutes the essential improvement of the invention. The nut element includes the usual body portion having an internally threaded opening for engagement with the threaded end of the reinforcing bar, and an additional abutment portion which may be dome-shaped surrounding the threaded opening. An opening in the top of the dome-shaped portion may then be coaxial with, and of smaller diameter than the threaded opening in the nut.Thus, as the end of the bar is threaded into the nut, threaded advance is limited by contact of the end of the bar with the inside of the dome-shaped portion.

With the nut engaged with the threaded end of the bar to the maximum extent allowed by the dome-shaped portion, rotational torque applied to the nut is transmitted to the bar, thereby causing the end within the hole to break the two-compartment resin cartridge and mix the contents thereof. When the resin has hardened sufficiently to prevent further rotation of the bar, normally less than a minute after mixing is complete, excess torque applied to the nut will displace the domeshaped portion, by fracture or distortion thereof, allowing continued screw threaded advance of the nut on the bar. The nut may then be tightly secured against the bearing plate, or washer placed between the nut and plate, and sufficient torque applied to produce the desired amount of tension in the resinanchored bar.

The invention will now be described by way of example with reference to the accompanying drawings, in which; Figure 1 is a section, showing the rock reinforcement system, including the novel nut element, being inserted in a drill hole in the rock formation to be supported; Figures 2 and 3 are further sectional views showing the reinforcement system at intermediate and final stages of installation, respectively; Figure 4 is a partial perspective view, partly in section, of the nut element in the form in which it is cast; and, Figure 5 is a perspective view of the nut element in its finished. form.

Figs. 1 to 3 show a cross section of a rock formation 10, such as the roof of a coal mine, in which a blind drill hole 1 2 has been formed with conventional drilling tools for the purpose of installing elements which will serve to support a surface 14 and generally reinforce the surrounding rock structure. An elongate steel bar 1 6 is of a commercially available type used in the reinforcement of concrete structures, and is commonly referred to as rebar. The length of rebar used in rock reinforcement is, of course, chosen in accordance with the depth of the drill hole, and the diameter is usually either 3/4" or 7/8". The bar 1 6 is provided with threads 1 8 extending from one end for a portion of its length.

A conventional resin cartridge 20 is formed in two compartments physically separating components 22 and 24 of a resin grouting mix. Such cartridges are commercially available from a variety or sources and include a polyester resin as one of the components and a reaction agent such as a catalyst or curing or hardening agent as the other. The two components remain in a semi-liquid or thixotropic phase until mixed, whereupon the resin begins to solidify. Curing and solidification continue until an extremely-strong bond is formed by the resin grout.

As seen in Fig. 1, the cartridge 20 has been placed in the drill hole 1 2 and is sup ported therein upon an end 26 of the rebar 1 6. A screw threaded end 1 8 extends outside the drill hole and passes through an opening in a support plate 28 which is carried upon a nut element 30, screwed onto the end of the rebar 16. The cartridge 20 is forced against the end of the hole 1 2 as the rebar 1 6 is moved upward, as indicated in Fig. 1 by arrow 32. The nut element 30 is engaged by a socket tool (not shown) such as employed in bolting machines commonly used in coal mines and elsewhere, which is power-driven to move the rebar 1 6 upwardly and rotate it at a high speed by rotation imparted to the nut element 30, as explained later in more detail.

Continued upward movement of the rebar 1 6 from the position of Fig. 1 will rupture the cartridge 20, allowing the components 22 and 24 to be mixed by rotation of the rebar 1 6. The mixture will fill the space within the hole 1 2 around the end 26 for a desired portion of the length of the rebar 1 6.

Before completing explanation of the rock reinforcement procedure, the structure of the nut element 30 will be described in detail.

The element 30 is preferably formed as a casting of malleable iron. As seen in Fig. 4, the casting includes a base 38 of circular cross section, a body portion 40 of polygonal, in this case square, cross section, and an upstanding flange portion 42, all of which are integrally formed as a single casting. The maximum cross sectional dimension of the portion 40 is no more than the diameter of the base 38. An opening 44 extends through the base and body portions 38 and 40 and is subsequently tapped to provide internal screw threads in the usual manner. It is preferred that the casting be so formed and dimensioned that the opening 44 may be tapped to either 3/4" or 7/8" diameter. For convenience in casting, the opening 44 is initially tapered, having diameters of, for example, 3/8" and 5/8" at is lower and upper ends, respectively, prior to tapping.The castings are preferably annealed to achieve the necessary hardness, and then tapped to the desired thread diameter.

After tapping, the nut element 30 is subjected to a further shaping operation, namely the forming of the ring section or flange 42 into a radially inwardly projecting curved annular lip defining a dome-shaped portion, as shown in Fig. 5 and indicated by reference numeral 46. This may be accomplished with a die defining hemispherical cavity which is forced downwardly over the flange 42 with a controlled stroke to the surface of the body portion 40 adjoining the outer periphery of the flange, resulting in the dome-shaped portion 46 having an opening 48 in the top. The wall which forms the flange portion 42 is so dimensioned in the casting that the maximum diameter of the opening 48 is approximately 9/16", thus being about 3/16" smaller than a 3/4" tapped opening and 5/16" smaller than a 7/8" tapped opening in the nut element 30.By way of illustration, a nut element having the desired characteristics and dimensions may be formed of malleable iron, ASTM A47 grade 32510, with a flange having a height of 7/16", external diameter of 1 3/32" and internal diameters at the top or free end of 5/16" and at the bottom or junction with body portion 40 of 7/8".

Returning now to Fig. 1, it may be seen that the nut element 30 has been advanced onto the screw threads 1 8 of the rebar 1 6 until the terminal end of the rebar has engaged the inside of the dome-shaped portion 46. Since the opening 48 is of smaller diameter than the end of the rebar 16, screw threaded advance is thus limited and rotation imparted to the nut element 30 will be transmitted to the rebar 16, permitting mixing of the resin components as indicated in Fig.

2. The bolting machine is held in supporting engagement with the nut element 30 for a time sufficient to allow the resin mixture to harden enough to prevent further rotation of the rebar 1 6 which for many common resin packages will be only 10 or 1 5 seconds.

After the resin grouting has hardened to the point of preventing further rotation of the rebar 16, excess torque applied to the nut element 30 will cause the dome-shaped portion 46 to crack or fracture around the opening 48. This, of course, permits further screw threaded advance of the nut element 30 on the screw threads 18, urging the plate 28 into tight engagement with the surface 14 of the rock formation 10 and producing a desired degree of tension in the rebar 1 6. In practice, drill hole diameters of 1" and 11/18" have been used for 3/4" and 7/8" thread diameters, respectively, when constructed substantially in accordance with the example set forth earlier herein, and 15" to 18" of resin.

Torque transmitted through the nut to the rebar is sufficient to break the resin cartridge and mix the contents without danger or cracking or breaking dome-shaped portion 46, which is easily fractured to allow screw thread take-up by application of necessary excess torque after the resin hardens sufficiently to prevent further rotation of the rebar 1 6. Although not shown in the present drawing, a hardened steel washer is usually employed between the nut element 30 and the plate 28. If desired, after tapping and forming the dome-shaped portion 46, the nut element 30 may be pearlitized for further hardening.

It may thus be seen that the nut element provides an effective means of transmitting rotation to a rebar for mixing the components of a resin grouting and then tensioning the bar by application of excess torque after the resin hardens. The nut element is not significantly more expensive than a conventional nut of similar dimensions, requiring only the additional step of pressing the concave die over the flange or ring to form the dome-shaped portion. Since the breakable or frangible portion of the element is spaced outwardly from the end of the tapped portion of the nut, there is no adverse effect on the threads after cracking or breaking by application of excess torque.

Claims

1. A nut element comprising a body portion having an internally screw threaded opening extending therethrough to accept, in use, into one end an externally screw threaded member of predetermined diameter; an abutment portion fixed to the body portion and extending radially inwardly to obstruct an extension of the externally threaded member extending, in use, through the screw threaded opening at a position spaced from the other end of the threaded opening, whereby screw threaded advance of the externally threaded member, in use, through the body portion is limited by contact of the end thereof with the abutment portion; and the abutment portion being so constructed and arranged that application of a predetermined torque to one of the nut element and the externally screw threaded member while holding the other stationary permits further relative screw threaded advance by forcing the abutment portion out of its obstructing position.

2. An element according to claim 1, wherein the abutment portion is integrally formed with the body portion.

3. An element according to claim 2, wherein the body portion and the abutment portion are a unitary metal casting.

4. An element according to any one of the preceding claims, wherein the abutment portion encircles the other end of the opening through the body portion.

5. An element according to claim 4 wherein the abutment portion includes an annular lip extending radially inwardly to provide the obstruction.

6. An element according to any one of the preceding claims, wherein the body portion includes a first part of circular cross section extending from the one end to an integral connection with a second part of polygonal cross sectional shape, extending to the other end of the screw threaded opening, the diameter of the first part being at least as great as the largest cross sectional dimension of the second part.

7. An element according to any one of the preceding claims, wherein the abutment portion comprises a dome-shaped portion fixed at its base to the body portion and surrounding the other end of the screw threaded opening.

8. An element according to claim 7, wherein the dome-shaped portion is open at its top to a diameter smaller than that of the screw threaded opening.

9. An element according to claim 8, wherein the open top of the dome-shaped portion is coaxial with the screw threaded opening.

10. A nut element substantially as described with reference to the accompanying drawings.

11. A method of forming a nut element, the method comprising forming an integral, malleable, metal casting including a body portion having an opening extending therethrough between first and second ends; tapping the opening to form internal screw threads extending from the first to the second end; and mechanically deforming a second portion of the casting to extend radially inwardly across a cylindrical plane passing through and of the same diameter as the body portion opening on the second end side of the body portion; whereby the second portion, after being deformed, is capable of limiting screw threaded advance of an externally screw threaded member through the internal screw threads from the first to the second end thereof until application of a predetermined torque sufficient to move the second portion of the cylindrical plane and allow the exter nally screw threaded member to pass there through.

1 2. A method according to claim 11, wherein the second portion is deformed to extend across the cylindrical plane at a posti ion spaced from the second end of the body portion opening.

1 3. A method according to claim 11 or claim 12, wherein the second portion is formed in the casting as a cylindrical wall surrounding the second end of the body por tion opening, and is mechanically deformed to a dome-shape terminating in an annular lip of diameter less than that of the cylindrical plane.

14. A method according to anyone of claims 11 to 13, further including the step of annealing the casting prior to the tapping thereof.

1 5. A method of forming a nut element substantially as described with reference to the accompanying drawings.

1 6. A method of reinforcing a rock forma tion wherein a blind hole is drilled in the rock face; a two component resin grout cartridge is inserted into the hole; an elongate bar having a screw threaded outer end is inserted into the hole with its outer end projecting from the hole; the bar is manipulated to rupture the cartridge and rotated to mix the components by means of a tool applied to a nut element according to any one of claims 1 to 10 which has been screwed onto the outer end of the bar until the end of the bar engages the abutment portion; and, after the resin grout has set, additional torque is applied to the nut element to cause the outer end of the bar to force its way past the abutment portion and hence the nut element to be screwed further onto the bar and to force a member which is threaded onto the bar into hard engagement 'with the rock face and thus tension the bar.

1 7. A method of reinforcing a rock struc ture substantially as described with reference to the accompanying drawings.