CABLE BOLTS

Abstract

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 percent 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 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.

Claims

1. 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 percent 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 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.

2. A cable bolt according to claim 1, it further comprises 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.

3. A cable bolt according to claim 1 or 2, wherein said fixture comprises 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.

4. A cable bolt according to any one of the preceding claims, wherein said multi-strand cable is partially covered with a sleeve.

5. A cable bolt according to any one of the preceding claims, wherein at least one of the steel wires has a corrosion resistant coating.

6. A cable bolt according to any one of the preceding claims, wherein at least one of the steel wires has surface deformation.

7. A cable bolt according to any one of the preceding claims, wherein the cable bolt has a deformation at fracture of at least 10 cm/m.

8. A cable bolt according to any one of the preceding claims, wherein the diameter of said multi-strand cable is in the range of 10 to 40 mm.

9. A cable bolt according to any one of the preceding claims, wherein said multi-strand cable has a preselected length of at least 6 m.

10. A cable bolt according to any one of the preceding claims, wherein said multi-strand cable is in the form of seven steel wire having a central steel wire and six outer steel wires.

11. A cable bolt according to claim 10, wherein the diameter of the central steel wire is larger than the diameter of the outer steel wires.

12. 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 percent 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 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.

13. A process of manufacturing a multi-strand cable bolt having energy absorption of at least 20 KJ/m for a multi-strand cable bolt having a diameter of about 15.4 mm, and at least 30 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 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 percent to 1.0 weight percent, a sulphur and phosphor content being limited to 0.025 weight percent, the remainder being iron and unavoidable impurities, 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.

14. A process of manufacturing a multi-strand cable bolt having energy absorption of at least 20 KJ/m for a multi-strand cable bolt having a diameter of about 15.4 mm, and at least 30 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 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 percent to 1.0 weight percent, a sulphur and phosphor content being limited to 0.025 weight percent, the remainder being iron and unavoidable impurities, 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.

Description

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

[0032] FIG. 1 is a schematic illustration of a typical stress-strain curve of a metal.

[0033] FIG. 2 is a cross-section view of a multi-stand cable used for the cable bolt according to the present invention.

[0034] FIG. 3 shows a partial sectional view in side elevation according to an example of a cable bolt of the present invention.

[0035] 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

[0036] 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.

[0037] The steel wire is treated by various steps of the process as follows: [0038] a first austenitizing step during which the steel wire stays in a furnace at about 950 C. during 120 seconds, [0039] a second quenching step for partial martensite transformation at a temperature between 20 C. and 280 C. during less than 25 seconds; [0040] 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 [0041] a fourth cooling step at room temperature during 20 or more seconds.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

TABLE-US-00001 TABLE 1 Comparation of the properties of rock bolts Max. possible Load Deformation Energy Dia. Mass Length capacity capacity absorption (mm) (kg/m) (m) (ton) (cm/m) (KJ/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 Rebar 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

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.