Composite yieldable rock anchor with improved deformation range

Abstract

The present invention pertains to a yieldable rock anchor (10), comprising an elongated tendon (12) extending longitudinally along a tendon axis (A) from a proximal end (16) to a distal end (18), wherein the tendon includes a substantially non-yielding rigid first anchor portion (20) at or near said distal end (18) and extending towards said proximal end (16), and at least one plastically deformable axially yielding portion (26) intermediate said non-yielding rigid first anchor portion (20) and said proximal end (16). The first anchor portion (20) may be a hollow bar member, and the first anchor portion (20) and the at least one yielding portion (26) are integrally joined or coupled to one another to form at least part of said elongated tendon (12).

Claims

1. A yieldable rock anchor, comprising: a non-unitary elongated tendon extending longitudinally along a tendon axis from a proximal end to a distal end, the tendon including a substantially non-yielding rigid first anchor portion at or near said distal end and extending towards said proximal end, and at least one plastically deformable axially yielding portion intermediate said non-yielding rigid first anchor portion and said proximal end, wherein both the first anchor portion and the at least one yielding portion are hollow bar members, and wherein the first anchor portion and the at least one yielding portion are integrally joined to one another in an end-to-end manner by welding or gluing to form at least part of said elongated tendon, wherein strength tolerance bands of the substantially non-yielding rigid anchor portion and the at least one plastically deformable axially yielding portion do not overlap, and wherein the at least one axially yielding portion has a deformation range of at least 100 mm/m.

2. The yieldable rock anchor according to claim 1, wherein the rock anchor is self-drilling.

3. The yieldable rock anchor according to claim 1, wherein the tendon includes at least one substantially non-yielding rigid further anchor portion intermediate said proximal end and said at least one yielding portion.

4. The yieldable rock anchor according to claim 3, wherein the tendon includes a plurality of substantially non-yielding rigid further anchor portions and a plurality of plastically deformable axially yielding portions in alternating arrangement.

5. The yieldable rock anchor according to claim 3, wherein the at least one substantially non-yielding rigid further anchor portion is provided with a plurality of anchoring members protruding from an outer surface of the respective further anchor portion.

6. The yieldable rock anchor according to claim 1, wherein the non-yielding rigid first anchor portion is provided with a plurality of anchoring members protruding from an outer surface of the first anchor portion.

7. The yieldable rock anchor according to claim 6, wherein the plurality of anchoring members include protuberances selected from the group of ribs, grooves, slots, indentations, threads, bosses and studs.

8. The yieldable rock anchor according to claim 1, wherein the at least one yielding portion has a generally smooth outer surface.

9. The yieldable rock anchor according to claim 1, wherein the at least one yielding portion has an outer diameter that is smaller than an outer diameter of the first rigid anchor portion.

10. The yieldable rock anchor according to claim 1, wherein the first anchor portion has a higher strength than the at least one yielding portion.

11. The yieldable rock anchor according to claim 1, wherein the at least one yielding portion and the first anchor portion are steel hollow bar members, the steel of the hollow bar members constituting the at least one yielding portion being different from the steel of the hollow bar members constituting the first anchor portion.

12. The yieldable rock anchor according to claim 1, wherein the tendon includes an externally threaded portion at or near said proximal end.

Description

(1) Currently preferred embodiments of a yieldable rock anchor according to the present invention will now be described in more detail with reference to the accompanying schematic figures.

(2) FIG. 1 shows a side view of a first embodiment of a yieldable rock anchor according to the present invention,

(3) FIG. 2 shows a side view of a second embodiment of a yieldable rock anchor according to the present invention, and

(4) FIG. 3 shows a side view of a third embodiment of a yieldable rock anchor according to the present invention including multiple yielding portions.

(5) FIG. 1 shows a side view of a first embodiment of a yieldable rock anchor, or rock bolt, generally designated at 10. The rock anchor 10 includes an elongated tendon 12 extending longitudinally along a tendon axis A from a proximal end 16 to a distal end 18. The tendon 12 is preferably fabricated from metal, such as a steel bar, however other bar constructions are possible. While the tendon 12 will usually have an at least essentially circular cross section, the present invention is not limited to tendons having a circular cross section.

(6) At or near the distal end 18 and extending in the direction of the proximal end 16, the tendon 12 has a substantially non-yielding rigid first anchor portion 20 intended to firmly anchor the tendon in place. The first anchor portion 20 is made from a hollow bar member and is provided, on an outer surface thereof, with a plurality of anchoring members 22 which in the embodiment shown take the form of ribs, such as on a conventional rebar, but may also take the form of grooves, slots, indentations, threads, bosses and studs. The anchoring members 22 are provided to enhance a fixing action between the first anchor portion, a bonding agent such as grout or resin, and the surrounding rock structure. As is well-known to skilled persons in the field to which the present invention pertains, grout or resin is used to fasten a rock anchor in a borehole.

(7) In the embodiment shown, the rock anchor 10 is of the self-drilling variety, and to this end includes a drilling head 24 mounted to the distal end 18 of the tendon 12.

(8) Extending from the substantially non-yielding rigid first anchor portion 20 towards the proximal end 16 is a plastically deformable axially yielding portion 26 which is also made from a hollow bar member but has a generally smooth outer surface. In the embodiments shown, both the hollow rigid first anchor portion 20 and the hollow yielding portion 26 are made of steel and are welded to one another at weld 28a to thus form at least a part of the elongated tendon 12. Alternatively, the hollow rigid first anchor portion 20 and the hollow yielding portion 26 may be glued to one another. In the embodiment shown, the yielding portion 26 has essentially the same outer diameter as the first anchor portion 20, but it is also possible for the yielding portion to have an outer diameter that is smaller than the outer diameter of the rigid anchor portion 20.

(9) A proximal externally threaded end portion 30 includes the proximal end 16 and is joined, either integrally or by welding as shown at 28b, to the yielding portion 26. In use, the proximal end 16 projects outwardly beyond a rock face and is configured, by way of the externally threaded portion 30, to cooperate with an anchor plate 32 and a threadingly engagable nut 34 used to clamp the proximal end of the rock anchor 10 against the rock face and to apply a suitable pretension, if desired. The externally threaded portion 30 of the tendon 12 may in use act as a substantially non-yielding rigid further anchor portion 36 to the extent it is located within a borehole (not shown), that is in an area extending approximately from the anchor plate 32 towards the distal end 18. Accordingly, the yieldable rock anchor 10 shown in FIG. 1 includes the at least substantially non-yielding first anchor portion 20 closest to the toe of the borehole, i.e. closest to the distal end 18 of the tendon 12, as well as the plastically deformable axially yielding portion 26 located intermediate the first anchor portion 20 and the proximal end 16, and an at least substantially non-yielding rigid further anchor portion 36 located intermediate the proximal end 16 and the yielding portion 26.

(10) In order for the rock anchor 10 to operate as intended, both the first anchor portion 20 and the further anchor portion 36, regardless of the material used to make them, must have a strength which is sufficiently higher than a strength of the yielding portion 26 to ensure that it is the plastically deformable yielding portion 26 which lengthens or elongates axially with dilating rock, to thereby accommodate and absorb the rock forces without having the rock anchor 10 fail. Considering strength tolerance bands of the first anchor portion and any further anchor portion as well as of the at least one yielding portion, it is important to ascertain that the tolerance bands of the anchor portions and the yielding portion(s) do not overlap. Also, it is necessary that in use the yielding portion 26, or any yielding portion for that matter, forms a debonded section whereas the first anchor portion and any further anchor portion forms a bonded section, i.e. bonded to the surrounding rock structure. To this end, the yielding portion 26 has a generally smooth outer surface, thus facilitating debonding. To further enhance debonding capabilities, the outer surface of the yielding portion 26, or any yielding portion, may be suitably coated to reduce friction, for example by applying a thin layer of oil or another non-stick material.

(11) There are different ways to ensure that the strength of the first anchor portion and any further anchor portion is sufficiently higher than that of the or any yielding portion. For example, if anchoring members 22 in the form of ribs or threads are provided on anchor portions 20, 36, they may be formed onto the outer surface of the respective anchor portion by using a cold rolling process (or any other suitable press forming operation), by which the strength of the hollow bar member forming an anchor portion is increased and its plasticity reduced when compared to a hollow bar member which consists of the same type of steel and has not been subjected to a cold rolling process. Further, hollow bar members intended to form yielding portions may be heat-treated, thus increasing their capability to plastically deform by decreasing strength. Still further, different types of steel may be used as anchor portions and as yielding portions, respectively, with steel used for yielding portions having a greater elongation and lower strength than steel used for anchor portions. Combinations of those possibilities are also possible, allowing to custom tailor the tendon 12 to particular applications. Tests conducted by the applicant have shown that 22MnB5 steel is particularly suitable to be used in the present invention for forming both anchor portions and yielding portions.

(12) In tests conducted by the applicant, yieldable rock anchors 10 according to the present invention were found to allow elongations of between 120 mm/m and 150 mm/m, corresponding to an axial deformation of between 12% and 15%, respectively. This is considered a huge improvement over prior art yieldable rock anchors which allow axial deformations in the range of approximately 0.3% to 2%.

(13) FIG. 2 shows a side view of a second embodiment of a yieldable rock anchor 10a which differs from the rock anchor 10 of FIG. 1 in that the substantially non-yielding rigid first anchor portion 20 and the plastically deformable axially yielding portion 26 are not welded to one another, but are instead coupled to one another using a separate coupling element, which in the embodiment shown is an internally threaded steel sleeve 38. Likewise, the axially yielding portion 26 is not welded to the further anchor portion 36 as in FIG. 1, but is instead coupled to the further anchor portion 36 using another coupling element, embodied here as another internally threaded steel sleeve 38. As shown in FIG. 2, both sleeves 38 are formed as end pieces of the axially yielding portion 26 having a larger outer diameter than the remainder of the axially yielding portion, and are integrally joined to the axially yielding portion by welding, but could also be embodied as separate members, e.g. as steel sleeves which are not integral end pieces of the axially yielding portion 26 (not shown). Here, both sleeves 38 are made of a type of steel that is different from the steel used for forming the axially yielding portion 26 in order to ensure that any yielding is restricted to the axially yielding portion 26. Alternatively, the coupling elements could be sleeves made from fiber reinforced plastics material (not shown), and instead of using internally threaded sleeves, crimped sleeves could for example be used. Any other coupling element capable of reliably joining or coupling a non-yielding anchor portion to an axially yielding portion may be used in the context of the present invention.

(14) FIG. 3 shows a side view of a third embodiment of a yieldable rock anchor 10b according to the present invention, which is similar to the first embodiment shown in FIG. 1, but includes, in addition to the substantially non-yielding rigid first anchor portion 20, a plurality of substantially non-yielding rigid further anchor portions 36 as well as a plurality of plastically deformable axially yielding portions 26 in alternating arrangement. In FIG. 3, two non-yielding further anchor portions 36 and two axially yielding portions 26 are shown, but any number of such further anchor portions and axially yielding portions is contemplated. Just like the embodiment shown in FIG. 1, the anchor portions and axially yielding portions of FIG. 3 are welded (or glued) to one another at welds 28a-28d, but could also be coupled to one another by means of coupling elements, as shown in FIG. 2.

(15) Typically, an axially yielding portion 26 may have a length of one meter, but may also be shorter or longer. Also, the length and/or diameter of both the anchor portions and the yielding portions forming the tendon 12 may vary, that is, each anchor portion and each the yielding portion need not have the same length and/or diameter.