CA2480729C - Yieldable rock fastener system and method - Google Patents
Yieldable rock fastener system and method Download PDFInfo
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- CA2480729C CA2480729C CA2480729A CA2480729A CA2480729C CA 2480729 C CA2480729 C CA 2480729C CA 2480729 A CA2480729 A CA 2480729A CA 2480729 A CA2480729 A CA 2480729A CA 2480729 C CA2480729 C CA 2480729C
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- rock
- sleeve
- cable bolt
- cable
- fastener system
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- E—FIXED CONSTRUCTIONS
- E21—EARTH DRILLING; MINING
- E21D—SHAFTS; TUNNELS; GALLERIES; LARGE UNDERGROUND CHAMBERS
- E21D21/00—Anchoring-bolts for roof, floor in galleries or longwall working, or shaft-lining protection
- E21D21/0026—Anchoring-bolts for roof, floor in galleries or longwall working, or shaft-lining protection characterised by constructional features of the bolts
- E21D21/0033—Anchoring-bolts for roof, floor in galleries or longwall working, or shaft-lining protection characterised by constructional features of the bolts having a jacket or outer tube
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- E—FIXED CONSTRUCTIONS
- E21—EARTH DRILLING; MINING
- E21D—SHAFTS; TUNNELS; GALLERIES; LARGE UNDERGROUND CHAMBERS
- E21D21/00—Anchoring-bolts for roof, floor in galleries or longwall working, or shaft-lining protection
- E21D21/0026—Anchoring-bolts for roof, floor in galleries or longwall working, or shaft-lining protection characterised by constructional features of the bolts
- E21D21/006—Anchoring-bolts made of cables or wires
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- Engineering & Computer Science (AREA)
- Mining & Mineral Resources (AREA)
- Structural Engineering (AREA)
- Life Sciences & Earth Sciences (AREA)
- General Life Sciences & Earth Sciences (AREA)
- Geochemistry & Mineralogy (AREA)
- Geology (AREA)
- Piles And Underground Anchors (AREA)
Abstract
A yieldable rock fastener system has a pre-stressed seven-strand cable bolt grouted inside a borehole in a rock face of a mine or tunnel. A steel sleeve is press-fitted onto the cable bolt either outside the rock face or inside the borehole. When the sleeve is external, the sleeve is designed to yieldably slip relative to the cable bolt under a load exceeding a predetermined threshold force induced by a rock burst or other rock displacement. When the sleeve is internal, the sleeve is grouted inside the borehole so that the cable bolt yieldably slips relative to the internal sleeve when the predetermined threshold force is exceeded. The yieldable rock fastener system thus absorbs and controls rock bursts and other rock strata movements, thereby inhibiting cave-ins and collapses.
Description
YIELDABLE ROCK FASTENER SYSTEM AND METHOD
TECHNICAL FIELD
The present invention relates generally to rock support devices for mining and tunneling and particularly to yieldable rock fasteners far controlling rock bursts or other unwanted displacements of rock strata.
In order to provide a safe environment for miners working underground, rock support mechanisms are installed at regular intervals to secure the roof and/or walls of a tunnel. The selection of a suitable support mechanism is dictated by factors such as geological characteristics, the dimensions of the excavation, the time that the support is required, and the expected rock stresses (which generally increase with mine depth).
Stress realignment induced by mining activity can, in some cases, provoke a violent release of energy and a sudden rock strata displacement, known as a rock burst.
Controlling rock bursts is crucial to protecting personnel and equipment. In addition to rock bursts, there is also a phenomenon known as "closure" where the walls, back, and floor of an underground excavation slowly creep into the mine opening.
There are several support mechanisms that are well known and commonly used in underground mining and tunneling, such as bulged cable bolts (see Canadian Patent 1,059,351 to Villgren) and anchoring devices with ribs and shear-off threads (see U.S. Patent 4,904,122 to Herbst et al.). Other designs include rock bolts incorporating an expansion anchor; resin and cement-grouted rebar-type bolts; and friction stabilizer bolts which are driven into position by the percussive action of the drill or expanded with high-pressure water to conform to the hole diameter.
Also known are load-indicating devices for grouted anchors, as taught in Canadian Patent 2,035,786 to Ischebeck.
Several techniques have also been developed to provide a gradual yielding to inhibit rock bursts and closure. A
yielding support mechanism absorbs and dissipates rock energy in a controlled and secure manner. If the rock support mechanism is unable to yield, stresses can build up and the device may fail violently and without warning.
Thus, the yielding support mechanism not only cushions a rock burst but also yields to accommodate closure. Examples of yieldable support mechanisms are disclosed in Canadian Patent 1,197,387 to Powondra, U.S. Patent 3,967,455 to Conway, and U.S. Patent 5,882,148 to Mraz. As disclosed in Conway and Mraz, the prior-art yieldable rock fasteners typically employ a yieldable element such as a collar or sleeve which is mounted between the elongated structural members (e. g. the bolt or rod) and the bored hole.
Other yielding techniques involve cable lacing whereby cable is strung through a series of anchor bolts. Another yieldable system uses a cone bolt which is designed to plough through the anchoring resin. As appreciated by persons of ordinary skill in the art, friction stabilizers also provide a degree of yieldability.
One of the shortcomings of these prior-art yieldable rock fasteners is that the yieldable element is mounted within the bored hole. The ability of the yieldable rock fastener to yield is thus limited by the hole and the elongated structural member around which the yieldable element is mounted. In other words, the yieldable element only permits yielding over a relatively short distance.
Furthermore, since the yieldable element is mounted inside the hole, no visual cue is provided to miners when the rock support begins to yield.
Yet another example of a prior-art yielding rock bolt is disclosed in PCT Application WO 03/021081 Al (Maltby) in which a widened portion of a cable bolt is pulled through a collar with a bore smaller than the widened portion. The collar is anchored within the borehole so that rock displacements are resisted by the threshold force required to pull the widened portion of the cable bolt through the small bore of the collar. Although this rock bolt provides yieldable support, it suffers from the disadvantage that it is more expensive to manufacture due to its widened portion. Furthermore, the rock bolts are manufactured for unique configurations, i.e., each collar must be anchored at a predetermined depth within the hole corresponding to the point where the cable bolt widens.
Accordingly, it would be highly desirable to provide an improved yieldable rock fastener system that overcomes one or more of the deficiencies of the prior art.
SiTI~IARY OF THE INVENTION
Accordingly, an object of the present invention. is to provide a yieldable rock fastener system and a method for securing rock walls and rock roofs in mines and tunnels that overcomes at least some of the deficiencies of the prior art. The yieldable rock fastener has an elongated structural member, which is preferably a cable bolt having a plurality of interwoven strands. The cable bolt is inserted into a hole bored into the rock face of a mine or tunnel and bonded to the rock with grout, such as resin. A
TECHNICAL FIELD
The present invention relates generally to rock support devices for mining and tunneling and particularly to yieldable rock fasteners far controlling rock bursts or other unwanted displacements of rock strata.
In order to provide a safe environment for miners working underground, rock support mechanisms are installed at regular intervals to secure the roof and/or walls of a tunnel. The selection of a suitable support mechanism is dictated by factors such as geological characteristics, the dimensions of the excavation, the time that the support is required, and the expected rock stresses (which generally increase with mine depth).
Stress realignment induced by mining activity can, in some cases, provoke a violent release of energy and a sudden rock strata displacement, known as a rock burst.
Controlling rock bursts is crucial to protecting personnel and equipment. In addition to rock bursts, there is also a phenomenon known as "closure" where the walls, back, and floor of an underground excavation slowly creep into the mine opening.
There are several support mechanisms that are well known and commonly used in underground mining and tunneling, such as bulged cable bolts (see Canadian Patent 1,059,351 to Villgren) and anchoring devices with ribs and shear-off threads (see U.S. Patent 4,904,122 to Herbst et al.). Other designs include rock bolts incorporating an expansion anchor; resin and cement-grouted rebar-type bolts; and friction stabilizer bolts which are driven into position by the percussive action of the drill or expanded with high-pressure water to conform to the hole diameter.
Also known are load-indicating devices for grouted anchors, as taught in Canadian Patent 2,035,786 to Ischebeck.
Several techniques have also been developed to provide a gradual yielding to inhibit rock bursts and closure. A
yielding support mechanism absorbs and dissipates rock energy in a controlled and secure manner. If the rock support mechanism is unable to yield, stresses can build up and the device may fail violently and without warning.
Thus, the yielding support mechanism not only cushions a rock burst but also yields to accommodate closure. Examples of yieldable support mechanisms are disclosed in Canadian Patent 1,197,387 to Powondra, U.S. Patent 3,967,455 to Conway, and U.S. Patent 5,882,148 to Mraz. As disclosed in Conway and Mraz, the prior-art yieldable rock fasteners typically employ a yieldable element such as a collar or sleeve which is mounted between the elongated structural members (e. g. the bolt or rod) and the bored hole.
Other yielding techniques involve cable lacing whereby cable is strung through a series of anchor bolts. Another yieldable system uses a cone bolt which is designed to plough through the anchoring resin. As appreciated by persons of ordinary skill in the art, friction stabilizers also provide a degree of yieldability.
One of the shortcomings of these prior-art yieldable rock fasteners is that the yieldable element is mounted within the bored hole. The ability of the yieldable rock fastener to yield is thus limited by the hole and the elongated structural member around which the yieldable element is mounted. In other words, the yieldable element only permits yielding over a relatively short distance.
Furthermore, since the yieldable element is mounted inside the hole, no visual cue is provided to miners when the rock support begins to yield.
Yet another example of a prior-art yielding rock bolt is disclosed in PCT Application WO 03/021081 Al (Maltby) in which a widened portion of a cable bolt is pulled through a collar with a bore smaller than the widened portion. The collar is anchored within the borehole so that rock displacements are resisted by the threshold force required to pull the widened portion of the cable bolt through the small bore of the collar. Although this rock bolt provides yieldable support, it suffers from the disadvantage that it is more expensive to manufacture due to its widened portion. Furthermore, the rock bolts are manufactured for unique configurations, i.e., each collar must be anchored at a predetermined depth within the hole corresponding to the point where the cable bolt widens.
Accordingly, it would be highly desirable to provide an improved yieldable rock fastener system that overcomes one or more of the deficiencies of the prior art.
SiTI~IARY OF THE INVENTION
Accordingly, an object of the present invention. is to provide a yieldable rock fastener system and a method for securing rock walls and rock roofs in mines and tunnels that overcomes at least some of the deficiencies of the prior art. The yieldable rock fastener has an elongated structural member, which is preferably a cable bolt having a plurality of interwoven strands. The cable bolt is inserted into a hole bored into the rock face of a mine or tunnel and bonded to the rock with grout, such as resin. A
yielding sleeve is press-fitted onto the cable bolt outside the rock face such that the sleeve will only yield under a load exceeding a predetermined threshold force. In the event of a rock burst or a substantial displacement of rock, rock surrounding the mine or tunnel will push against a washer plate sandwiched between the rock face and the sleeve. Tf the load exceeds the threshold force, the sleeve will controllably yield by slipping over the cable bolt until the sleeve abuts a stopper mounted to the cable bolt. The yieldable rock fastener system thus absorbs and controls rock bursts and other rock strata movements, thereby inhibiting cave-ins and collapses.
Therefore, a first aspect of the invention provides a rock fastener system for yieldable support of a rock wall or roof in a mine or tunnel. The rock fastener system includes an elongated structural member having a rock-penetrating portion adapted to extend into a hole bored in the rock and a protruding~portion adapted to protrude from the hole. The rock fastener system further includes grout for bonding the rock-penetrating portion of the elongated structural member to the rock. The rock fastener system also includes a sleeve press-fitted onto the protruding portion of the elongated structural member into a high-strength interference fit that yields only if a load exceeding a threshold force is applied to the sleeve. The rock fastener system also has a stopper fixed to a protruding portion of the elongated structural member and spaced apart from the sleeve to define a yielding distance over which the sleeve can be resistibly displaced under a load exceeding the threshold force until the sleeve abuts the stopper.
Therefore, a first aspect of the invention provides a rock fastener system for yieldable support of a rock wall or roof in a mine or tunnel. The rock fastener system includes an elongated structural member having a rock-penetrating portion adapted to extend into a hole bored in the rock and a protruding~portion adapted to protrude from the hole. The rock fastener system further includes grout for bonding the rock-penetrating portion of the elongated structural member to the rock. The rock fastener system also includes a sleeve press-fitted onto the protruding portion of the elongated structural member into a high-strength interference fit that yields only if a load exceeding a threshold force is applied to the sleeve. The rock fastener system also has a stopper fixed to a protruding portion of the elongated structural member and spaced apart from the sleeve to define a yielding distance over which the sleeve can be resistibly displaced under a load exceeding the threshold force until the sleeve abuts the stopper.
A second aspect of the invention provides a method of yieldably fastening rock in a mine or underground tunnel.
The method includes the steps of boring a hole in the rock;
inserting grout into the hole; inserting an elongated structural member into the grouted hole; press-fitting a sleeve onto the elongated structural member to create a high-strength interference fit which only yields under an applied force exceeding a predetermined threshold force;
and affixing a stopper to the elongated structural member.
The stopper is spaced apart from the sleeve to define a yielding distance through which the sleeve can yield when a load exceeding the predetermined threshold force is exerted on the sleeve.
A third aspect of the present invention provides a rock fastener system for yieldable support of a tunnel or mine, the rock fastener system including an elongated structural member having a rock-penetrating portion adapted to extend into a hole bored in the rock and a protruding portion adapted to protrude from the hole. The system also includes a, sleeve press-fitted onto the elongated structural member in a high-strength interference fit that only yields if a load exceeding a predetermined threshold force is applied to the sleeve.
A fourth aspect of the present invention provides a rock fastener system for yieldable support of a tunnel or mine. The rock fastener system includes an elongated structural member having a rock-penetrating portion adapted to extend into a hole bored in the rock; and a protruding portion adapted to protrude from the hole. The system also includes an internal sleeve press-fitted onto the rock-penetrating portion of the elongated structural member in a high-strength interference fit that only yields if a load exceeding a first predetermined threshold force is applied to the elongated structural member; and an external sleeve press-fitted onto the protruding portion of the elongated structural member in a high-strength interference fit that only yields if a load exceeding a second predetermined threshold force is applied to the external sleeve.
Preferably, the rock fastener system includes a cable bolt to which an internal and/or external sleeve is press-fitted. Where the sleeve is mounted internal to the borehole, the sleeve is grouted permanently into position so that the cable bolt can yieldably slip relative to the grouted-in internal sleeve. Where the sleeve is mounted external to the borehole, the sleeve can yieldably slip relative to the cable bolt which is itself grouted inside the borehole.
The various embodiments of the present invention overcome at least some of the deficiencies of the prior art. In the first embodiment, where an external sleeve is press-fitted on a protruding portion of the cable bolt, the rock fastener provides a useful visual cue to miners that a rock shift has caused the rock fastener to yield.
Furthermore, the threshold force at which the fastener will yield can be predetermined. The predetermined threshold force can thus be tailored to specific applications based upon rock conditions. The yieldable rock fastener therefore permits significant controlled strata movement over a predetermined yielding distance without fully stressing the cable bolt. The yieldable rock fastener yields slowly and controllably while maintaining a predetermined resistive force on the shifting rock strata.
The yieldable fastener controls both gradual rock movements due to closure as well as violent displacements due to rock bursts.
The second, third and fourth embodiments, where an internal sleeve is grouted inside the borehole, provide a versatile, relatively inexpensive, easy-to-manufacture rock bolt system that can be tailored to a variety of applications where smooth, predictable yielding performance is desirable.
BRIEF DESCRIPTION OF THE DRAWINGS
Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which:
FIG. 1 is a schematic side view of a rock fastener system in accordance with an embodiment of the present invention;
FIG. 2 is a partial view of the bulged cable bolt used in the rock fastener system shown in FIG. 1;
FIG. 3 is a perspective view of the components of a stopper for use in the embodiment of FIG. 1, namely a cable grip having a pair of conical wedges and a barrel which forces the conical wedges to grasp the cable bolt;
FIG. 4 is a schematic side view of the rock fastener system in its initial, "unyielded" configuration;
FIG. 5 is a schematic side view of the rock fastener system after yieldably displacing to the stopper;
FIG. 6 is a schematic side view of a rock fastener system in accordance with a second embodiment of the present invention;
FIG. 7 is a schematic side view of a rock fastener system in accordance with a third embodiment of the present invention; and FIG. 8 is a schematic side view of a rock fastener system in accordance with a fourth embodiment of the present invention.
It will be noted that throughout the appended drawings, like features are identified by like reference numerals. The drawings are not to scale.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIGS. 1 to 5 illustrate a yieldable rock fastener system and method in accordance with an embodiment of the present invention. In general, the yieldable rock fastener system has a pre-stressed seven-strand cable bolt with bulges at regular intervals. The cable bolt is inserted into a hole bored into the rock face of a mine or tunnel.
The cable bolt is banded to the rock with grout. A sleeve is press-fitted onto the cable bolt outside the rock face such that the sleeve will only yield under a load exceeding a threshold force. In the event of a rock burst or a substantial displacement of rock, the rock will push outward against a washer plate sandwiched between the rock face and the sleeve. If the load exceeds the threshold force, the sleeve will controllably yield by slipping over the cable bolt until the sleeve abuts a stopper mounted to the cable bolt. The yieldable rock fastener system thus absorbs and controls rock bursts and other rock strata movements, thereby inhibiting cave-ins and collapses.
_ g _ FIG. 1 illustrates a yieldable rock fastener system generally designated by reference numeral 1 in accordance with an embodiment of the present invention. The rock fastener system 1 has an elongated structural member 2 which is inserted into a hole 4 bored into the rock of a tunnel or mine. The elongated structural member 2 is bonded to the surrounding rock with grout 6 as will be described below.
The elongated structural member 2 is preferably a cable bolt having seven strands, although persons of ordinary skill in the art will readily appreciate that cable bolts having a greater number of strands (or fewer strands) may be used. Furthermore, although a cable bolt represents the best mode known to the applicant of implementing this yieldable rock fastener system, other types of elongated structural members, such as rebar or rock bolt, may be utilized for lower-stress applications.
Preferably, the cable bolt is pre-stressed, degreased, plain or galvanized Grade 270 ASTM A-416 having a total diameter (i.e., including all seven strands) of 0.6'° (or alternatively of 0.5") and having an ultimate tensile strength of about 29-30 tons. When the cable bolt is to be anchored with resin grout, the cable bolt is typically 6' to 20' in length and is bulged to mix the resin. When the cable bolt is anchored with cement grout, the cable bolt is typically 10' to 30' in length, but could be much longer because the cement grout is pumped into the drill hole.
Cable bolts that are anchored with cement grout can be plain, have deformed portions, or have buttons (sleeves) pressed onto the cable bolt along the length. A person skilled in the art will appreciate that these dimensions could be varied as could the type of steel used.
The cable bolt is preferred because it has an ultimate tensile strength of 29-30 tons in comparison to a rock bolt which wi7_1 yield at about 8-9 tons or a 3/4"
rebar bolt which will yield at approximately 12-14 tons.
Even if the sleeve yieldably slides into abutment with the stopper, the fastener still retains 30 tons of rock-supporting capacity. Furthermore, the cable bolt is preferred because its surface is smoother than a rebar bolt whose surface tends to inhibit smooth slippage of the press-fitted sleeve. Though rock bolt material is smooth, it does not provide a suitable means to mix the resin grout unlike the bulges of the cable bolt. Therefore, not only does cable bolt exhibit superior strength, as compared to rebar or rock bolt, but the bulges on the cable bolt also facilitate mixing of the high-strength resin grout.
Moreover, the surface characteristics of mufti-strand cable bolt allow the sleeve to slip in a manner that is much more predictable than slippage over rebar or rock bolt.
As shown in FIG. 1, the cable bolt 2 has a rock-penetrating portion 3 which is inserted into the rock and a protruding portion 5 that protrudes from the rock and to which are attached various components, which will be described below. The rock-penetrating portion 3 has a sharply cut forward end 8 to facilitate insertion of the rock-penetrating portion 3 of the cable bolt 2 into the hole 4. The forward end 8 is preferably cut to an angle of about 45 degrees.
As illustrated in FIG. 2, the cable bolt 2 has a plurality of bulges 10 at regular intervals along the length of the rock-penetrating portion 3 of the cable bolt 2, although cable bolts without bulges may also be used.
These bulges are, however, preferred because they help to mix the grout, or "grouting agent", in order to enhance bonding between the cable bolt and the surrounding rock.
Preferably, the grout 5 is a high-strength resin, such as the Ground-LokT'" resin manufactured and sold by Ground Control (Sudbury) Ltd. of Sudbury, Ontario, Canada. When a cable bolt is installed in a borehole using resin in cartridge form, the cable bolt must be spun to mix the resin. Alternatively, the cable bolt may be bonded inside the borehole using cement grout. The cement grout is pumped into the borehole after the cable bolt has been inserted. The cement grout is transferred to the borehole via a grout tube and. is pumped into the borehole to totally fill the void surrounding the cable bolt. In most installations, the borehole is completely filled with the grouting agent, be it resin grout or cement grout.
However, when cement grout is used, there is generally no need to spin the cable bolt or to cut the end of the cable.
As will be described in more ample detail below, it is sometimes advantageous to "uncouple" a portion of the cable bolt from the grouting agent surrounding the cable bolt using grease or a plastic sheath.
Referring back to FIG. 1, the yieldable rock fastener system 1 includes a cylindrical steel sleeve 20 which is press-fitted to the protruding portion of the cable bolt 2 using a 600-ton hydraulic press. In other words, the sleeve (or "button") 20 is plastically compressed onto the cable bolt, thereby creating a high-strength interference fit. After press-fitting, the sleeve 20 can only slip over the cable bolt 2 if the rock exerts a large enough force on the sleeve, i.e., a "rock-pressure"
force greater than a predetermined threshold force. The threshold force is "predetermined" in that a predictable relationship exists between (i) the compressive force used fox press-fitting the sleeve and the sleeve's mechanical properties and dimensions and (ii) the force that will be required to dislodge the press-fitted sleeve over the cable bolt.
Preferably, the sleeve 20 is made of 12814 steel.
The sleeve 20 preferably has a length of 2 inches, a diameter of 2 inches and a bore of 43/64" (before press-fitting). When press-fitted onto the cable bolt with a 600-ton hydraulic press, the sleeve 20 will predictably dislodge and begin to slip over the cable bolt (i.e.
"yield") at an applied load of approximately 12 tons. As will be appreciated by persons skilled in the art; the outer shape of the sleeve need not be cylindrical and the outer dimensions may be varied.
After the sleeve is press-fitted to the cable bolt (which is previously done by the manufacturer at an assembly plant to meet the end user's specifications), a washer plate 13 is slid over the cable bolt 2 and the cable bolt 2 is inserted into the bore hole 4. The cable bolt 2 is then grouted into position inside the bore hole 4 using resin grout or cement grout. The washer plate l3 is then secured against a rock face 12 so that the washer plate is sandwiched between the sleeve 20 and the rock face 12. The washer. plate 13 has a central aperture with a diameter large enough to permit the washer plate 13 to be slid onto the cable bolt 2 prior to press-fitting the sleeve 20. The washer plate 13 transfers force from the rock face 12 to the sleeve 20. Although the washer plate 13 is preferably a 6-inch square plate with a thickness of 0.25 inches, a washer plate having other dimensions may be, of course, be used provided it is sufficiently strong to withstand the stresses imposed by the adjacent rock.
As shown in FIG. 1, a cylindrical stopper 14 is mounted to the protruding portion 5 of the cable bolt 2.
As shown in FIG. 1, the stopper 14 is spaced apart from the sleeve 20 thereby defining a yielding distance 22 through which the sleeve may slip. The yielding distance 22 corresponds to the maximum permissible longitudinal rock displacement that is tolerated by the rock fastener system 1 before the cable bolt 2 begins to bear the full tensile load.
The stopper 14 preferably includes a cable grip 17 and a mating barrel 15. As shown in FIG. 3, the barrel 15 is a cylinder with a conical cavity 16. The cable grip 17.
has two symmetrical conical wedges 17a, although three or more conical wedges could be employed. Since both the conical wedges 17a and the conical cavity 16 have an angle of 7.5 degrees, the barrel may be snugly fitted over the conical wedges 17a to ensure that the cable grip 17 tightly grasps the cable bolt 2. When the rock fastener system 1 yields, the sleeve slips from the position shown in FIG. 4 to the position shown in FIG. 5, that is, the sleeve displaces into abutment with the stopper. Due to the design of the stopper, the barrel exerts a force on the conical wedges of the cable grip. Because of the angled interface between the barrel and the conical wedges, a vector component of the force acting on the conical wedges is radially inward, thereby forcing the conical wedges of the cable grip against the cable bolt. Thus, the more longitudinal force exerted by the sleeve on the stopper, the more tightly the cable grip grasps the cable bolt. The design of the stopper ensures that the stopper is not displaced by the force exerted by the sleeve on the stopper. As shown in FIG. 4 and FIG. 5, a retaining ring 19 may be affixed to the conical wedges 17a of the cable grip 17. Persons of .ordinary skill in the art will appreciate that the stopper need not be cylindrical.
Moreover, the stopper need not have a barrel and conical cable grip as described above; other components can be substituted to provide a stopper or abutment for the sleeve.
In operation, when a dynamic load due. to a rock burst or a static load due to closure imparts a force on the washer plate 13, the sleeve 20 will only yield ("dislodge" or "slip") if the "rock-pressure" force exceeds the predetermined threshold force. FIG. 4 shows the rock fastener system 1 in its initially installed (°'unyielded") position. FIG. 5 shows the rock fastener system 1 after it has yielded, or "slipped". The sleeve 20 will slip until it abuts the stopper 14 at which point the cable bolt 2 will bear the full tensile load. If the load generated by the rock burst or closure exceeds the ultimate tensile strength of the cable bolt, then the cable bolt will fail.
As shown in FIG. l, a tool attachment point 18 is provided at the end of the protruding portion 5 of the cable bolt 2. The tool attachment point 18 enables a tool to be connected to the cable bolt 2 so that the cable bolt 2 can be rotated through the resin 6 in order to ensure as complete intermingling of the resin grout 6 around the cable bolt 2. The separated strands in the various bulges 10 of the cable bolt further enhance bonding. The tool attachment point may be designed to connect to any one of a number of known tools, e.g. via threads, hexagonal or square head, or any other means that permit a tool to be detachably connected for "spinning" (i.e. rotating) the cable bolt through the resin grout. A drill or rotational impact tool can be used to spin the cable bolt.
The rock fastener system 1 can be used in a method of yieldably fastening rock in a mine or underground tunnel.
First, the hole 4 of suitable diameter and depth is drilled into the rock face 7. Second, the grout 6 is inserted into the hole 4. Typically, the resin grout is contained in one or more resin cartridges that are stuffed into the hole 4.
The cartridges may contain two compartments, each containing a complementary ingredient,' which combine to form the resin grout 6. Alternatively, the grout 6 may be pre-mixed and pumped into the hole 4 around the cable bolt 2.
As noted abave, the yieldable rock fastener system 1 is preassembled to include the bulged cable bolt 2 and the pressed-on sleeve 20 based on customers' specifications. In the field, the customer (end user) simply inserts the :preassembled cable bolt into the drill hole and anchors it in the hole using resin grout or cement grout. The washer plate is slid over the cable bolt prior to inserting the cable bolt into the drill hole.
The rock-penetrating portion 3 of the cable bolt 2 is then inserted into the hole 4 until the washer plate 13 abuts the rock face 12. The sharply cut forward end 8 of the cable bolt 2 helps the rock-penetrating portion 3 penetrate into the hole 4, puncturing and tearing the compartments of the resin cartridges, thereby enabling the components of the resin grout 6 to mix. The cable bolt is spun to enhance mixing and bonding of the grout. The rock fastener system is then held still while the resin cures, forming a solid bond between the rock-penetrating portion 3 of the cable bolt 2 and the rock surrounding the hole 4.
The yieldable rock fastener system 1 allows yielding over a long range, thus accommodating substantial displacements of rack strata. Furthermore, since the sleeve is external to the rock face, slippage or "yielding"
of the sleeve 20 in response to a load in excess of the predetermined threshold force will be visible to miners.
In other words, the slipped sleeve will serve as an indicator of strain so that a dangerous situation may be redressed by further buttressing the tunnel or evacuating personnel from the tunnel. Optionally, the sleeve displacement can be made more visible by adding visual cues, such as colours or other markings on the cable bolt in the yielding distance 22 between the sleeve 20 and the stopper 14. Optionally, a displacement sensor or transducer may be added to generate and transmit an electrical signal to a computerized monitoring system or to an alarm. For example, the alarm may be sounded if the sleeve reaches the abutment or if the rate of yielding is too rapid.
The yieldable rock fastener system thus provides an easily installed ground support system that yields controllably and gradually when a predetermined threshold force has been exceeded. The component dimensions, materials, and other system parameters can be tailored to provide a predetermined threshold force and yielding distance that optimize mine safety for a variety of rock conditions.
Further embodiments of the present invention are illustrated in FIGS. 6-8 in which a bulged or plain ("unbluged") cable bolt is designed to yieldably slide relative to a pressed-on sleeve that is, in turn, grouted inside the hole (or "borehole"). In other words, the embodiments shown in FIGS. 6-8 utilize an internally secured sleeve grouted inside the borehole (in lieu of, or in addition to, a movable sleeve pressed onto the cable bolt outside the borehole, such as was illustrated with respect to the embodiment of FIGS. 1-5.) However, the degree of "yieldability" in all four of these embodiments is primarily determined by the press-fit (or interference fit) between the sleeve and the cable bolt as well as the sleeve's dimensions, which determine the load at which the sleeve or cable bolt slips relative to the other and the rate at which such slippage occurs.
FIG. 6 illustrates schematically a second embodiment of a yieldable rock fastener system 1 having an elongated structural member such as, for example, a seven-stranded, plain cable bolt 2. As was the case with the embodiment of FIGS. 1-5, the cable bolt 2 has a rock-penetrating portion adapted to extend into a hole bored in the rock and a protruding portion adapted to protrude from the hole. The rock fastener system 1 has a washer plate 13 installed against a rock face 12 so that the washer plate 13 is sandwiched between an externally mounted stopper 14 and the rock face 12. The washer plate 13 has a central aperture with a diameter large enough to permit the washer plate 13 to be slid onto the cable bolt 2 prior to attaching the stopper. The washer plate l3 transfers force from the rock face 12 to the stopper 14. Although the washer plate 13 is preferably a 6-inch square plate with a thickness of 0.25 inches, a washer. plate having other dimensions may be, of course, be used provided it is sufficiently strong to withstand the stresses imposed by the adjacent rock.
As described above with reference with FIG. 3, the stopper l4 has a cable grip 1'7 and a mating barrel 15. The barrel 15 is a cylinder with a conical cavity 16. The _ 1~._ cable grip 17 has two symmetrical conical wedges 17a, although three or more conical wedges could be employed.
As, shown in FIG. 6, a retaining ring 19 may also be affixed to the conical wedges 17a of the cable grip 17. While the cable grip represents the best mode of implementing a stopper for use in the present invention, persons of ordinary skill in the art will readily appreciate that other types of stoppers may be utilized without departing from the spirit and scope of the present invention.
Still referring to FIG. 6, the rock fastener system 1 has an internally secured sleeve 24 which is permanently grouted inside the borehole 4 with grout or resin 6 (as was described above). The internal sleeve 24 is press-fitted onto the rock-penetrating portion of the cable bolt 2 using a 600-ton hydraulic press. It should be noted that the internal sleeve 24 of this second embodiment is, in most cases, different from the (external) sleeve 20 of the first embodiment. Although the material of the internal sleeve 24 and the material of the (external) sleeve 20 are generally the same, the length, outer diameter and inner diameter of the internal sleeve 24 are usually different from those of the (external) sleeve 20. The dimensions of the internal sleeve 24 may be varied in order to increase or decrease the predetermined threshold force at which the cable bolt begins to yieldably slip relative to the grouted-in sleeve. For example, as the inner diameter of the internal sleeve 24 is reduced, the predetermined threshold force is increased. Likewise, as the length of the internal sleeve 24 is increased, the predetermined threshold force is also increased. The outer diameter of the internal sleeve 24 is selected to optimize the grouting of the sleeve to the surrounding rock in the bore hole 4.
_ 18 -As shown in FIG. 6, a slip agent such as a plastic sheath 26 (or alternatively a coating of grease or inert resin) can be used to '°uncouple" the cable bolt from the bore hole to facilitate displacement of the cable bolt within the borehole when the cable bolt 2 yieldably slips relative to the internal sleeve 24. In operation, after a bore hole 4 is drilled, the cable bolt 2 (with internal sleeve 24 already pressed on) is inserted into the bore hole 4 and grouted permanently into position using pre-inserted resin grout packages in the, manner already described above. The washer plate 13 is then sandwiched between the rock face 12 and the stopper 14 whose cable grip 17 and barrel 15 are made snug against the washer plate. As described above, the more force applied to the barrel, the more the barrel bears against the cable grip, the more the cable grip grasps the cable bolt 2.
Therefore, the "gripping force" of the cable grip and barrel is designed to exceed the predetermined threshold force above which the internal sleeve 24 begins to yieldably slip relative to the cable bolt 2. In other words, the rock fastener system 1 is designed to translate forces acting on the washer plate l3 and stopper 14 into a "pulling force" on the cable bolt 2 which, when it exceeds the predetermined threshold force, causes the cable bolt 2 to be pulled through the internal sleeve 24. In summation, therefore, when a rock burst or other displacement of rock occurs, the cable bolt 2 will yieldably slip relative to the grouted-in internal sleeve 24. As noted above, the cable bolt will only begin to yieldably slip when the forces exerted on the washer plate via the rock face exceed the predetermined threshold force.
FIG. 7 illustrates schematically a third embodiment of the present invention in which a pair of internal sleeves 24 are grouted with resin grout 6 inside the bore hole 4. As in the second embodiment, the rock fastener system 1 of this third embodiment uses a plastic sheath 26 (or grease or inert resin) at various sections along the rock-penetrating portion of the cable bolt to "uncouple" the cable bolt 2 from the bore hole 4. As in the second embodiment, the stopper 14 (having cable grip 17 and barrel 15) snugly presses the washer plate 13 against the rock face 12.
When a rock burst or other rock displacement occurs, the rock-displacement forces are transferred through the washer plate 13 to the cable bolt 2 via the barrel 15 and cable grip 17. Consequently, the rock displacement is resisted by the cable bolt 2 anchored within the borehole 4. When the force of the rock face 12 acting on the washer plate l3 and stopper 14 (and thus the force pulling on the cable' bolt 2) exceeds the predetermined threshold force of both internal sleeves 24, the cable bolt will begin to yieldably slip relative to the immobilized internal sleeves 24. When the cable bolt is retained by both internal sleeves 24, the predetermined threshold force is thus a function of the total length of both internal sleeves 24. However, when the cable bolt 2 slips through the first of the two internal sleeves, the force retaining the cable bolt drops because only the second internal sleeve is resisting pull-out of the cable bolt. The force resisting pull-out of the cable bolt drops linearly as the cable bolt is pulled through each successive internal sleeve.
Although the internal sleeves 24 are shown to be identical in FIG. 6, it is to be expressly understood that the internal sleeves 24 need not be identical.
Furthermore, persons of ordinary skill in the art will readily appreciate that the two internal sleeves of this third embodiment can be extrapolated into other embodiments having three or more internal sleeves, which may or may not be identical and which may or may not be equally interspersed within the bore hole. Regardless of the number of internal sleeves, the dimensions can be altered to augment or diminish the total predetermined threshold force resisting pull-out of the cable bolt.
FIG. 8 illustrates schematically a fourth embodiment of the present invention in which both an internal sleeve 24 and an external sleeve 20 are press-fitted onto the cable bolt 2. This configuration represents a hybrid of the first embodiment and the second embodiment and therefore provides for compound motion, meaning that the yieldability of this system is a function of both the slippage of the external sleeve relative to the cable bolt and the slippage of the cable bolt relative to the internal sleeve. As will be appreciated by those of ordinary skill in the art, the predetermined threshold force required to the cause slippage of the cable bolt relative to the internal sleeve is not necessarily the same as the predetermined threshold force required to cause slippage of the external sleeve relative to the cable bolt.
In other words, the predetermined threshold force corresponding to the internal sleeve and that corresponding to the external sleeve may be tailored to provide desired yielding characteristics. For example, the rock fastener system could be designed such that the cable bolt yieldably slips relative to the internal sleeve at the same time as the external sleeve yieldably slips relative to the cable bolt. Alternatively, the rock fastener system could be designed so that first the external sleeve yieldably slips relative to the cable bolt until the external sleeve abuts the stopper 14 at which point the cable bolt begins to yieldably slip relative to the internal sleeve.
As a further variant, one or more of the bulges 10 of the bulged cable bolt 2 could be pre-filled W th a compressible filler which would prevent the grout 6 from entering the void (visible in FIG. 2) that is created when the cable bolt is bulged. The void could be pre-filled with one of a variety of fillers exhibiting various degrees of compressibility. The bulge could then act as a restriction which is forcibly compressed as the bulge is dragged or pulled through the internal sleeve 24. If the void in the bulge becomes filled with grout, the grouted bulge, being practically incompressible, obstructs the slippage of the cable bolt relative to the internal sleeve as soon as the bulge encounters the internal sleeve. If, however, the bulge is pre-filled with a compressible filler, the filler prevents the grout from entering the void in the bulge. The bulge is thus able to constrict to its unbulged diameter for controlled slippage through the internal sleeve. In this variant, the predetermined threshold force above which the cable bolt will yieldably slip relative to the internal sleeve will vary not only with the strength of the press fit and the dimensions of the sleeve, but also with the compressibility of the compressible filler and the resistance of the bulge to be constricted as it is pulled through the internal sleeve.
The resistance of the bulge to being constricted is a function of a variety of factors, including the modulus of elasticity of the strands of the cable bolt, their orientation as well as the degree of plastic deformation incurred during the bulging process.
In summary, therefore, the rock fastener system can be configured with an external sleeve press-fitted onto a plain or bulged cable bolt with a yield distance limited by a barrel and cable grip stopper assembly.
Alternatively, the rock fastener system can be configured with an internal sleeve press-fitted onto a plain or bulged cable bolt such that the internal sleeve is grouted in the borehole while a barrel and cable grip stopper assembly is attached external to the borehole. Finally, the rock fastener system can be configured with both an internal sleeve and an external sleeve press-fitted onto a plain or bulged cable bolt.
It will be apparent to those skilled in this art that various modifications and variations may be made to the embodiment disclosed herein without departing from the spirit and scope of the present invention. Accordingly, the embodiments of the invention described above are intended to be exemplary only. Those skilled in the art will therefore appreciate that the forgoing description is illustrative only, and that various alternatives and modifications can be devised without departing from the spirit of the present invention. Accordingly, the present is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended 2S claims.
The method includes the steps of boring a hole in the rock;
inserting grout into the hole; inserting an elongated structural member into the grouted hole; press-fitting a sleeve onto the elongated structural member to create a high-strength interference fit which only yields under an applied force exceeding a predetermined threshold force;
and affixing a stopper to the elongated structural member.
The stopper is spaced apart from the sleeve to define a yielding distance through which the sleeve can yield when a load exceeding the predetermined threshold force is exerted on the sleeve.
A third aspect of the present invention provides a rock fastener system for yieldable support of a tunnel or mine, the rock fastener system including an elongated structural member having a rock-penetrating portion adapted to extend into a hole bored in the rock and a protruding portion adapted to protrude from the hole. The system also includes a, sleeve press-fitted onto the elongated structural member in a high-strength interference fit that only yields if a load exceeding a predetermined threshold force is applied to the sleeve.
A fourth aspect of the present invention provides a rock fastener system for yieldable support of a tunnel or mine. The rock fastener system includes an elongated structural member having a rock-penetrating portion adapted to extend into a hole bored in the rock; and a protruding portion adapted to protrude from the hole. The system also includes an internal sleeve press-fitted onto the rock-penetrating portion of the elongated structural member in a high-strength interference fit that only yields if a load exceeding a first predetermined threshold force is applied to the elongated structural member; and an external sleeve press-fitted onto the protruding portion of the elongated structural member in a high-strength interference fit that only yields if a load exceeding a second predetermined threshold force is applied to the external sleeve.
Preferably, the rock fastener system includes a cable bolt to which an internal and/or external sleeve is press-fitted. Where the sleeve is mounted internal to the borehole, the sleeve is grouted permanently into position so that the cable bolt can yieldably slip relative to the grouted-in internal sleeve. Where the sleeve is mounted external to the borehole, the sleeve can yieldably slip relative to the cable bolt which is itself grouted inside the borehole.
The various embodiments of the present invention overcome at least some of the deficiencies of the prior art. In the first embodiment, where an external sleeve is press-fitted on a protruding portion of the cable bolt, the rock fastener provides a useful visual cue to miners that a rock shift has caused the rock fastener to yield.
Furthermore, the threshold force at which the fastener will yield can be predetermined. The predetermined threshold force can thus be tailored to specific applications based upon rock conditions. The yieldable rock fastener therefore permits significant controlled strata movement over a predetermined yielding distance without fully stressing the cable bolt. The yieldable rock fastener yields slowly and controllably while maintaining a predetermined resistive force on the shifting rock strata.
The yieldable fastener controls both gradual rock movements due to closure as well as violent displacements due to rock bursts.
The second, third and fourth embodiments, where an internal sleeve is grouted inside the borehole, provide a versatile, relatively inexpensive, easy-to-manufacture rock bolt system that can be tailored to a variety of applications where smooth, predictable yielding performance is desirable.
BRIEF DESCRIPTION OF THE DRAWINGS
Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which:
FIG. 1 is a schematic side view of a rock fastener system in accordance with an embodiment of the present invention;
FIG. 2 is a partial view of the bulged cable bolt used in the rock fastener system shown in FIG. 1;
FIG. 3 is a perspective view of the components of a stopper for use in the embodiment of FIG. 1, namely a cable grip having a pair of conical wedges and a barrel which forces the conical wedges to grasp the cable bolt;
FIG. 4 is a schematic side view of the rock fastener system in its initial, "unyielded" configuration;
FIG. 5 is a schematic side view of the rock fastener system after yieldably displacing to the stopper;
FIG. 6 is a schematic side view of a rock fastener system in accordance with a second embodiment of the present invention;
FIG. 7 is a schematic side view of a rock fastener system in accordance with a third embodiment of the present invention; and FIG. 8 is a schematic side view of a rock fastener system in accordance with a fourth embodiment of the present invention.
It will be noted that throughout the appended drawings, like features are identified by like reference numerals. The drawings are not to scale.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIGS. 1 to 5 illustrate a yieldable rock fastener system and method in accordance with an embodiment of the present invention. In general, the yieldable rock fastener system has a pre-stressed seven-strand cable bolt with bulges at regular intervals. The cable bolt is inserted into a hole bored into the rock face of a mine or tunnel.
The cable bolt is banded to the rock with grout. A sleeve is press-fitted onto the cable bolt outside the rock face such that the sleeve will only yield under a load exceeding a threshold force. In the event of a rock burst or a substantial displacement of rock, the rock will push outward against a washer plate sandwiched between the rock face and the sleeve. If the load exceeds the threshold force, the sleeve will controllably yield by slipping over the cable bolt until the sleeve abuts a stopper mounted to the cable bolt. The yieldable rock fastener system thus absorbs and controls rock bursts and other rock strata movements, thereby inhibiting cave-ins and collapses.
_ g _ FIG. 1 illustrates a yieldable rock fastener system generally designated by reference numeral 1 in accordance with an embodiment of the present invention. The rock fastener system 1 has an elongated structural member 2 which is inserted into a hole 4 bored into the rock of a tunnel or mine. The elongated structural member 2 is bonded to the surrounding rock with grout 6 as will be described below.
The elongated structural member 2 is preferably a cable bolt having seven strands, although persons of ordinary skill in the art will readily appreciate that cable bolts having a greater number of strands (or fewer strands) may be used. Furthermore, although a cable bolt represents the best mode known to the applicant of implementing this yieldable rock fastener system, other types of elongated structural members, such as rebar or rock bolt, may be utilized for lower-stress applications.
Preferably, the cable bolt is pre-stressed, degreased, plain or galvanized Grade 270 ASTM A-416 having a total diameter (i.e., including all seven strands) of 0.6'° (or alternatively of 0.5") and having an ultimate tensile strength of about 29-30 tons. When the cable bolt is to be anchored with resin grout, the cable bolt is typically 6' to 20' in length and is bulged to mix the resin. When the cable bolt is anchored with cement grout, the cable bolt is typically 10' to 30' in length, but could be much longer because the cement grout is pumped into the drill hole.
Cable bolts that are anchored with cement grout can be plain, have deformed portions, or have buttons (sleeves) pressed onto the cable bolt along the length. A person skilled in the art will appreciate that these dimensions could be varied as could the type of steel used.
The cable bolt is preferred because it has an ultimate tensile strength of 29-30 tons in comparison to a rock bolt which wi7_1 yield at about 8-9 tons or a 3/4"
rebar bolt which will yield at approximately 12-14 tons.
Even if the sleeve yieldably slides into abutment with the stopper, the fastener still retains 30 tons of rock-supporting capacity. Furthermore, the cable bolt is preferred because its surface is smoother than a rebar bolt whose surface tends to inhibit smooth slippage of the press-fitted sleeve. Though rock bolt material is smooth, it does not provide a suitable means to mix the resin grout unlike the bulges of the cable bolt. Therefore, not only does cable bolt exhibit superior strength, as compared to rebar or rock bolt, but the bulges on the cable bolt also facilitate mixing of the high-strength resin grout.
Moreover, the surface characteristics of mufti-strand cable bolt allow the sleeve to slip in a manner that is much more predictable than slippage over rebar or rock bolt.
As shown in FIG. 1, the cable bolt 2 has a rock-penetrating portion 3 which is inserted into the rock and a protruding portion 5 that protrudes from the rock and to which are attached various components, which will be described below. The rock-penetrating portion 3 has a sharply cut forward end 8 to facilitate insertion of the rock-penetrating portion 3 of the cable bolt 2 into the hole 4. The forward end 8 is preferably cut to an angle of about 45 degrees.
As illustrated in FIG. 2, the cable bolt 2 has a plurality of bulges 10 at regular intervals along the length of the rock-penetrating portion 3 of the cable bolt 2, although cable bolts without bulges may also be used.
These bulges are, however, preferred because they help to mix the grout, or "grouting agent", in order to enhance bonding between the cable bolt and the surrounding rock.
Preferably, the grout 5 is a high-strength resin, such as the Ground-LokT'" resin manufactured and sold by Ground Control (Sudbury) Ltd. of Sudbury, Ontario, Canada. When a cable bolt is installed in a borehole using resin in cartridge form, the cable bolt must be spun to mix the resin. Alternatively, the cable bolt may be bonded inside the borehole using cement grout. The cement grout is pumped into the borehole after the cable bolt has been inserted. The cement grout is transferred to the borehole via a grout tube and. is pumped into the borehole to totally fill the void surrounding the cable bolt. In most installations, the borehole is completely filled with the grouting agent, be it resin grout or cement grout.
However, when cement grout is used, there is generally no need to spin the cable bolt or to cut the end of the cable.
As will be described in more ample detail below, it is sometimes advantageous to "uncouple" a portion of the cable bolt from the grouting agent surrounding the cable bolt using grease or a plastic sheath.
Referring back to FIG. 1, the yieldable rock fastener system 1 includes a cylindrical steel sleeve 20 which is press-fitted to the protruding portion of the cable bolt 2 using a 600-ton hydraulic press. In other words, the sleeve (or "button") 20 is plastically compressed onto the cable bolt, thereby creating a high-strength interference fit. After press-fitting, the sleeve 20 can only slip over the cable bolt 2 if the rock exerts a large enough force on the sleeve, i.e., a "rock-pressure"
force greater than a predetermined threshold force. The threshold force is "predetermined" in that a predictable relationship exists between (i) the compressive force used fox press-fitting the sleeve and the sleeve's mechanical properties and dimensions and (ii) the force that will be required to dislodge the press-fitted sleeve over the cable bolt.
Preferably, the sleeve 20 is made of 12814 steel.
The sleeve 20 preferably has a length of 2 inches, a diameter of 2 inches and a bore of 43/64" (before press-fitting). When press-fitted onto the cable bolt with a 600-ton hydraulic press, the sleeve 20 will predictably dislodge and begin to slip over the cable bolt (i.e.
"yield") at an applied load of approximately 12 tons. As will be appreciated by persons skilled in the art; the outer shape of the sleeve need not be cylindrical and the outer dimensions may be varied.
After the sleeve is press-fitted to the cable bolt (which is previously done by the manufacturer at an assembly plant to meet the end user's specifications), a washer plate 13 is slid over the cable bolt 2 and the cable bolt 2 is inserted into the bore hole 4. The cable bolt 2 is then grouted into position inside the bore hole 4 using resin grout or cement grout. The washer plate l3 is then secured against a rock face 12 so that the washer plate is sandwiched between the sleeve 20 and the rock face 12. The washer. plate 13 has a central aperture with a diameter large enough to permit the washer plate 13 to be slid onto the cable bolt 2 prior to press-fitting the sleeve 20. The washer plate 13 transfers force from the rock face 12 to the sleeve 20. Although the washer plate 13 is preferably a 6-inch square plate with a thickness of 0.25 inches, a washer plate having other dimensions may be, of course, be used provided it is sufficiently strong to withstand the stresses imposed by the adjacent rock.
As shown in FIG. 1, a cylindrical stopper 14 is mounted to the protruding portion 5 of the cable bolt 2.
As shown in FIG. 1, the stopper 14 is spaced apart from the sleeve 20 thereby defining a yielding distance 22 through which the sleeve may slip. The yielding distance 22 corresponds to the maximum permissible longitudinal rock displacement that is tolerated by the rock fastener system 1 before the cable bolt 2 begins to bear the full tensile load.
The stopper 14 preferably includes a cable grip 17 and a mating barrel 15. As shown in FIG. 3, the barrel 15 is a cylinder with a conical cavity 16. The cable grip 17.
has two symmetrical conical wedges 17a, although three or more conical wedges could be employed. Since both the conical wedges 17a and the conical cavity 16 have an angle of 7.5 degrees, the barrel may be snugly fitted over the conical wedges 17a to ensure that the cable grip 17 tightly grasps the cable bolt 2. When the rock fastener system 1 yields, the sleeve slips from the position shown in FIG. 4 to the position shown in FIG. 5, that is, the sleeve displaces into abutment with the stopper. Due to the design of the stopper, the barrel exerts a force on the conical wedges of the cable grip. Because of the angled interface between the barrel and the conical wedges, a vector component of the force acting on the conical wedges is radially inward, thereby forcing the conical wedges of the cable grip against the cable bolt. Thus, the more longitudinal force exerted by the sleeve on the stopper, the more tightly the cable grip grasps the cable bolt. The design of the stopper ensures that the stopper is not displaced by the force exerted by the sleeve on the stopper. As shown in FIG. 4 and FIG. 5, a retaining ring 19 may be affixed to the conical wedges 17a of the cable grip 17. Persons of .ordinary skill in the art will appreciate that the stopper need not be cylindrical.
Moreover, the stopper need not have a barrel and conical cable grip as described above; other components can be substituted to provide a stopper or abutment for the sleeve.
In operation, when a dynamic load due. to a rock burst or a static load due to closure imparts a force on the washer plate 13, the sleeve 20 will only yield ("dislodge" or "slip") if the "rock-pressure" force exceeds the predetermined threshold force. FIG. 4 shows the rock fastener system 1 in its initially installed (°'unyielded") position. FIG. 5 shows the rock fastener system 1 after it has yielded, or "slipped". The sleeve 20 will slip until it abuts the stopper 14 at which point the cable bolt 2 will bear the full tensile load. If the load generated by the rock burst or closure exceeds the ultimate tensile strength of the cable bolt, then the cable bolt will fail.
As shown in FIG. l, a tool attachment point 18 is provided at the end of the protruding portion 5 of the cable bolt 2. The tool attachment point 18 enables a tool to be connected to the cable bolt 2 so that the cable bolt 2 can be rotated through the resin 6 in order to ensure as complete intermingling of the resin grout 6 around the cable bolt 2. The separated strands in the various bulges 10 of the cable bolt further enhance bonding. The tool attachment point may be designed to connect to any one of a number of known tools, e.g. via threads, hexagonal or square head, or any other means that permit a tool to be detachably connected for "spinning" (i.e. rotating) the cable bolt through the resin grout. A drill or rotational impact tool can be used to spin the cable bolt.
The rock fastener system 1 can be used in a method of yieldably fastening rock in a mine or underground tunnel.
First, the hole 4 of suitable diameter and depth is drilled into the rock face 7. Second, the grout 6 is inserted into the hole 4. Typically, the resin grout is contained in one or more resin cartridges that are stuffed into the hole 4.
The cartridges may contain two compartments, each containing a complementary ingredient,' which combine to form the resin grout 6. Alternatively, the grout 6 may be pre-mixed and pumped into the hole 4 around the cable bolt 2.
As noted abave, the yieldable rock fastener system 1 is preassembled to include the bulged cable bolt 2 and the pressed-on sleeve 20 based on customers' specifications. In the field, the customer (end user) simply inserts the :preassembled cable bolt into the drill hole and anchors it in the hole using resin grout or cement grout. The washer plate is slid over the cable bolt prior to inserting the cable bolt into the drill hole.
The rock-penetrating portion 3 of the cable bolt 2 is then inserted into the hole 4 until the washer plate 13 abuts the rock face 12. The sharply cut forward end 8 of the cable bolt 2 helps the rock-penetrating portion 3 penetrate into the hole 4, puncturing and tearing the compartments of the resin cartridges, thereby enabling the components of the resin grout 6 to mix. The cable bolt is spun to enhance mixing and bonding of the grout. The rock fastener system is then held still while the resin cures, forming a solid bond between the rock-penetrating portion 3 of the cable bolt 2 and the rock surrounding the hole 4.
The yieldable rock fastener system 1 allows yielding over a long range, thus accommodating substantial displacements of rack strata. Furthermore, since the sleeve is external to the rock face, slippage or "yielding"
of the sleeve 20 in response to a load in excess of the predetermined threshold force will be visible to miners.
In other words, the slipped sleeve will serve as an indicator of strain so that a dangerous situation may be redressed by further buttressing the tunnel or evacuating personnel from the tunnel. Optionally, the sleeve displacement can be made more visible by adding visual cues, such as colours or other markings on the cable bolt in the yielding distance 22 between the sleeve 20 and the stopper 14. Optionally, a displacement sensor or transducer may be added to generate and transmit an electrical signal to a computerized monitoring system or to an alarm. For example, the alarm may be sounded if the sleeve reaches the abutment or if the rate of yielding is too rapid.
The yieldable rock fastener system thus provides an easily installed ground support system that yields controllably and gradually when a predetermined threshold force has been exceeded. The component dimensions, materials, and other system parameters can be tailored to provide a predetermined threshold force and yielding distance that optimize mine safety for a variety of rock conditions.
Further embodiments of the present invention are illustrated in FIGS. 6-8 in which a bulged or plain ("unbluged") cable bolt is designed to yieldably slide relative to a pressed-on sleeve that is, in turn, grouted inside the hole (or "borehole"). In other words, the embodiments shown in FIGS. 6-8 utilize an internally secured sleeve grouted inside the borehole (in lieu of, or in addition to, a movable sleeve pressed onto the cable bolt outside the borehole, such as was illustrated with respect to the embodiment of FIGS. 1-5.) However, the degree of "yieldability" in all four of these embodiments is primarily determined by the press-fit (or interference fit) between the sleeve and the cable bolt as well as the sleeve's dimensions, which determine the load at which the sleeve or cable bolt slips relative to the other and the rate at which such slippage occurs.
FIG. 6 illustrates schematically a second embodiment of a yieldable rock fastener system 1 having an elongated structural member such as, for example, a seven-stranded, plain cable bolt 2. As was the case with the embodiment of FIGS. 1-5, the cable bolt 2 has a rock-penetrating portion adapted to extend into a hole bored in the rock and a protruding portion adapted to protrude from the hole. The rock fastener system 1 has a washer plate 13 installed against a rock face 12 so that the washer plate 13 is sandwiched between an externally mounted stopper 14 and the rock face 12. The washer plate 13 has a central aperture with a diameter large enough to permit the washer plate 13 to be slid onto the cable bolt 2 prior to attaching the stopper. The washer plate l3 transfers force from the rock face 12 to the stopper 14. Although the washer plate 13 is preferably a 6-inch square plate with a thickness of 0.25 inches, a washer. plate having other dimensions may be, of course, be used provided it is sufficiently strong to withstand the stresses imposed by the adjacent rock.
As described above with reference with FIG. 3, the stopper l4 has a cable grip 1'7 and a mating barrel 15. The barrel 15 is a cylinder with a conical cavity 16. The _ 1~._ cable grip 17 has two symmetrical conical wedges 17a, although three or more conical wedges could be employed.
As, shown in FIG. 6, a retaining ring 19 may also be affixed to the conical wedges 17a of the cable grip 17. While the cable grip represents the best mode of implementing a stopper for use in the present invention, persons of ordinary skill in the art will readily appreciate that other types of stoppers may be utilized without departing from the spirit and scope of the present invention.
Still referring to FIG. 6, the rock fastener system 1 has an internally secured sleeve 24 which is permanently grouted inside the borehole 4 with grout or resin 6 (as was described above). The internal sleeve 24 is press-fitted onto the rock-penetrating portion of the cable bolt 2 using a 600-ton hydraulic press. It should be noted that the internal sleeve 24 of this second embodiment is, in most cases, different from the (external) sleeve 20 of the first embodiment. Although the material of the internal sleeve 24 and the material of the (external) sleeve 20 are generally the same, the length, outer diameter and inner diameter of the internal sleeve 24 are usually different from those of the (external) sleeve 20. The dimensions of the internal sleeve 24 may be varied in order to increase or decrease the predetermined threshold force at which the cable bolt begins to yieldably slip relative to the grouted-in sleeve. For example, as the inner diameter of the internal sleeve 24 is reduced, the predetermined threshold force is increased. Likewise, as the length of the internal sleeve 24 is increased, the predetermined threshold force is also increased. The outer diameter of the internal sleeve 24 is selected to optimize the grouting of the sleeve to the surrounding rock in the bore hole 4.
_ 18 -As shown in FIG. 6, a slip agent such as a plastic sheath 26 (or alternatively a coating of grease or inert resin) can be used to '°uncouple" the cable bolt from the bore hole to facilitate displacement of the cable bolt within the borehole when the cable bolt 2 yieldably slips relative to the internal sleeve 24. In operation, after a bore hole 4 is drilled, the cable bolt 2 (with internal sleeve 24 already pressed on) is inserted into the bore hole 4 and grouted permanently into position using pre-inserted resin grout packages in the, manner already described above. The washer plate 13 is then sandwiched between the rock face 12 and the stopper 14 whose cable grip 17 and barrel 15 are made snug against the washer plate. As described above, the more force applied to the barrel, the more the barrel bears against the cable grip, the more the cable grip grasps the cable bolt 2.
Therefore, the "gripping force" of the cable grip and barrel is designed to exceed the predetermined threshold force above which the internal sleeve 24 begins to yieldably slip relative to the cable bolt 2. In other words, the rock fastener system 1 is designed to translate forces acting on the washer plate l3 and stopper 14 into a "pulling force" on the cable bolt 2 which, when it exceeds the predetermined threshold force, causes the cable bolt 2 to be pulled through the internal sleeve 24. In summation, therefore, when a rock burst or other displacement of rock occurs, the cable bolt 2 will yieldably slip relative to the grouted-in internal sleeve 24. As noted above, the cable bolt will only begin to yieldably slip when the forces exerted on the washer plate via the rock face exceed the predetermined threshold force.
FIG. 7 illustrates schematically a third embodiment of the present invention in which a pair of internal sleeves 24 are grouted with resin grout 6 inside the bore hole 4. As in the second embodiment, the rock fastener system 1 of this third embodiment uses a plastic sheath 26 (or grease or inert resin) at various sections along the rock-penetrating portion of the cable bolt to "uncouple" the cable bolt 2 from the bore hole 4. As in the second embodiment, the stopper 14 (having cable grip 17 and barrel 15) snugly presses the washer plate 13 against the rock face 12.
When a rock burst or other rock displacement occurs, the rock-displacement forces are transferred through the washer plate 13 to the cable bolt 2 via the barrel 15 and cable grip 17. Consequently, the rock displacement is resisted by the cable bolt 2 anchored within the borehole 4. When the force of the rock face 12 acting on the washer plate l3 and stopper 14 (and thus the force pulling on the cable' bolt 2) exceeds the predetermined threshold force of both internal sleeves 24, the cable bolt will begin to yieldably slip relative to the immobilized internal sleeves 24. When the cable bolt is retained by both internal sleeves 24, the predetermined threshold force is thus a function of the total length of both internal sleeves 24. However, when the cable bolt 2 slips through the first of the two internal sleeves, the force retaining the cable bolt drops because only the second internal sleeve is resisting pull-out of the cable bolt. The force resisting pull-out of the cable bolt drops linearly as the cable bolt is pulled through each successive internal sleeve.
Although the internal sleeves 24 are shown to be identical in FIG. 6, it is to be expressly understood that the internal sleeves 24 need not be identical.
Furthermore, persons of ordinary skill in the art will readily appreciate that the two internal sleeves of this third embodiment can be extrapolated into other embodiments having three or more internal sleeves, which may or may not be identical and which may or may not be equally interspersed within the bore hole. Regardless of the number of internal sleeves, the dimensions can be altered to augment or diminish the total predetermined threshold force resisting pull-out of the cable bolt.
FIG. 8 illustrates schematically a fourth embodiment of the present invention in which both an internal sleeve 24 and an external sleeve 20 are press-fitted onto the cable bolt 2. This configuration represents a hybrid of the first embodiment and the second embodiment and therefore provides for compound motion, meaning that the yieldability of this system is a function of both the slippage of the external sleeve relative to the cable bolt and the slippage of the cable bolt relative to the internal sleeve. As will be appreciated by those of ordinary skill in the art, the predetermined threshold force required to the cause slippage of the cable bolt relative to the internal sleeve is not necessarily the same as the predetermined threshold force required to cause slippage of the external sleeve relative to the cable bolt.
In other words, the predetermined threshold force corresponding to the internal sleeve and that corresponding to the external sleeve may be tailored to provide desired yielding characteristics. For example, the rock fastener system could be designed such that the cable bolt yieldably slips relative to the internal sleeve at the same time as the external sleeve yieldably slips relative to the cable bolt. Alternatively, the rock fastener system could be designed so that first the external sleeve yieldably slips relative to the cable bolt until the external sleeve abuts the stopper 14 at which point the cable bolt begins to yieldably slip relative to the internal sleeve.
As a further variant, one or more of the bulges 10 of the bulged cable bolt 2 could be pre-filled W th a compressible filler which would prevent the grout 6 from entering the void (visible in FIG. 2) that is created when the cable bolt is bulged. The void could be pre-filled with one of a variety of fillers exhibiting various degrees of compressibility. The bulge could then act as a restriction which is forcibly compressed as the bulge is dragged or pulled through the internal sleeve 24. If the void in the bulge becomes filled with grout, the grouted bulge, being practically incompressible, obstructs the slippage of the cable bolt relative to the internal sleeve as soon as the bulge encounters the internal sleeve. If, however, the bulge is pre-filled with a compressible filler, the filler prevents the grout from entering the void in the bulge. The bulge is thus able to constrict to its unbulged diameter for controlled slippage through the internal sleeve. In this variant, the predetermined threshold force above which the cable bolt will yieldably slip relative to the internal sleeve will vary not only with the strength of the press fit and the dimensions of the sleeve, but also with the compressibility of the compressible filler and the resistance of the bulge to be constricted as it is pulled through the internal sleeve.
The resistance of the bulge to being constricted is a function of a variety of factors, including the modulus of elasticity of the strands of the cable bolt, their orientation as well as the degree of plastic deformation incurred during the bulging process.
In summary, therefore, the rock fastener system can be configured with an external sleeve press-fitted onto a plain or bulged cable bolt with a yield distance limited by a barrel and cable grip stopper assembly.
Alternatively, the rock fastener system can be configured with an internal sleeve press-fitted onto a plain or bulged cable bolt such that the internal sleeve is grouted in the borehole while a barrel and cable grip stopper assembly is attached external to the borehole. Finally, the rock fastener system can be configured with both an internal sleeve and an external sleeve press-fitted onto a plain or bulged cable bolt.
It will be apparent to those skilled in this art that various modifications and variations may be made to the embodiment disclosed herein without departing from the spirit and scope of the present invention. Accordingly, the embodiments of the invention described above are intended to be exemplary only. Those skilled in the art will therefore appreciate that the forgoing description is illustrative only, and that various alternatives and modifications can be devised without departing from the spirit of the present invention. Accordingly, the present is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended 2S claims.
Claims (10)
1. A method of yieldably fastening rock in a mine or underground tunnel, the method comprising the steps of:
a. boring a hole in the rock;
b. inserting grout into the hole;
c. installing a washer plate over an elongated structural member;
d. inserting said elongated structural member into the grouted hole wherein the elongated structural member has a protruding portion outside of the hole, said protruding portion being previously press-fitted with a sleeve in a high-strength interference fit which yields under an applied force exceeding a predetermined threshold force and wherein said sleeve is disposed behind said waster plate;
and, e. affixing a stopper to the protruding portion a predetermined distance behind said sleeve.
a. boring a hole in the rock;
b. inserting grout into the hole;
c. installing a washer plate over an elongated structural member;
d. inserting said elongated structural member into the grouted hole wherein the elongated structural member has a protruding portion outside of the hole, said protruding portion being previously press-fitted with a sleeve in a high-strength interference fit which yields under an applied force exceeding a predetermined threshold force and wherein said sleeve is disposed behind said waster plate;
and, e. affixing a stopper to the protruding portion a predetermined distance behind said sleeve.
2. The method as claimed in claim 1 wherein the step of using the elongated structural member comprises the step of using a cable bolt.
3. The method as claimed in claim 2 further comprising the step of rotating the cable bolt to mix the grout to enhance bonding between the cable bolt and the rock.
4. The method as claimed in claim 1 wherein said distance is a yielding distance through which the sleeve can yield when a load exceeding the predetermined threshold force is exerted on the sleeve.
5. The method as claimed in claim 4 wherein the step of affixing said stopper further comprises the steps of:
a. Measuring said desired yielding distance from the sleeve to the stopper;
b. At the yielding distance attaching a cable grip having conical wedges; and c. forcing a barrel over said cable grip so that the cable grip grasps the cable bolt.
a. Measuring said desired yielding distance from the sleeve to the stopper;
b. At the yielding distance attaching a cable grip having conical wedges; and c. forcing a barrel over said cable grip so that the cable grip grasps the cable bolt.
6. A rock fastener system for yieldable support of a tunnel or mine, the rock fastener system comprising:
a. an cable bolt having:
i. a rock-penetrating portion adapted to penetrate into a hole bored in said rock; and ii. a protruding portion adapted to protrude from said hole;
b. at least one sleeve press-fitted onto said rock penetrating portion in a high-strength interference fit having a predetermined and variable yield strength that yields when a load exceeding said predetermined and variable yield strength is applied, wherein said at least one sleeve is secured on the rock penetrating portion within the hole;
c. a washer plate disposed over said protruding portion and abutting against the rock; and, d. a sleeve pressed onto the protruding portion of the cable bolt and abutting the washer plate;
e. a stopper mounted onto the protruding portion a predetermined yield distance behind said sleeve; and, f. a suitable slip agent for application to the rock-penetrating portion with the exception of the sleeve fit portion.
a. an cable bolt having:
i. a rock-penetrating portion adapted to penetrate into a hole bored in said rock; and ii. a protruding portion adapted to protrude from said hole;
b. at least one sleeve press-fitted onto said rock penetrating portion in a high-strength interference fit having a predetermined and variable yield strength that yields when a load exceeding said predetermined and variable yield strength is applied, wherein said at least one sleeve is secured on the rock penetrating portion within the hole;
c. a washer plate disposed over said protruding portion and abutting against the rock; and, d. a sleeve pressed onto the protruding portion of the cable bolt and abutting the washer plate;
e. a stopper mounted onto the protruding portion a predetermined yield distance behind said sleeve; and, f. a suitable slip agent for application to the rock-penetrating portion with the exception of the sleeve fit portion.
7. The rock fastener system as claimed in claim 6 wherein the stopper comprises a cable grip and mating barrel.
8. The rock fastener system as claimed in claim 6 wherein the cable bolt is plain.
9. The rock fastener system as claimed in claim 6 wherein the cable bolt is bulged.
10. The rock fastener system as claimed in claim 6 wherein the cable bolt has at least one bulge prefilled with a compressible filler.
Priority Applications (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA2820010A CA2820010A1 (en) | 2004-06-30 | 2004-09-07 | Yieldable rock fastener system and method |
| CA2480729A CA2480729C (en) | 2004-06-30 | 2004-09-07 | Yieldable rock fastener system and method |
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA2,472,705 | 2004-06-30 | ||
| CA002472705A CA2472705A1 (en) | 2004-06-30 | 2004-06-30 | Yieldable rock fastener system and method |
| CA2480729A CA2480729C (en) | 2004-06-30 | 2004-09-07 | Yieldable rock fastener system and method |
Related Child Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA2820010A Division CA2820010A1 (en) | 2004-06-30 | 2004-09-07 | Yieldable rock fastener system and method |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2480729A1 CA2480729A1 (en) | 2005-12-30 |
| CA2480729C true CA2480729C (en) | 2013-07-02 |
Family
ID=35589021
Family Applications (2)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA2820010A Abandoned CA2820010A1 (en) | 2004-06-30 | 2004-09-07 | Yieldable rock fastener system and method |
| CA2480729A Expired - Fee Related CA2480729C (en) | 2004-06-30 | 2004-09-07 | Yieldable rock fastener system and method |
Family Applications Before (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA2820010A Abandoned CA2820010A1 (en) | 2004-06-30 | 2004-09-07 | Yieldable rock fastener system and method |
Country Status (1)
| Country | Link |
|---|---|
| CA (2) | CA2820010A1 (en) |
Families Citing this family (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| KR20110036827A (en) * | 2008-07-25 | 2011-04-11 | 가포드 피티와이 엘티디 | A method of encasing a yielding rock bolt shaft |
| CN102022129A (en) * | 2010-10-15 | 2011-04-20 | 山西潞安环保能源开发股份有限公司五阳煤矿 | Anchorage length limit device for grouting anchor cable |
| CN102434179B (en) * | 2011-11-18 | 2013-12-04 | 山东科技大学 | Deformation anchor cable capable of increasing resistance continuously |
| US9677399B2 (en) | 2013-07-12 | 2017-06-13 | Minova International Limited | Yieldable rock anchor |
| CN103526754A (en) * | 2013-09-30 | 2014-01-22 | 中国建筑第八工程局有限公司 | Method for constructing head-expanded anchoring rod in sand-rich decomposed rock |
| CN106150532A (en) * | 2016-06-28 | 2016-11-23 | 冯飞燕 | A kind of top board based on two numbers support subsides dredging method |
| CA3075116A1 (en) | 2017-09-08 | 2019-03-14 | Dywidag-Systems International Pty Limited | Encapsulation system and method of installing a rock bolt |
| RU184871U1 (en) * | 2018-04-17 | 2018-11-13 | Федор Александрович Анисимов | Flexible cable anchor with support element |
-
2004
- 2004-09-07 CA CA2820010A patent/CA2820010A1/en not_active Abandoned
- 2004-09-07 CA CA2480729A patent/CA2480729C/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| CA2480729A1 (en) | 2005-12-30 |
| CA2820010A1 (en) | 2005-12-30 |
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| EEER | Examination request | ||
| MKLA | Lapsed |
Effective date: 20180907 |