INTRODUCTION

The invention relates to an energy absorbing, yielding rock anchor for supporting a wall of a rock face.

BACKGROUND TO THE INVENTION

It will be appreciated by those engaged in the art that mining excavations, particularly (although not limited to) underground mining excavations, create conditions of increased stresses in surrounding rock formations. This may cause cracks in a rock structure, which can propagate and coalesce, eventually causing rock failures such as rock bursts and large deformations. Rock bolts and cable anchors, collectively referred to herein as rock anchors, are used in the mining industry to support and stabilise a roof and side walls in mining tunnels and shafts against rock falls. Such a rock anchor is inserted into and secured within a drill hole drilled into a rock face.

Traditional rock bolts do not, however, effectively consolidate rock formations, as they cannot accommodate progressive failure of a rock mass, such as occurs under seismic conditions, which leads to snapping of the rock bolt, or slippage between the rock bolt and rock wall, causing rock failures. Mechanical cable anchors have a significantly higher load-bearing capacity compared to rock bolts, can be installed in excavations with low head room, and can easily cater for increased support length demands (e.g., in large excavations applications). Due to the perceived supremacy that mechanical cable anchors offer owing to its high load bearing capacity, numerous operations religiously installed these units to secure the long-term stability of excavations. This practise inadvertently created a significant safety risk as traditional mechanical cable anchors were not designed for excavations that may be subjected to high deformation and/or dynamic loading conditions (e.g., seismicity, stressed abutments, squeezing ground, etc.).

The high stresses associated with deep rock excavations requires rock anchors to yield under a dynamic force - that is extend or compress in length between its anchor point within a drill hole and the rock face it is supporting, failing which the anchor snaps/breaks under seismic activity. Prior art in the use of deforming, energy absorbing tubes to serve as a yielding device in rock anchors are disclosed, inter alia, in the following patent documents, namely CN105822332, CN110905571 , CN111794792 and EP0204927.

The roof anchor market is a hugely competitive market and is extremely price sensitive, where cents on the dollar can make a difference. The current invention aims to provide a simple, easy to install, cost effective, user friendly, high strength, static- and dynamic capable yielding rock anchor that can sustain load under deformation or dynamic loading conditions, not sacrifice its high Ultimate Tensile Strength (UTS) to introduce any yielding, be suitable to various environments, can produce a transverse expansion effect when axial tension and deformation of the rock anchor occurs, and is tightly secured within a drill hole to achieve optimal energy absorption and consolidation.

SUMMARY OF THE INVENTION

According to the invention there is provided an energy absorbing, yielding rock anchor for supporting a wall of a rock face, the rock anchor comprising - an elongate anchor cable which is insertable into a drill hole of a rock face, the anchor cable terminating at one end thereof in an anchor head, and at an opposite end thereof in an anchor base; a cable anchor unit that is concentrically secured to the anchor cable approximate the anchor head; a hole anchor unit that is connected to the anchor cable in longitudinally spaced-apart relationship to the cable anchor unit, the hole anchor unit at least in part being longitudinally displaceable on the anchor cable relative to the cable anchor unit and configured to secure the anchor cable in the drill hole; a compression-absorbing yielding unit extending between the cable anchor unit and the hole anchor unit and being deformable under compression; and a bearing plate unit secured to the anchor cable approximate the anchor base and dimensioned to press against the rock face for securing the anchor cable within the drill hole, such that the anchor cable, the cable anchor unit, the hole anchor unit and the intermediate yielding unit are secured within the drill hole with only the bearing plate unit and anchor base extending outside of the drill hole. The anchor cable may be a solid rod, such as is used in rock bolts; or it may be a steel strand or steel cable, which comprises a number of cables/ strands/ rods that are twisted together to form the steel strand or steel cable.

The anchor cable extends through the cable anchor unit such that the cable anchor unit circumferentially engages the anchor cable. The cable anchor unit comprises a tapered wedge with a patterned cylindrical axial bore that grips onto the anchor cable; and wedge receiver with a tapered axial bore which is complimentarily configured to the tapered wedge and through which the anchor cable extends, such that the tapered wedge is axially displaceable into the tapered bore of the wedge receiver for securing the cable anchor unit onto the anchor cable.

The tapered wedge may include a larger external diameter one end and a smaller external diameter opposite end with the patterned cylindrical bore extending through the wedge. The tapered wedge may include a number of tapered wedge segments that are connectable to each other approximate the larger diameter end of the tapered wedge such that the opposite free ends of the wedge segments are radially outwardly displaceable relative to each other at the smaller diameter end of the tapered wedge. In particular, the wedge segments may be connectable to each other through a spring wire approximate the larger diameter end of the tapered wedge. More particularly, each wedge segment may include a spring-receiving groove extending parallel to the larger diameter end such that, when assembled, the spring-receiving grooves of the wedge segments are aligned to form a continuous spring-receiving groove extending concentrically about the anchor cable and in which the spring wire is located to connect the wedge segments to each other. The hole anchor unit comprises - a tapered cone with a larger external diameter one end and a smaller external diameter opposite end with a cylindrical bore extending through the cone and arranged coaxially on the anchor cable, the tapered cone being axially displaceable on the anchor cable; a mechanical expansion anchor comprising a ring-like base portion of substantially uniform thickness concentrically arranged about the anchor cable between the anchor head and the cable anchor unit; and at least two elongate leafcarrying arms extending integrally from the base portion in a common direction and concentrically arranged about the anchor cable, each of the leaf-carrying arms terminating at its free ends in a leaf with slots intermediate each leaf, the leaves lying in a common plane perpendicular to the anchor cable to define a radially expansible shell concentrically about the tapered cone; and an elongate compression spring coaxially arranged about the anchor cable between the cable anchor unit and the ring-like base portion of the mechanical expansion anchor of the hole anchor unit.

The mechanical expansion anchor may include four elongate leaf-carrying arms extending integrally from the base portion with each of the leaf-carrying arms terminating at its free ends in a leaf with slots intermediate each leaf, such that the expansible shell is defined by four radially expansible leaves.

The tapered cone is connected to the cable anchor such that the smaller diameter end of the cone is inserted into an opening defined by the free ends of the leaves with the anchor cable extending through the tapered cone and the expansible shell. After insertion into the drill hole, the anchor cable is pulled at the anchor base to move the tapered cone axially between the leaves, the latter thus being expanded radially from their free ends and bent outwardly from their opposite ends which remain integrally attached to the ring-like base portion.

In the unexpanded condition the expansible shell defines a tapered central bore which is complimentarily configured to the tapered cone, and an external diameter which is very close to that of the drill hole so that the expansible shell fictionally engages the drill hole and is restrained from axial displacement as the anchor cable is pulled at the anchor base, thereby moving the tapered cone axially on the anchor cable and expanding the shell into tightly gripping engagement with the drill hole. The cable anchor is then tensioned to a desired degree by applying a required amount of pulling force at the anchor base while biasing this force against the bearing plate unit, forcing the bearing plate unit into engagement with the rock face around the entrance to the drill hole.

The compression-absorbing yielding unit comprises a thin-walled tube coaxially arranged about the anchor cable and pressing at one end thereof against the wedge receiver of the cable anchor unit and pressing at an opposite end thereof against the tapered cone of the hole anchor unit. The compression-absorbing yielding tube is configured to undergo compression deformation under seismic forces when the tapered cone is axially displaced on the anchor cable towards the cable anchor unit. The compression-absorbing yielding tube may include one or a number of holes, slits or indentations extending through or into the tube wall to create weak points where deformation can be initiated in order to eliminate a peak force.

The bearing plate unit comprises a conical bearing plate having a base and an axial bore extending through its apex; a cylindrical wedge-receiving barrel including a tapered bore which is insertable into the axial bore of the conical bearing plate; and a tapered wedge which is complimentarily configured to and insertable into the tapered bore of the wedge-receiving barrel such that the anchor cable extends through the tapered wedge, wedge-receiving barrel and conical bearing plate.

The conical bearing plate may include at least one, but preferably a number of, breather holes extending through the plate for accommodating grout and/or breather tubes. The conical bearing plate further may include a flange extending either radially inwardly from the base or extending radially outwardly from the base.

The cylindrical wedge-receiving barrel may terminate at its one end in a rounded nose which is located in the axial bore of the conical bearing plate and may include a narrow washer-receiving neck adjacent the rounded nose. The neck may house a rubber or silicone force-indicator washer which collapses or is expelled when the anchor cable reaches a predetermined tension force to serve as a visual indicator that the anchor cable has been tensioned optimally.

The tapered wedge may include a larger external diameter one end and a smaller external diameter opposite end with a patterned cylindrical axial bore extending through the wedge and such that the bore grips onto the anchor cable. The tapered wedge may include a number of tapered wedge segments that are connectable to each other approximate the larger diameter end of the tapered wedge such that the opposite free ends of the wedge segments are radially outwardly displaceable relative to each other at the smaller diameter end of the tapered wedge. In particular, the wedge segments may be connectable to each other through a spring wire approximate the larger diameter end of the tapered wedge. More particularly, each wedge segment may include a spring-receiving groove extending parallel to the larger diameter end such that, when assembled, the spring-receiving grooves of the wedge segments are aligned to form a continuous spring-receiving groove extending concentric about the anchor cable and in which the spring wire is located to connect the wedge segments to each other.

SPECIFIC EMBODIMENT OF THE INVENTION

Without wishing to be bound thereto, the invention will now further be described and illustrated with reference to the following non-limiting examples in which -

FIGURE 1 is a side view of the rock anchor of the invention;

FIGURE 2 is an exploded view of the bearing plate unit;

FIGURE 3 is a sectional side view of the rock anchor of Figure 1 ;

FIGURE 4 is a sectional view of the cylindrical wedge-receiving barrel of the bearing plate unit;

FIGURE 5 is a side view of an alternative embodiment of the energy-absorbing yielding tube; FIGURE 6 is a sectional side view of a second embodiment of the conical bearing plate including a radially inwardly extending flange;

FIGURE 7 is a perspective view from below of the conical bearing plate of Figure 6;

FIGURE 8 is a perspective view from below of a third embodiment of the conical bearing plate including a radially outwardly extending flange.

An energy absorbing, yielding rock anchor for supporting a wall of a rock face is generally designated by reference numeral (10). The rock anchor (10) comprises an elongate anchor cable (12) which is insertable into a drill hole (not shown) of a rock face, with the anchor cable (12) terminating at one end thereof in an anchor head (14), and at an opposite end thereof in an anchor base (16). In the illustrated embodiment of the invention, the anchor cable (12) is a steel strand or steel cable, which comprises a number of cables/ strands/ rods that are twisted together to form the steel strand or steel cable.

The rock anchor (10) further comprises a cable anchor unit (18) that is concentrically secured to the anchor cable (12) approximate the anchor head (14); and a hole anchor unit (20) that is connected to the anchor cable (12) in longitudinally spaced-apart relationship to the cable anchor unit (18). The hole anchor unit (20) is at least in part longitudinally displaceable on the anchor cable (12) relative to the cable anchor unit (18) and is configured to secure the anchor cable (12) in the drill hole.

The rock anchor (10) further comprises a compression-absorbing yielding unit (22) extending between the cable anchor unit (18) and the hole anchor unit (20) and being deformable under compression. The compression-absorbing yielding unit (22) comprises a thin-walled tube (22) coaxially arranged about the anchor cable (12) and pressing at one end thereof against the cable anchor unit (18) and pressing at an opposite end thereof against the hole anchor unit (20). The compression-absorbing yielding tube (22) is configured to undergo compression deformation under seismic forces when the hole anchor unit (20) is axially displaced on the anchor cable (12) towards the cable anchor unit (18).

The compression-absorbing yielding tube (22) may include one or a number of holes, slits or indentations (23) (as illustrated in Figure 5) extending through or into the wall of the yielding tube (22) to create weak points where deformation can be initiated in order to eliminate a peak force. The compression-absorbing yielding tube (22) may have any cross-sectional shape, including round, square or rectangular. It may also be any structure associated with thin-walled energy absorbers, such as composite structures and filled structures and composite filled structures, examples of which are well known in the art and published widely.

The rock anchor (10) further comprises a bearing plate unit (24) secured to the anchor cable (12) approximate the anchor base (16) and dimensioned to press against the rock face for securing the anchor cable (12) within the drill hole. The rock anchor (10) is configured such that the anchor cable (12), the cable anchor unit (18), the hole anchor unit (20) and the intermediate yielding unit (22) are secured within the drill hole with only the bearing plate unit (24) and anchor base (16) extending outside of the drill hole. The anchor cable (12) extends through the cable anchor unit (18) such that the cable anchor unit (18) circumferentially engages the anchor cable (12). The cable anchor unit (18) comprises a tapered wedge (26A) with a patterned cylindrical axial bore (28) to grip onto the anchor cable (12); and a wedge receiver (30) with a tapered axial bore (32) which is complimentarily configured to the tapered wedge (26A) and through which the anchor cable (12) extends. The tapered wedge (26A) is axially displaceable into the tapered bore (32) of the wedge receiver (30) for securing the cable anchor unit (18) onto the anchor cable (12).

The tapered wedge (26A) of the cable anchor unit (18) is similar in design to the tapered wedge (26B) (described hereunder) of the bearing plate unit (24) and is described with reference to Figure 2. The tapered wedge (26A; 26B) includes a larger external diameter one end (34) and a smaller external diameter opposite end (36) with a patterned cylindrical bore (38) extending through the wedge (26A; 26B). The tapered wedge (26A; 26B) includes a number of tapered wedge segments (26.1 ; 26.2) that are connectable to each other approximate the larger diameter end (34) of the tapered wedge (26A; 26B) such that the opposite free ends of the wedge segments (26.1 ; 26.2) are radially outwardly displaceable relative to each other at the smaller diameter end (36) of the tapered wedge (26A; 26B). In particular, the wedge segments (26.1 ; 26.2) are connectable to each other through a spring wire (40) approximate the larger diameter end (34) of the tapered wedge (26A; 26B). More particularly, each wedge segment (26.1 ; 26.2) includes a spring-receiving groove (42) extending parallel to the larger diameter end (34) such that, when assembled, the spring-receiving grooves (42) of the wedge segments (26.1 ; 26.2) are aligned to form a continuous spring-receiving groove (42) extending concentrically about the anchor cable (12) and in which the spring wire (40) is located to connect the wedge segments (26.1 ; 26.2) to each other.

The hole anchor unit (20) comprises a tapered cone (44) with a larger external diameter one end (46) and a smaller external diameter opposite end (48) with a cylindrical bore (50) extending through the cone (44) and arranged coaxially on the anchor cable (12). The tapered cone (44) is axially displaceable on the anchor cable (12).

The hole anchor unit (20) further comprises a mechanical expansion anchor (52) comprising a ring-like base portion (54) of substantially uniform thickness concentrically arranged about the anchor cable (12) between the anchor head (14) and the cable anchor unit (18); and four elongate leaf-carrying arms (56) extending integrally from the base portion (54) in a common direction and concentrically arranged about the anchor cable (12). Each of the leaf-carrying arms (56) terminates at its free end in a leaf (58) with slots (60) intermediate each leaf (58), the leaves (58) lying in a common plane perpendicular to the anchor cable (12) to define a radially expansible shell concentrically about the tapered cone (44).

The hole anchor unit (20) further comprises an elongate compression spring (62) coaxially arranged about the anchor cable (12) between the cable anchor unit (18) and the ring-like base portion (54) of the mechanical expansion anchor (52) of the hole anchor unit (20). The tapered cone (44) is connected to the anchor cable (12) such that the smaller diameter end (48) of the cone (44) is inserted into an opening defined by the free ends of the leaves (58) with the anchor cable (12) extending through the tapered cone (44) and the expansible shell. After insertion into the drill hole, the anchor cable (12) is pulled at the anchor base (16) to move the tapered cone (44) axially between the leaves (58), the latter thus being expanded radially from their free ends and bent outwardly from their opposite ends which remain integrally attached to the ring.

In the unexpanded condition the expansible shell defines a tapered central bore (64) which is complimentarily configured to the tapered cone (44), and an external diameter which is very close to that of the drill hole so that the expansible shell fictionally engages the drill hole and is restrained from axial displacement as the anchor cable (12) is pulled at the anchor base (16), thereby moving the tapered cone (44) axially on the anchor cable (12) and expanding the shell into tightly gripping engagement with the drill hole. The anchor cable (12) is then tensioned to a desired degree by applying a required amount of pulling force at the anchor base (16) while biasing the pulling force against the bearing plate unit (24), forcing the bearing plate unit (24) into engagement with the rock face around the entrance to the drill hole.

The bearing plate unit (24) comprises a conical bearing plate (66) having a base (68) and an axial bore (70) extending through its apex; a cylindrical wedge-receiving barrel (72) including a tapered bore (74) which is insertable into the axial bore (70) of the conical bearing plate (66); and a tapered wedge (26B) which is complimentarily configured to the tapered bore (74) of the wedge-receiving barrel (72) and which is insertable into the tapered bore (74) of the barrel (72) such that the anchor cable (12) extends through the tapered wedge (26B), wedge-receiving barrel (72) and conical bearing plate (66).

The conical bearing plate (66) includes at least one, but preferably a number of, breather holes (78) extending through the bearing plate (66) for accommodating grout and/or breather tubes (not shown). The conical bearing plate (66) further includes a flange (80) extending either radially inwardly from the base (68) (as illustrated in Figures 6 and 7) or extending radially outwardly from the base (68) (as illustrated in Figure 8).

When the conical bearing plate (66) is under compression in use between its base (68) and bore (70), the area around the bore (70) is in compression and the area around the base (68) in tension with a gradual transition from compression to tension in between. To be cost effective the conical bearing plate (66) needs to be as light as possible while still being able to handle the maximum required load. The areas in compression stress can exceed the yield strength of the steel in use, but the areas in tension cannot exceed the yield strength. Figures 6 and 7 present an optimized conical bearing plate (66) design where the cone (44) wall thickness (T) has been minimized and the base (68) have been strengthened by the radially inwardly extending flange (80) to reduce tension stress in the base (68) to be within the yielding stress of the material being used. Figure 8 presents a conical bearing plate (66) with the flange (80) extending radially outwardly from the base (68). The conical bearing plate (66) and the two flange embodiments can be manufactured by a metal casting process or manufactured by pressing the conical bearing plate (66) out of plate metal and then welding the flange to the base (68). The cylindrical wedge-receiving barrel (72) terminates at its one end in a rounded nose (82) which is located in the axial bore (70) of the conical bearing plate (66) and includes a narrow washer-receiving neck (84) adjacent the rounded nose (82). The neck (84) houses a rubber or silicone force-indicator washer (86) which collapses or is expelled when the anchor cable (12) reaches a predetermined tension force, when neck (84) collapses under the compression force, to serve as a visual indicator that the anchor cable (12) has been tensioned optimally.

In the assembled configuration, the anchor cable (12) extends from the anchor base (16), through the base plate unit (24), through the hole anchor unit (20), through the compression-absorbing yielding tube (22), and through the cable anchor unit (18) where it terminates in the anchor head (14). In the event of the anchor cable (12) comprising a number of cables/ strands/ rods that are twisted together to form the steel strand or steel cable, the anchor head (14) includes a ferrule (88) for binding the cables/ strands/ rods together at the anchor head (14).

The thin-walled, compression-absorbing yielding tube (22) presses at one end thereof against the wedge receiver (30) of the cable anchor unit (18) and presses at an opposite end thereof against the tapered cone (44) of the hole anchor unit (20). The yielding tube (22) optionally may be welded to the wedge receiver (30) and tapered cone (44) respectively. The compression spring (62) of the hole anchor unit (20) bears against the tapered wedge (26A) of the cable anchor unit (18), biasing the tapered wedge (26A) away from the ring-like base portion (54) of the mechanical expansion anchor (52) under a compression force, thereby biasing the tapered wedge (26A) of the cable anchor unit (18) into the tapered bore (32) of the wedge-receiving unit (30). This causes the patterned bore (28) of the tapered wedge (26A) to grip onto the anchor cable (12).

Simultaneously, the biasing force of the compression spring (62) against the ring-like base portion (54) of the mechanical expansion anchor (52) causes tension in the leafcarrying arms (56), causing axial displacement of the leaves (58) over the tapered cone (44) and a resultant radially outward displacement of the leaves (58) until they grip the drill hole. The anchor cable (12) passes through the smooth bore (50) of the tapered cone (44), which is displaceable on the anchor cable (12) under a seismic force.

The conical bearing plate (66) of the bearing plate unit (24) is biased against the rock face surrounding the drill hole.

The rock anchor (10) is tensioned by a hydraulic jack (not shown) bearing against the tapered wedge (26B) and/or the rear of barrel (72) of the bearing plate unit (24) while pulling the anchor base (16) to place the anchor cable (12) under a tension force between the tapered wedge (26B) of the bearing plate unit (24) and the tapered wedge (26A) of the cable anchor unit (18), such that the same tension force is created between the conical bearing plate (66) of the bearing plate unit (24) and the drill hole where the radially expanding leaves (58) grip the drill hole. In practice the drill hole may be several meters deep, with the anchor cable (12) similarly being several meters long, which means that the yielding tube (22) may be several meters away from the bearing plate unit (24) within the drill hole. During seismic activity the bearing plate unit (24) may be displaced from the hole anchor unit (20), which might have resulted in failure/snapping of the anchor cable (12) when the tension force exceeds its breaking strength. However, in order to prevent this failure, the thin-walled energy-absorbing tube (22) deforms in a controlled fashion under a so-called mean crushing force (MCF) that is less than the breaking strength of the anchor cable (12). Deformation of the tube (22) may be any of the modes (ring, diamond or mixed) known in the art of thin-walled energy absorbers and as described in, for example, Ahmad Baroutaji, Mustafa Sajjia, Abdul-Ghani Olabi, “On the crashworthiness performance of thin -walled energy absorbers: recent advances and future developments"', and G. Lu and T. X. Yu, “ Energy Absorption of Structures and Materials", Elsevier, 2003. Deformation of the tube (22) is guided externally by the drill hole and internally by the anchor cable (12) to prevent unwanted global buckling.

It is common practice for a thin-walled energy absorber to initially reach a peak force before deformation commences under a mean crushing force, which in the case of a properly-designed thin-walled energy absorber remains fairly constant. The peak force may be multiple orders higher than the mean crushing force and is therefore an unwanted feature since the anchor cable (12) can snap under this initial peak force. To eliminate this peak force, tube (22) includes indentations (23) which initiate the controlled deformation at the MCF.

It will be appreciated that alternative embodiments of the invention are possible without departing from the spirit or scope of the invention as defined in the claims.