REINFORCEMENT OF EARTH STRUCTURES

Abstract: A method and system for reinforcing, confining and stabilising an earth formation wherein steel tubes (3) are driven into spaced bore holes (1) in an earth formation and, subsequently, restraining elements (4) are anchored within the tubes (3) by a chemical or cementitious grout. The restraining elements comprise an insertable limb (5) located within tube (3) and a transverse portion (7) which lies adjacent the earth formation. Adjacent restraining elements are linked in the region of the mouth of the bore hole to provide a linked linear or mesh-like structure against the surface of the earth formation for restraint and confinement of the tubular members providing stabilisation.

Description: REINFORCEMENTOFEARTHSTRUCTURES This invention is concerned with a method and apparatus for reinforcing and confining, earth structures. There are many anchoring systems available for stabilizing, and/or reinforcing and/or confining earth structures in underground openings such as mine shafts, tunnels and the like. Most of these anchoring systems employ tensionable rock bolts on their own or in conjunction with other reinforcing and confining members such as steel or timber beams, steel or concrete arches and props and in some cases, reinforced cementitious liners. Generally speaking, the choice of stabilizing and/or reinforcing and/or confining system is made depending upon the nature of the earth formation and the cost effectiveness of the system. With rapidly increasing materials costs, miners are becoming concerned with less expensive systems than those hitherto employed but nevertheless exhibit similar or even better characteristics of strength and safety. One such alternative system which is rapidly gaining favour amongst miners is a friction rock stabilizer colloquially known in the art as a "splitset". The friction rock stabilizer comprises a hollow elongate steel body of generally constant cross section and having a slot through its wall thickness extending longitudinally over the entire length of the body. The cross sectional dimensions of the body are chosen to be greater than a bore hole of predetermined diameter. Insertion of such a body into the bore hole in an earth formation causes substantial circumferential compression of the body such that when inserted, the outer periphery of the body frictionally engages the wall of the bore hole. The degree of frictional engagement between the rock stabilizer and the bore hole is a function of both area of engagement and the forces attempting to restore the circumferential compression of the body. At least limited tension may be applied to the rock stabilizer by inserting a wedge through aligned openings in the exposed end of the stabilizer body immediately beneath a plate or washer adjacent the bore hole opening. The main function of such rock stabilizers is to stabilize the earth formation against lateral shifting of bedding planes. The hollow construction enables partial collapse and deformation at a split plane under strains which could otherwise cause failure in shear of prior art roof bolts. Once deformed in this manner, the holding power of the stabilizer is increased somewhat. Rock stabilizers of the type referred to above are described in Australian Patent Nos. 514,153 and 497,039 and the content of those specifications is incorporated herein by reference. Another type of stabilizer comprises a malleable steel tube having a sealed end therein. The tube is inserted into a bore hole and "inflated" by substantial hydraulic or pneumatic pressure until the wall expands sufficiently to frictionally engage the wall of the bore hole. Although quite effective against lateral earth movements the abovementioned rock stabilizers are not entirely satisfactory in reinforcing and confining earth formations against convergence. Particularly effective reinforcing confining systems have been described in Australian PatentApplication No. 91307/82 by the present Inventor and co-pending Australian Patent Application Nos. 16015/83 and PF 6836, by the present co-applicants. The descriptions of the above patent applications are also incorporated herein by reference. It has been found by combining a friction rock stabilizer of the type previously mentioned with a reinforcing and confining system of the type immediately abovementioned greatly improved results can be achieved. According to one aspect of the invention there is provided a system for reinforcing, confining and stabilizing an earth formation comprising:- a plurality of hollow members frictionally engaged in pre-prepared bore holes in an earth formation by a circumferential compression of said hollow members; a plurality of restraining units, at least some of which have at least one retaining limb inserted into the interior of a respective hollow member and at least one transverse portion adjacent the surface of said earth formation, the transverse portions defining a linked arrangement anchored at least partly within said hollow members. Preferably said restraining unit comprises a substantially L-shaped member. Most .preferably said restraining unit comprises a substantially U-shaped member bent intermediate its length to form a retaining limb portion and a transverse portion. Preferably said retaining limb portion is anchored within said hollow member by a chemical grout or a cementitious grout. Alternatively said retaining limb portion is anchored ■ within said hollow member by mechanical means such as wedge means. According to another aspect of the invention there is provided a method of stabilizing, reinforcing and confining an earth formation by means of a plurality of restraining units, the restraining units having at least one retaining limb portion for insertion into the earth formation and a transverse portion for positioning adjacent the surface of the earth formation, wherein:- a plurality of spaced bore holes are formed in the e ar th f o rmat i on ; friction elements are inserted into the spaced bore holes, the friction elements comprising hollow members adapted to frictionally engage a bore hole wall by circumferential compression of said hollow members; and, the retaining limb portions of said restraining units are anchored within the interior of said hollow members and the transverse portions are arranged to define a linked arrangement adjacent the surface of said earth formation. A preferred embodiment of the invention will now be described with reference to the accompanying drawings in which:- FIG. 1 illustrates an assembly of components showing a linkage therebetween. FIG. 2 illustrates a single assembly of FIG. 1 subjected to shear strain within an earth formation. In FIG. 1, bore holes 1 are drilled into an earth formation 2 such as the wall or roof structure of a mine shaft, tunnel or other underground opening. Into the bore holes 1 are inserted friction rock stabilizers 3 of the type generally described in Australian Patent No. 497,039. The stabilizers, comprising a length of steel tubing of generally circular cross section with a slot extending longitudinally thereof are frictionally engaged within the bore hole by circumferential compression. A reinforcing and confining element 4 comprising a generally U-shaped length of steel reinforcing rod is bent intermediate its length to form an L-shaped member in side elevation as shown. That leg 5 of the L-shaped member having the free ends of the steel rod is inserted into the hollow interior of the friction stabilizer and is anchored at least adjacent its upper end 6 by a chemical or cementitious grout 10. The transverse portion 7 of the L-shaped member having a loop 8 at its remote end lies adjacent the surface 9 of the earth formation. The loop 8 extends around leg 5 of an adjacent L-shaped reinforcing element in the region of the bend. Thus it can be seen that linked structures as generally described in our co-pending Patent Applications are readily constructed. A major advantage of the above system is that excavation of the tunnel or shaft can continue at a high rate of advance without having to wait for the relatively slow installation of prior art reinforcing and confining systems. With the present system bore holes are drilled at a predetermined spacing and the frcition stabilizers are driven into place. Although the reinforcing strength of such friction stabilizers is not high, the reinforcing effect is immediate and sufficient to enable excavation to continue. If required, at least a limited amount of tension may be applied to the stabilizer by inserting a wedge through aligned apertures in the exposed end of the stabilizer and immediately below a plate or washer. This limited tension applies proportionately limited compression to the earth mass to reinforce it to some degree. With prior art systemsemploying a chemical anchor for say a tensionable rock bolt, it is necessary to carry out the additional steps of inserting a grout cartridge before the bolt, spinning the bolt to burst the cartridge and mix the contents thereof, waiting for the initial cure of the grout, fitting the plate washer and nut and then tensioning the nut. These additional steps can substantially impede the rate of roof or wall reinforcement and thus the rate of excavation advance, With the present system, however, it is possible to maintain a high.rate of insertion of friction stabilizers and allow say a second team of miners to insert the reinforcing and confining system. As there is no immediate urgency as to the rate of curing of the anchoring grout, the present system enables the use of inexpensive cementitious grout cartridges to anchor the retaining limb portions 5. Cementitious grout cartridges are described in our copending Australian Patent Application No. PF 8573 Accordingly, it will be seen that the present invention enables substantial advantages in cost and time savings over the other priot art structures without sacrificing performance strength or safety. FIG. 2 illustrates a further advantage of the present invention. One of the alleged advantages of friction stabilizers is than when lateral movement of earth planes occurs about a shear plane 11 the stabilizer is deformable. This deformation not only prevents failure in shear as with a solid rock bolt but enhances the frictional engagement between the stabilizer and the earth mass. As the reinforcing rod of the restraining units 4 is of relatively small dimension, it is able to accommodate the shape of deformed region 12 of the friction stabilizer without shearing. In addition, the tensile structure formed by the linked transverse members 7 substantially resists such lateral strain by deployment of force through the linked structure. The lateral members 7 when placed tinder tensile stress provide substantial support against convergence ' of the laterally sheared mass 13 by placing this mass into compression. A further advantage is that instead of using say an 8mm reinforcing rod which has a six ton tensile rating at loop 8, it is possible to use a 3 x 3mm stranded mild steel wire which has a four ton tensile rating at the loop. These tensile ratings are considered quite adequate in terms of potential load support for reinforcing and confining earth formations. Although the above description of the preferred embodiment is limited to a longitudinally split metal tube, it will be clear to a skilled addressee that non-split tubes may be utilized. All that is required of the tube is that it is capable of being frictionally engaged with the wall of a bore hole.

Claims: CLAIMS : 1. A system for reinforcing confining and stabilizing an earth formation comprising:- a plurality of hollow members adapted for frictional engagement in pre-prepared bore holes in an earth formation by a circumferential compression of said hollow members; a plurality of restraining units, at least some of which have at least one retaining limb inserted into the interior of a respective hollow member and at least one transverse portion adjacent the surface of said earth formation, the transverse portions defining a linked arrangement anchored at least partly within said hollow members. 2. A system as claimed in claim 1 wherein said restraining unit comprises a substantially L-shaped member. 3. A system as claimed in claim 1 or claim 2 wherein said restraining unit comprises a substantially U-shaped member bent intermediate its length to form a retaining limb portion and a transverse portion. 4. A system as claimed in any preceding claim wherein said retaining limb portion is anchored within said hollow member by a chemical grout or a cementitious grout. 5. A system as claimed in any one of claims 1-3 wherein said retaining limb portion is anchored within said hollow member by mechanical means such as wedge means. 6. A system as claimed in any preceding claim wherein said tube is expandable under the influence of an internally applied fluid pressure to frictionally engage the wall of a bore hole. 7. A system as claimed in any preceding claim wherein said hollow members comprise metal tubes having a non- circular cross section. 8. A system as claimed in any preceding claim wherein said hollow members comprise metal tubes having a longitudinally extending slit therein. 9. A system as claimed in any preceding claim wherein said hollow members are closed at a normally insertable end. 10. A system as claimed in any preceding claim wherein said transverse members define a linked linear support structure. 11. A system as claimed in any one of claims 1-9 wherein said transverse members define a linked mesh-like support structure. 12. A method of stabilizing, reinforcing and confining an earth formation by means of a plurality of restraining units, the restraining units having at least one retaining limb portion for insertion into the earth formation and a transverse portion for positioning adjacent the surface of the earth formation, wherei :- a plurality of spaced bore holes are formed in the earth formation; friction elements are inserted into the spaced bore holes, the friction elements comprising hollow members adapted to frictionally engage a bore hole wall by circumferential compression of said hollow members; and, the retaining limb portions of said restraining units are anchored within the interior of said hollow members and the transverse portions are arranged to define a linked arrangement adjacent the surface of said earth formation.

Inventors: TUCKER MICHAEL CHARLES, TUCKER, MICHAEL, CHARLES
Applicants: GEARHART AUSTRALIA, TUCKER MICHAEL C, GEARHART AUSTRALIA LIMITED, TUCKER, MICHAEL, CHARLES
Application year: 1984
Main classification: E21D20
IPC codes: E21D11, E21D20, E21D21