GROUT PACK RESTRAINING ENVELOPE

Abstract

A grout pack [1] restraining envelope is provided which includes a plurality of restraining rings [7, 8] shaped to extend circumferentially around a grout bag [2] in a vertically spaced arrangement up at least a part of the height of the grout bag to provide support against lateral deformation of an installed grout pack as it occurs under axial load. At least some of the restraining rings are of differing tensile strengths or are configured to yield at differing tensions. The grout pack may have a collapsible polymer or wire mesh [3] positioned on the inside of the restraining rings to which at least some of the restraining rings are fastened. Restraining rings [7] of relatively higher tensile strength are provided at vertical positions at which relatively greater lateral deformation of the grout bag is expected, and rings [8] of relatively lower tensile strength are provided at vertical positions at which relatively less lateral deformation of the grout bag is expected. The restraining rings may be of the yielding or non-yielding type.

Description

FIELD OF THE INVENTION

This invention relates to a support for underground mining operations and, in particular, to a restraining system for supporting an inflatable grout bag that, in use, is filled with a cementitious or other settable mix, typically, but not necessarily, including pulverised rock or backfill of processed or barren mined material.

The term grout bag as used herein is intended to mean a generally permeable bag that is used to contain slurry pumped into it under pressure and to retain the solids while allowing excess moisture to permeate through the grout bag wall that is typically made of a suitable textile material such as a geotextile material.

The term grout pack as used herein is intended to mean a grout bag that is used in combination with a restraining envelope encircling the generally upright walls of the grout bag, the restraining envelope including either or both of a reinforcing mesh and a series of elongate tension elements typically in the form of confinement rings or bands encircling the grout bag.

BACKGROUND TO THE INVENTION

Depending on the type and quality of rock being supported, the depth of mining, the prevalent field stresses, seismicity, stoping width and a number of other factors, stope support can utilise a vast range of materials, configurations and systems, including, gum poles, timber and composite packs, steel props, unmined ore pillars, rock anchors and granular (tailings) types of supports.

Among the granular support media, cemented grout packs are increasingly being utilized as combination support products, consisting essentially of a support column formed by cured cemented backfill or a similar cured cementitious grout, contained within a geotextile bag and stiffened against lateral deformation under axial load with an external restraining envelope of either or both of a reinforcing mesh and a series of tension rings or bands encircling the grout bag, typically both. The mesh is typically a reinforcement of wire or polymer netting and the restraining rings are usually made of steel wire.

In a non-yielding type of grout pack restraining rings that encircle the mesh are made to fail at a minimum of a predetermined tension. In the instance of yielding grout packs the restraining rings or ring assemblies are able to yield under load thereby maintaining a measure of restraining force before their eventual failure.

Under vertical (axial) load the grout pack reduces in length and dilates laterally according to the Poisson's ratio of the grout material. Besides the cohesion of the cemented material, the geotextile bag, the surrounding mesh, as well as the restraining rings all contribute in some measure to the support resistance of the pack in that they restrain the lateral dilation of the grout column.

The geotextile material is usually woven or knitted from low tenacity polymer fibres and offers little lateral confinement as it stretches easily under load. Its primary function is to provide suitable containment for the fluent grout slurry when it is introduced with optimal drainage and filtering properties.

The mesh basically forms a support structure for the geotextile material, preventing excessive bulging (which would be accompanied by increased solids losses through the pores that become enlarged consequent on stretching of the fabric) under hydrostatic loading of the uncured fluent grout slurry. To add some degree of yieldability to the cured pack, the netting wires (or fibres) are usually oriented at 45 degrees to the axis of the pack allowing the mesh to stretch in the horizontal direction whilst providing some additional lateral confinement to the pack.

The circumferential restraining rings are the major structural confinement of the grout pack and their strengths contribute directly and significantly to the support resistance of the pack. It is these rings that largely control the compression behaviour of the grout pack.

It has been noticed in practice that grout packs yield by expanding and disintegrating from the top downwards. The restraining rings near the top of the grout pack are therefore generally the first to break or to experience excessive yield.

The term “tensile strength” as used herein refers to the totality of material tensile properties including yield stress, ultimate stress and associated strain values.

The term “ring” is used in a broad sense and includes bands straps and other tension resisting members.

OBJECT OF THE INVENTION

It is an object of this invention to provide a grout pack restraining envelope which better controls lateral dilation of the pack, in use.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a grout pack restraining envelope which includes a plurality of restraining rings shaped to extend circumferentially around a grout bag in a vertically spaced arrangement up at least a part of the height of the grout bag to provide support against lateral deformation of an installed grout pack as it occurs under axial load, the restraining envelope being characterised in that at least some of the rings are of differing tensile strengths or are configured to yield at differing tensions.

Further features of the invention provide for the restraining envelope to include a collapsible polymer or wire mesh positioned on the inside of the restraining rings to which at least some of the restraining rings are fastened.

In accordance with one aspect of the invention, rings of different tensile strengths are employed, the tensile strengths of the rings being selected so that rings of relatively higher tensile strength are provided at vertical positions at which relatively greater lateral deformation of the grout bag is expected, and rings of relatively lower tensile strength are provided at vertical positions at which relatively less lateral deformation of the grout bag is expected.

Further features of the invention provide for restraining rings at positions further up the height of the grout bag to be of relatively greater tensile strength than rings lower down the height of the grout pack. In one embodiment of the invention, rings of three different tensile strengths are used; an upper set of rings of highest tensile strength, a middle set of rings of intermediate tensile strength and a lower set of rings of lowest tensile strength.

The rings may be made from metal wire and may have differing tensile strength by virtue of having differing cross-sectional areas. Alternatively, the rings may have differing tensile strengths by virtue of having different material properties.

In one embodiment of the invention, at least some of the rings carry secondary outer rings of greater diameter, the outer rings being configured to provide restraint to the expansion of the grout pack prior to failure of the rings of lesser diameter, thereby providing a progressive yield of the grout pack. The outer rings may be of different tensile strength from the inner rings.

In accordance with a second aspect of the invention, components in the form of rings suitable for inclusion in a grout restraining envelope are provided that are configured to yield at differing tensions, the tension at which the rings are configured to yield being selected so that rings which yield at higher tension are provided at vertical positions at which relatively greater lateral deformation of the grout bag is expected and rings which yield at relatively lower tension are provided at vertical positions at which relatively less lateral deformation of the grout bag is expected.

The yielding rings may be formed by elongate elements that have first and second opposing ends connected to each other in an overlapping configuration to form an expansion zone so that the rings yield by controlled extension of their length when the tension applied to the rings by lateral deformation of the bag exceeds a predetermined tension force. The opposing ends may be connected by means of at least one collar that provides a predetermined swaging force thereby creating a predetermined frictional or shear resistance to movement between the first and second opposing ends.

The rings may be evenly spaced apart up the height of the grout pack or may be spaced in a non-uniform arrangement in which the rings are more closely spaced at vertical positions at which relatively greater lateral deformation of the grout bag is expected.

The invention extends to a grout pack comprising a grout bag and a restraining envelope encircling the grout bag, where the restraining envelope is as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:—

FIG. 1 is a partly broken away elevation of a first embodiment of a grout pack having a restraining envelope according to the invention; and

FIGS. 2 to 7 are each a schematic isometric view of the restraining rings of first, second, third, fourth, fifth and sixth embodiments of grout pack restraining envelopes according to the invention.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

Referring firstly to FIG. 1 of the drawings, a grout pack, generally indicated by numeral [1], is of generally known construction and has, in the operative position, a geotextile bag [2] confined by means of a generally cylindrical mesh [3] that is used to retain a cementitious grout mixture [4] within the bag. Encircling the mesh is a plurality of vertically spaced restraining rings that, in all of the embodiments of the invention described below, form a restraining envelope together with the mesh and geotextile bag.

In use, the grout pack assembly is suspended in a mining excavation from the hanging wall [5] to contact the foot wall [6] and, with the assembly properly installed, liquid cementitious grout is pumped into the grout bag to fill it to its maximum capacity. During this procedure, the lateral hydraulic dilation is confined by the restraining envelope composed of the mesh and restraining rings.

Once the grout has cured, the grout pack is able to take axial load from the descending rock hanging wall. The dilation of the cured cemented grout (due to the material's Poisson's ratio) is then impeded primarily by the pack's restraining rings, thereby stiffening the pack's load-deformation response.

Turning now to the first embodiment of the invention that is illustrated both in FIG. 1 and in FIG. 2, fixed diameter restraining rings [7, 8] are threaded into the periphery of the mesh of the envelope. The restraining rings are, in this embodiment of the invention, evenly distributed over the height of the grout pack, from the top to the bottom. The three upper restraining rings [7] are fixed high strength rings, whilst the three lower restraining rings [8] are three fixed medium strength rings. The spacing of the rings in this case may be considered to be the normal ring spacing.

In the second embodiment of the invention the restraining rings of which are illustrated in FIG. 3, there are five fixed high strength restraining rings [9], positioned within the upper part of the grout pack, and six fixed medium strength restraining rings [10] positioned in the lower part of the grout pack. All the rings are spaced approximately equally up the height of the grout pack. With almost double the number of restraining rings, this grout pack is substantially stronger than the one described with reference to FIG. 1.

In the third embodiment of the invention the restraining rings of which are illustrated in FIG. 4, three fixed high strength restraining rings [11] are positioned at the upper part of the grout pack, and three fixed medium strength restraining rings [12] are positioned in the lower part of the grout pack. However, in this instance, in addition to the fixed ring complement, a set of floating restraining rings of larger diameter is provided with two floating high strength restraining rings [13] being suspended from the top two fixed restraining rings [11] and a single floating medium strength restraining ring [14] suspended from the third fixed restraining ring from the top. The floating restraining rings are positioned in between the fixed rings in the vertical direction.

Under load, the inner fixed restraining rings restrain the initial dilation of the pack and, once the top fixed restraining rings fail, the floating restraining rings take over the restraining duty, thereby extending the yield range of the grout pack substantially. This ring set illustrates how the compressive strength and the yield performance of a grout pack can be modified, utilizing various combinations of restraining rings.

In the fourth embodiment of the invention the restraining rings of which are illustrated in FIG. 5, a restraining envelope has unequal spacing of two different restraining ring types. In this instance there are three fixed high strength restraining rings [15], positioned in the upper part of the grout pack, and only two fixed medium strength restraining rings [16], positioned in the lower part of the grout pack. Indeed the arrangement of the restraining rings in the lower part of the grout pack is depleted in that one of the rings that would have been positioned between the two rings present, has been omitted.

Compared to the upper, high strength part of the pack, the lower part is substantially weakened, not only by using medium strength rings, but also by the omission of one of those rings. It is envisaged that in many applications the lower half of a grout pack is never dilated by vertical stress and an arrangement such as that illustrated could be adequate as well as more cost-effective.

In a fifth embodiment of the invention the restraining rings of which are illustrated in FIG. 6, the restraining ring arrangement includes two fixed high strength restraining rings [17] positioned in the top part of the envelope; two fixed medium strength intermediate restraining rings [18] positioned in the central part of the envelope; and two fixed low strength restraining rings [19] positioned in the lower part of the envelope. This grout pack is therefore partitioned into three restraint zones; a high strength zone; a medium strength zone; and a low strength zone.

Finally, in a sixth embodiment of the invention the restraining rings of which are illustrated in FIG. 7, a similar arrangement of restraining rings to that described with reference to FIG. 6 is provided. However, in this instance the uppermost two high strength restraining rings [20]; the two intermediate medium strength intermediate restraining rings [21] and the two lower low strength rings [22] are each made as yielding rings formed by elongate elements that have first and second opposing ends connected to each other in an overlapping configuration to form an expansion zone so that the rings yield by controlled extension of their length. In this instance the opposing ends are connected by means of a pair of spaced collars [23] that provide a predetermined swaging force thereby creating a predetermined frictional resistance to movement between the first and second opposing ends.

It will be understood that numerous variations may be made to the embodiments of the invention described above without departing from the scope hereof. In particular, the restraining rings may be of any desired configuration with different diameters and different vertical arrangements and different arrangements of any floating rings that may be present. The arrangement may be configured to provide different positions of strong restraint zones within a grout pack.

Claims

1. A grout pack restraining envelope which includes a plurality of restraining rings shaped to extend circumferentially around a grout bag in a vertically spaced arrangement up at least a part of the height of the grout bag to provide support against lateral deformation of an installed grout pack as it occurs under axial load, wherein at least some of the rings are of differing tensile strengths or are configured to yield at differing tensions.

2. A grout pack restraining envelope as claimed in claim 1 in which the restraining envelope includes a collapsible polymer or wire mesh positioned on the inside of the restraining rings to which at least some of the restraining rings are fastened.

3. A grout pack restraining envelope as claimed in the claim 1 in which restraining rings of relatively higher tensile strength are provided at vertical positions at which relatively greater lateral deformation of the grout bag is expected, and rings of relatively lower tensile strength are provided at vertical positions at which relatively less lateral deformation of the grout bag is expected.

4. A grout pack restraining envelope as claimed in claim 1 in which restraining rings at positions further up the height of the grout bag are of relatively greater tensile strength than restraining rings lower down the height of the grout pack.

5. A grout pack restraining envelope as claimed in claim 1 in which restraining rings of three different tensile strengths are used; an upper set of restraining rings of highest tensile strength, a middle set of restraining rings of intermediate tensile strength and a lower set of restraining rings of lowest tensile strength.

6. A grout pack restraining envelope as claimed in claim 1 in which the restraining rings are made from metal wire and may have differing tensile strengths by virtue of having differing cross-sectional areas.

7. A grout pack restraining envelope as claimed in claim 1 in which the restraining rings have differing tensile strengths by virtue of having different material properties.

8. A grout pack restraining envelope as claimed in claim 1 in which at least some of the restraining rings carry secondary outer restraining rings of greater diameter, the outer restraining rings being configured to provide restraint to the expansion of the grout pack prior to failure of the restraining rings of lesser diameter, thereby providing a progressive yield of the grout pack.

9. A grout pack restraining envelope as claimed in claim 1 in which any yielding restraining rings are formed by elongate elements that have first and second opposing ends connected to each other in an overlapping configuration to form an expansion zone so that the restraining rings yield by controlled extension of their length when tension applied to the restraining rings consequent on lateral deformation of the bag exceeds a predetermined tension force.

10. A grout pack restraining envelope as claimed in claim 9 in which the opposing ends of said yielding restraining rings are connected by means of at least one collar that provides a predetermined swaging force thereby creating a predetermined frictional or shear resistance to movement between the first and second opposing ends.

11. Components in the form of restraining rings suitable for inclusion in a grout pack restraining envelope as claimed in claim 1 wherein at least some restraining rings are adapted to yield at differing tensions.