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(11) | EP 3 121 369 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | CABLE BOLTS |
| (57) 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 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 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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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.
Disclosure of Invention
[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 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 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.
[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 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 are 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 (σ) and strain (ε) 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 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 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 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;
- 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,
- f) twisting the quenched and partitioned steel wires into a multi-strand cable,
- 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,
- 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,
- f) 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.
[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.
Fig. 2 is a cross-section view of a multi-stand cable used for the cable bolt according to the present invention.
Fig. 3 shows a partial sectional view in side elevation according to an example of a cable bolt of the present invention.
Fig. 4 shows a partial sectional view in side elevation according to another example of a cable bolt of the present invention.
Mode(s) for Carrying Out the Invention
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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 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.
[0029] 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.
[0030] 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.
| Dia. (mm) | Mass (kg/m) | Max. possible Length (m) | Load capacity (ton) | Deformation capacity (cm/m) | Energy absorption (KJ/m) | |
| Standard cable bolt | 15.4 | 1.10 | >8 | 27 | 7 | 15.5 |
| D-bolt 1 | 20 | 2.5 | <3 | 19 | 15 | 26.0 |
| D-bolt 2 | 22 | 3.0 | <3 | 23 | 15 | 32.0 |
| Rebar 1 | 22 | 2.98 | <6 | 16.5 | 16 | 22 |
| Rebar 2 | 25 | 3.85 | <6 | 21.5 | 15.5 | 28.5 |
| Inventive cable bolt 1 | 15.4 | 1.10 | >8 | 20 | 15.5 | 28.0 |
| Inventive cable bolt 2 | 17.8 | 1.43 | >8 | 27 | 15 | 37.0 |
[0031] 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.
[0032] 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.
[0033] 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.
[0034] 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.
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 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 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.
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 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.
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,
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,
f) twisting the quenched and partitioned steel wires into a multi-strand cable,
g) cutting the multi-strand cable into a preselected length,
h) adding a fixture to an end portion of said multi-strand cable.
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,
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,
f) 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.