Note: Descriptions are shown in the official language in which they were submitted.
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Cable bolts
Description
Technical Field
[0001] The invention relates to cable bolts, in particular to cable bolts used
for
burst prone areas in mining operations and its production method.
Background Art
[0002] In mining or construction operations, safety is of paramount
importance.
Various support methods are employed in mining tunnels to protect
workers from small blocks and loose rocks which may fall from the roof or
sidewall, such as mechanical rock bolts, screen, shotcrete, grouted rebar
and cable bolts. Among them, cable bolts can reach far into the rock mass
and reinforce large volumes of rock to prevent separation along planes of
weakness such as joints. Cable bolts can be installed remotely in long
boreholes to reach the planned stope boundary and provide pre-
reinforcement to the otherwise inaccessible walls. Cable bolts are one of
the effective options for support of inaccessible rock faces for stability and
dilution control since cable strands can bend around fairly tight radii,
making installation of long bolts from confined working places possible.
Cable bolts help to maintain a continuum nature within the rock mass,
thereby improving overall stability. In addition, by supporting blocks of rock
at the excavation surface, the remaining rock mass is prevented from
loosening and weakening. Cable bolts thus restrict the dangerous and
costly effects of progressive instability and failure.
[0003] When the underground excavations are deeper, or for larger spans in
major intersections, large underground chambers or in active mining
stopes, there are rocks displacement related to squeezing rock behaviour
and/or areas prone to rock burst. In these critical conditions, the
conventional cable bolts are not able to dissipate the energy impacts due
to the movement of the rock mass. There is a demonstrated increasing
need for effective support at these areas.
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Disclosure of Invention
[0004] It is an object of the invention to avoid the disadvantages of the
prior art.
[0005] It is also an object of the invention to provide a cable bolt for
effective
support and balance to rock mass.
[0006] It is another object of the present invention to provide a cable bolt
for
reacting to movement of rock mass and absorbing high energy impact.
[0007] According to a first aspect of the present invention, there is provided
a
cable bolt for providing support and balance to a rock mass, comprising:
a multi-strand cable having a plurality of steel wires being twisted together,
said multi-strand cable having a first end portion for anchoring in a
borehole of rock mass and a second end portion for being positioned
adjacent to the opening of the borehole,
a fixture secured to the second end portion of said multi-strand cable,
wherein said cable bolt has energy absorption of at least 20 KJ/m for a
cable bolt having a diameter of about 15.4 mm, and at least 30 KJ/m for a
cable bolt having a diameter of about 17.8 mm, and
wherein at least one of the plurality of steel wires is made from steel
having as steel composition:
a carbon content ranging from 0.20 weight percent to 0.95 weight percent,
a silicon content ranging from 0.5 weight percent to 2.0 weight percent,
a manganese content ranging from 0.40 weight percent to 1.0 weight
percent,
a chromium content ranging from 0.0 weight per cent to 1.0 weight
percent,
a sulphur and phosphor content being limited to 0.025 weight percent, the
remainder being iron and unavoidable impurities, and wherein the sum of
the weight fractions of all the elements in the steel is equal to 100%,
and said steel has as metallurgical structure:
a volume percentage of retained austenite ranging from 4 percent to
25 percent, the remainder being tempered primary martensite and
untempered secondary martensite.
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[0008] The cable bolt according to the present invention may further comprise
a
plate for placement between the rock mass and said fixture for tensioning
said multi-strand cable relative to the rock mass, said plate defining a plate
opening for the passage of said multi-strand cable through said plate.
[0009] The fixture of the cable bolt can be used for tensioning said multi-
strand
cable relative to the rock mass. The fixture may comprise a wedge portion
and a corresponding head portion, wherein said wedge portion engages
said multi-strand cable and secures said multi-strand cable within said
head portion as said wedge portion engages said corresponding head
portion for tensioning said multi-strand cable.
[0010] As an example, the multi-strand cable of the cable bolt may be
partially
covered with a sleeve. The sleeve may be in a form of tube and cladded
on at least one portion of the multi-strand cable. The sleeve may be made
from metal material and preferably the same material of the cable.
Alternatively, polymers or plastic materials, e.g. polypropylene sheath can
be applied. The portion of multi-strand cable covered by the sleeve is
intended to be free to deform since it is not bound by the grout. Therefore,
the multi-strand cable can present high elongation or energy dissipation at
fracture.
[0011] According to the present invention, the cable bolt is a multi-strand
cable
bolt and the multi-strand cable comprises a plurality of steel wires being
twisted together. Preferably, at least one of the steel wires has a corrosion
resistant coating, e.g. zinc or zinc alloy. More preferably, all the steel
wires
have the corrosion resistant coating. The corrosion resistant coating on
each of the steel wires secures a longer life time of the multi-strand cable
bolts particularly in corrosive environments e.g. in coal mines.
[0012] As an option, at least one of the steel wires has surface deformations,
e.g.
indentations formed by rolling. Preferably, all the steel wires at the outer
surface of the multi-strand cable have surface deformations. The desired
deformations can increase the penetration of grout to the cable bolt and
thus enhance the anchorage of the cable bolt to the rock mass.
[0013] The cable bolt, or in the other word the multi-strand cable of the
cable bolt
according to the present invention may have a preselected length of less
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than 6 m. However, thanks to its flexibility, the cable bolt can have a
preselected length of more than 6 m, e.g. more than 8 m. The maximum
possible preselected length of cable bolts is larger than other underground
support means like D-bolts and rebars. This means cable bolts can reach
deeper into the rock mass and reinforce larger volumes of rock.
[0014] As an example, the multi-strand cable may be in the form of seven steel
wire having a central steel wire and six outer steel wires. The diameter of
the central steel wire may be larger than the diameter of the outer steel
wires. As another example, the multi-strand cable is in the form of six steel
wires having a central steel wire and five outer steel wires. Thanks to the
multi-strand cable construction, compared with D-bolts and rebars, the
cable bolts having a similar diameter are lighter and more flexible.
[0015] For rock bolts used for the support of underground mines or
excavations,
both the deformation at fracture and the tensile strength may be important
properties. More importantly, the energy absorption ability of the bolts
presents the bolt performance in dynamic environment. The capacity of
energy absorption of a rock bolt can be estimated from an engineering
stress-strain curve. An engineering stress-strain curve is typically
constructed from the load deformation measurements. In the test a
specimen is subjected to a continually increasing uniaxial tensile force
while simultaneous observations are made of the deformation of the
specimen. Deformation is the change in axial length divided by the original
length of the specimen. A typical stress-stain curve of a metal is illustrated
in Fig. 1. The relationship between the stress (a) and strain (c) that a
particular material displays is known as that particular material's stress¨
strain curve. As indicated by the shaded area in Fig. 1, the energy
absorption (also called energy dissipation) is the integrated area under the
entire stress-strain curve to the break or fracture point (as indicated by
point F in the curve of Fig. 1) where the test specimen is fractured.
[0016] The cable bolts according to the present invention have good energy
absorption, which is not a character of conventional cable bolts. The cable
bolt of the present invention may have a deformation at fracture of at least
7 cm/m, preferably at least 10 cm/m, and more preferably at least 15
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cm/m. The diameter of the multi-strand cable is in the range of 10 to 40
mm, preferably in the range of 10 to 20 mm, e.g. about 15.4 mm and about
17.8 mm. Since the multi-strand cable is made by several wires, the
diameter of multi-strand cable may deviate much from standard design.
For instance, herein a multi-strand cable having a diameter of about 15.4
mm may include a multi-strand cable in practice in the range of 14.4 mm to
16.4 mm. The cable bolt of the present invention preferably have energy
absorption of at least 20 KJ/m, and more preferably at least 25KJ/m, for a
cable bolt having a diameter of about 15.4 mm. The cable bolt of the
present invention preferably have energy absorption of at least 30 KJ/m,
and more preferably at least 35 KJ/m, for a cable bolt having a diameter
of about 17.8 mm. The high energy absorption of the cable bolts according
to the present invention makes it possible to elongate or deform with the
movement of the rock mass and absorb high energy impact. Such cable
bolts are suitable for areas prone to rock burst in mines.
[0017] According to a second aspect of the present invention, it is provided a
multi-strand cable having a plurality of steel wires being twisted together,
the diameter of said multi-strand cable being in the range of 10 to 40 mm,
wherein said multi-strand cable has energy absorption of at least 20 KJ/m
for a cable having a diameter of about 15.4 mm, and at least 30 KJ/m for a
cable having a diameter of about 17.8 mm, and
wherein at least one of the plurality of steel wires is made from steel
having as steel composition:
a carbon content ranging from 0.20 weight percent to 0.95 weight percent,
a silicon content ranging from 0.5 weight percent to 2.0 weight percent,
a manganese content ranging from 0.40 weight percent to 1.0 weight
percent,
a chromium content ranging from 0.0 weight per cent to 1.0 weight
percent,
a sulphur and phosphor content being limited to 0.025 weight percent,
the remainder being iron and unavoidable impurities, and wherein the sum
of the weight fractions of all the elements in the steel is equal to 100%,
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and said steel has as metallurgical structure:
a volume percentage of retained austenite ranging from 4 percent to 25
percent, the remainder being tempered primary martensite and
untempered secondary martensite.
[0018] According to a third aspect of the present invention, it is provided a
process of manufacturing a multi-strand cable bolt having an energy
absorption of at least 20 KJ/m, preferably at least 25 KJ/m, for a multi-
strand cable bolt having a diameter of about 15.4 mm, and at least 30
KJ/m, preferably at least 35 KJ/m, for a multi-strand cable bolt having a
diameter of about 17.8 mm, wherein said process comprising the following
steps:
a) selecting a steel wire with steel composition:
a carbon content ranging from 0.20 weight per cent to 0.95 weight
percent,
a silicon content ranging from 0.5 weight per cent to 2.0 weight percent,
a manganese content ranging from 0.40 weight per cent to 1.0 weight
percent,
a chromium content ranging from 0.0 weight per cent to 1.0 weight
percent,
a sulphur and phosphor content being limited to 0.025 weight percent,
the remainder being iron and unavoidable impurities, and wherein the
sum of the weight fractions of all the elements in the steel is equal to
100%,
b) austenitizing said steel wire above Ac3 temperature between 920 C and
980 C during a period less than 120 seconds,
c) quenching said austenitized steel wire between 20 C and 280 C during
a period less than 60 seconds,
d) partitioning said quenched steel wire between 320 C and 500 C during
a period ranging from 10 seconds to 600 seconds,
e) cooling down the partitioned steel wire to room temperature,
0 twisting the quenched and partitioned steel wires into a multi-strand
cable,
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g) cutting the multi-strand cable into a preselected length,
h) adding a fixture to an end portion of said multi-strand cable.
[0019] According to a fourth aspect of the present invention, it is provided a
process of manufacturing a multi-strand cable bolt having energy
absorption of at least 20 KJ/m, preferably at least 25 KJ/m, for a multi-
strand cable bolt having a diameter of about 15.4 mm, and at least 30
KJ/m, preferably at least 35 KJ/m, for a multi-strand cable bolt having a
diameter of about 17.8 mm,
said process comprising the following steps:
a) selecting a steel wire with steel composition:
a carbon content ranging from 0.20 weight per cent to 0.95 weight percent,
a silicon content ranging from 0.5 weight per cent to 2.0 weight percent,
a manganese content ranging from 0.40 weight per cent to 1.0 weight
percent,
a chromium content ranging from 0.0 weight per cent to 1.0 weight
percent,
a sulphur and phosphor content being limited to 0.025 weight percent,
the remainder being iron and unavoidable impurities, and wherein the sum
of the weight fractions of all the elements in the steel is equal to 100%,
b) twisting said steel wires into a multi-strand cable;
c) austenitizing said multi-strand cable above Ac3 temperature between
920 C and 980 C during a period less than 120 seconds,
d) quenching said austenitized multi-strand cable between 20 C and
280 C during a period less than 60 seconds,
e) partitioning said quenched multi-strand cable between 320 C and
500 C during a period ranging from 10 seconds to 600 seconds,
0 cooling down the partitioned multi-strand cable to room temperature,
g) cutting the quenched and partitioned multi-strand cable into a
preselected length,
h) adding a fixture to an end portion of said multi-strand cable.
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[0020] After the quenching step, which occurs between Ms, the temperature at
which martensite formation starts and Mf, the temperature at which
martensite formation is finished, retained austenite and martensite has
been formed. During the partitioning step, carbon diffuses from the
martensite phase to the retaining austenite in order to stabilize it more.
The result is a carbon-enriched retained austenite and a tempered
martensite.
[0021] After the partitioning step, the partitioned steel wire is cooled down
to room
temperature. The cooling can be done in a water bath. This cooling down
causes a secondary untempered martensite, next to the retained austenite
and the primary tempered martensite.
[0022] The austenitizing step occurs at temperatures ranging from 920 C to
980 C, and preferably between 930 C and 970 C. Preferably, the
partitioning step d) occurs at relatively high temperatures ranging from
400 C to 500 C, more preferably from 420 C to 460 C. The inventor has
experienced that these temperature ranges are favourable for the stability
of the retained austenite in the final steel wire.
[0023] Preferably, the diameter of the multi-strand cable is in the range of
10 to
40 mm and the preselected length of said multi-strand cable is at least 6
m. The multi-strand cable may be in the form of seven steel wire having a
central steel wire and six outer steel wires.
Brief Description of Figures in the Drawings
[0024] Fig. 1 is a schematic illustration of a typical stress¨strain curve of
a metal.
[0025] Fig. 2 is a cross-section view of a multi-stand cable used for the
cable bolt
according to the present invention.
[0026] Fig. 3 shows a partial sectional view in side elevation according to an
example of a cable bolt of the present invention.
[0027] Fig. 4 shows a partial sectional view in side elevation according to
another
example of a cable bolt of the present invention.
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Mode(s) for Carrying Out the Invention
[0028] A cable bolt according to the present invention comprises a multi-
strand
cable. The multi-strand cable is made by twisting at least two steel wires.
As an example, the steel wire has as a steel composition: a carbon
content of 0.55 weight percent, a silicon content of 1.2 weight percent, a
manganese content of 0.7 weight percent, a chromium content of 0.6
weight percent and the remainder being iron. The starting temperature of
martensite transformation Ms of this steel is about 280 C.
The steel wire is treated by various steps of the process as follows:
- a first austenitizing step during which the steel wire stays in a furnace
at
about 950 C during 120 seconds,
- a second quenching step for partial martensite transformation at a
temperature between 20 C and 280 C during less than 25 seconds;
- a third partitioning step for moving carbon atoms from the martensite
phase to the austenite phase to stabilize this at a temperature around 460
C during about 15 seconds; and
- a fourth cooling step at room temperature during 20 or more seconds.
The steel wire produced through above process has as metallurgical
structure: a volume percentage of retained austenite of about 20 percent,
the remainder being tempered primary martensite and untempered
secondary martensite.
[0029] As an embodiment, inventive cable 1 has a diameter of about 15.4 mm
and 1+6 configuration. The central wire or king wire has a diameter of
about 5.4 mm and each outer wire has a diameter of about 5.0 mm.
[0030] As another embodiment, inventive cable 2 has a diameter of about 17.8
mm and 1+6 configuration. The central wire or king wire has a diameter of
about 6.10 mm and each outer wire has a diameter of about 5.85 mm.
[0031] The cross-section of a multi-strand cable 20 having 1+6 configuration
is
shown in Fig. 2. The six outer steel wires 22 are twisted around the central
wire 24. Fig. 3 shows a partial sectional view in side elevation of a cable
bolt in an example. As shown in Fig. 3, the first end of multi-strand cable
31 is inserted in a borehole 32 and the second end 33 is attached with a
fixture 34 secured to the end of the multi-strand cable for tensioning the
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multi-strand cable relative to the rock mass. The fixture 34 of the cable bolt
may comprise a wedge portion (not shown in Fig. 3) and a corresponding
head portion, wherein said wedge portion engages said multi-strand cable
and secures said multi-strand cable within said head portion as said
wedge portion engages said corresponding head portion for tensioning
said multi-strand cable. The cable bolt may further comprise a plate 35
placed between the rock mass and the fixture 34. The plate 35 has an
opening for the passage of the multi-strand cable through the plate.
[0032] Upon sufficient insertion of the cable 31, the first end of cable
contacts the
bonding agent cartridge 36, such as an uncured resin enclosed in a bag
and separated from a catalyst which is provided in the inner part of the
borehole. This causes the bonding agent to flow around and along the
length of the multi-strand cable 31 to secure the multi-strand cable 31
within the borehole by e.g. cured resin 37.
[0033] The properties, i.e. the diameter (Dia.), the mass, the maximum
possible
length which can be installed in a borehole of a mine, the maximum load
or load capacity, the deformation at fracture or deformation capacity, and
the energy absorption of the multi-strand cable bolt according to the
present invention are compared with the properties of standard cable bolt
and commercially available D-blots and rebars in Table 1.
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[0034] Table 1 Comparation of the properties of rock bolts
Max.
Load Deformation Energy
Dia. Mass possible
capacity capacity absorption
(mm) (kg/m) Length
(ton) (cm/m) (KJ/m)
(m)
Standard
15.4 1.10 >8 27 7 15.5
cable bolt
D-bolt 1 20 2.5 <3 19 15 26.0
D-bolt 2 22 3.0 <3 23 15 32.0
Reber 1 22 2.98 <6 16.5 16 22
Rebar 2 25 3.85 <6 21.5 15.5 28.5
Inventive
15.4 1.10 >8 20 15.5 28.0
cable bolt 1
Inventive
17.8 1.43 >8 27 15 37.0
cable bolt 2
[0035] Rebar, also known as reinforcing steel, is a steel bar used as a
tension
device to strengthen and hold the rock mass or concrete in tension.
Rebar's surface is often patterned to form a better bond with the grout or
concrete. D-Bolt is a smooth steel bar with a number of anchors along its
length. It is anchored in a borehole with either resin or cement grout. The
D-Bolt is only fixed with the grout in the anchors' positions, while the
smooth sections between the anchors can freely deform when subjected
to rock dilation. D-bolts and rebars are commonly used for underground
supporting. As shown in table 1, the cable bolts generally have lighter
mass than the D-bolts and rebars. In addition and importantly, the flexibility
of cable bolts is much better than D-bolts and rebars. The cable bolts can
be installed with a preselected length of more than 8 m, while the D-bolts
and rebars typically have a preselected length of less than 3 m and 6 m
respectively due to their limited flexibility. In addition, the cable bolts
can
withstand a relatively high load i.e. 20 tons and even more. However, the
deformation at fracture of the D-bolts and rebars is about two times of that
of the standard cable bolt.
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[0036] The inventive cable bolt 1 has a same diameter (15.4 mm) and
configuration as the standard cable bolt except the composition and
thermal treatment of steel wires are different. The maximum load which
the inventive cable bolt 1 can suffer is slightly lower than the standard
cable bolt (20 tons vs. 27 tons). On the other hand, the deformation at
fracture of the inventive cable bolt 1 is about 15.5 cm/m, which is more
than double the value of the standard cable bolt (7 cm/m in table 1). The
energy absorption of the inventive cable bolt 1 is thus significantly higher
than that of the standard cable bolt (28 KJ/m vs. 15.5 KJ/m). For inventive
cable bolt 2 having a diameter of 17.8 mm, the load capacity is the same
as the standard cable bolt (27 tons) while the deformation at fracture is
more than two times of the load capacity of standard cable bolt (15 cm/m
vs. 7 cm/m). As shown in table 1, the energy absorption of the inventive
cable bolt 2 is about 37 KJ/m, which is significantly higher than the energy
absorption of standard cable bolt and even higher than the studied D-bolts
and rebars.
[0037] It can be seen, compared with conventional rock supporting means, the
inventive cable bolts are attractive means for supporting mining operations
in particular for areas prone to burst because the inventive cable bolt has
less in materials and mass, has more in flexibility and ductility, and
importantly has higher energy absorption.
As another example as shown in Fig. 4, a plurality of polymer sleeves 42 are
applied at selected positions along the length of the multi-strand cable 41.
The plurality of polymer sleeves 42 may be applied by cladding. The
sleeves are intended to protect the covered portions of the multi-strand
cable from grout. Thus, the covered portions can freely deform when
subjected to rock movement or dilation. The anchored portions, which are
not covered by sleeves, make the bolt anchored to the rock mass. In this
configuration, the failure at one portion does not affect the other portions.
Each portion works independently, only a fraction of the load transferred to
the cable bolt plate. This type of cable bolt is strong, tough, reliable and
easy to install with standard equipment.
