Rock bolt assembly with failure arrestor

A rock anchor assembly includes: a resiliently radially deformable tubular member longitudinally extending between leading and trailing ends and which has an arrestor formation integral with, or engaged to, a trailing end part of the member; an elongate element longitudinally extending through the member between first and second ends and which attaches to the tubular member at spaced distal and proximal load points and which has a failure arrestor fixed at a point within the member; and a faceplate on the tubular member or the elongate member. When the assembly is inserted in a rock hole, with the faceplate bearing against the rock face, and load is applied along the elongate element that will cause the element to sever above the point at which the arrestor is fixed, the failure arrestor engages the arrestor formation, arresting the ejectment of a proximal portion of the elongate element from the hole.

BACKGROUND OF THE INVENTION

The invention relates to a rock anchor assembly.

In a dynamic load support environment, a rock anchor prevents catastrophic failure of the rock wall, which the anchor supports, by absorbing the energy of the rock movement by stretching. A problem arises in an ungrouted application when the steel material of the rock anchor deforms to its maximum tensile capacity, whereafter the anchor is prone to snap. As the anchor is in tension, the moment the anchor breaks, its proximal severed section has a tendency to eject from the rock hole at great force. This creates a projectile which poses a great danger to mine workers in the vicinity.

The invention aims to overcome the problem by providing a mechanism to arrest the detached portion of steel as it attempts to eject from the support hole.

The present invention at least partially addresses the aforementioned problem.

SUMMARY OF INVENTION

The invention provides a rock anchor assembly which includes:

a resiliently radially deformable tubular member which longitudinally extends between a leading end and a trailing end and which has an arrestor formation integral with, or engaged to, a trailing end part of the member;

an elongate element which longitudinally extends through the member between a first end and a second end and which attaches to the tubular member at spaced distal and proximal load points and which has a failure arrestor fixed at a point within the member;

a faceplate on the tubular member or the elongate member;

wherein, when the assembly is inserted in a rock hole, with the faceplate bearing against the rock face, and load is applied along the elongate element that will cause the element to sever above the point at which the arrestor is fixed, the failure arrestor engages the arrestor formation to arrest the ejectment of a proximal portion of the elongate element from the rock hole.

The arrestor formation may be the trailing end part of the tubular member which has been swaged to taper towards the trailing end. Alternatively, the arrestor formation may be an element, for example a collar or bush, which is engaged with an inner surface of the trailing end portion to reduce the internal diameter of the member.

The elongate element may be an elongate element which is made of a suitable steel material which has a high tensile load capacity.

The elongate element may be adapted with a break formation, for example a notch or an annular groove, between the failure arrestor and the first end, about which the element breaks.

The point at which the failure arrestor is fixed on the elongate element may be predetermined on allowing elongation of the elongate element, to its tensile load capacity, without the failure arrestor coming into contact with the arrestor formation.

The failure arrestor may be a nut, or the like, which is threadedly engaged to the elongate element. Alternatively, the failure arrestor may be a deformation which deforms the elongate element in at least one radial direction, for example a paddled deformation.

The assembly may include a first load bearing formation engaged with the elongate element and the tubular member at the proximal load point.

The arrestor formation may be the first load bearing formation.

The assembly may include an expansion element engaged, or integrally formed, with the elongate element at the distal load point.

The assembly may include a load applicator means engaged with the elongate element between the proximal load point and the second end which is actuable to preload the elongate element in the rock hole between the distal load point and the faceplate.

DESCRIPTION OF PREFERRED EMBODIMENTS

A rock anchor assembly10according to a first embodiment of the invention is depicted inFIGS. 1 to 3of the accompanying drawings.

The rock anchor assembly10has a resiliently radially deformable sleeve11having a generally tubular body12that longitudinally extends between a leading end14and a trailing end16. Within the sleeve body, a cavity18is defined. The body12has a slit20extending along the body from a point of origin towards the trailing end16and ending at the leading end14. The slit provides for radial compression of the tubular sleeve body as the body is inserted into a rock hole as will be described in greater detail below.

The sleeve body12has a slightly tapered leading portion24that tapers toward the leading end14to enable the sleeve11to be driven into a rock hole having a smaller diameter than the body. At an opposed end, the sleeve body has a tapered trailing portion25, the function of which will be described below. Between the leading and trailing tapered portions (24,25), the sleeve body has a consistent internal diameter

In this example, the rock anchor assembly10includes an elongate element26which longitudinally extends between a first end28and a second end30. The elongate element is located partly within the cavity18of the sleeve body and has a proximal portion32which, at least part of which extends the trailing end16of the sleeve body. The proximal portion is threaded. The elongate element is exemplified as a steel rod.

An expansion element34is mounted on the first end28of the rod26at a first end28. In this example, the expansion element34is threadingly mounted onto a threaded leading portion36of the rod26, which rod is received in a blind threaded aperture (not illustrated) of the expansion element34. The expansion element34takes on the general frusto-conical form, with an engagement surface40which tapers towards the leading end14of the sleeve body. The maximum diameter of the expansion element is greater than the internal diameter of the sleeve body12.

The rock anchor assembly10further includes a load application means42mounted on the proximal portion32of the rod26, towards the rod's second end30. In this example, the means42includes a hexagonal nut44, which is threadedly engaged to the portion32, and a spherical seat46, which has a central bore for mounting on the proximal portion32of the rod. A last component of the means42is a domed face plate50which engages with the projecting portion32, between the seat and the sleeve's trailing end16.

The rock anchor assembly10also includes a retaining fitting52. In this embodiment, the fitting is a barrel shaped element which press fits into the annular space between the rod26and the sleeve11to frictionally retain the sleeve in position on the rod. The fitting52maintains an initial positioning of the sleeve body12relatively to the elongate element26, with the leading end14abutting the expansion element40. In use of the assembly10, the fitting becomes load bearing.

The assembly10further includes a failure arrestor54which is, in this embodiment, a nut which threadedly engages to the proximal portion32of the rod, within the sleeve12. Initially, on assembly of the anchor assembly10, the arrestor54is spaced at a distance, designated X onFIG. 1A, from the sleeve trailing end16. This distance is a predetermined distance, the considerations in this pre-determination are explained below.

Between the failure arrestor24and the first end28of the rod26, the rod is formed with an annular rebate55about which the rod is designed to break in circumstances described below.

In use, the assembly10is installed in a rock hole56predrilled into a rock face58behind which adjacent rock strata layers require stabilization. SeeFIG. 2. The rock hole will be of a diameter that is slightly smaller than the diameter of the body12of the sleeve10, although greater than the maximum diameter of the expansion element34to allow unhindered insertion of the assembly into the rock hole. Facilitated by the slit20, the sleeve body12compressively deforms, to accommodate passage into the rock hole. Initially, the frictional forces resulting from the interference fit between the sleeve body12and the rock hole walls retain the rock anchor assembly10in the hole, and allow for the transfer of proportional load from the rock strata about the rock face58to the sleeve body12.

The assembly10is fully and operationally installed in the rock hole54when both the sleeve is wholly contained therein, but with a length of the projecting portion32of the elongate element26extending from the rock hole54. On this length, the face plate50, the nut44and the spherical seat46are located, initially with the face plate50free to move axially on the rod between the rock face56and the trailing position of the barrel46.

Active anchoring of the sleeve body12in the rock hole50, additional to that provided passively by frictional fit, is achieved by pull through of the expansion element34into and through the sleeve body12. This provides a point anchoring effect. The expansion element is caused to move by actuating the load application means42by applying a drive means (not shown) to spin and then torque the hex nut44. Initially the nut is spun into contact with the face plate50and then to push the faceplate into abutment with the rock face58. Due to opposed thread direction on a leading end portion and the projecting portion32of the rod, this rotation does not lead to disengagement of the elongate element with the expansion element.

Torqueing of the hex nut44, now abutting the faceplate50, will draw the threaded projecting portion32of the elongate element26through the nut and pull the attached expansion element34against the leading end14of the sleeve body12. Reactively, as the hex nut44is torqued, the faceplate50is drawn and held in progressive and proportional load support with the rock face58.

Before the expansion element34moves into the cavity18, the element contacts the leading end14of the sleeve body12in bearing engagement which causes the trailing end of the sleeve to reactively engage the fitting52. The fitting52, now in load support of the sleeve12, prevents the sleeve11from giving way axially relatively to the elongate element26due to ingress of the expansion element34.

With the sleeve11held stationary relatively to the elongate element26, the expansion element engages the sleeve body12at the leading end and forces the body12at this end into radially outwardly deformation. Ultimately, the expansion element34is caused to be drawn fully into the tapered leading portion24of the sleeve body12, as illustrated inFIGS. 2 and 3, which radially outwardly deforms along the path of ingress to accommodate the passage of the element34. The radial outward deformation forces the sleeve body12into frictional contact with walls of the rock hole56. This action achieves anchoring of the sleeve body12, and thus the anchor assembly10, within the rock hole.

The faceplate50is in load support of the rock face58and is thus subjected to a moving face (illustrated inFIG. 2) due to quasi-static or seismic loading, whilst the first end28of the elongate element26is anchored within the sleeve which in turn is anchored within the rock hole. Anchored at one end, and pulled at the other, the rod26elongates thereby absorbing the energy of the static and seismic forces.

The failure arrestor54will move with the rod26, as it stretches, through the sleeve towards the trailing end. The initial spacing X is pre-set so that the rod is allowed to stretch to close to its maximum tensile capacity, absorbing maximum energy, without the arrestor coming into contact with the diametrically reduced tapered trailing portion25of the sleeve. At the point where the elongate element26breaks, at maximum loading, the arrestor will be positioned just short of the start of the tapered trailing portion25(seeFIG. 2A).

When the rod finally breaks, at the rebate55, the proximal portion32of the elongate element26separates from a remaining part60(seeFIG. 3) of the rod. The arrestor54, being diametrically larger than the width of the internal diameter of portion25, will come into resistive contact with the walls of this portion, arresting the proximal portion32from being ejected from the hole56by the static or seismic forces. This is shown inFIG. 3.

Frictional interaction of the arrestor54with the tapered portion25provides a load carrying structure secondary to the primary load carrying structure provided by the interaction of the expansion element34with the sleeve body12along the leading tapered portion24. This allows a mine worker to return and rehabilitate the rock mass that was subjected to static deterioration or seismic damage in a manner described below.

With static deterioration or seismic damage, the rock strata underlying the rock face58will fragment and scale from the rock face. But, due to the arrested projecting portion32of the elongate element, and the space now created between the faceplate50and the sleeve, there is a capacity to re-tension the assembly10by spinning the nut44, the faceplate50is driven back into contact with a now retreated rock face58. Torqueing the nut will ensure that tension is reinstated in the assembly10between the arrestor54and the faceplate, thereby reintroducing some supporting reactionary force through the faceplate50to the rock face58.

A second embodiment of the rock anchor assembly10A is illustrated inFIG. 4. In describing this embodiment, like features bear like designations. Only the differences over the earlier embodiment are described.

The assembly10A includes an arrestor element62, such as a collar of bush, which is welded to the inside surface of the proximal portion25of the sleeve11. Although a tapered proximal portion is illustrated in this figure, this tapering is not essential and, instead, the sleeve diameter reduction is achieved with the arrestor element.

It is against this element that the failure arrestor comes into contact. In this embodiment, the failure arrestor54A is a paddle shaped adaptation of the rod26.

In the embodiments described above, the sleeve11and the elongate element26are made of structural grade steel. This is non-limiting to the invention as it is envisaged that at least the sleeve11and the elongate element26can also be made of a fibre reinforced plastic (FRP) such as, for example, pultruded fibreglass. It is further anticipated that all of the components of the components of the rock anchor assembly (10,10A) can be made off a FRP.