Technical field of the invention

The present invention relates to an assembly for an at least partially removable anchor device with an anchoring body, comprising at least one tension member and an induction generating device. Furthermore, the invention also relates to an anchor device.

Background art

Anchor devices like pre-stressed anchors, grouted soil or rock anchors are frequently used during construction of buildings, bridges, tunnels and the like. Sometimes, the anchors are used permanently to stabilise the construction with respect to the surrounding building ground. Such permanent anchors can be frequently seen in tunnel portals, near bridgeheads and at steep slopes, in particular in difficult terrain (like poor or sliding subsoil). Here, the anchor is mounted and remains in the ground for a long time, usually for the lifetime of the building or structure that needs to be stabilised.

However, during construction works sometimes temporary anchors are needed. For example, if a building is constructed in an urban area, a building pit has to be excavated, for example for providing a cellar and/or an underground parking. Here, the construction pit has to be stabilised by temporary pit wall linings that have to be anchored in the surrounding building ground. However, when construction advances, the forces that act on the temporary pit walls can successively be taken over by the construction that is built (in particular by concrete slabs or the like). Since, particularly in urban areas, the piece of land the construction is built on has to be regularly used as efficiently as possible, the anchoring devices will usually extend into the building ground of the neighbouring property. This is usually accepted, as long as this intervention with the neighbouring property is only of a temporary nature. For this purpose, anchors are needed that can be at least partially removed when they are not needed any more. Pre-stressed anchors (irrespective of whether they are permanent or temporary) are usually made of a bar, a strand or a bunch of parallely arranged wires or strands that are usually made of steel that can be stressed. These tensile elements act as tension members. Near the supporting structure that has to be stabilised, a so-called anchor head is connected to the tension members) that also rests on the structure. Typically, an anchor plate is arranged between the anchor head and the supporting structure to distribute the forces more evenly. Inside the anchor in the section that is adjacent to the anchor head, the tension member(s) are initially guided through a single or common jacket duct, surrounding the tension member(s), so that the tension member(s) do not contact the surrounding soil directly. In this section, the tension members) can be freely elongated, so that an appropriate tensioning force can be provided. At the other end of the anchor, opposite of the anchoring head, the tension member(s) extend into an anchoring body, which can comprise, with regard to the force transmission, none, one or more anchoring units. The anchoring body is used to transfer the forces from the tension member(s) into the soil. Typically, the anchoring body is made of tension member(s) bonding to a filling material which is transferring the force to the ground. The filling material can be an injected grout that has been pressed into the borehole and the surrounding soil.

Essentially, there are two typical designs of an anchor device. According to a first design, the tension member(s) are connected to the anchoring body over the whole length of the anchoring body. According to a second design, the jacket duct(s) run through a major part of the anchoring body, so that the tension member(s) contact the anchoring body only at the very last tip of the anchoring body. Depending on the tensioning force, several anchoring units could be distributed over the length of the anchoring body to realize a smooth force transmission into a transition zone of the soil.

If the anchor is intended to be removable, it is necessary to cut the tension member(s) after use. This has to be preferably done in the vicinity of the transition zone where the tension member(s) leave the jacket duct and come into direct contact with material of the anchoring body. Since this section lies deep inside the borehole, special means have to be provided so that the cutting of the tension member(s) can be performed.

The anchoring body, that inevitably remains within the soil, is not problematic, since such an anchoring body is typically of the size of a frequently found piece of rock. Therefore, if the neighbouring property is intended to be covered with a building in the future, no serious problems will be encountered.

According to the state of the art, several suggestions for doing this have already been proposed. For example, according to German patent document DE 195 00 091 C1 it has been proposed to remove the tension member(s) of an anchor by providing predetermined breaking points by mechanical or thermal weakening of the tension member(s) over a certain length. For removing the anchor, the tension member(s) can be torn off by increasing the tensile load on the tension member(s). However, this proposal has severe disadvantages. For example, the (intentional) loss of strength of the tension member(s) has to be compensated by additional tension members that are weakened accordingly. Another disadvantage is that the weakened tension member(s) typically tear off abruptly and do not show a sufficient elongation right before breakage. It is obvious that such behaviour of an anchor is undesired for safety reasons.

Another suggestion has been made in DE 34 00 350 A1 , where it is suggested to cut the tension member(s) by heating and melting using thermite or a similar reaction mixture. Since the device with the reaction mixture has to be provided when inserting the anchor, the amount of reaction mixture is somewhat limited according geometrical facts. It can happen that due to inho- mogeneities in the soil, the typically provided amount of reaction mixture will not be sufficient. This can have the consequence that the tension member(s) are not completely cut off. This is, of course, not desired.

Another suggestion was made in Swiss patent document CH 603 919 A5. Here it is suggested to heat up the tension member(s) by means of an induction coil, until, due to the induction, the tension member(s) are cut through and can be removed from the borehole. According to the document, the indue- tion coil is arranged between a thermal isolation hose and an outer thermal isolation covered with a sealing bandage. Furthermore, asbestos cement is provided at both axial ends of the induction coil. However, a disadvantage with the described design is that the induction coil is placed and integrated on the end of the common jacket duct and thus a volume remains between the tension members) and the common jacket duct. Unless special means for sealing are provided, water can accumulate in the volume between the tension member(s) and the common jacket duct in the vicinity of the induction coil. This has the effect that, when the induction coil is engaged, firstly water has to be evaporated, before the tension member(s) will eventually heat up to a higher temperature. This way, a much longer time is needed for the removal of the anchor, where the time needed for removing the anchor can change erratically or in fact, a sufficient heating of the tension member(s) becomes impossible.

To improve the removal of the anchor even further, according to European patent application EP 0 583 725 A1 , it has been suggested to provide a metal tube between the induction coil and the tension member(s), wherein the metal tube consists of austenitic material. By using the additional metal tube between the tension member(s) and the induction coil, it is possible to heat up the tension member(s) by induction till up to the Curie temperature (768°C). A disadvantage with this design is that the design is more costly and more complicated. Furthermore, the heat-up of the tension member(s) by thermal conductivity between the additional metal tube and the tension member(s) is very inhomogeneous. Therefore, the tension member(s) that are lying in the middle of the cross-section of the bundle of tension members are heated up with a delay, resulting in an uneven breaking failure of the tension member(s). In addition the anchor according to EP 0 583 725 A1 does not comprise an inner sealing. Thus the same problems of accumulated water in the anchor body appear and a sufficient heating of the tension member(s) may become impossible.

Finally the Swiss patent document CH 702 926 B9 discloses an anchor that is partly removable from the soil, wherein an induction coil is arranged around a supporting tube, which is non-electrically conductive. The area between the induction coil and a breaking point of the tension member(s) remains free of an electrically conductive metal tube or sleeve. Again, an inhomogene- ous heating of the tension mennber(s) is possible and can complicate the removal of the anchor. Furthermore, this assembly is costly and its installation takes a long time since the supporting tube with the induction coil needs to be hold in place while the injection material is solidifying.

Summary of the invention

Therefore, there is still a need for an improvement of removable anchors. In particular, it is desired to have an anchor that is less complex and less costly in design. Furthermore, it is desired to have removable anchors, where the removal can be effectuated more reliable.

It is therefore an object of the invention to improve an assembly for an at least partially removable anchor device in a way that it shows advantageous features over common means or devices for at least partially removable anchors that are known in the state of the art. It is another object of the invention to improve an anchor device, in particular an at least partially removable anchor device that shows advantageous features over anchor devices (at least partially removable anchor devices) that are known in the state of the art.

It is suggested to design an assembly for an at least partially removable anchor device with an anchoring body, comprising at least one tension member and an induction generating device in a way that said induction generating device is arranged at least partially directly adjacent to said at least one tension member. The tension member or tension members can in particular comprise a bar, a wire or an assembly of wires, such as a strand. By the term "directly adjacent" or alternatively "directly neighbouring" it is particularly meant that no additional device, which is a structural part of an anchor device, is arranged between the induction generating device and the tension member or tension members. In particular, no support tube, no additional metal tube or sleeve and no jacket duct separates the induction generating device and the tension member or tension members. In other words, the induction generating device can directly contact (some of) the tension member or tension members. However, it is possible that a distance or a play is provided between the induction generating device and the tension member or tension members, at least in an "initial phase" where the two pieces are assembled and mounted in a borehole. The distance or play, filled with grout in a later phase of installation, respectively by the injection of the anchor device, can for example realize a thermal insulation between the tension member or tension members and parts of the induction generating device. The assembling can be done either at the factory, at a storage space at a construction site, or immediately before mounting the anchor into a borehole right at the construction site. However, it is possible that during a "preparatory step" before implanting the anchor, or in the process of implanting the anchor into the borehole, the tension member or tension members are modified in a way that the induction generating device and the tension member or tension members are brought into direct contact with each other. Thus the position of the induction generating device on the tension member or tension members can be fixed, for example.

It should be noted that the induction generating device can comprise some stabilising elements. For example, it is possible that the induction generating device, in particular in case it is designed as an electric conductor loop, a coil or the like, comprises some kind of tightening element. This tightening element could be a tape, a thermoshrink sleeve or tape, a cable-tie, or the like. The tightening element may be arranged around and/or on the outside and/or on the radial inside of the electric conductor loops. Also it is possible that the induction generating device comprises some stabilising resin with which the electric conductors are connected to each other. Of course, it is additionally or alternatively possible that the stiffeness of the electric conductors of the induction generating device is chosen in a way that the induction generating device is mechanically stable.

In particular, the induction generating device can comprise one or several electric conductor loops. Electric conductor loops have proven to be a particularly simple and effective means for generating an electric induction. Furthermore, the shape of an electric conductor loop is suggested by the outer form of a typical tension member or a bundle of tension members, which has usually an approximately circumferential or even a circular shape. Additionally it is suggested to design an assembly for an at least partially removable anchor device, comprising at least one tension member and an induction generating device in a way that said induction generating device is designed as a separate device, in particular separately from said at least one tension member. Preferably, the induction generating device is designed as a separately manageable device, independent of other means of the anchor device. Of course the induction generating device may be arranged with a connection and sealing with electric supply lines. Advantageously electric conductor loops of the induction generating device may be realized by a power supply wire which is directly mounted on the tension member, place holders, a distance holder for keeping distance between the induction generating device and the surrounding soil of the borehole and/or a spreading device for spreading the tension members of a bundle of a tension members.

In particular, the induction generating device can be designed separate from a jacket duct, a support tube or the like. This way, the design of the overall assembly can be particularly simple and the arrangement can be completed at any time, including immediately before mounting the anchor into the borehole right at the construction site. A relative positioning of the induction generating device on the tension member or a bundle of tension members can be realised by mechanical contact between both of them, e.g. by spreading the tension members by a spreading device. Also, the positioning may be realized by mechanical contact with additional elements, like distance pieces or place holders. In particular, the contact can be in form of a force-fit contact. If the induction generating device is arranged inside an anchoring body, the anchoring body, for example injected grout that hardens after injection, can even be used for sealing the volume area of the induction generating device from the outside, in particular for hindering water to get into the vicinity of the induction generating device. A particular preferred aspect is that the material of the anchoring body (for example grout) can come into direct contact with the induction generating device. This can support a thermal insulation between the tension member or the tension members and the induction generating device. Also a particularly simple and effective handling of the anchor device can be realised. It should be particularly mentioned that due to the missing support tube and/or due to the arrangement of the induction generating device at a position slightly away from the end of the jacket duct on the side of the anchoring body or directly in the end sealing of the common jacket duct the design is simpler and more efficient.

Preferably, said induction generating device comprises an electric conductor loop, preferably an electric coil. In other words, one could also say that a (plurality of) electric conductor loop(s) is designed as an electric coil (this way, a plurality of loops is provided). With such an arrangement, the efficiency of the assembly can be enhanced. In particular, a large induction with a comparatively small electric current can be realised. This has the advantageous effect that the cross-section of the electric conductors, particularly the electric conductors for supplying the induction generating device with electric energy, like the above mentioned supply wire, can show a comparatively small cross- section without a significant risk of damage of said conductors due to an over- current. This way the reliability can be enhanced and/or the cost for the assembly can be reduced. There are lower material costs for the electric conductors; copper which is regularly used for such electric conductors is comparatively expensive.

Furthermore, with respect to the anchor force, it is suggested to design the assembly with one or a plurality of electric conductor loops (that can be arranged on top of each other, in particular to design it with an electric coil comprising a plurality of layers). This way it is possible to introduce a strong electric induction field into a small volume, in particular over a short length of the tension member or the bundle of tension members. This way a reliable cut-off of the tension member or the tension members can be performed with a relatively small amount of electrical power. The electric conductor for the loops can be of essentially any type. In particular it is possible to use heat resistant electrical wires. However, if a protection material (e.g. grout) to the conductor loops is provided, it is also possible to use standard out-of-the-shelf electrical wires with a plastic insulation surrounding the electrical wire, for example made of PVC.

It is possible to design the assembly with tension members as commonly known, wherein one tension member comprises a plurality of wires. Generally a tension member comprises seven wires e.g. of pre-stressing steel that are stranded (rope-woven) with each other.

It is also possible to design the assembly with at least one placeholder element, in particular for substituting a tension member. Sometimes, the load capacity has to be individually adapted with respect to the construction site. This can be done by simply varying the number of tension members within an anchor device. To be able to maintain a symmetrical arrangement of a tension member within a bundle of tension members in the area of the induction coil, the "missing" tension member can be replaced by substituting elements. Such a substituting element can be in particular a device that is non-magnetic and has a significantly lower mechanical load capacity, in particular with respect to tensile forces. However, this substituting element should still have a sufficiently high load capacity with respect to its stability in maintaining its cross- section. As an example, such a substituting element can comprise, or be constructed of, plastic material, glass fibre materials or the like. The substituting element can be massive or hollow in design.

It is particularly useful to design the assembly in a way that said induction generating device is at least partially designed to be shape retaining, in particular by using a tightening element and/or a stiff electrical conductor and/or a stabilising resin. This way, the assembling of the induction generating device can be facilitated, in particular if no special tools should be used for the arrangement.

Preferably, the assembly is designed in a way that said induction generating device is arranged with a play with respect to said tension member or tension members. This way, placing the induction generating device on the tension member or tension members can be significantly facilitated. Of course, it is possible that at a later stage the induction generating device can come into contact with the tension member or tension members, in particular in a force-fit contact. This can particularly occur if in a bundle of tension members the tension members are spread for purposes of attachment to an anchoring body. It is further preferred if the assembly for an at least partially removable anchor device and the anchor device, respectively, are designed in a way that they comprise at least one jacket duct. Using a common jacket duct, the tension member or tension members can be put under tensile force in a homogeneous way, even if the anchor device is mounted into the borehole. This is because no direct interaction between the grout and the tension member or tension members is occurring over the length of the jacket duct. With such an arrangement, the resulting anchor device can transmit high tensile forces and furthermore the reliability of the anchor device can be improved. The common jacket duct can be advantageously also used as a duct for electrical supply lines, or even supply lines of a different purpose, by acting as a duct for said lines, e.g. if they are arranged inside the common jacket duct. But even if the respective electrical supply lines or other supply lines are arranged on the outside of the common jacket duct, the common jacket duct can act as some kind of a "stabilising device" for said supply lines. This way, the stability of the respective supply lines can also be enhanced. In a further preferred embodiment, an end sealing can be provided at the extremity of the common jacket duct on the side of the anchor body of the anchor device.

It is further preferred if the assembly and the anchor device, respectively, are designed in a way that they comprise a plurality of tension members and at least one spreading device for spreading these tension members. Preferably, but not necessarily, each of the individual tension member is positioned within a single jacket duct which protects each individual tension member from adverse exterior conditions. The advantage of the spreading device and the spreading of the tension members is that each of the tension members can be separated and spaced apart from each other. Thus, the mechanical stability of the tension members inside an anchoring body can be enhanced, in particular with respect to the forces that can be transmitted. At the same time, such spreading can be used for a force-fit arrangement and/or a centring of the induction generating device on the respective part of the tension member or a bundle of tension members. This way, a defined relative position of the induction generating device and the tension member or a bundle of tension members can be reached. The spreading can be particularly realised by bending the individual tension members in a radially outward way. In particular, the spreading of the individual tension members of a woven tension arrangement comprising a plurality of tensioning members can be undone along the length of the anchoring body. Several spreading devices may be used along the length of the anchoring body as will be explained below. Furthermore, in between two expansion areas along the length of the anchoring body, each comprising a spreading device, a bunching area can be realized by a bunching device, that bundles the tensioning members.

It is preferred if the assembly and the anchor device, respectively, are designed in a way that said spreading device is arranged adjacent to said induction generating device, wherein preferably the spreading of the tension members results in a centring of said induction generating device with respect to the tension member or members. This way a particularly strong mechanical fixation of the tension members within the anchoring body can be achieved. At the same time, a particularly large amount of the resulting removable anchor device can be removed once the induction generating device is used.

Furthermore it is also preferred that the assembly and the anchor device, respectively, are designed with a distance element, like a distance piece or distance holder, arranged around the induction generating device. The distance element basically surrounds the induction generating device circumferentially around the tension member or bundle of members, to keep some distance to the surroundings, like the borehole wall. The distance element may be realized as a cage-like grid that sits on the tension member or bundle of tension members and comprises an enlarged middle part with an enlarged diameter.

In particular, the assembly and the anchor device, respectively, can be designed in a way that one end of said tension member or tension members comprises an anchor head and an anchor plate and at the other end an anchor body is provided as explained above. This way, it is very easy to hold a (temporary) side wall lining for a construction pit in place. Furthermore, the resulting anchor device can be used just like anchor devices, previously known in the state of the art, or for example also for compression type anchors as will be explained below. It is furthermore advantageous to design the assembly and the anchor device, respectively, in a way that said tension member or bundle of tension members on one end are designed for connection with at least one compression anchoring unit. In this case the anchor device is designed as a compression type anchor. The compression anchoring units are part of the same anchoring body. A compression anchoring unit provides an anchor head for the fixation of the tension member or tension members and a device for the force application into the anchoring body. For example a compression fitting or a wedge anchoring and an anchor plate can be used as commonly known. Preferably the anchoring body of such a compression type anchor comprises several compression anchoring units, wherein each of the units fixes only a fraction of the number of tension members of the anchor device. Thus several compression anchoring units are aligned behind each other in tension direction, preferably (with respect to the soil conditions) equally distanced to each other, and distribute the tension force at several areas into the soil. Each compression anchoring unit comprises an anchor part, like an anchor plate, a special shaped casting body or the like, that transmits the forces from the tension member or tension members to the anchoring body. The induction generating device, particularly an electric conductor loop, is advantageously applied close to or at the anchor part of each of the compression anchoring unit. Thus after disconnecting the tension member or tension members, only the compression anchoring unit and the hardened grout of the anchoring body remain in the building ground.

During mounting of the anchor device grouting material enters between the induction generating device and its surroundings so that a thermal insulation of the induction generating device to the tension member or tension- members is realized. Also the induction generating device is protected against penetration of water. Furthermore, although not necessary and generally not used with this type of anchors, it is not fully excluded that a distance element (similar to that used with the other type of anchors) is also used for centring the induction generating device in the borehole so that the grouting can be equally arranged around the induction generating device. Furthermore spacers may be provided to keep some space between the induction generating device and the tension member or tension members to provide some thermal insulation between the inner surface of the induction generating device and the tension member or tension members. This is particularly advantageous in case electrical wires with a PVC covering can be used.

Furthermore, when the anchoring body of an anchor device comes to lie in an area with loose or friable soil or rock and/or in flowing or standing groundwater, the assembly with the induction generating device and a tension member or tension members may, if necessary, be wrapped with a separating material. In particular, this separating material (e.g. a sewed geotextil fabric), can be used over the length of the anchoring body to hinder the lost of consistency of the grout of the anchoring body during the injection and the hardening.

A further aspect of the present invention refers to the anchor device comprising a device for an at least partially removable anchor according to the preceding description. In particular, the anchor device is designed as an at least partially removable anchor device. The resulting anchor device can show the same or at least similar features and advantages as the previously described device. Furthermore, the anchor device can be modified in the light of the previously given information.

Brief description of the drawings

The invention will become clearer when considering the following description of embodiments of the present invention, together with the enclosed figures. The figures are showing:

Fig. 1 : a first possible embodiment of a pre-stressed anchor device comprising an induction coil in a schematic cross-section;

Fig. 2: an enlarged view of the area around the induction coil of the pre-stressed anchor according to Fig. 1 in a schematic cross-section; Fig. 3: a second possible embodiment of a pre-stressed anchor device comprising several induction coils and compression anchoring units in a schematic cross-section;

Fig. 4: an enlarged view of the area around the induction coil and the compression anchoring unit of the pre-stressed anchor according to Fig. 3 in a schematic cross-section;

Fig. 4a - 4d: possible arrangements of induction coils relative to a bundle of tension members in schematic cross-sections; and

Fig. 5a - 5b: possible arrangements for tension members in a common jacket duct and for supporting tubes and/or lines in schematic cross- sections.

Detailed description of the preferred embodiments

In Fig. 1 a first possible embodiment of a partially removable pre- stressed anchor device 1 in a pre-stressed condition comprising an induction generating device in form of an induction coil 2 is shown in a schematic cross- section. The pre-stressed anchor device 1 comprises a bundle 3 of tension members 4 in its centre. At this place, it is important to stress that the bundle 3 of tension members 4 can be a conventional tendon made of strands, i.e. an assembly of several wires of pre-stressing steel that are stranded together. However, another bundle 3 of tension members 4 can also be imagined such that the invention is not only limited to strands. The bundle 3 of tension members 4 connects an anchor head 5 and an anchoring body 7 with each other. The bundle 3 of tension members 4 is under a tensile stress, so that anchor head 5 and anchoring body 7 are pulled towards each other. This way, the anchor head 5 that rests on an anchor plate 6 can exert a force onto a supporting structure to be stabilised, in this case a temporary side wall lining 8 of a construction pit. The anchoring body 7 mostly consists of grout that has been injected in liquid or paste-like state through a borehole 9 that has been prepared using an appropriate drill. As soon as the side walls of the construction pit do not have to be secured anymore, for example due to advances in constructing the building itself, the side wall linings 8 together with at least major parts of the pre-stressed anchor devices lose their necessity and parts of the pre-stressed anchor devices may be removed.

The total length I of the pre-stressed anchor device 1 can be divided into the free anchor length and the anchoring length l ₂ . In the section of the anchor device 1 within the free anchor length , the tension members 4 are guided within a single and/or a common jacket duct 15, 10, so that it is assured that the tension members 4 do not interact with the hardened grout that has been injected for forming the anchoring body 7. Due to this design with single jacket ducts 15 and/or a common jacket duct 10, the tensile force is distributed approximately evenly and homogeneously over the free length of the pre- stressed anchor device 1 , a functionality that is regularly desired. In addition to the common jacket duct 10, each of the individual tension members 4 can be positioned additionally inside a single jacket duct 15, as represented in Fig. 2.

In the section of the pre-stressed anchor device 1 within the anchoring length l ₂ , however, the tension members 4 come into direct contact with the anchoring body 7. Of course, some transitions are foreseen, so that the end of the common jacket duct 10 is sealed by an end sealing 22, made preferably of heated bitumen, to avoid a penetration of injection liquid. The common jacket duct 10 can protrude into the anchoring body 7 for a short distance (as indicated in Fig. 1 ). If necessary, to provide for a mechanically more stable contact between the tension member 4 and the anchoring body 7, the bundle 3 of tension members 4 can be spread into individual tension members 4, as can be seen in Fig. 1 . For performing this spreading, a spreading device 1 1 is used.

Furthermore, as indicated in Fig. 1 , the induction coil 2 rests directly on (some of) the tension members 4 of the bundle 3 of tension members 4. Originally, for mounting the induction coil 2, there is a play between the inner radial surface of the induction coil 2 and the outer radial surface of the bundle 3 of tension members 4. However, in the ready-state of the anchor device 1 , the individual tension members 4 of the bundle 3 are already spread to some extent even before the spreading device 1 1 . In effect, the spreading device 1 1 can be placed at a certain distance from the induction coil 2 during installation of the anchor device 1 in a way that there will be a direct force-fit contact between the induction coil 2 and the tension members 4. Therefore, the position of the induction coil 2 on the tension members 4 is fixed and furthermore centred.

As represented in Fig. 1 and Fig. 2, the induction generating device 2 can be mounted slightly away from the extremity of the common jacket duct 10 and/or the single jacket ducts 15 on the side of the anchor body 7. However, it is also imaginable to mount the induction generating device 2 directly within the the common jacket duct 10 alone or in conjunction the end sealing 22 of the common jacket duct 10.

Figure 2 shows a detail of the general set up of the anchor device of Figure 1 . Additionally to the embodiment of Figure 1 , but not compulsory, a distance element 20 is arranged around the induction coil 2 for keeping distance between the induction coil 2 and its surroundings, like the wall of the borehole 9. Also, the distance element 20 centres the bundle 3 of tension members 4 and the induction coil 2 within the borehole 9. The space around the induction coil 2 between the bundle 3 of tension members 4 and the inner side of the distance element 20 can be filled with grout. This provides a sealing so that no water penetrates in direction of the bundle 3 of tension members 4 from the outside and provides a thermal insulation towards the surroundings of the induction coil 2. The distance element 20 can be made of a grid-like element with a bulgy middle part of large diameter and end parts of small diameter, that rest e. g. on the jacket duct 10 and the tension members 4. The grout can enter the interior of the distance element 20 through the grid. Furthermore, when the anchor body 7 lies in a loose or friable soil or rock area and/or in flowing or standing groundwater, it can be, totally or only some parts, additionally wrapped in a separating material that is disposed to prevent a loose of grout due to trickle or wash away. This separating material can preferably be a sewed geotextil fabric to form a bag closed at one end or the like but other forms and designs are also possible.

When parts of the anchor device 1 are to be removed (see description above), an electric current is introduced into the induction coil 2. For example, depending on the number of tension members, an electric current of a frequency of 0.1 kHz to 40 kHz may be used. For the supply of the induction coil 2, electrical conductors 12 are provided that are arranged in the presently shown embodiment within the common jacket duct 10. But other arrangements are possible as well. By application of such a frequency, the metal of the tension members 4 is heated up by electrical induction. Since the maximum mechanical load that a tension member 4 can receive without breaking depends strongly on the temperature, and considering that a significant tensile force is present on the tension members 4, a comparatively low temperature is sufficient to cause a breaking of the tension members 4 in the vicinity of the induction coil 2. After breakage of the tension members 4, the major part of the anchor device 1 can be removed, for example the complete free anchor length of the tension members 4, the anchor head 5 and the anchor plate 6. The anchoring body 7 can remain in position and does not have to be removed in this embodiment. However, the cross section of an anchoring body 7 typically does not exceed the size of a typical stone, so that this will pose no notable problems for construction works that might be performed later on in this area.

As an example, experiments have shown that a temperature of 300 C is typically sufficient to result in a breakage of the pre-stressed tension members 4 under realistic conditions. This is still way lower than the Curie- temperature that ranges around 760°C for pre-stressing steel of a composition that is usually used for tension members.

As can be seen in the embodiment of Fig. 1 (and likewise from Fig. 2) the common jacket duct 10 ends well before the induction coil 2. Therefore, the induction coil 2 will be completely enclosed by the hardened grout of the anchoring body 7. This will also be the case for the embodiment of Fig. 2, comprising the distance element 20 around the induction coil 2 for providing some filling space around the induction coil 2, that during the injection this part will be filled with grout. It has to be realised that this is very advantageous, since the hardened grout of the anchoring body 7 effectively seals the volume around the induction coil 2 against water influx, for example due to groundwater present. Furthermore, a thermal insulation is provided as mentioned before. Therefore, the arrangement of the induction coil 2 directly on the tension members 4 (and without a support tube and/or without an arrangement on top of the common jacket duct 10) is not only simpler in design, but also shows advantageous features.

In Fig. 3, another possible embodiment of a pre-stressed anchor device 13 is shown, namely a so-called pressure body anchor as a compression type anchor. The pressure body anchor device 13 has a similar design as the pre-stressed anchor 1 that is shown in Figs. 1 and 2. The essential difference to the anchor of Figs. 1 and 2 is the introduction of the anchor force into the anchoring body 7 by a compression anchoring unit 21 comprising an induction generating device which allows for separating the tension members at this point, and therefore allowing for removal of the full length of the tension members 4. Therefore, it can be guaranteed that no or nearly no (pre-stressing) steel remains in the anchor body 7 respectively in the building ground. The pressure body anchor device 13 in Fig. 3 comprises three compression anchoring units 21 , 21 ', 21 " which are, with regard to the soil condition, equally distanced along the anchoring length l ₂ of the anchoring body 7. Of course, another number of compressing anchoring units (e.g. even one) is also imaginable. Each of the compression anchoring units 21 , 21 ', 21 " holds a fraction of the overall number of tension members 4 of the bundle 3. In this way the tension force can be distributed along the anchoring length l ₂ and transferred into the surrounding soil along this length.

Each of the tension members 4 in a bundle 3 of tension members is protected by a single jacket duct 15 over its whole length . The single jacket duct 15 ends at the front of the compression anchoring unit to which the respective tension member 4 is connected. Each compression anchoring unit 21 , 21 ', 21 " comprises an induction generating device 2, 2', 2" in form of an induction coil, an anchor plate 32 with or without a spiral 31 or a special shaped casting and one or more anchor heads 33 as visible in Fig. 4. Each induction coil 2, 2', 2" is positioned shortly before or in the inner part of the compression anchoring unit 21 , 21 ', 21 " on the tension member or tension members 4 which are spread and attached to the anchor plate 32 or the casting. A further possible design is to position the induction coils 2, 2', 2" directly on the anchor head 33 (or the anchor heads if more than one are used). Those tension members 4 which are not attached to the pressure anchoring unit 21 run over it to the next pressure anchoring unit 21 ' and so on. The pressure anchoring body is designed in a way to get an optimal force transmission into the anchoring body 7. Preferably, it is designed as a casting which may comprise a main flange as an anchor plate and one or several ribs for a partial force introduction into the anchoring body 7. It is shaped with space for the running trough tension members to the next pressure anchoring units 21 ', 21 " and so on. If needed, one or more holes in the casting provide a de-airing of its inner part to obtain a full-filling with grout. The casting can be made of steel, aluminium, fiber reinforced plastic or (cement) grout in a high compressive strength with or without a reinforcement. Of cause, if necessary, a spiral 31 can be added to enhance the compressive strength of the hardened grout of the anchoring body 7 (as illustrated in Fig. 4). By applying an electric current of a frequency of 0.1 kHz to 40 kHz to the respective induction coil 2, 2', 2", the pre-stressed tension member or tension members 4 who are connected to the respective compression anchoring unit 21 , 21 ', 21 " will be heated up and break in this area. This way the full length of the respective tension member or tension members 4 can be removed.

In Fig. 4a to 4d, possible relative arrangements of the induction gen- eratig device 2 with respect to the tension members 4 are shown. As one can further see from the embodiments shown in Fig. 4a to 4d, each tension member 4 comprises a plurality of stranded (rope-woven) wires 14, wherein seven wires 14 for each tension member 4 are provided as commonly used for pre-stressing strands. Likewise, depending on the anchor force, one or a plurality of tension members 4 for the complete bundle 3 are provided. Also the distance element 20 is shown, wherein the distance element 20 is surrounded by hardened grout of the anchoring body 7.

Figure 4a shows a first variation of the assembly for at least partially removable temporary anchor device, wherein the induction generating device in form of the induction coil 2 sits tight on the bundle 3 of tension members 4. In this variation, the induction generating device is a readymade cylindrical induction coil 2 comprising induction coil loops made of high heat-resistant electric cable. As shown, the bundle 3 together with the induction coil 2 is centred in the distance element 20 and the borehole 9. The distance element 20 protects the induction coil 2 against damage during mounting in to the borehole. The grout surrounding the induction coil 2 within the distance element 20 provides a sealing against penetration of water and further a thermal insulation, whereby the heat generated by the induction coil 2 can heat up the pre-stressed tension members 4 equally to generate a safe breakage of the tension members 4.

Figure 4b shows a second variation of the assembly similar to figure 4a. In this variation the induction generating device is a readymade cylindrical induction coil 2 comprising induction coil loops made of common electric cable with PVC insulation material. Spacers 18 are provided on some areas of the outer circumference of the bundle 3 of tension members 4. The spacers 18 provide some spacing between the induction coil 2 and each of the tension members 4, which can be filled with grout. After hardening of the grout, the grout provides a heat shield for the PVC material of the induction coil 2. The spacers made of non-magnetic material are arranged in between the induction coil 2 and the tension members 4. Their shape is designed in respect of the number of tension members 4 in a bundle 3 of tension members 4 to provide enough space between the tension members 4 and the induction coil 2 for filling with grout during the injection of grout and thus to obtain a heat-shield for the induction coil 2 by the hardened grout. The role of the distance element 20 is the same as described in Figure 4a.

Figure 4c again shows a third variation of the device for at least partially removing temporary anchor device. The induction coil 2 is made by loops of the electrical conductor 12, which provides electrical current to the induction generating device. The induction coil 2 is tightened to the bundle 3 of tension members by a force-fitting engagement, which is the result e. g. of spreading the tension members 4 of the bundle 3 by a spreading device. In the shown variation, the loops of the induction coil 2 are distanced from the bundle 3 by spacers 18. However, depending on the material of the conductor 12, the spacers 18 can also be omitted.

Figure 4d shows an embodyment of the induction generating device 2 similar to that of Fig. 4a, but with an additional duct surrounding the whole arrangement. In this variation, the induction generating device 2 and a tension member 4 or the bundle 3 of tension members 4 are surrounded by a plastic duct or hose 19; 10 made of PE, PP or PVC to provide protection against corrosion. For most permanent anchors, preferably, a corrugated jacket duct 19 is used, in which case the corrugated jacket duct 19 is grouted internally and externally to transfer the load from the tension member(s) 4 to the duct and then from the corrugated duct to the ground. However, it is also possible to use the common jacket duct 10 or a shrinking hose in the area of the induction generating device 2. The induction generating device 2 in form of a readymade cylindrical induction coil comprising induction coil loops made of high heat-resistant electric conductor sits tight on the of the tension members 4. A filling duct 16 for grouting the inner part of the anchor device is housed within the bundle 3, instead of one of the tension members 4. The distance element 20 protects the anchor device during mounting and centers it in the borehole, which will be grouted afterwards, as already explained.

The designs shown in Fig. 4a to 4d (and other, presently not shown designs) can be used in combination with the pre-stressed anchor 1 according to Fig. 1 , as well as with the pressure body anchor 13, as shown in Figs. 3 and 4 (or even with different types of anchoring devices, presently not explicitly shown).

In Figures 5a and 5b a section of the bundle 3 of tension members 4 along the free anchoring length is shown, where the bundle 3 is still surrounded by the common jacket duct 10. The individual tension members 4 each may be enveloped by a single jacket duct 15 (as in Fig. 5a), but it is not compulsory (as in Fig. 5b). The jacket ducts 10 and 15 provide the free elongation of the tension members 4 by the application the pre-stressing force and protection against corrosion. Also the electrical conductors 12 for providing the induction generating device with an electric current can be surrounded by a conductor duct 17 and housed within the common jacket duct 10. Alternatively or additionaly, the electrical conductors 12 could be guided with or without a conductor duct 17 arranged along the outside of the common jacket duct 10. The common jacket duct can be omitted if not used.

Although in the illustrated embodiment the number of tension members 4 is always chosen to be seven, it is of course possible to add and/or to remove tension members 4 to increase and/or to decrease the number of tension members 4 within the bundle 3. This way, the mechanical capacity of the respective anchor 1 and 13 can be appropriately adapted. Only as a matter of completeness it should be mentioned that the number of individual wires 14 within a single tension member 4 can be different as well. It is even possible to "mix" tension members 4 with a (partially) varying number of individual wires 14