EP0111512A1

EP0111512A1 - Reinforcing and confining earth formations

Info

Publication number: EP0111512A1
Application number: EP83901798A
Authority: EP
Inventors: Michael Charles Tucker
Original assignee: Gearhart Australia Ltd
Current assignee: Gearhart Australia Ltd
Priority date: 1982-06-16
Filing date: 1983-06-08
Publication date: 1984-06-27
Also published as: ZA834289B; EP0111512A4; WO1984000048A1

Abstract

Renforcement et délimitation de formations de terrain comprenant une pluralité d'organes de support présentant une partie de retenue (21) permettant l'ancrage dans un trou de forage dans la formation de terrain et une partie exposée possédant au moins deux extrémités libres (22, 23) situées sensiblement contre la formation de terrain en formant entre elles un angle compris entre 90o et 180o, les extrémités libres possédant des boucles fermées pour l'interconnexion directe avec des organes de support dans des trous de forage adjacents pour définir une structure en treillis au-dessus de la surface de la formation de terrain. Celle-ci peut être renforcée en mettant sous tension la partie ancrée d'un organe de support dans le trou de forage et/ou les extrémités libres.Reinforcement and delimitation of land formations comprising a plurality of support members having a retaining part (21) allowing anchoring in a borehole in the land formation and an exposed part having at least two free ends (22, 23) located substantially against the formation of terrain by forming an angle between 90o and 180o between them, the free ends having closed loops for direct interconnection with support members in adjacent boreholes to define a trellis structure above the surface of the terrain formation. This can be reinforced by tensioning the anchored part of a support member in the borehole and / or the free ends.

Description

REINFORCINGANDCONFININGEARTHFORMATIONS This invention is concerned with devices for reinforcing and consolidating earth structures such as mine shafts, tunnels and other underground structures and to improvements in rock bolts. Rock bolts are used either individually or in association with beam members and mesh members to reinforce a mine roof or wall structure to prevent or ameliorate the possibility of collapse of the roof or wall.

The most common type of rock bolt comprises a helically threaded shaft which is inserted into a pre- drilled hole and retained, in situ by a chemical grout such as a polyester resin or the like. The shaft diameter is somewhat smaller than the hole diameter so that it is essential for the chemical grout to have a rapid curing time (between 15 seconds to 1 minute) in order firstly to retain the bolt in the hole unsupported and secondly to enable the bolt to be tensioned by a nut. The helical thread extending over at least the upper part of the bolt shaft serves to increase the surface area of the bolt to enhance bonding thereto and also in an endeavour to prevent the liquid resin from flowing away from the region between the drill hole walls and the bolt itself.

To ameliorate the problem of low initial bolt retention and resin flow, rapidly curing resins with excess . catalyst and/or accelerator are generally employed. A difficulty with this is that excess heat is generated with the exothermic curing reaction and the resultant effects are initial reduction in resin viscosity and increased flow. A further problem is encountered with post curing shrinkage as the resin cools after cross-linking and shrinks away from both the bolt and the drill hole thus reducing structural integrity.

The problem of initial bolt retention has been overcome to some degree by the use of split end bolts and wedges. By ramming the bolt upwardly the split ends of the

OMPI bolt are forced outwardly by engagement of the wedge with the upper end of the drill hole. By using a mechanical retention means it^* is possible then to use a considerably less expensive cementitious grout to bond the rock bolt into its bore hole.

Rock bolts are, however, quite expensive to manufacture due to the considerable amount of steel in each bolt, nut and support plate as well as the machining required to shape eac item. In a mine shaft, tunnel or the like, it is not uncommon to have rock bolts at 1 metre centres on a surface (roof or wall) to be supported. In more difficult situations, it is common to support a welded steel mesh or steel beams against the earth face by rock bolts. This adds substantially to the cost of reinforcing and confining the earth structure.

In co-pending Australian Patent Application No. 91307/82 by the same Inventor, a mesh-like structure comprised of steel rod members was proposed. These members comprised a variety of shapes including U-shape, M-shape with leg portions for insertion in a bore hole and a transverse portion for lying against an earth surface. The mesh structure was obtained by inserting into prepared bore holes, the legs of the members to obtain a linked, mesh-like arrangement. The main difficulties associated with this system were the need for accurate bore hole spacing, the inability to use conventional chemical grout cartridges because it was not possible to spin the members in a bore hole and there was no direct link between adjacent members making up the mesh structure. The members were linked by providing a plurality of legs in each bore hole, which legs were subsequently bonded by a cementitious grout in the bore hole.

The degree of tensioning of a rock bolt in a bore hole will vary depending on th.e nature of the earth formation. For example in a coal mine, the coal seam is a heavily fractured and layered mass and many miners prefer to apply the greatest degree of tension possibly, say, up to 12 tonnes. By contrast, in hard rock mining conditions, the degree of tensioning may be substantially reduced, particularly if the purpose of the rock bolt is to support a mesh liner or the like.

If the mining conditions are such that the rock bolt or the like does not require immediate tensioning, it is possible to use less expensive, slower curing cementitious grouts provided that the bolt may have some initial means for retention in the bore hole.

It is an aim of the present invention to overcome or alleviate the problems of prior art earth reinforcing and confining structures and to provide an apparatus and a method adapted for rapid installation of a reinforcing and confining mesh-like structure for earth formations.

According to one aspect of the invention there is provided a support member for earth formations comprising:- a retaining portion for anchoring in the bore hole and a normally exposed portion having at least two free ends for direct connection to respective support members situated in adjacent bore holes, said exposed portion being adapted to be bent to form at least two transverse limbs in use lying substantially against the earth formation. Preferably said support member comprises a closed loop on each of said at least two free ends.

Preferably said rod-like elements comprise elongate substantially U-shaped members, the free ends of the legs forming the insertable portion of the support member. Preferably the support member includes means to enable tensioning of said support member within a bore hole.

According to another aspect of the invention there is provided a system for reinforcing and confining an earth formation, the system comprising a plurality of restraining units each having a retaining portion anchored to a

OMPI respective bore hole in an earth formation and at least two transverse limbs lying substantially against the^' earth formation, the free ends of said transverse limbs being interlinked directly to respective restraining units in adjacent bore holes to define a mesh-like structure. Preferably said system is so constructed and arranged whereby at least some of said transverse limbs lie against the earth formation at an angle of substantially 180° to each other. Preferably said system is so constructed and arranged whereby at least some of said transverse limbs lie against the earth formation at an angle of substantially 90° to each other.

Preferably said system is tensioned to provide a tension in the retaining portion of said support members. Preferably said system is tensioned to provide a tension in the transverse limbs of said support members. According to yet another aspect, of the invention there is provided a method of reinforcing and confining an earth formation by means of a plurality of rod-like support members, the members each having a retaining portion for anchoring in a prepared bore hole in the earth formation and a normally exposed portion having at least two free ends adapted to lie against the earth formation at an angle of between 180 -90° to each other, wherein the retaining portions are anchored in prepared bore holes and the exposed portion is arranged to lie against the earth formation as transverse limbs to define a mesh-like structure having the transverse limbs linked directly to respective support members in adajcent bore holes.

Preferably the retaining portion of at least some of the support members is tensioned.

Preferably the transverse limbs of at least some of the support members is tensioned. As used herein, the expression "rod-like member"

OMPI includes steel rod of round, square, polygonal or any other cross section, and the rod may include undulations or may have a textured or deformed surface such as helical threads and the like. Various preferred embodiments of the invention will now be described with reference to the accompanying drawings in which:-

FIG.^■ 1 illustrates a collar for a mine roof support member. FIG. 2 illustrates a mine roof support member.

FIG. 3 illustrates an alternative embodiment of one aspect of the invention.

FIG. 4 illustrates one form of a mesh-like mine roof^'support structure. FIG. 5 illustrates an alternative mesh-like structure

FIG. 6 illustrates a single support member of the arrangement shown in FIG. 5.

FIG. 7 shows the interlinked structure according to the invention. FIG. 8 shows a means for tensioning the support members.

FIGS. 9, 10 and 11 show alternative tensioning means. FIGS. 12, 13 and 14 show alternative configurations o support members according to the invention. FIG. 1 illustrates one form of a mine roof support member according to the invention. The member comprises a pair of elongate U-shaped elements 1, 2 formed from steel rod. The rods are preferably retained in relative juxtaposition as a bundle by frictional engagement of a collar 3 as illustrated in FIG. 2. The collar is not considered essential to this embodiment as the elements could be retained at least for handling and installation purposes by a steel, plastic or rubber band or the like. A wedge 4, comprising a short length of steel rod of the same or different diameter to the elements 1, 2 is at least partially inserted into the cavity between the four adjacent ends of the U-shaped elements and is retained therein by frictional engagement.

Installation of the mine roof support member of FIG. 1 is effected by employing an adaptor 5 with a cavity 6 shaped to receive the lower ends of the U-shaped elements 1,2. The lower end of adaptor 5 has a square section spigot adapted to engage a corresponding socket of a conventional mine drill. In a manner analogous to the installation of conventional mine roof bolts, a bore hole of desired diameter and depth is first formed. A sachet of chemical grout is placed into the bore hole and then the assembly is inserted into the bore hole. With the aid of the mine drill and adaptor 5, the assembly is rotated briefly within the bore hole to release and mix the chemical grout. Before curing of the grout commences, the rotation of the drill is ceased and upward pressure is exerted on the assembly by the hydraulic lift of the drill to engage wedge 4 against "the upper extremity of the bore hole. As upward pressure .is exerted on the assembly, wedge 4 is driven between the ends of the U-shaped elements to force them into firm engagement with the walls of the bore hole. If required, the degree of wedging may be altered by adjusting the position of collar 3 (or equivalent) closer to or further from the region of the wedge. Any tendency of the collar to be moved downwardly as the upper rod ends move apart under the influence of the wedge is resisted by engagement between the lower flared end of the collar and the wall of the bore hole.

In FIG. 2, the collar comprises a length of thin- walled steel tubing 9. One end 10 of the tubing is flared or otherwise expanded to have an external diameter greater . than that of the other end 11. Flaring may be achieved by conventional metal shaping processes but conveniently it may be achieved by providing a slit 12 adjacent the end to be flared and then opening the slit 12 to the desired degree, On the opposite side of slit 12 and extending the entire length of the tube is a further slit 13. The dimensions of the tube 9 are selected or otherwise adjusted by expanding or contracting slit 13 whereby the collar may be firmly frictionally engaged on a mine roof support member. The dimensions of the flared end 2 are chosen or otherwise adjusted to permit a light frictional engagement between the flared portion of the collar and the wall of a pre-prepared drill hole. It will be readily apparent at this stage that the collar would be equally applicable to conventional rock bolts.

In a fairly typical situation employing a 24mm outside diameter roof bolt or an equivalent diameter assembly of FIG. 1 and a 28mm inside diameter bore hole, a collar of any suitable length with an outside diameter of 25mm at its unflared end and 28mm at its flared end may be employed. Depending on the volume of chemical grout intended to be employed, the sleeve is placed at a suitable position remote from the upper end of the bolt. After insertion of the bolt into the borehole with the grout package and a short mixing cycle, the drill can be removed immediately as the flared end of the collar serves to retain the bolt in place. The flared end, being a friction fit within the bore hole, also serves to retain the resinous grout in the upper region of the bolt by preventing downflow. In this manner, it is anticipated that less expensive, slower curing (and thus flowable) grouts may be employed by use of this aspect of the invention. It is also possible that even less expensive very slow curing cementitious grouts may be employed with this aspect of the invention.

In an alternative method of securing the mine roof support member, a member comprising a plurality of rod-like elements is inserted into a bore hole with a sachet of chemical grout as described above. Also as previously described, the member is rotated to puncture the sachet to release and then mix the contents. Instead of ceasing rotation of the structure with the drill before the grout commences curing, rotation is continued until the torque clutch of the drill slips, thus indicating cure of the grout. With the torque clutch set at an appropriate torque, the plurality of rod-like elements are twisted into a configuration comprising a multi-start helix. By appropriate torque adjustment and, if required, upward compression by the hydraulic drill extension, the effective diameter of the roof support member is increased by the helical twisting action by either plastic or elastic deformation of the rod¬ like elements (or a combination thereof) . In this manner, a substantial portion of the length of the support member is caused to firmly engage the wall of the bore hole thereby increasing the degree of retention in the earth formation. As shown in FIG. 3, the bore hole 14 is formed with a flared or tapered opening 15. A support member, comprising a pair of U-shaped elements 16, 17 chosen so that the length of the support member, when helically twisted corresponds substantially with that of the bore hole. Prior to insertion of the U-shaped- elements in the bore hole, a support plate 18 is placed over the free ends of the rod-like elements and positioned adjacent the ooped or outer ends. The support plate 18 may comprise a flat plate with a centrally located aperture (preferably circular) . Most preferably the portion of the plate in the region of the aperture is formed as a frusto-conical projection 19 as shown. When a support member of FIG. 1 or the like is rotated in the bore hole with a suitable rapid curing grout, the grout, as it cures, retains the free ends of the U-shaped members thus permitting the members to be twisted into a helical configuration. As twisting continues the effective length of the U-shaped members reduces until the sides of the "eye" 20 are tensioned upwardly against the inner face of frusto-conical projection 19. Thus the support members of FIG. 1 or the like may be

OMP employed in a manner analogous to a conventional tensionable rock bolt. An advantage is that an "eye" is formed at the free exposed end through a cable which may be laced, to form a mesh-like structure. Although it is preferred to use the wedge and collar as described above, it is considered that for certain earth formations, either or both of these support member retention devices may be dispensed with. The frictional engagement of the multi-start helical configuration of rod-like elements within the bore hole may alone give sufficient retention, either with or without the further retention of the grout.

FIG. 4 illustrates one way in which roof or wall support members, as described herein and with particular reference to FIG. 1, may be installed in a mine roof or wall structure to form a wholly or partially interconnected mesh structure. A support member is inserted and secured into a bore hole as described above with portion of the lower ends of the U-shaped elements extending outwardly from the bore hole. The two U-shaped legs are then bent outwardly from each other at 180 until the bent portions lie closely abutting the earth surface. If required, a further adaptor may be attached to the mine drill to separate the legs and guide them into closely abutting relationship with the earth surface by operation of the hydraulic extension of the mine drill. Additional bore holes may be drilled at predetermined intervals from the initial bore hole to enable adjacent support members to mechanically interconnect. Alternatively, once the legs are bent flat against the earth surface, the drill bit is placed within the loops adjacent their outer extremity and further bore holes drilled. In this manner, a substantially continuous, interlocking mesh structure may be constructed over a predetermined area of a mine roof or wall surface. In areas where earth or rock confinement is considered to be a particular problem, several overlapping layers of leg portions may be arranged to achieve any one of a number of mesh patterns of suitable strength characteristics. FIG. 5 shows an alternative and most preferred mesh array. A support member 21 is anchored in a bore hole and the free ends 22, 23 of the U-shaped members are then bent to lie against the earth surface at about 90 to each other. Bore holes are then drilled within the loops of the U-shaped ends to accept additional support members which are then bent as described above. Repetition of these steps results in a two dimensional, substantially regular mesh array over the surface of the earth structure to be reinforced and confined. The mesh size may be varied by altering the length of the support members or simply by altering the depth of the bore hole to leave an exposed portion of support member to a suitable length. If additional confinement is required, a further mesh-like array may be arranged over the top of the first array to give a smaller mesh size. FIG. 6 shows a single support member of the array of

FIG. 5.

It will be clear to a skilled addressee that the resultant interlinked mesh-like structure gives a substantiall continuous two dimensional tensile web over- the surface of the earth formation. The web is anchored to the earth formation at the intersections of the mesh. Accordingly, a localized failure in the earth formation may be supported and confined by dissipation of forces throughout the two dimensional tensile web which in turn is supported at the earth face by the anchored portions.

FIG. 7 shows a cross section through an earth formation in which an array of FIG. 5 is anchored.

In some earth formations, e.g. coal mines, where the structure is very friable or laminar in nature, it is advantageous to apply a tension to the support members either

OMPI in the anchored portion or the transverse portion (or both) to reinforce the earth structure. Reinforcement is achieved when the earth mass is compressed by the application of a tensile force in the support member between the anchored end and the earth surface. By suitable spacing of the bore holes for adjacent support members, it is possible to obtain overlapping stress "envelopes" when the insertable portions of the members are tensioned, thus enabling a stressed reinforcement of the earth formation. This is well known in conventional rock bolt technology.

A particular advantage of this invention resides in the interconnection of anchored members to provide what amounts to a substantially continuous tensile web over the surface of the earth formation. Thus, if the lateral element forming the interconnected array are also tensioned, this enables compression of the earth mass in a direction normal to that applied by tensioning the insertable portions alone.

FIG. 8 shows one method for tensioning the array. The drawing shows the legs of U-shaped members 24 arid 25 inserted in a bore hole 26. The free looped ends 27,28 are bent at 90 as shown generally in FIGS. 5 and 6. An interlinked intersection between adjacent mesh elements occurs where the free looped end 29 of an adjacent support member passes around and captures members 24 and 25 in the region of the bend between insertable and transverse portions.

The tensioning means comprises a flat steel plate 30, preferably an indentation or recess 31 to receive the free end of tensioning bolt 32. Bolt 32 is threadedly engaged with a channel section member 33, through which channel the legs of U-shaped member 24 lie. By tightening bolt 32 it can be seen that a tension will be applied to both the insertable leg portions as well as the transverse leg portions. A certain amount of additional tension is applied at the intersections when adjacent support members are tensioned. It will be seen that by placing the tension means between adjacent bore holes rather than close to the bore hole itself, the majority of the tensioning force will be applied in the transverse direction through the mesh array.

FIG. 9 shows an alternative embodiment of a tensioning means. The device comprises a base plate 33 for positioning against an earth surface. The plate 33 includes a depression or recess 34 to receive the free end of bolt 35 threadedly engaged in plate 36. One end of plate 36 includes a tab 37 slit in the end of the plate and bent downwardly to provide a curved surface of suitable radius. The operation of this device is substantially the same as that shown in FIG. 8 except that the transverse legs are retained in the recess formed by shoulders 38, 39 and tab 37. The plate 36 is fulcrumed for tilting movement at its rear edge 40 which engages base plate 33. By positioning the device of FIG. 9 close to a bore hole it is possible to apply considerable tension to the insertable portion of a support member.

FIG. 10 shows yet another alternative tensioning means suitable for tensioning both insertable leg portions of a support member comprising two U-shaped members with transverse portions bent at 90 to each other. The means comprises a truncated triangular base plate 41 which, if required, may have a concavely curved nose portion 42 to enable the base plate to closely abut a support member in a bore hole. Like the base plate of FIGS. 8 and 9 the base plate 41 may have an indentation or recess (not shown) to receive a free end of bolt 43 threadedly engaged in channel member 44. The upturned side flanges 45, 46 are disposed approximately at right angles to locate the transverse limbs of a support member as shown generally in FIGS. 5 and 6. When the bolt is tensioned, both the insertable portions and the 90 splayed transverse limbs of the support member will

OMPI be tensioned to reinforce the earth mass.

FIG. 11 shows a further variation on the tensioning means which is adapted for a support member with 180 splayed transverse limbs. The support member comprises a pair of U-shaped elements 45, 46 with a threaded shank 47 attached thereto intermediate their length by welding or the like. The support member is anchored in a bore hole to a depth such that the end of the threaded shank 47 extends a short distance from the bore hole opening. A base plate 48 with a central aperture 49 is placed over the looped ends of the U-shaped members protruding from the bore hole and the transverse limbs 50, 51 are then bent at 180 to each other to be against the earth surface. An inverted channel member 52 with a central aperture 53 is placed over the shank 47 to contact the base plate 48 with the transverse limbs within the channel. A threaded nut 54 is then engaged with shank 47 and torqued to tension the insertable portion of members 45 and 46. It will be clear of course that a simple timber, plastic or steel wedge may be inserted between the transverse limbs to tension at least the transverse members.

FIG. 12 illustrates an alternative embodiment of the support members according to the invention. The member comprises an insertable shank 55 with a screw threaded end 56. Attached to the shank (by welding or the like) below the threaded portion are limbs 57, 58 having at their remote ends closed eyes 59, 60 shaped by any suitable means such as forging, bending followed by welding, etc. Limbs 57, 58 are bent to be against the earth surface when the remote end of shank 55 is anchored in a bore hole with the threaded portion 56 protruding. Tensioning is effected in a manner similar to that shown in FIG. 11.

FIG. 13 is a variation on the embodiment shown in FIG. 12 wherein the transverse portions are formed by bending down two U-shaped loops 61, 52 which are attached to

OMPI a threaded shank 63 in a manner similar to that of FIG. 12.

FIG. 14 is a further variation comprising crook-shaped elements 64,65 with threaded portions 66, 67 at the ends of the crooked loops 68, 69. Bars 70, 71 having apertures adjacent each end are placed over the elements 64, 65 and the respective ends of the crooked loops 68, 69 and retained in position by threaded nuts 72, 73. Open or unclosed loops are not considered satisfactory for the practice of the present invention as the unclosed loops can open under tension on the transverse limbs. It is important to the working of the invention that the structural integrity be maintained to establish a substantially continuous tensile mesh.

It will be readily apparent to a skilled addressee that many modifications and variations may be made to the various aspects of the invention without departing from the spirit and scope thereof.

Claims

CLAIMS :

1. A support member for earth formations comprising:- a retaining portion for anchoring in the bore hole and a normally exposed portion having at least two free ends for direct connection to respective^' support members situated in adjacent bore holes, said exposed portion being adapted to be bent to form at least two transverse limbs in use lying substantially against the earth formation.

2. A support member as claimed in claim 1 including a closed loop on each of said at least two free ends.

3. A support member as claimed in claim 1 or claim 2 wherein said rod-like elements comprise elongate substantially U-shaped members, the free ends of the legs forming the insertable portion of the support member.

4. A support member as claimed in any preceding claim wherein the support member includes means to enable tensioning of said support member within a bore hole.

5. A system for reinforcing and confining an earth formation-, the system comprising a plurality of restraining units each having a retaining portion anchored to a respective bore hole in an earth formation and at least two transverse limbs lying substantially against the earth formation, the free ends of said transverse limbs being interlinked directly to respective restraining units in adjacent bore holes to define a mesh-like structure.

6. A system as claimed in claim 5 wherein at least some of said transverse limbs lie against the earth formation at an angle of substantially 180° to each other.

7. A system as claimed in claim 5 wherein at least some of said transverse limbs lie against the earth formation at an angle of substantially 90° to each other.

8. A system as claimed in any one of claims 5 to 7 wherein at least portion of the mesh-like structure is tensioned to provide a tension in at least some of the retaining portions of said support members.

9. A system as claimed in any one of claims 5-8 wherein at least portion of the mesh-like structure is tensioned to provide a tension in at least some of the transverse limbs of said support members.

10. According to yet another aspect of the invention ther is provided a method of reinforcing and confining an earth formation by means of a plurality of rod-like support members, the members each having a retaining portion for anchoring in a prepared bore hole in the earth formation and a normally exposed portion having at least two free ends adapted to lie against the earth formation at an angle of between 180 - 90 to each other, wherein the retaining portions are anchored in prepared bore holes and the exposed portion is arranged to lie against the earth formation as transverse limbs to define a mesh-like structure having the transverse limbs linked directly to respective support members in adjacent bore holes.

11. A method as claimed in claim 10 wherein the retaining portion of at least some of the support members is tensioned.

12. A method as claimed in claim 10 or 11 wherein the transverse limbs of at least some of the support members is tensioned.

13. A support member for earth formations substantially as hereinbefore described with reference to the accompanying drawings.

14. A system for reinforcing and confining an earth formation substantially as hereinbefore described.

15. A method of reinforcing and confining an earth formation substantially as hereinbefore described.