WO2006018656A1

WO2006018656A1 - Improved anchorage head assembly

Info

Publication number: WO2006018656A1
Application number: PCT/GB2005/003254
Authority: WO
Inventors: Richard David Neilson; Andrew Joe Starkey; Ana Ivanovic; Brian Jackson; Andy Ritson
Original assignee: Aberdeen University; Amec Group Limited
Priority date: 2004-08-19
Filing date: 2005-08-19
Publication date: 2006-02-23
Also published as: EP1789634B1; GB0418545D0; DE602005013250D1; ATE425312T1; EP1789634A1

Abstract

The present invention relates to a ground anchorage head assembly (10) for an anchorage tendon (15), the head assembly comprising a bearing plate (11); and a resilient coupling (11, 12), for coupling the bearing plate to said tendon, said resilient coupling having a non-linear stiffness characteristic.

Description

Improved Anchorage Head Assembly

The present invention relates to an improved anchorage head assembly and in particular to such a head assembly used for monitoring the condition of ground anchorages. In this respect, such ground anchorages are commonly used in the support of engineering structures such as tunnels, retaining walls, mines, dry docks, dams, and prestressed structures.

Ground anchorages of this sort can be subdivided into a number of different types. First, there are rock bolt assemblies, where there is usually a single tendon, in the form of a solid bar. Figure 1 shows such a rock bolt assembly. As shown, the rock bolt assembly includes a single tendon 1, in the form of a solid bar, having a free anchor length 2 namely, one that is free from the surrounding rock 3, and a fixed anchor length 4 which is set into the surrounding rock. The bolt may be bonded using resin or cement grout or can be mechanically locked into place using an expanding sleeve. A domed bearing plate 6 includes a hemispherical washer 5 and nut that is threadingly engaged with a threaded section 7 of the tendon 1. The washer 6 rests in a recess in the bearing plate 6 so that the washer and the plate effectively provide a ball joint so that the plate can align with the rock surface without distorting the bolt. This arrangement serves to mitigate mis-alignment of the plate 5 relative to the tendon.

Secondly, there are strand anchorages of the sort shown in Figure 2, where the tendon 1' is a twisted multi-strand cable, often comprising a core surrounded by for example 6 outer wires. These types of anchorage assemblies may be single tendons, but in large structures they often comprise a number of tendons enclosed in a common sheath and sharing a common bearing plate 5 ' .

In this respect the applicants of the present invention have previously devised a system for use in the monitoring of ground anchorages, known as the GRANIT (Trade Mark) system. As explained in their associated patent, no. EP0754263, this system, as depicted in Figure 3, operates by applying an axial impulse load of low magnitude and predetermined characteristics to the head of the anchorage assembly, using a specially developed loading device 17, which is attached to the protruding length of the anchorage. An accelerometer 9 is attached to the surface of the impact device in order to measure the resulting vibration response of the anchorage head. The response signatures obtained are then transferred to a laptop computer for further processing. A datum response signature is normally recorded for the intact anchorage immediately after installation. The response signatures are subsequently obtained both after any blasting and/or at a later date. Any differences between the responses indicates a potential change in the characteristics of the anchorage system.

GRANIT has been shown to work effectively^ to assess load in rock bolt anchorages during a number of projects. Preliminary results on single strand anchorages have also shown that, if the anchorage head characteristics are suitable, GRANIT can be used effectively with single strand tendons. The basis for load prediction is reliant on the non¬ linear stiffness of the anchorage system which is not designed into the anchorage per se but is a product of the contact between the rock/soil surface and the elastic deformation of the bearing plate and lock-off wedges/barrels or nut/washer assembly.

Moreover, it has become apparent from testing using the GRANIT system and other test procedures that there are effectively two categories of anchorage:

1. Class I - anchorages which have relatively flexible heads which results in one or more of the natural frequencies changing appreciably with load and therefore making the anchorage diagnosable.

2. Class II - anchorages whose head assemblies are so stiff that there is no appreciable change in the natural frequencies after a small load has been applied and therefore are not amenable to load diagnosis using the GRANIT system. This is often the case with single and multi-strand anchorages with large, stiff heads.

Hence, in the case of such Class II anchorages, the applicants came to recognise that the stiffness of their anchorage head assemblies prevents the condition of the anchorage assembly being diagnosed effectively, making it necessary to carry out more invasive and destructive monitoring procedures in order to assess its condition.^'

The applicants of the present invention thus sought to address such problems of current anchorage head assemblies. According to an aspect of the present invention there is provided a ground anchorage head assembly for an anchorage tendon, the head assembly comprising a bearing plate; and a resilient coupling, for coupling the bearing plate to said tendon, said resilient coupling having a non-linear stiffness characteristic. Such an arrangement allows condition diagnosis of head assemblies previously impervious to such monitoring.

Preferably, the resilient coupling comprises a non-linear

,resilient element in contact with the bearing plate, and a coupling element for securing the resilient coupling to the tendon.

Preferably, said resilient coupling comprises a non¬ linear washer. The combination of the non-linear washer and bearing plate affords a stable and readily standardisable platform for monitoring measurement.

Alternatively, said resilient coupling comprises a spring member, and more preferably a helical spring. In this connection, the spring member acts to decouple the means for securing a tendon from the bearing plate.^'

In preferred embodiments, said spring member comprises a substantially tubular member having a helical slot. The variance in slot gap, i.e. the distance -between facing surfaces of the slot in the spiral helical construction of the spring, may thus be used for direct measurement of the load applied by the tendon to the barrel.

The incline of the helical slot may, for example, be substantially 75 degrees to the longitudinal axis of the spring member and the helical slot may, in certain embodiments extend for between 560 and 650 degrees of revolution.

In preferred embodiments, the stiffness of the resilient coupling increases as it deflects ' to a greater extent. In other words, the flexibility of the resilient coupling reduces, the further the resilient coupling deflects. In preferred embodiments, said coupling element comprises a barrel and one or more wedge members. The barrel may be configured to seat reliably with the resilient element so as to ensure a stable, readily reproducible assembly can be formed. Conveniently, said bearing plate comprises a linear washer having a thickness substantially equal to its radial width. The relatively great thickness of the bearing plate lifts the resilient element away from the surface from which the tendon protrudes, thereby keeping the active part of the assembly clear of detritus and discontinuities at the surface'. In preferred embodiments, the ground anchorage head assembly comprises a plurality of coupling elements for a plurality of tendons, and a unitary resilient element. ^'Alternatively, the ground anchorage head assembly comprises a plurality of resilient elements, for a plurality of tendons .

Conveniently, a distribution plate is provided between said plurality of coupling elements and said resilient element, for distributing load between said plurality of coupling elements and the resilient element.

According to a further aspect of the present invention there is provided a resilient coupling for use in a ground anchorage head assembly for an anchorage tendon, the resilient coupling comprising:- a spring member for mounting between a tendon and a bearing plate for decoupling the two; wherein relative movement between elements of the spring member can be used for measuring load applied thereto. Examples of the present invention will now be described, with reference to the accompanying drawings of which:-

Figures 1 and 2 show known anchorage head assembly arrangements;

Figure 3 shows a schematic drawing of the known GRANIT (Trademark) diagnostic system;

Figure 4 shows a first embodiment of a ground anchorage head assembly of the present invention in cross section;

Figure 5 shows in cross-section a second embodiment of the ground anchorage head assembly of the present invention; Figure 6 shows, in cross-section a third embodiment of a ground anchorage assembly head of the present invention;

Figure 7 shows a further embodiment of a resilient coupling for use in a ground anchorage head assembly of the present invention; Figure 8 shows a perspective view of the coupling of Figure 7; and

Figure 9 shows a cross-sectional view of a ground anchorage head assembly incorporating the resilient coupling of Figures 7 and 8

Figure 4 shows a ground anchorage head assembly 10 comprising a bearing plate 11, a barrel 13, wedge members 14, and a resilient coupling having a resilient element 12 in the form of a non-linear washer.

The non-linear washer has a non-liner stiffness characteristic, such that its stiffness increases as it deflects. Put another way, the flexibility -of the non-linear washer decreases, the further it deflects. This is to be contrasted with a conventional "linear" spring which has a constant stiffness, Graph A below showing a comparison between a conventional "linear" spring and a spring having a non¬ linear characteristic.

Graph A

Non-linear Stiffening Force/Deflection Characteristic

0.1 0.2 0.3 0.4 0.S 0.6 0.7

Deflection (mm)

In Figure 4, a strand or tendon 15 is shown emerging from a rock surface 16 and is coupled to the head assembly 10 via the connection of the wedge members 14 in barrel 13.

The introduction of the non-linear washer 12 allows the force/deflection characteristics of the anchorage head assembly 10 to be pre-determined by the geometry and material properties of the washer while still permitting the use of a large "stiff" anchorage head assembly to react the load back into the surrounding rock/soil/structure.

This arrangement of head assembly complements the existing GRANIT technology and ensures that anchorages assembled with a nonlinear washer can be assessed for load easily at any point in their operational lives. The use of typical corrosion protection procedures will ensure that the washer functions effectively over long periods . Further, the relative stiffness of the non-linear washer

12 compared to the strand tendon will ensure that the deflection of the non-linear washer will have little effect on the overall stiffness of the final anchorage assembly, since the majority of the deflection will occur in the tendon. Also the non-linear washer 12 can be used for rock bolt type anchorages provided that the relative stiffness of the non-linear washer to the bolt is arranged to ensure that the majority of the deflection occurs in the bolt.

As shown, the bearing plate comprises a linear washer 11 having a thickness substantially equal to its radial width. The relatively large or great thickness of the bearing plate 11 lifts the resilient element away from the surface 16 of the rock from which the tendon protrudes, thereby keeping the active part of the assembly clear of detritus and discontinuities at the surface.

In Figure 5, a multi-tendon anchorage head assembly 20 of a second embodiment is shown. In this regard, for multi- tendon strand anchorage assemblies, the load carrying mechanism is more complex. Currently, with known arrangements the load from each strand in a multi-tendon anchorage reacts through the barrels used to lock off each of the individual tendons and onto a common bearing plate. This means that the bearing plate experiences the total load i.e. the sum of the loads in the individual tendons making any form of condition diagnosis impracticable. The applicants of the present invention have realised that their concept proposed above for the single tendon anchorage can be applied to a multi-tendon anchorage by placing a large resilient element in the form of nonlinear washer/plate 22 between a distribution plate 28 and bearing plate 21 of the anchorage head assembly. The- tendons 15 are coupled to the barrels- 23 using wedge members 24. This provides a reading of the total load in the anchorage, when the GRANIT system is used to excite the whole head assembly.

In a further arrangement as exemplified in the third embodiment of the present invention shown in Figure 6, each tendon 15 is provided with its own resilient element in the form of a non-linear washer 32 placed under barrel 33 containing wedge members 34.

From the perspective of using the GRΑNIT system, this effectively converts each tendon into a separate single tendon Class I anchorage, each of which may be monitored individually to give the distribution of load in the individual tendons, and therefore, can also generate a value for the total load in the whole anchorage assembly. This allows assessment of the condition of the anchorage as a whole in terms of load, but also can identify any failure in any particular tendon or group of tendons. With all the above embodiments, the design of the resilient element washer can be optimised to provide a progressive shift in the anchorage's natural frequency with load. The washers 'can be designed to fit within the space restrictions of typical anchorage heads and a range can be designed to meet most of the regularly encountered applications. Since the relative stiffness of the bearing plate, and therefore in this case the non-linear washer, to the tendon is important, the stiffness characteristic of the washer can be designed for particular tendon geometries and load levels to provide the optimal frequency characteristics for diagnosis using the GRANIT system. Care needs to be taken to ensure that the flexibility of the non-linear washer does not comprise any corrosion protection measures required by British Standards to BS8081. This is not seen as a major problem, however, because typical deflections on suitable bearing plates have typically been substantially less than lmm at a load of 5OkN. The extension of a typical 15.2mm diameter 4m long strand tendon under the same tensile loading would be about βmm.

An example according., to the first embodiment was subjected to testing. In this respect two sets of GRANIT (Trade Mark) tests were undertaken on a single strand cable anchorage. In the first set of tests the anchorage was assembled as per standard working practice with a large (standard) bearing plate and a barrel with wedges. A hydraulic jack was used to stress the anchorage assembly to various levels. At each level the anchorage assembly was locked off and the jack removed. The GRANIT system was then attached and a number of tests undertaken. Additional confirmation tests were also undertaken with a commercial impact hammer. The GRANIT equipment was then removed and the anchorage stressed to the next level. For the second set of tests the same test procedure was used but in this case the anchorage was assembled with the assembly of the first embodiment. Figure A shows the results of the first set of tests with the original head configuration which results in Class II characteristics. As can be seen from the four load levels, after a shift in the frequencies from OkN to 15kN there is no further shift and so diagnosis of the load on the basis of frequency would be impossible beyond this load value.

Class II anchor head frequency response

Frequency (Hz)

Figure 1: Results from original Class II anchorage head

Figure B (below) shows the results from the modified anchorage head with the nonlinear washer. In this case there is a clear progressive shift in frequency over the four load levels which allows diagnosis by the GRANIT system.

The use of individual washers under each barrel of a multi strand anchorage would allow individual strands to be monitored and diagnosed. ' Class I anchor head frequency response

Frequency (Hz)

Figure B: Results from modified anchorage head with nonlinear washer providing Class I anchorage head characteristics

The • test shows clearly that the introduction of a assembly according to the present invention can modify an anchorage from Class II characteristics to Class I and so make it diagnosable by the GRANIT (Trade Mark) system.

Figure 9 shows a ground anchorage head assembly 40 comprising a bearing plate 41, and a resilient element 42 in the form of spring member, a barrel 43 and wedge members^' 44. Referring to Figures 7 and 8, the spring member is of a helical type, comprising a tubular member 45 having a helical slot 46. In the embodiment shown, it is provided integrally with a barrel member 47 having a conical, inwardly tapering section 48. It should be noted that the helical slot is shown slightly exaggerated in Figure 8 for illustrative purposes.

The spring has a non-linear stiffness characteristic. In this respect, its stiffness can be arranged to increase as it deflects to a greater extent; in other words, its flexibility reduces, the further it deflects. This characteristic may be induced by progressive winding or providing increased closure between certain coils.

As such with the wedge members 44 in place, as shown in Figure 9, a tendon 49 is clamped in position within the head assembly 40. Even though the tendon is clamped in position, the spring member 42 acts to decouple the tendon clamping mechanism from the bearing plate 41. In this connection, the conical section provided within the barrel member terminates in relation to the beginning of the helical slot so as to avoid undesirable interference between the wedge members 44 and the slot 46.

The spring member 42 is hence provided between the wedge barrel and the bearing plate and provides a reaction to the imposed load on the barrel. In this regard, the variance in slot gap, i.e. the distance between facing surfaces of the slot in the spiral helical construction of the spring, may be used for direct measurement of the load applied by the tendon to the barrel, since this distance will vary according to the load on the tendon or strand. This allows the loads on individual strands of a multi strand anchor to be separately analysed, which is not possible with conventional integral head plate and wedge assemblies. Depending upon the nature of the spring, it can, if required, provide a non-linear reaction to the imposed load.

The spring member 42 may, as shown, be formed as part of the barrel, but alternatively could be a ^• separate unit installed in connection with the barrel. The introduction of the spring member 42 allows the force/deflection characteristics of the anchorage head assembly 40 to be pre-determined by the geometry and material properties of the spring while still permitting the use of a large "stiff" type anchorage head assembly, to react the load back into the surrounding rock/soil/structure.

Claims

Claims : -

1. A ground anchorage head assembly for an anchorage tendon, the head assembly comprising a bearing plate; and a resilient coupling, for coupling the bearing plate to said tendon, said resilient coupling having a non-linear stiffness characteristic.

2. A ground anchorage head assembly according to claim 1, wherein the resilient coupling comprises a resilient element in contact with the bearing plate, and a coupling element for securing the resilient coupling to the tendon.

3. A ground anchorage head assembly according to claim 1 or 2, wherein said resilient coupling comprises a non-linear washer.

4. A ground anchorage head assembly according to claim 1 or 2, wherein said resilient coupling comprises a non-linear spring member.

5. A ground anchorage head assembly according to claim 4, wherein said resilient coupling comprises a helical spring.

6. A ground anchorage head assembly according to claim 4 or 5, wherein said spring member comprises a substantially tubular member having a helical slot.

7. A ground anchorage head assembly according to any preceding claim, wherein the stiffness of the resilient coupling increases as it deflects to a greater extent.

8. A ground anchorage head assembly according to claim 2, wherein said coupling element comprises a barrel and one or more wedge members.

9. A ground anchorage head assembly according to any preceding claim, wherein said bearing plate comprises a linear washer having a thickness substantially equal to its radial width.

10. A ground anchorage head assembly according to claim 2, wherein the ground anchorage head assembly comprises a plurality of coupling elements for a plurality of tendons, and a unitary resilie.nt element.

11. A ground anchorage head assembly -according to claim 2, wherein ' the ground anchorage head assembly comprises a plurality of resilient elements, for a plurality of tendons.

12. A ground anchorage head assembly according to claim 10, wherein a distribution plate is provided between said plurality of coupling elements and said resilient element, for distributing load between said plurality of coupling elements and the resilient element.

13. A resilient coupling for use in a ground anchorage head assembly for an anchorage tendon, the resilient coupling comprising:- a spring member for mounting between a tendon and a bearing plate for decoupling the two; wherein relative movement between elements of the spring member can be used for measuring load applied thereto.

14. A ground anchorage substantially as hereinbefore described with reference to Figure 4 to 9 of the accompanying drawings. '