METHOD OF STABILIZING A ROCK STRUCTURE

Abstract: A rock bolt in the form of a steel tube (11) is expanded against the rock by means of an expandable body (15) inside it. The expandable body (15) comprises a rubber hose which is temporarily pressurized to a pressure that is usually 50-100 MPa so that the bolt anchors in the borehole. The expandable body (15) is part of a mounting tool and it is withdrawn immediately when depressurized.

Description: This invention relates to a method of stabilizing a rock structure comprising boring a hole, inserting in the borehole a tubular stabilizer that is slimmer than the borehole and expanding the tubular stabilizer to anchor in the borehole. In US patent 4459067, a rock stabilizer is shown which comprises a closed longitudinally folded tube which is pressurized to expand to anchor in the borehole. This bolt provides an outstanding anchoring and the accepted diameter range of the holes is extremely wide. However, the bolt is comparatively expensive. In Canada patent 1171310, a longitudinally folded rock stabilizer is shown which is expanded in the borehole by means of a mandrel that is forced into the stabilizer. The expansion of the stabilizer is comparatively complicated and requires a comparatively high force. In US patent 3922867 and US 4012913 rock stabilizers are shown which comprise a tube with a longitudinal slot. The stabilizers are initially wider than the borehole and they are forced into the borehole. The insertion requires a force that is of the same magnitude as the anchoring, and the allowed diameter range for the holes is very narrow. In US patent 3349567, a rock stabilizer is shown which comprises a tube that is inserted in the borehole and then expanded at discrete points by pulsed magnetic fields induced by high voltage pulses in a coil in a probe that is temporarily inserted in the stabilizer. The anchoring will probably be poor. It is an object of the invention to provide a method of stabilizing a rock structure; a method that is fast and simple and makes a low total cost for stabilizers as anchored in the rock. The invention will be described with reference to the drawings. Fig 1 is a transverse section along lines 1-1 in Fig 4 through a borehole in the rock in which a stabilizer or rock bolt is inserted. Inside the stabilizer is a expandable body. Fig 2 is a section corresponding to Fig 1 and taken along lines 2-2 in Fig 5 but showing the stabilizer when being expanded to anchor in the borehole. Fig 3 is a section corresponding to Figs 1 and 2 and taken along lines 3-3 in Fig 6 but showing the stabilizaer anchored in the borehole and the expandable body removed. Figs 4-6 are longitudinal sections corresponding to Figs 1-3. Fig 7 is a view showingan operator mounting a stabilizer in a roof of a rock cavity e.g. a tunnel. Figs 8-10 correspond to Figs 1-3 resp. but show a modified form of the stabilizer. Figs 11-13 correspond to Figs 1-3 resp. but show another modified form of the stabilizer. Figs 14-17 show in crossection four other forms of a stabilizer. Fig 18 is a fragmentary view seen as indicated by the arrows 18 in Fig 15. Fig 19 show in a longitudinal section a stabilizer with a shoulder for supporting a plate. The rock stabilizer shown in Figs 1-7 comprises a tube 11 of metal for example steel and preferably mild steel. One of its ends is formed as a flange 12 that forms a support for a rock supporting plate 13. An expansion body comprises an elastic tube 15 e.g. a hose of reinforced rubber that is part of a mounting tool 14 that is best shown in Figs 4-7. The rubber hose 15, is mounted on a base 16 and its ends are sealed to the base 16. The base 16 is mounted on a rod 18 as can be seen in Fig 7. Through a hose 17 which is coupled to a pump 20 through a supply valve 21 as shown in Fig 7, the elastic tube 15 of the mounting tool 14 can be pressurized to expand radially. In Fig 7 the mounting of a stabilizer 11 is shown. The mounting tool 14 is first inserted in the stabilizer 11 and used to insert the stabilizer in a borehole 23 as shown in Figs 7, 4, and 1. Then, the valve 21 is actuated to pressurize the elastic tube 15 to expand so that the tube 15 forces the stabilizer 11 against the borehole at such a force that the stabilizer is deformed plastically to expand against the borehole and to transmit a force to the borehole which widens the borehole by elastic deformation of the rock as shown in Figs 5, 7, and 2. Further, the stabilizer 11 is plastically deformed to adjust to the irregularities of the borehole as shown in an exaggerated manner in Fig 5. Then, the elastic tube 15 of the mounting tool 14 is depressurized and the mounting tool 14 is removed, leaving the stabilizer 11 anchored in the borehole as shown in Figs 3 and 6. The elastically deformed rock shrinks more than the plastically deformed stabilizer 11 and there will be a shrinkage fit between the stabilizer and the borehole which anchors the bolt by friction. The plastic adjustment to the irregularities increases the anchoring. The stabilizer 11 can advantageously be made of mild steel and the hydraulic pressure can for example be 50-100 Mpa (500-1000 bar). It should be noted that a borehole wall is never smooth, and often the borehole is not completely straight but somewhat in spiral. It is also not very difficult to bore a hole that is less straight and has less smooth a surface than usual. The plastic adjustment of the stabilizer to the irregularities of the borehole increases the anchoring. In some rocks, the rock will be plastically deformed as well. The stabilizer 11 can for example be 1-3 m long or longer and used in a borehole with a diameter of for example 25-45 mm. In all the figures but Fig 7, the stabilizer 11 is shown shortened. The expansion body 15 of the mounting tool 14 can be about as long as the stabilizer 11 so that it can expand the entire length of the stabilizer as illustrated. It can also be shorter than the stabilizer and it can be used to expand a part of the stabilizer and then depressurized and moved in the stabilizer to expand another part of the stabilizer so that the entire stabilizer will eventually be expanded. Sometimes it might be desirable to expand only a part of the stabilizer 11 for example the part of the stabilizer adjacent the bottom of the borehole in order to get a top anchored bolt. In Figs 8-10, which correspond to Figs 1-3, an alternative design of the stabilizer 11 is shown. The stabilizer comprises a corrugated steel tube 11. Fig 8 shows the stabilizer before expansion, Fig 9 shows the stabilizer during expansion and Fig 10 shows the stabilizer anchored in the borehole. Figs 11-13 correspond also to Figs 1-3 too but they show another alternative design of the stabilizer 11. The tubular stabilizer 11 has a flat 31 and a slot 32 opposite the flat so that the two wings 33, 34 are formed. The flat 31 is forced against the borehole by the expandable hose 15 as shown in Fig 12 and when the hose is depressurized, the area of the original flat 31 will act as a spring to force the wings 33, 34 outwardly and improve the anchoring by friction. There will probably be a clearance between the stabilizer 11 at the area of the original flat 31 and the rock when the stabilizer is anchored. In Figs 14-17 modified cross section designs of slotted stabilizers 11 are shown. In Fig 15, the tube 11 is circular in cross section. The slot may either be straight as in the embodiment shown in Figs 11-13 or it may be designed as shown in Fig 18. One edge 35 is then ondulating and the other edge 36 is serrated. There will always be teeth of the serrated edge 36 that engage with the ondulating edge 35 to prevent shrinkage and thereby increase the anchoring force. The tube 11 in Fig 16 has overlapping longitudinal edges. The tube 11 in Fig 14 has bent edges that contact each other. There may also be an open slot between the edges. In Fig 17, three different ways of making the outer surface of the tube 11 rough are shown. The metal strip forming the tube 11 can have protruding weld spots 40; it can be punched to form knobs 41 or it can be knurled as shown at 42. By making the surface rough in any illustrated or non-illustrated way, the pull-out force of the stabilizer will usually be increased. In Fig 19, an alternative to the flange 12 is shown. A cylinder 37 is friction welded to the tube 11 in order to form a support for the rock engaging plate 13. It is not necessary that the expanding body is part of a mounting tool, the stabilizer can instead first be inserted in the borehole and then, the expanding body can be inserted in the stabilizer.

Claims: 1. Method of stabilizing a rock structure comprising boring a hole (22), inserting in the borehole a tubular stabilizer (11) that is slimmer than the borehole and expanding the tubular stabilizer (11) to anchor in the borehole,characterized in that a body (15), expandable by pressure fluid, is pressurized to expand within the stabilizer (11) so that the stabilizer (11) is expanded to anchor in the borehole, whereafter the expandable body (15) is depressurized and removed from the stabilizer (11) which is left anchored in the borehole. 2. Method according to claim 1,characterized in that the stabilizer (11) is first mounted on said expandable body (15) and then inserted in the borehole. 3. Method according to claim 1 or 2,characterized in that the stabilizer (11) is expanded to anchor in the borehole (22) over substantially its entire length. 4. Method according to any one of the preceding claims,characterized in that the expandable body is expanded to plastically deform the stabilizer (11) to extend its periphery and to elastically deform the rock to provide a shrinkage fit between the rock and the stabilizer (11). 5. Method according to claim 4,characterized in that a stabilizer (11) is used which has a closed cross-section. 6. Method according to claim 5,characterized in that the periphery of the stabilizer is extended. 7. Method according to any one of the claims 1-3,characterized in that a stabilizer (11) is used which has an axial slot (32) and the stabilizer is expanded substantially without its periphery being extended.

Inventors: LORNE, ROBERT HERRON
Applicants: ATLAS COPCO AKTIEBOLAG
Application year: 1986
Main classification: E21D20
IPC codes: E21D20, E21D21