AU2009227874B2

AU2009227874B2 - Method of supporting ground using a combined rock bolt, and such a combined rock bolt

Info

Publication number: AU2009227874B2
Application number: AU2009227874A
Authority: AU
Inventors: Charlie Chunlin Li
Original assignee: Normet International Ltd
Current assignee: Normet International Ltd
Priority date: 2009-10-20
Filing date: 2009-10-20
Publication date: 2015-10-01
Anticipated expiration: 2029-10-20
Also published as: AU2009227874A1

Abstract

Abstract The invention comprises a method for supporting rock and a combined rock bolt. The steps of the method are: - drilling a borehole in the rock, - arranging a hollow friction bolt (1) generally in contact with the borehole wall and extending generally from the rock surface and along at least a portion of the borehole, - arranging grout in said borehole. The two previous steps may be interchanged. - arranging a solid bolt (2) with at least one smooth stem portion (3) with an anchor (4) near an inner end of said solid bolt (2), said solid bolt (2) extending within said hollow friction bolt (1) in the borehole, said solid bolt (2) generally extending throughout the length of said hollow friction bolt (1), - allowing said grout to set and harden. The method may comprise arranging an inner stem portion (3) with said anchor (4) of said solid bolt (2) extending beyond an inner end of said hollow friction bolt (1). (Fig. 2a for being published with the abstract) Ci))

Description

AUSTRALIA Patents Act COMPLETE SPECIFICATION (ORIGINAL) Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority Related Art: Name of Applicant: Dynamic Rock Support AS Actual Inventor(s): Charlie Chunlin Li Address for Service and Correspondence: PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA Invention Title: METHOD OF SUPPORTING GROUND USING A COMBINED ROCK BOLT, AND SUCH A COMBINED ROCK BOLT Our Ref: 869133 POF Code: 485675/494583 The following statement is a full description of this invention, including the best method of performing it known to applicant(s): - 1- 2 Field of the invention The present invention relates to a rock bolt for use particularly in the mining industry. More specifically, the present invention is a modification to hollow so-called friction bolts such split-set bolts, i.e. hollow tubes with a longitudinal slit and provided with a surface flange ring for holding a face plate. Background art A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims. Where the terms "comprise", "comprises", "comprised" or "comprising" are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereto. Hollow tubes inserted as friction bolts in boreholes, of which a particular type is called Split set bolts, may be used for reinforcing ground. Particularly, split set bolts of 46 mm may be pressed into a borehole of diameter slightly less, such as 41-45 mm, and will sit fixed due to the friction. Split set bolts are used in boreholes down to 35 mm diameter. An illustration of a so called split set bolt is shown in Fig. 6a. The most commonly used sizes of Split set are 46 and 39 mm. The borehole diameter usually recommended is 41-45 mm for Split set 46 and 35 - 38 mm for Split set 39. The split set bolt commonly used may not withstand much tensile force; it will usually start to slip at about 5 tonnes, 3 i.e. about 50 kN. Further, the shear resistance of split set bolts is relatively low and if subject to shear forces from lateral shear deformation of hard foliated rocks the split set bolt may be locked in the longitudinal direction of the hole and thus easily fail in shear. Such hollow tube bolts, also called tube stabilisers, for instance the so-called Split set, are used to secure rocks in some underground openings. It is anchored in a borehole via the friction and mechanical interlock on the contact interface between the tube and the borehole wall. Tube stabilisers are characterised of their ductility, that is, they accommodate large rock deformations. However, the load-bearing capacity of tube stabilisers, for instance Split set, is very low. It is known to reinforce a hollow tube bolt with a rebar bolt grouted within the hollow tube. A patent application on such a hybrid bolt (US2008/0219775 Al) describes how to enhance the load-bearing capacity of the Split set. The hybrid bolt is composed of a Split set bolt and a rebar bolt that is fully grouted within the hollow Split set bolt, as shown in Fig. 7. The hybrid bolt significantly enhances the shear strength of the bolt compared to the Split set bolt. The pull load capacity of the hybrid bolt is also higher than Split set. The hybrid bolt still absorbs deformation energy in the same manner as Split set, that is, through the friction at the tube rock interface. The frictional resistance of the hybrid bolt is higher than the frictional resistance of the Split set bolt alone. Therefore, the hybrid bolt would absorb more energy as the rock is deformed than what the Split set bolt alone does. This significantly increases both the tolerance to axial load and significantly improves the lateral (shear) load tolerance, too. However, in case of extensive rock dilatation parallel to the borehole axis direction, the split set bolt may easily break and then the rock deformation load is transferred to the rebar bolt, which then easily breaks due to the overall rigid fixation of the 4 rebar in the grout. Thus a local dilatation of the rock which exceeds the tensile strength of the rebar bolt may further break the bolt because the rebar bolt is incapable of redistributing the stress longitudinally, thus incurring stress-concentration necking of the rebar bolt. Summary of the invention According to the present invention there is provided a combined rock bolt (1, 2) for being grouted in a borehole, comprising: - a hollow friction split bolt (1) for being arranged generally in contact with the borehole wall and for extending from generally the rock surface and at least extending in a portion of the borehole, - a solid bolt (2) for being arranged within said hollow friction split bolt (1) and extending at least throughout said hollow friction split bolt (1), - said solid bolt (2) is provided with at least one smooth stem portion (3) and at least one anchor (4) near an inner end of said solid bolt, wherein said solid bolt is arranged to be inserted into the borehole through said hollow friction split bolt with the inner end first, until said at least one anchor is arranged beyond said friction split bolt. A significant advantage of the present invention is the capability of the solid bolt with its smooth stem portion to take up much energy and which is allowed to be strained over a long section between the anchors, in this simplest embodiment the anchor near the inner end and a surface anchor. In case of extensive rock dilatation parallel to the borehole axis direction, the split set bolt may relatively easily slip or/and break. However, a significant portion of rock deformation load is loaded on the solid bolt between its anchored positions. Thus a local 5 dilatation of the rock which would otherwise exceed the tensile strength of a rebar bolt would not easily break the bolt of the invention because the combined bolt of the invention is capable of redistributing the stress longitudinally, thus preventing stress concentration and further preventing or significantly delaying necking of the solid sleek bolt. In another aspect of the invention is a method for supporting rock, comprising the steps of: - drilling a borehole in the rock, - arranging a hollow friction split bolt (1) generally in contact with the borehole wall and extending generally from the rock surface and along at least a portion of the borehole, - arranging grout in said borehole, - inserting a solid bolt (2) with at least one smooth stem portion (3) with an anchor (4) near an inner end of said solid bolt (2), into the borehole through said hollow friction split bolt with the inner end first, until said at least one anchor (4) is arranged beyond said friction split bolt (1) - allowing said grout to set and harden. Alternatively the method may be conducted in a slightly different order, comprising arranging grout in the borehole before forcing in the hollow friction bolt (1) and then the solid bolt (2). In an advantageous aspect of the invention the method comprises arranging an inner stem portion (3) with said anchor (4) of said solid bolt (2) extending beyond an inner end of said hollow friction bolt (1). In such an embodiment the inner anchor (4) of the solid bolt will be loaded directly, and in case no other anchors exist on the solid bolt (2), except for the surface anchor, the forces on the solid bolt (2) due to rock dilatation 5a will be distributed over generally the entire length of the solid bolt (2). The method comprises setting at least an inner anchor (4) in the borehole in grout. In an advantageous aspect of the invention at least one anchor (4) within said hollow friction bolt (1) in the borehole is set in grout. In a preferred embodiment, at least a near-surface anchor (4) is set in grout within the split set bolt (1), thus at least the inner and the outer anchor may take up load along the solid bolt stem extending between the two anchors. This may unload the split set bolt or take over the entire role as rock reinforcement when the split set bolt slips/breaks, and represent a significant long strain tolerant bolt which may take up far more rock deformation as compared to a split set bolt reinforced by a rebar bolt.

6 Brief description of the drawings The invention and some background art is illustrated in the drawings. Fig. la is a schematic illustration of a combined rock bolt according to the invention comprising a hollow cylindrical split set bolt and a solid bolt with at least one anchor arranged near an inner end on an otherwise smooth bolt. The combined rock bolt is illustrated as installed with grout in a borehole and comprises a face plate and a nut together constituting a rock surface anchor. It is illustrated that the resin or cement does not intrude between the borehole wall and the split set bolt, although some resin or cement may leak through the slit into that very tight annular space. Fig. lb illustrates shear load on the surface of the split set bolt of Fig. la, and Fig. lc illustrates the axial tensile load in the solid bolt of Fig. la as a function of distance from the surface. Fig. 2a is similar to Fig. la with the difference that according to an embodiment of the invention the solid bolt is provided with another anchor arranged near the surface end of the bolt, but still within the resin or cement filled portion within the hollow bolt. Fig. 2b is similar to Fig. lb and illustrates shear load on the surface of the split set bolt of Fig. 2a. Fig. 2c illustrates the axial tensile load in the solid bolt of Fig. 2a, please notice the drop in axial load in the solid bolt exterior of the near surface anchor. Fig. 3a is similar to Fig. 2a with the difference being a simplified embodiment without a nut and a face plate mounted on 7 the solid bolt, only the split set bolt face plate arranged. Fig. 3b illustrates the shear load on the surface of the split set bolt of Fig. 3a, and Fig. 3c illustrates the axial load in the solid bolt of Fig. 3a as installed without a rock surface anchor on the solid bolt. Fig. 4a illustrates two further features of embodiments of the invention. Firstly, a further anchor is arranged between the inner anchor and the near-surface anchor. Secondly, a weak link is formed on purpose on the hollow friction bolt by making a transversal slot cut on the hollow friction bolt. Fig. 4b illustrates the shear load on the split set bolt, and Fig. 4c illustrates the axial tensile load in the solid bolt under axial rock dilatation. The inset Fig. 4d shows a side view and a cross-section view of a very simple embodiment of the weak link. Fig. 5a is similar to Fig. 4a with the main difference being the absence of a main stem surface anchor. Fig. 5b and Fig. 5c show the resulting shear load along the split set bolt and the resulting tensile load in the solid bolt, which is reduced to practically nil outside of the position of the near-surface anchor. Fig. 6a shows a longitudinal cross section of a split set bolt of the prior art arranged in a borehole in a rock. Fig. 6b illustrates the surface shear load along the split set bolt in that situation of which rock dilatation has set in. Fig. 7 is an image taken on the surface of an underground tunnel wall with a surface support of steel net held by plates of split set bolts, and in which the split set bolts are reinforced by rebar bolts with surface plates. One of the rebar bolts in this 8 image extends by error out from the surface and is shown here only to substantiate the fact that it is a rebar bolt. Fig. 8 is a cross-section of prior art friction bolts of the expandable type which are inserted into the borehole and inflated by internal pressure through the part of the bolt extending out of the borehole. Embodiments of the invention The invention is illustrated in the attached drawing Figures. The most basic mechanical embodiment of the invention is illustrated in Fig. la and is a combined rock bolt (1, 2) for being grouted in a borehole, comprising a hollow friction bolt (1) arranged for being forced into the borehole generally in contact with the borehole wall and for extending from generally the rock surface and at least extending inwards through a portion of the length of the borehole, with a solid bolt (2) arranged for being put within said hollow friction bolt (1) and extending at least throughout said hollow friction bolt (1). The solid bolt (2) is provided with at least one sleek stem portion (3, 3f) and at least one anchor (4, 4f) near an inner end of the solid bolt (2). Here, the strength of the surface anchor's plate link to the rock surface must be larger than the strength of the solid bolt (2). In a preferred embodiment of the invention, the combined rock bolt is provided with an inner stem portion (3f) with said anchor (4f) of said solid bolt (2) extending beyond an inner end of the hollow friction bolt (1). This will assure that the solid bolt is subject to tension loading once the grout has hardened and general rock dilatation occurs. The rock dilatation starts immediately during blasting and excavation of a cavity in the rock, such as forming a drift tunnel or during a drift during mining or underground construction.

9 According to a preferred embodiment of the invention, the hollow friction bolt is provided with a surface anchor (11), here called a friction bolt surface anchor portion (11). This friction bolt surface anchor portion (11) should comprising a flange (12) and a face plate (13) for holding a steel net for retaining the rock surface, possibly covered with shotcrete. In a preferred embodiment of the invention such as shown in Fig. la, Fig. 2a and Fig. 4a the solid bolt (2) is provided with a solid bolt surface anchor portion (21). The solid bolt surface anchor portion (21) usually comprises a face plate (22) and an outer threaded portion (23) with a nut (24). Alternatively the threaded portion and the nut may be replaced by a so-called barrel and wedge lock to fix the face plate to the solid bolt. In an embodiment of the invention the solid bolt (2) is provided with two or more smooth stem portions (3) separated by anchors (4), the number of anchors (4) being two or more. This situation is shown in Figs. 2a, 3a, and 4a. One of the anchors (4n) may be arranged near the rock surface portion of the solid bolt (2). This will change the load pattern on the solid bolt (3). It will change the load pattern from what is illustrated in Fig. Ic, in which the entire part of the smooth stem length (3) between the inner anchor (4f) (also named the "far" anchor) and the surface anchor (21) is loaded, to what is illustrated in Fig. 2c in which generally the stem portion between the inner anchor (4f) and the near-surface anchor (4n) is loaded. This leaves the main rock dilatation load to the solid stem, and may leave any possible surface net to be held only by the surface anchor of the split set bolt (1). In a preferred embodiment of the invention, the anchors are integrated with the stem and formed as paddle-shapes along the bolt. If a near-surface anchor (4n) is arranged in grout within the hollow pipe, and a surface anchor (22, 23, 24) is used on the 10 solid stem, the strength requirements to the surface anchor may be reduced. A significant advantage to such an embodiment is that the threaded portion of the stem does not need to be as strong as the remainder of the solid stem, because the stem outside of the near surface anchor (4n) is less loaded than the stem portion further in. In an embodiment of the invention shown in Fig. 4a (and also in Fig. 5a), the hollow friction bolt (1) may be provided with a weak link portion (16), or several weak link portions, such as transversal slots, furrows or cuts. These weak links are arranged for breaking during rock dilatation exceeding the tensile strength of the weak link portions. The effect of the weak link portions (16) breaking during dilatation of the rock is that the solid stem (2) takes over the load and allows the load to be distributed over a longer material length along a smooth stem portion (3, 3f) between two successive anchors (4, 4f, 4n). The solid bolt solid anchor portion (21) may in other embodiments comprise a so-called "barrel & wedge" connection for locking the face plate, or an integral bolt head. In an embodiment of the invention the surface portion of the solid bolt is not anchored but free in the outer portion, and an integrated anchor on the stem may be arranged some distance in from the surface, set in grout within the split set bolt (1). In a particular embodiment of the invention, the hollow friction bolt (1) may be of the so called split-set bolt or of the expandable pipe "Swellex (TM)" type, which must be further prepared before inserting the solid bolt (1). In a method aspect of the invention, it is a method for supporting rock, comprising the steps of: - drilling a borehole in the rock, - arranging a hollow friction bolt (1) generally in contact with 11 the borehole wall and extending generally from the rock surface and along at least a portion of the borehole, - arranging grout in the borehole, - arranging a solid bolt (2) with at least one smooth stem portion (3) with an anchor (4n) near an inner end of the solid bolt (2). The solid bolt (2) is arranged extending within the hollow friction bolt (1) in the borehole, with the solid bolt (2) generally extending throughout the length of the hollow friction bolt (1), and - allowing the grout to set and harden. Alternatively the method may be conducted in a slightly different order, comprising arranging grout in the borehole before forcing in the hollow friction bolt (1) and then the solid bolt (2). If setting the split set bolt (1) and then grouting before setting the solid bolt, some grout may leak through the slit anyway. The most important fixation of the hollow friction bolt results from the split set bolt spanning against the circular wall, and additionally the resin or cement binding to the borehole wall through the slit along the entire resin or cement filled portion of the split set bolt. The method of the invention covers both having the split set bolt covering an extensive outer part of the borehole so that the inner anchor (4f) is fixed in grout within the split set bolt (1), which would work fine when set under rock dilatation tension, or comprising, in the preferred embodiment of the invention, arranging an inner stem portion (3) with said anchor (4) of said solid bolt (2) extending beyond an inner end of said hollow friction bolt (1), which would load the solid stem immediately tension is formed. Anyway, it is necessary setting at least one inner anchor (4f) in the borehole in grout in order to transfer load to the solid stem. Further, as shown in Fig. 2, at least one anchor (4, 4n) within said hollow friction bolt (1) in the 12 borehole may be set in grout. Fig. 4a illustrates two further features of embodiments of the invention. Firstly, a further anchor is arranged between the inner anchor (4f) and the near-surface anchor (4n), the further anchor (4) also arranged within the hollow tube. Secondly, a weak link (16) is formed on purpose on the hollow friction bolt by making a transversal slot cut on the hollow friction bolt. Fig. 4b illustrates the shear load on the split set bolt, and Fig. 4c illustrates the axial tensile load in the solid bolt under axial rock dilatation. The inset Fig. 4d shows a side view and a cross-section view of a very simple embodiment of the weak link. Imagining that the rock dilatation is not linear but increases toward the surface, the weak link will break rather early. The load on the solid stem portion (3) will be greatest between the outer, near-surface anchor (4n) and the subsequent anchor (4). The load on the stem portion (3) between the outer near-surface anchor (4n) and the surface anchor (22) will be significantly reduced. Thus the strength of the thread/bolt and the surface plate may be lower than the strength of the un-threaded portions of the shank. The embodiment illustrated in Fig. 4a will thus constitute a very reliable anchoring both with regard to load distribution and energy capacity and prevent loosing surface anchor control. Fig. 5a is similar to Fig. 4a with the main difference being the absence of a main stem surface anchor. Fig. 5b and Fig. 5c show the resulting shear load along the split set bolt and the resulting tensile load in the solid bolt, which is reduced to practically nil outside of the position of the near-surface anchor. Fig. 6a shows a longitudinal cross section of a split set bolt of the prior art arranged in a borehole in a rock. Fig. 6b 13 illustrates the surface shear load along the split set bolt in that situation of which rock dilatation has set in. Fig. 7 is an image taken on the surface of an underground tunnel wall with a surface support of steel net held by plates of split set bolts, and in which the split set bolts are reinforced by rebar bolts with surface plates. One of the rebar bolts in this image extends by error out from the surface and is shown here only to substantiate the fact that it is a rebar bolt.

Claims

1. A combined rock bolt for being grouted in a borehole, comprising - a hollow friction split bolt for being arranged generally in contact with the borehole wall and for extending from generally the rock surface and at least extending in a portion of the borehole, - a solid bolt for being arranged within said hollow friction split bolt and extending at least throughout said hollow friction split bolt, - said solid bolt provided with at least one smooth stem portion and at least one anchor near an inner end of said solid bolt, wherein said solid bolt is arranged to be inserted into the borehole through said hollow friction split bolt with the inner end first, until said at least one anchor is arranged beyond said friction split bolt.

2. The combined rock bolt according to claim 1, with an inner stem portion with said anchor of said solid bolt extending beyond an inner end of said hollow friction split bolt.

3. The combined rock bolt according to claim 1 or 2, said hollow friction split bolt provided with a friction bolt surface anchor portion (11).

4. The combined rock bolt according to claim 3, said friction bolt surface anchor portion further comprising a flange and a face plate.

5. The combined rock bolt according to any one of claims 1 to 4, said solid bolt comprising a solid bolt surface anchor portion. 15

6. The combined rock bolt according to claim 5, said solid bolt surface anchor portion further comprising a face plate and an outer threaded portion with a nut.

7. The combined rock bolt according to any one of claims 1 to 6, said solid bolt provided with two or more smooth stem portions separated by anchors, the number of anchors being two or more.

8. The combined rock bolt according to claim 7, one of said anchors arranged near the rock surface portion of said solid bolt.

9. The combined rock bolt according to claim 7 or 8, said hollow friction bolt arranged with weak link portions formed by slots, furrows or cuts arranged for breaking during rock dilatation.

10. A method for supporting rock, comprising the steps of: - drilling a borehole in the rock, - arranging a hollow friction split bolt generally in contact with the borehole wall and extending generally from the rock surface and along at least a portion of the borehole, - arranging grout in said borehole, - inserting a solid bolt with at least one smooth stem portion with an anchor near an inner end of said solid bolt, into the borehole through said hollow friction split bolt with the inner end first, until said at least one anchor is arranged beyond said friction split bolt, - allowing said grout to set and harden.

11. The method according to claim 10, comprising arranging an inner stem portion with said anchor of said solid bolt extending beyond an inner end of said hollow friction split bolt.

12. The method according to claim 10 or 11, comprising setting at least one inner anchor in the borehole in grout. 16

13. The method of according to claim 12, comprising setting at least one anchor within said hollow friction split bolt in the borehole in grout.