Bolt assembly
A bolt assembly for insertion into a rock wall including a hole having a diameter an a method thereof are provided. The assembly includes a resilient cylinder for insertion into the hole, a shaft for insertion into the cylinder, a grouting material for retaining the shaft within the cylinder and stiffening the assembly, and coupling means for coupling the extremity of the cylinder to the extremity of the shaft. The cylinder has a diameter which is greater than the diameter of the hole when the cylinder is uncompressed. In this manner, the cylinder is radially compressed when inserted into the hole and thereby frictionally engages the hole. The cylinder has an extremity extending outside the hole when it is inserted therein. The shaft includes an extremity which extends outside the hole when the shaft is inserted into the cylinder in an operating position.
The present invention relates to bolt assemblies, and more specifically to bolt assemblies for insertion into a rock wall and a method thereof.
BACKGROUND OF THE INVENTIONIt is known in engineering construction to stabilise rock elements through use of mechanical bolting elements. More specifically in the art of underground mining, it is known to maintain the stability of walls, openings and excavated areas by drilling holes into the rock wall and installing support systems, such as metal bolts, therein in order to hold the ground together. An anchor, for example a plate extending outward from an exposed extremity of the bolt, may be provided which forms a link between the outer surface of the rock wall and the bolt and prevents the support system from translating inwards in the event of rock movement. Surface support may also be provided in the form of wire mesh screens which are placed across the wall's surface and can be held in place by the support system's anchors.
Many problems are encountered in rock engineering, particularly, when mining at depth. In difficult ground conditions, such as yielding ground, the stresses present in the rock exceed the rock strength and movement occurs. In underground mining, this movement is often referred to as closure, as the walls or back and floor of the excavations tend to converge. In some instances, these movements can be quite large.
In order to stabilise underground excavations, several common stabilising systems are known in the art.
Conventional friction bolts typically comprise a metal tube which is inserted into a hole of a slightly smaller diameter. By forcing the friction bolt into such a hole, the friction bolt is radially compressed and retained by the resultant frictional forces created therebetween. A friction bolt can be provided with a slotted opening the length of its tubular body in order to ease its compression. Friction bolts are able to slide with respect to the hole in converging ground, however, such bolts typically have a relatively low pull-out force and resistance in shear and tension.
Alternatively, reinforcement bar, commonly referred to as rebar, can be retained in a hole using a resin, cement or any other viscous liquid or semi-liquid grouting medium which is capable of hardening. However, such systems are unsuitable for use in converging ground. Due to their very high anchorage capacity, the stiff resin- or cement-grouted rebar becomes loaded to failure very quickly, even under very small displacements.
Similar to the above-noted usage of grouted rebar, cablebolts are typically formed by retaining a cable in a hole using a resin or cement grouting material. So-called yielding cable bolts are similarly constructed but place a portion of the cable inside a tubular sleeve. Under loading, the portion of the cable within the sleeve is free to deform with respect to the surrounding rock while the remainder is firmly anchored in place by the resin or cement. Both cablebolts and yielding cablebolts can be used in ground prone to closure, but only up to a certain point. Even though the sleeved portion can provide some allowance for closure, such systems are unable to accommodate larger deformations. Eventually, either the cable or its anchor at the rock wall's surface fails, much like regular grouted cables.
Conebolts and modified conebolts, for their part, provide a rod with a conical section extending from its interior extremity. The rod is grouted into the hole and the conical section serves to radially compress the grouting material in response to a longitudinal force. Conebolts have been proven to be very effective in dealing with projections of pieces of rock, commonly referred to as rockbursts. However, in certain ground conditions early deformations in the rock mass may “lock” a conebolt in place and prevents it from yielding, effectively making them useless in the event of a rockburst.
Known in the art are the following U.S. patents which disclose prior art relating to security systems and the like.
U.S. Pat. No. 6,390,735, issued May 21, 2002 to Gaudreau et al., describes a modified conebolt.
U.S. patent application Ser. No. 10/3368,842, published Aug. 19, 2004 under publication 2004/0161316 and filed by LOCOTOS et al., describes a mine bolt for a bore hole wherein a cylindrical tube is inserted into a hole which is larger in size. The tube is anchored in the rock wall by resin. In use, a resin cartridge is inserted the hole, followed by the tube. As noted in paragraph [0047], the end of the tube which is inserted into the hole “is closed so resin 32 from the resin cartridge 56 will not enter the interior 18 of the tube 16 but flow along the outer surface of the tube 16”, thereby retaining the tube within the hole. This system therefore suffers from many of the same problems as the grouted rebar, cablebolt and conebolt systems mentioned above.
As such, these existing systems either: a) do not yield enough to accommodate large displacements; b) lose their ability to yield only a few months after installation due to the nature of the movement involved, for example internal wall shearing; or c) yield at a low tonnage, providing inadequate confinement to the rock mass. Therefore there is a need for a device capable of yielding over its entire length, at a constant high tonnage, while retaining its ability to yield over time as it survives shearing.
Furthermore, in the event of a rock failure near the face of an excavation the sudden release of energy can result in the projection of pieces of rock. Such occurrences represent a risk to both equipment and personnel when mining at depth. Therefore there is also a need for a device allowing energy dissipation during a seismic event and being effective under dynamic load in the field.
Also known in the art are the following patents and published applications which also describe security systems and the like: SU 1176090; U.S. Pat. No. 2,667,037; U.S. Pat. No. 3,222,873; U.S. Pat. No. 3,234,742; U.S. Pat. No. 3,301,123; U.S. Pat. No. 3,379,016; U.S. Pat. No. 3,379,019; U.S. Pat. No. 3,695,045; U.S. Pat. No. 3,837,258; U.S. Pat. No. 3,925,996; U.S. Pat. No. 3,971,177; U.S. Pat. No. 3,987,635; U.S. Pat. No. 4,193,715; U.S. Pat. No. 4,528,792; U.S. Pat. No. 5,127,769; U.S. Pat. No. 5,919,006; US 2003/0219316; US 2005/0158127; and US 2006/0153645.
SUMMARY OF THE INVENTIONAn aspect of the present invention is to provide a bolt assembly which, by virtue of its design and components, satisfies at least some of the above-mentioned needs and is thus an improvement on other related devices known in the prior art.
Another aspect of the present invention is to provide a bolt assembly having superior stabilising characteristics in difficult and hazardous situations, such as in yielding ground and in ground prone to rockbusting.
In accordance with one aspect of the present invention, there is provided a bolt assembly for insertion into a rock wall including a hole having a diameter. The assembly includes a resilient cylinder for insertion into the hole, a shaft for insertion into the cylinder, a grouting material for retaining the shaft within the cylinder and stiffening the assembly, and coupling means for coupling the extremity of the cylinder to the extremity of the shaft. The cylinder has a diameter which is greater than the diameter of the hole when the cylinder is uncompressed. In this manner, the cylinder is radially compressed when inserted into the hole and thereby frictionally engages the hole. The cylinder has an extremity extending outside the hole when it is inserted therein. The shaft includes an extremity which extends outside the hole when the shaft is inserted into the cylinder in an operating position.
The invention is also directed to a method of installing a bolt assembly in a rock wall including a hole having a diameter. The method includes the steps of providing a compressible cylinder having a diameter which is greater than the diameter of the hole when the cylinder is uncompressed, inserting the cylinder into the hole until an extremity of the cylinder remains outside the hole thereby compressing the cylinder such that the cylinder frictionally engages the hole, inserting a shaft into the cylinder until an extremity of the shaft remains outside the cylinder, providing a grouting material for retaining the shaft within the cylinder and stiffening the assembly, and coupling the extremity of the cylinder and the extremity of the shaft with a coupling means.
As can be appreciated, a bolt assembly according to the present invention can be used in a wide variety of situations which would normally require multiple specialised tools and can be highly work-intensive and costly to install. The bolt assembly as described herein can be used either in slow, static loading situations (yielding ground) or in violent, dynamic loading situations (burst-prone rock) while retaining its ability to yield over its entire length at high and constant tonnage and dissipate energy in the event of a rockburst.
Additional advantages and other features of the present invention will become more apparent upon reading of the following non-restrictive description of a preferred embodiment thereof, given for the purpose of exemplification only, with reference to the accompanying drawings.
In the context of the present description, the expression “bolt” includes different types of devices that can be used according to the present invention. Moreover, although the present invention was primarily designed for insertion into a rock element and for mining, it could be inserted into concrete or into other materials and could be used for other applications.
In the following description, similar features in the drawings have been given similar reference numerals and, for clarity, some elements are not referred to in some figures if they were already identified in a preceding figure.
With reference to
The cylinder 10 has a cylindrical body and is made of a stiff, resilient material such as a metal. The cylinder 10 is chosen to have a diameter which is greater than the diameter of the hole 2 such that it is radially compressed when it is inserted into the hole 2. The cylinder 10 is thereby frictionally retained within the hole 2 by the forces exerted between the inner wall of the hole 2 and the outer surface of the cylinder 10. An extremity 18 of the cylinder, also referred to as the head, remains extending outside the hole 2.
The shaft 30 is similarly made of a stiff material, such as a metal. Preferably, the shaft 30 is formed from a section of rebar. An outer extremity 34 of the shaft 30 similarly remains extending outside the hole 2, while an inner extremity 32 is plunged within the cylinder 10. A coupling means 38 couples the extremity 34 of the shaft 30 to the extremity 18 of the cylinder 30.
With specific reference to
In the illustrated embodiment, the cartridges 14 have been provided with a conventional resin 14 and a supplemental cartridge 14a has been provided with a quicker-acting “fast” resin. As will be apparent to one of ordinary skill in the art, other grouting materials and mediums are well within the scope of the present invention, as are other cartridge arrangements, so long as the grouting material is operable to bond the shaft 30 within the cylinder 10.
The shaft 30 is preferably shorter in length than the cylinder 10 so that inner extremity 32 of the shaft 30 does not extend beyond the cylinder 10 and into the hole 2, which could negatively impact the ability of the assembly 5 to perform in a yielding ground. In such a situation, the grouting material 14 would bond a portion of the shaft 30 directly to the rock wall 1 at the inner extremity of the hole 2. In order to prevent unnecessary wastage of the grouting material 14, the inner extremity of cylinder 10 may be physically closed.
The cylinder 10 is preferably of the split-set type which comprises a longitudinal opening extending its length, as is known in the art, so as to facilitate its compression within the hole 2. However, it will be appreciated that a cylinder 10 without such a slot may similarly be press-fit within the hole 2. Similarly, a balloon-type cylinder 10 which is inflated upon being inserted to the hole 2 may be used as part of the present invention.
The outer surface of the cylinder 10, i.e. that which engages rock wall within the hole 2, is relatively smooth so as to be able to slide with respect to any moving or yielding ground. The bolt assembly 5 is preferably designed to yield within the hole 2 at just below the system's ultimate strength. As such, the assembly 5 will advantageously translate within the hole 2 before it breaks. This yield point is also referred to as its anchoring capacity or pull-out resistance
The hardened resin 14 within the cylinder 10 is advantageously utilized to both hold the shaft 30 within the cylinder 10 and strengthens the assembly 5 as a whole. The presence of the resin 14 within the cylinder 10 increases the radial strength cylinder 10, which in turn can increase the pull-out resistance of the assembly 5 by as much as two to three times that of a conventional cylinder. Moreover, the compressive strength of the grouting material, and hence the pull-out resistance of a given bolt assembly 5, could be selected in order to give more or less radial strength to the cylinder 10.
Providing the combination of the cylinder 10 which is frictionally retained within the hole 2 and the shaft 30 which itself is retained within the cylinder 10 by the grouting material 14 not only provides a stronger bolt assembly 5 with a greater, more controllable pull-out resistance which is capable of yielding at high tonnage, but also allows for a simplified and more reliable installation. In use, the resin cartridges 14 are preferably pre-positioned within the cylinder 10 prior to its insertion into the hole 2. This arrangement avoids the need to insert resin cartridges 14 directly into the hole 2, which can puncture the cartridges when placed in broken ground thereby rendering them useless. Herein, the cylinder 10 protects the grouting material 14 during insertion, thereby preventing an unintentional and premature bonding of the material 14. While in many cases it is also possible to first insert the cylinder 10 into the hole 2 prior to positioning the grouting material 14 therewithin, it is especially preferable to combine the cylinder 10 and grouting material 14 first when doing so in heavily broken ground, such as gravel and the like, which may enter the empty cylinder 10 during its insertion into the hole 2 and damage any subsequently positioned cartridges.
The extremity 34 of the shaft 30 is provided with an externally threaded portion. Preferably the coupling means 38 comprises a corresponding internally threaded portion and functions as a nut which can be screwed onto the shaft 30 and tightened so as to couple, and thereby strengthen, the extremities 18 and the 34 of the cylinder 10 and shaft 30, respectively. For example, a resin-grouted rebar 30 would provide a substantial increase in strength to the outer extremity 18 of a split-type cylinder 10, thereby allowing for higher confinement of the rockmass through the yielding process. Strengthening the extremity 18 of the cylinder 10 in this manner is advantageous because such extremities are often the weak points in cylinders 10 which are retained by frictional force. However other types of coupling means 38 could result in similar performances. For example, forged-head rebar has been used successfully in this type of application in the past. Also, a shorter rebar 30 could be used, provided that the grouting medium 14 used does not need to be mixed by that rebar 30 as described above. Furthermore, larger or smaller diameter cylinders 10 and shafts 30 could be used to adjust the pull-out resistance. The use of bigger, stronger components is likely to produce an end result with increased capacity.
Preferably, the assembly 5 is provided with a first plate 20 which extends radially from the outer extremity 18 of the cylinder 10 and rests against the surface 3 of the rock wall 1. The plate 20 is positioned around the cylinder 10 and may be fixed to its extremity 18 or may comprise an aperture through which is passed the cylinder 10 as it is inserted into the hole 2.
A second plate 36 can be positioned around the shaft 30 between the coupling means 38 and the extremity 18 of the cylinder 10. The second plate 36 may comprise an aperture through which is passed the shaft 40 or alternatively may be fixed to coupling means 38. The second plate 38 may engage the extremity 18, the first plate 20, or both. A wire-mesh screen for supporting the rock wall 2 may be clamped between the plate 20 and the rock wall surface 3, or alternatively between the plates 20 and 36.
The bolt assembly 5 can be installed in the rock wall 1 as described herein below. The compressible cylinder 10, which has a diameter that is greater than the diameter of the hole 2 it is uncompressed is inserted into the hole 2 until its extremity 18 remains outside the hole. In a specific example, a Ø37 mm hole 2 is drilled into the rock wall 1, and a 2.0 m long, Ø39 mm cylinder 10 is inserted therein. By inserting the cylinder 10 into the hole 2, it is radially compressed and remains frictionally engaged therewithin.
The shaft 30 is next inserted into the cylinder 10 until its respective extremity 38 remains outside the cylinder 10, or at least proximate to the extremity 18. In a specific example, a 1.9 m long, Ø22 mm section 30 of rebar is inserted into the cylinder 10. The grouting material 14 is provided for retaining the shaft 30 within the cylinder 10 and for stiffening the assembly 5. In a specific example, the grouting material 14 may be a series of resin cartridges 14, 14a which are pre-positioned therewithin in order to prevent broken fragments of rock from entering and hampering the rest of the installation. The rebar 30 is 15 then inserted and spun in order to puncture the cartridges and initiate bonding of the resin 14, 14a therein following the specific requirements of the resin used, as seen in
The extremity 18 is provided with the plate 20 to hold an eventual surface support screen and the rebar 30 may also be provided with its own plate 36 in order to prevent early failure of the extremity 18 of the cylinder 10.
The extremity 18 of the cylinder 10 and the extremity 34 of the shaft 30 are then coupled with the coupling means 38. In a specific example, the coupling means 38 is a rebar nut which is tightened to push the plate 36 against the extremity 18 of the cylinder 10.
As apparent for a person skilled in the art, various adjustments to this process are possible and many component sizes are conceivable as well.
The bolt assembly 5 can be used in any engineering construction involving the stabilization of rock elements through the use of mechanical bolting elements. It can be used either in slow, static loading situations, as for example yielding ground, or in violent, dynamic loading situations, as for example burst-prone rock. It retains a good potential for slipping even after being subjected to massive shearing forces.
The bolt assembly 5, due to its ability to slip at a tonnage slightly lower than its ultimate strength, overcomes the problem of getting loaded very quickly to failure of the stiff resin- or cement-grouted rebar, by moving along with the walls of the excavation while providing the very high confinement normally associated with stiffer bolts.
The bolt assembly 5 overcomes the problem of cable or plate failure due to inability to accommodate large deformations in grounds prone to closure, by moving along with the walls of the excavation.
Furthermore, the bolt assembly 5 solves the major problem of dealing with rockbursts, where early deformations in the rock mass essentially “locks” conebolt and modified conebolt in place and prevents them from yielding, making them useless in the event of a rockburst. The ability of the bolt assembly to yield remains unaffected even after significant deformations along the bolt has occurred.
Due to its ability to yield at a high and constant tonnage, the bolt assembly solves many problems encountered in rock engineering, particularly, but not limited to, mining at depth. For example, the bolt assembly 5 solves the closure problem, where the walls or back and floor of the excavations tend to converge, because it is capable of yielding over its entire length, at a constant high tonnage, and it also retains its ability to yield over time as it survives shearing. It represents a significant improvement over standard support systems.
The bolt assembly 5 has also proven to be very effective under dynamic load in the field that occurs in the event a rock failure near the face of an excavation the sudden release of energy can result in the projection of pieces of rock.
The bolt assembly 5 provides immediate support in a wide variety of situations which would normally require multiple specialized support types, which are often work-intensive and costly to install. It uses low-cost existing elements that are already widely used in the industry, and its installation is very simple.
Tensioning the shaft 30 should not change the behaviour of the bolt assembly 5.
Furthermore, as apparent to a person in the art, different sizes of devices could be used according to the needs and other types of cylinder, grouting material, shaft or coupling means could also be used with this invention. It should be understood that the configuration of the preferred embodiment could also be modified in order to be inserted in other elements, as for example cement. The invention could also be used in other applications, as apparent to a person skilled in the art.
Claims
1. A bolt assembly for insertion into a rock wall comprising a hole having a diameter, the assembly comprising:
- a) a resilient cylinder for insertion into the hole, the cylinder having a diameter which is greater than the diameter of the hole when the cylinder is uncompressed so that the cylinder is radially compressed when inserted into the hole and thereby frictionally engages the hole, the cylinder having an extremity extending outside the hole when the cylinder is inserted into the hole;
- b) a shaft for insertion into the cylinder, the shaft comprising an extremity which extends outside the hole when the shaft is inserted into the cylinder in an operating position;
- c) a grouting material for retaining the shaft within the cylinder and stiffening the assembly; and
- d) coupling means for coupling the extremity of the cylinder to the extremity of the shaft.
2. The bolt assembly of claim 1, wherein the shaft comprises a threaded portion proximate its extremity and the coupling means comprises a correspondingly threaded portion such that the coupling means is screwable along the shaft in the operating position to couple the extremity of the cylinder to the extremity of the shaft.
3. The bolt assembly of claim 2, further comprising a plate capable of being positioned around the shaft between the coupling means and the extremity of the cylinder.
4. The bolt assembly of claim 2, further comprising a plate capable of being positioned around the cylinder proximate to the cylinder's extremity for contact with the rock wall.
5. The bolt assembly of claim 1, wherein the shaft comprises external ridges.
6. The bolt assembly of claim 1, wherein the grouting material is a resin contained in a cartridge, the cartridge being positioned within the cylinder and punctured by the shaft during its insertion into the cylinder.
7. The bolt assembly of claim 6, wherein the resin is a multi-part resin contained in separate chambers of the cartridge, the chambers being punctured by the shaft during its insertion into the cylinder, thereby mixing the multi-part resin and initiating the curing thereof.
8. The bolt of claim 1, wherein the cylinder further comprises a longitudinal opening extending its length.
9. A method of installing a bolt assembly in a rock wall comprising a hole having a diameter, the method comprising the steps of:
- a) providing a compressible cylinder having a diameter which is greater than the diameter of the hole when the cylinder is uncompressed;
- b) inserting the cylinder into the hole until an extremity of the cylinder remains outside the hole, thereby compressing the cylinder such that the cylinder frictionally engages the hole;
- c) inserting a shaft into the cylinder until an extremity of the shaft remains outside the cylinder;
- d) providing a grouting material for retaining the shaft within the cylinder and stiffening the assembly; and
- e) coupling the extremity of the cylinder and the extremity of the shaft with a coupling means.
10. The method of claim 9, wherein step a) comprises a step of inserting a resin cartridge into the cylinder prior to step b), and step c) comprises a step of puncturing the cartridge by means of the shaft.
11. The method of claim 9, wherein:
- in step b) the shaft comprises a threaded portion proximate to its extremity;
- in step e) the coupling means comprises a correspondingly threaded portion; and
- step e) further comprises the step of screwing the coupling means along the shaft to couple the extremity of the cylinder to the extremity of the shaft.
Type: Application
Filed: Oct 2, 2007
Publication Date: Sep 11, 2008
Inventors: Frederic Mercier-Langevin (Malartic), Mario Caron (Val-d' Or), Pascal Turcotte (Riviere-Heva)
Application Number: 11/905,622
International Classification: E21D 20/00 (20060101); E21D 21/00 (20060101);
