Note: Descriptions are shown in the official language in which they were submitted.
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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.
BACKGROUND OF THE INV13NTION
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
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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
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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
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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.
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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
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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
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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;
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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.
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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.
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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
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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
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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.
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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
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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.
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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
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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
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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
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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.
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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
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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.
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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
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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.
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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.
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