US3849992A

US3849992A - Reinforcing elements for stabilization of rocks

Info

Publication number: US3849992A
Authority: US
Inventors: J Murphy
Original assignee: Exchem Holdings Ltd
Current assignee: Exchem Holdings Ltd
Priority date: 1972-06-05
Filing date: 1973-05-23
Publication date: 1974-11-26
Anticipated expiration: 1991-11-26
Also published as: DE2327851A1; ES415818A1; CA970984A; ZA733570B; FR2188049A1; BE800497A; GB1384177A

Abstract

The stabilisation of weak rock formations, such as longwall coal faces in underground mines, is effected by inserting into a borehole drilled in the rock formation a reinforcing assembly in which a plurality of longitudinally extending annular segments are fixedly disposed in longitudinally staggered relationship about a coaxially arranged inner tube, feeding through the inner tube a bonding composition so as to cause it to flow back from the closed end of the borehole to the annular space at the mouth of the borehole, and allowing the bonding composition to harden so as to bond the reinforcing assembly to the rock formation.

Description

United States Patent Murphy REINFORCING ELEMENTS FOR STABILIZATION OF ROCKS John Michael Murphy, Mansfield, England Assignee: Exchem Holdings Limited, London,

England Filed: May 23, 1973 Appl. No.: 363,149

Inventor:

Foreign Application Priority Data June 5, 1972 Great Britain 26102/72 U.S. Cl. 611/45 B, 52/727 Int. Cl E02d 3/00, E02d 11/00 Field of Search 61/35, 45 B; 52/704, 744,

References Cited UNITED STATES PATENTS FOREIGN PATENTS OR APPLICATIONS 1,484,449 10/1969 Germany 61/45 B 1,182,185 11/1964 Germany 61/45 B 84,938 l/l955 Norway 61/45 B Primary ExaminerPaul R. Gilliam Assistant ExaminerAlexGrosz Attorney, Agent, or Firm-Larson, Taylor & Hinds [57] ABSTRACT The stabilisation of weak rock formations, such as longwall coal faces in underground mines, is effected by inserting into a borehole drilled in the rock formation a reinforcing assembly in which a plurality of Iongitudinally extending annular segments are fixedly disposed in longitudinally staggered relationship about a coaxially arranged inner tube, feeding through the inner tube a bonding composition :so as to cause it to flow back from the closed end of the borehole to the annular space at the mouth of the borehole, and allowing the bonding composition to harden so as to bond the reinforcing assembly to the rock formation.

13 Claims, 3 Drawing Figures WWWW REINFORCING ELEMENTS FOR STABILIZATION OF ROCKS This invention relates to the stabilisation of rock and is concerned with a method of stabilising or strengthening weak rock formations, such as in underground mines, and with a reinforcing assembly for use in said method.

It is well known to employ relatively short steel rockbolts, typically 5 to 7 feet long, around underground openings to stabilise and reinforce the strata immedi ately adjacent to the openings. Such rockbolts can achieve anchorage in the rock by means of expanding shells or wedging arrangements, or can be anchored at the inner end by means of a rapid setting cement usually based on an unsaturated polyester resin. Such end anchored rockbolts are tensioned by means of a nut and plate acting against the rock surface around the borehole containing the rockbolt. The stress within such a tensioned bolt provides a force restraining strata-bed separation or relative movement of adjacent blocks of broken rock and hence the use of such bolts in appropriately spaced locations can provide a stressed zone of reinforcement around a mine roadway or other underground opening.

Other types of reinforcement are also known in which a reinforcing element such as a rod of metal, wood or reinforced plastic is fully bonded, over its whole length, into a hole bored to receive it, using a suitable resinous or cementitious bonding composition known in the art. Such reinforcing elements are not tensioned but, since they are intimately bonded to the rock, provide maximum localised restraining forces about any interfaces where stresses tend to cause delamination or parting of the rock. The bonding composition for such reinforcing elements can be pumped into the borehole prior to insertion of the reinforcing element or more usually is contained in one or'more frangible capsules which are placed in the borehole and then ruptured and the contents caused to activate by rotation of the reinforcing element on insertion into the borehole. In practice, in the limited space normally available in mine roadways, the length of the reinforcing element is limited to six to eight feet although in special circumstances bolts up to twenty feet in length have been used around underground mine openings.

The relatively narrow band of rock which may be reinforced with the conventional reinforcing elements described above is normally adequate around a mine roadway but a much greater depth of temporary reinforcement is necessary to stabilise faulted zones on longwall coal faces. In practice it is desirable to provide 50 to 100 feet of forward reinforcement of the extractable seam and immediate roof strata to achieve optimum stabilisation. Efficient forward reinforcement to this depth has been achieved using interconnected wooden dowels, 36 mm. in diameter, in boreholes drilled to accommodate them. Using a technique of injection of resin cement around the exterior of the preinserted dowels from the open end of the borehole, it is necessary to provide a tube to vent the air from the back of the hole and hence allow the cement to fill all the annular space between the dowels and the rock. in practice, in order to insert the 36 mm. dowels and vent tube to a depth of 50 feet or more in a borehole drilled in weak rock, it is necessary to drill a borehole of at least 50 mm. and preferably 60 mm. diameter. The drilling of such boreholes poses certain practical difficulties and also they require a relatively large quantity of resin cement to fill the large annular space between the dowels and the wall of the borehole. It also necessitates the use of special drilling equipment, since the standard equipment normally available for shothole drilling in underground mines in the United Kingdom is only capable of drilling 43 mm. diameter boreholes.

It is therefore an object of the present invention to provide a method of reinforcing underground rock formations such as longwall coal faces which may be effected using the standard shothole drilling equipment and which can obviate the necessity of drilling relatively large diameter boreholes of the order of 50 mm. diameter and upwards.

Thus in accordance with the invention, there is provided a method of reinforcing an underground rock formation, which comprises drilling a borehole in the rock formation; inserting into the: borehole a reinforcing assembly comprising a plurality of longitudinally extending annular segments fixedly disposed in longitudinally staggered relationship about a coaxially arranged inner tube, the insertion of the reinforcing assembly being continued until the inner end thereof lies adjacent to the inner end portion of the borehole and the outer end of the reinforcing assembly lies adjacent the mouth of the borehole; feeding a bonding composition through the inner tube of the reinforcing assembly so as to cause the bonding composition to flow out of the inner end of the reinforcing assembly and back along the annular space between the assembly and the wall of the borehole; continuing the feeding of the bonding composition until the said annular space is substantially full of bonding composition; and thereafter allowing the bonding composition to harden so as to bond the reinforcing assembly to the rock formation.

In a first embodiment of the invention, the reinforcing assembly comprises a plurality of tubular members arranged in series, each formed from a plurality oflongitudinally extending annular segments fixedly disposed in longitudinally staggered relationship about a coaxially arranged inner tube, the inner tubes of adjacent tubular members communicating by means of a connecting memberjoining the end portions of said adjacent tubular members.

However in a second and preferred embodiment of the invention, the reinforcing assembly is formed from a continuous inner tube about which are fixedly disposed a plurality of longitudinally extending annular segments in longitudinally staggered relationship with respect to the inner tube.

The annular segments present in the reinforcing assembly may be formed from any suitable material, for example wood, metal, synthetic thermoplastics or thermosetting material, asbestos cement or resin-bonded glass fibre, whilst the inner tube may be formed, for example, from a synthetic plastics material such as, for example, polyethylene.

Preferably the annular segments present in the reinforcing assembly are in the shape: of semi-cylindrical annular segments. However it will be appreciated that the annular segments may have cross-sections other than semi-cylindrical.

The connecting member joining together adjacent tubular members in the aforementioned first embodiment of the invention may be, for example, a tubular sleeve into which the tubular members are push-fit, or

a spigot extending from the inner tube at one end of a tubular member and which is inserted into one end of the inner tube of the adjacent tubular member.

To lessen the danger of the inner end of the reinforcing assembly being blocked by residual drilling dust in the borehole during the insertion of the assembly into the borehole, it is advantageous to provide the inner end of the assembly with a tubular perforated ejection head having a closed front end. The head preferably has a generally cylindrical body which tapers towards the closed front end, to assist insertion of the reinforcement assembly into irregular boreholes. The ejection head is formed with a plurality of exit apertures through which the bonding composition can be ejected into the annular space surrounding the reinforcing assembly in the borehole, the exit apertures advantageously being substantially symmetrically disposed around the body of the ejection head.

For a better understanding of the invention and to show how the same may be carried into effect reference will now be made to the accompanying drawing in which:

FIG. I is a view of the end portion of a tubular member from which one embodiment of a reinforcing assembly can be produced;

FIG. 2 is a part longitudinal cross-section of a second embodiment of a reinforcing assembly; and

FIG. 3 is a view of an ejection head for a reinforcing assembly.

Referring to FIG. 1 of the drawing, a tubular member from a plurality of which can be formed a reinforcing assembly for use in the method of the invention comprises two longitudinally extending annular segments 2 of semi-cylindrical shape which are disposed in staggered relationship about a coaxially arranged inner tube 1.

In FIG. 2 there is shown a preferred reinforcing assembly for use in the method of the invention formed from a continuous inner tube 1 which is surrounded by a plurality of semi-cylindrical annular segments 2 which are in staggered locations with respect to the tube 1 so that at any point at least half of the ultimate tensile strength of the reinforcing assembly is available. In practice, the inner tube 1 may be formed from, for example, a flexible or semi-rigid plastics material or metal and fed from a coil thereof into the borehole in an unbroken length. The semi-cylindrical annular segments 2 can be supplied in convenient lengths, for example I to 2 metres, and can be positioned around the tube 1 by means of, for example, adhesive tape or metallic or plastics clips.

FIG. 3 shows an ejection head which can be attached to the reinforcing assembly of FIG. 1 or 2. The ejection head has a tubular body 1 which is tapered to form a closed front end 2. A plurality of apertures 3 are disposed substantially symmetrically over the surface of the body 1.

The bonding composition employed in the method of the invention may be a cementitious composition or preferably a resinous composition, advantageously one based on an unsaturated polyester resin.

The following Examples further illustrate the performance of the invention.

EXAMPLE I Tubular members as illustrated in FIG. 1 were made using a high density polyethylene tube ofinternal diameter 16 mm. and external diameter 19 mm. as the inner tube 1, and semi-circular section annular wooden strips of 19 mm. internal diameter and 36 mm. external diameter prepared from Kerning timber as the annular segments 2. The plastics tube and the timber strips were cut to 66 inches in length and then assembled with a longitudinal displacement so that the overall length of the assembled tubular member was 72 inches and each end of the inner tube was positioned midway between the respective ends of the two semi-circular wooden annular segments. A rapid hardening adhesive based on an unsaturated polyester resin was used to bond the inner tube to the annular segments to form the required tubular member. A high density polyethylene tube,

eight inches long, of 36 mm. internal diameter and having a wall thickness of 0.5 mm. was pushed on to one end of the tubular member and fastened with adhesive tape so that one inch of the polyethylene tube extended beyond the said one end of the tubular member, so forming a connecting member for another tubular member.

A borehole 43 mm. in diameter and 44 feet long was drilled horizontally into a faulted area on a longwall coal face. The tubular member as described above was fitted with a tapered ejection head as illustrated in FIG. 3. The ejection head was 36 mm. in diameter, 300 mm. in length and was perforated with 16 apertures, each 10 mm. in diameter, distributed approximately regularly over its surface. The tubular member with the ejection head fitted was inserted into the borehole and another similar tubular member was connected by insertion into the open end of the 36 mm. diameter connecting member. A further six similar tubular members were connected in like manner until the ejection head had reached the inner end of the borehole. The plastics connecting member was then removed from the end of the last tubular member projecting from the mouth of the borehole, which thus contained a reinforcing assembly in accordance with the invention. 1

A delivery pipe from a hand operated diaphragm pump was connected to the outer end of the plastics inner tube of the reinforcing assembly and an unsaturated polyester resin based bonding composition was pumped through the reinforcing assembly to the inner end of the borehole. The composition flowed through the ejection head and back along the annular space between the assembly and the wall of the borehole. Pumping was discontinued when the resin bonding composition appeared in the annulus at the mouth of the borehole. The resin based bonding composition contained, in addition to the unsaturated polyester resin, 50 percent by weight of inert filler and 1.0 per cent by weight of pyrogenic silica and was activated by a benzoyl peroxide/amine accelerator system of a type which is well known in the art.

A series of longhole reinforcing assemblies was placed as described above at approximately one metre intervals throughout the faulted area of the coalface. After hardening of the injected resinous bonding composition, the coal was cut by conventional mining machinery and very substantial improvement in ground conditions was confirmed due to the stabilisation effect of the reinforcing assemblies.

EXAMPLE 2 Coils of high density polyethylene tubing of 16 mm. internal diameter and 19 mm. external diameter were prepared in lengths of 45 feet. Semi-circular annular wooden segments of 19 mm. internal diameter and 36 mm. external diameter were prepared from Kerning' protruding from the borehole. A 6 feet length of semicircular annular wooden segment was then attached to a matching position and the composite advanced a further three feet into the borehole. Similar annular wooden segments were added sequentially. with three feet of advance after each addition until the ejection head reached the inner end of the borehole. The projecting portions of the wooden segments were sawn off, i

so that the borehole thus contained a reinforcing assembly of the type illustrated in FIG. 2. The outer end of the continuous polyethylene inner tube was then connected to the delivery pipe from a diaphragm pump as described in Example 1. A resin based bonding composition was then pumped through the inner tube and ejected into the inner end of the borehole until its appearance in the annulus at the mouth of the borehole. A pattern of reinforcing assemblies of the foregoing type was inserted into the broken roof above a coal seam, and was judged to have been mainly responsible for the improvement in roof conditions apparent when the coal seam was extracted.

1 claim:

1. A method of reinforcing an underground rock formation positioned above a coal seam being mined by the longwall system, which method comprises drilling into the rock formation a substantially horizontal borehole having a diameter less than 50 mm. and a length in excess of about feet; inserting into the borehole in stages a reinforcing assembly comprising a plurality of lengths of longitudinally extending annular segments fixedly disposed in longitudinally staggered relationship with respect to each other about a coaxially arranged inner tube, said assembly being made up from a plurality of the lengths of said annular segments while being inserted into the borehole, and the insertion of the reinforcing assembly being continued until the inner end thereof lies adjacent to the inner end portion of the borehole and the outer end of the reinforcing assembly lies adjacent to the mouth of the borehole; feeding a bonding composition through the inner tube of the reinforcing assembly so as to cause the bonding composition to flow out of the inner end of the reinforcing assembly and back along the annular space between the assembly and the wall of the borehole; continuing the feeding of the bonding composition until the said annular space is substantially full of bonding composition; and thereafter allowing the bonding composition to harden so as to bond the reinforcing assembly to the rock formation.

2. A method according to claim 1, wherein the reinforcing assembly comprises a continuous inner tube about which are fixedly disposed a plurality of lengths of longitudinally extending annular segments in longitudinally staggered relationship with respect to the inner tube.

3. A method according to claim 1, wherein each length of the annular segments is semi-cylindrical in shape.

4. A method according to claim 3, wherein each length of the annular segments is semi-cylindrical in shape.

5. A method according to claim 1, wherein the inner end of the reinforcing assembly is provided with a tubular perforated ejection head having a closed front end.

6. A method according to claim 5, wherein the ejection head has a generally cylindrical body which tapers towards the closed front end.

7. A method according to claim 6, wherein the ejection head is formed with a plurality of exit apertures which are substantially symmetrically disposed around the body of the ejection head.

8. A method according to claim .I, wherein the bonding composition is an unsaturated polyester resin based composition.

9. A longhole reinforcing assembly for use in a substantially horizontal borehole formed in an underground formation being mined by the longwall system, said assembly having a length in excess of about 40 feet, and comprising in combination a plurality of lengths of longitudinally extending annular segments fixedly disposed in longitudinally staggered relationship with respect to each other about a co-axially arranged inner tube providing an axial passage throughout the length of said reinforcing assembly for conducting a bonding composition into the annular space between said assembly and the wall of said borehole.

10. A reinforcing assembly claimed in claim 9, wherein that end of the reinforcing assembly which is to be positioned adjacent the closed end of the borehole is provided with a tubular perforated ejection head having a generally cylindrical body which tapers towards a closed front end.

11. A reinforcing assembly as claimed in claim 10, wherein the ejection head is formed with a plurality of exit apertures which are substantially symmetrically disposed around the body of the ejection head.

12. A reinforcing assembly as claimed in claim 9, wherein each length of the annular segments is semicylindrical in shape.

13. A reinforcing assembly as claimed in claim 9,

wherein said inner tube is a continuous tube.

Claims

1. A method of reinforcing an underground rock formation positioned above a coal seam being mined by the longwall system, which method comprises drilling into the rock formation a substantially horizontal borehole having a diameter less than 50 mm. and a length in excess of about 40 feet; inserting into the borehole in stages a reinforcing assembly comprising a plurality of lengths of longitudinally extending annular segments fixedly disposed in longitudinally staggered relationship with respect to each other about a coaxially arranged inner tube, said assembly being made up from a plurality of the lengths of said annular segments while being inserted into the borehole, and the insertion of the reinforcing assembly being continued until the inner end thereof lies adjacent to the inner end portion of the borehole and the outer end of the reinforcing assembly lies adjacent to the mouth of the borehole; feeding a bonding composition through the inner tube of the reinforcing assembly so as to cause the bonding composition to flow out of the inner end of the reinforcing assembly and back along the annular space between the assembly and the wall of the borehole; continuing the feeding of the bonding composition until the said annular space is substantially full of bonding composition; and thereafter allowing the bonding composition to harden so as to bond the reinforcing assembly to the rock formation.

8. A method according to claim 1, wherein the bonding composition is an unsaturated polyester resin based composition.

10. A reinforcing assembly as claimed in claim 9, wherein that end of the reinforcing assembly which is to be positioned adjacent the closed end of the borehole is provided with a tubular perforated ejection head having a generally cylindrical body which tapers towards a closed front end.

12. A reinforcing assembly as claimed in claim 9, wherein each length of the annular segments is semi-cylindrical in shape.

13. A reinforcing assembly as claimed in claim 9, wherein said inner tube is a continuous tube.