EP1980712A1

EP1980712A1 - Load cell for ground anchors

Info

Publication number: EP1980712A1
Application number: EP07007601A
Authority: EP
Inventors: Itzchak Lipsker; Amir Joram
Original assignee: Lipsker & Partners Engineering Services (1975) Ltd; Piletestcom Ltd
Current assignee: Lipsker & Partners Engineering Services (1975) Ltd; Piletestcom Ltd
Priority date: 2007-04-13
Filing date: 2007-04-13
Publication date: 2008-10-15

Abstract

There is provided a load cell for monitoring the residual forces in a pre-stressed soil and rock anchor, including a locking body having one hole through which a tensioning element passes, and an electronic bridge circuit containing strain-responsive elements affixed to the circumference of the locking body. The elements are oriented to detect changes in axial and circumferential dimensions of the locking body caused by changes in the residual forces of the soil and rock anchor,

Description

Field of the Invention

The present invention relates to a load cell for monitoring residual forces in soil and rock anchors.

Background of the Invention

Ground, e.g., soil and rock anchors are widely used in civil engineering to resist forces acting on retaining structures. Anchors installed for a long service term (say 50 years) are considered permanent, while those supposed to act over a short period only (say two years) are defined as temporary. As a rule, after installation, the anchors are pre-stressed to a specified tensile force, in order to minimize displacements of the retaining structures.
With time, the force in the anchor may decrease for various reasons such as relaxation and creep. As a result, anchored elements, such as foundations and retaining structures, may displace excessively and eventually fail. Therefore, it is extremely important to have suitable permanent means, which can monitor the residual force and verify that the anchor is functioning properly and according to design.
Load cells, whether electrical, hydraulic or pneumatic, for monitoring these forces exist, but suffer from several disadvantages and drawbacks. Not being integral parts of the anchor system, they have to be added to the system, consuming extra space, which makes the setup bulkier, difficult to approach and visually disturbing. Existing systems are also expensive, and their cost may exceed the cost of the anchor proper. This severely curtails the widespread application of anchor force monitoring. This situation, where only a minority of the anchors is monitored, may lead to failures with considerable losses in economic terms and even human lives.

Disclosure of the Invention

It is thus one of the objects of the present invention to overcome the disadvantages of the existing monitoring arrangements and to provide a load cell that is an integral component of a soil and rock anchor, and is inexpensive enough to allow the monitoring all, or at least most of, the anchors on a given site.
According to the invention, this is achieved by providing a load cell for monitoring the residual forces in a pre-stressed soil and rock anchor, comprising a locking body having at least one hole through which a tensioning element passes, and an electronic bridge circuit containing strain-responsive elements affixed to the circumference of said locking body, said elements being oriented to detect changes in axial and circumferential dimensions of the locking body caused by changes in the residual forces of said soil and rock anchor.

Brief Description of the Drawings

The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures, so that it may be more fully understood.
With specific reference now to the figures in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.
In the drawings:

Fig. 1: is a cross-sectional view of a soil and rock anchor system, according to the present invention;
Fig. 2: is a cross-sectional view of the mechanical part of the cell, according to the invention;
Fig. 3: illustrates a load cell, according to the present invention;
Fig. 4: is an electrical circuit of the strain gages, according to the present invention, and
Fig. 5: illustrates another embodiment of the load cell according to the invention.

Detailed Description of Preferred Embodiments

There is seen in Figs. 1 and 2 an anchor to be used with the load cell of the present invention. The anchor consists of a fixed or grouted portion 2 and a free portion 4. Multi-strand steel cables 6, advantageously four of them, with one of their ends fixedly attached to the grouted anchor portion 2, are lead through a central hole in a bearing plate 8 which is then brought into contact with the retaining structure 10. A locking body 12, a cross-sectional view of which is seen in Fig. 2, is then placed over bearing plate 8, with cables 6 passing through bores 14 in the split plugs 16 seated in the conical holes 18 in locking body 12. The conicity of the split plugs 16 fit the conicity of the conical holes 18 in the locking body 12, thus when the split plugs 16 accommodating cables 6 are driven into the locking body 12, the cables 6 are fixedly clamped into the locking body 12.
The free ends of cables 6 pass through a first hollow hydraulic jack 20, and then through a second hollow jack 22, and are then attached to a tensioning plate 24 seated on ram 26 of the second jack 22. The required tension can now applied to cables 6 by the second jack 22, after which split plugs 16 are forced into conical holes 18 in the locking body 12 by means of the first jack 20. Both jacks, 20 and 22, are then removed and the cable surplus is trimmed, rendering the anchor functional.
Referring to Fig. 3, there is illustrated a preferred embodiment of the present invention, according to which the locking body 12 is turned into a load cell 28, becoming an integral part of the anchor rather than an extraneous element introduced for monitoring the forces. The load cell 28 consists of a body 30 and strain-responsive elements, e.g., strain gages 32 forming part of an electronic circuit 34, illustrated in Fig. 4. Advantageously, the electronic circuit 34 is affixed in a groove 36 made in the circumference of the body 30 of the load cell 28. The circuit 34 can be affixed on the body 30 or in the groove 36 by using one of the bonding compounds manufactured for this purpose, e.g., M-Bond 43-B (Vishay). After the bonding procedure, the gages 32 are covered with a special protective coat. Other types of strain-responsive elements, such as hydraulic elements or devices, could also be utilized.
The electronic circuit 34 shown in Fig 4 is in the form of a four-arm double Wheatstone Bridge. Each arm of the Wheatstone Bridge contains two strain gages. Two opposite arms contain strain gages, S, S', while the other two opposite arms contain strain gages Sp, S'p. As further indicated in Fig. 4, the arms with the strain gages S, S' have their axes of sensitivity in the direction of the axial strain of the load cell 28, while the arms having strain gages Sp, S'p have their axes of sensitivity in the direction of the circumferential strain on the body cell 28. The strain gages S, S' and Sp, S'p, respectively, are connected in series. Thus, when a compressive load is applied to the load cell 28, it contracts along the longitudinal axis and expands in the circumferential direction (Poisson's Effect).
Typically, the resistance of each strain gage is 350 Ohm, and thus, the total resistance of each arm is 700 Ohm. For excitation, direct current with a stabilized voltage between 3 V and 10 V is applied. The circuit 34 is connected to a readout unit 38 which transforms the analog output of the load cell 28 into a digital one and connects via a suitable USB cable to a portable computer 40.
Before installation as part of the anchor, the load cell 28 is put in a suitable calibration device, and stressed to the maximum permissible load. The load is then relaxed in stages, and the readout for each load saved as a calibration table or curve.
After the anchor is installed, tested and locked, an initial reading of the device is performed. Further readings are later taken on demand. If these are identical to the initial reading, it means that there was no loss in the anchoring force. By use of the calibration curve, lower readings may be retransformed to obtain the residual anchor force.
Another embodiment of the load cell is shown in Fig. 5, in which cables 6 of the anchor are replaced by a single steel rod 42 with a threaded end, and the substantially cylindrical load cell 28 by a cylindrical nut 44. The latter may be of the self-locking type, or a counter-nut (not shown) may be applied. Also seen is the protective layer or coating 46 covering the gages affixed to the nut 44, or into a groove made therein.
It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrated embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims

A load cell for monitoring the residual forces in a pre-stressed soil and rock anchor, comprising:
a locking body having at least one hole through which a tensioning element passes, and

an electronic bridge circuit containing strain-responsive elements affixed to the circumference of said locking body, said elements being oriented to detect changes in axial and circumferential dimensions of the locking body caused by changes in the residual forces of said soil and rock anchor.
The load cell as claimed in claim 1, wherein said locking body is a cylindrical body.
The load cell as claimed in claim 1, wherein said locking body is a prismatic body.
The load cell as claimed in claim 1, wherein said bridge circuit is a Wheatstone Bridge comprising four arms.
The load cell as claimed in claim 4, wherein each arm of said Wheatstone Bridge includes two strain gages connected in series.
The load cell as claimed in claim 5, wherein the strain gages of one arm have their axes of sensitivity in the direction of the axial strain applied to said locking body, and the strain gages of the other two arms have their axes of sensitivity in the direction of the circumferential strain applied to said locking body.
The load cell as claimed in 1, wherein said locking body comprises a circumferential groove and said electronic bridge circuit is affixed in said groove.
The load cell as claimed in claim 1, further comprising readout instruments converting analogue output signals from said strain gages into digital signals.
The load cell as claimed in claim 1, wherein said electronic circuit is covered by a protective layer.
The load cell as claimed in claim 1, wherein said locking body contains at least one axial conical hole and further comprises tapered locking means consisting of a conical and split plug, wherein the conicity of said hole and of said plug is matching.
The load cell as claimed in claim 10, wherein said soil and rock anchor further comprising:
at least one tensioning element having one free end, the other end of which is fixedly attached to the soil-side end of said anchor;

said locking body being mounted on a bearing plate making contact with a structure to be anchored, and

said locking means fixedly lock at least one tensioning element inside said locking body after a predetermined pre-stressing force has been applied to said tensioning means.
The load cell as claimed in claim 11, wherein said hole tapers towards the face of said locking body that is mounted on said bearing plate.
The load cell as claimed in claim 11, wherein said tensioning element is a steel cable.
The load cell as claimed in claim 11, wherein said tensioning element is a steel rod, at least one end portion of which is threaded.
The load cell as claimed in claim 11, wherein said locking means is a self-locking nut.