GB2158482A - Device for separating the tension member of a prestressed pressure-grouted anchor by forming a predetermined breaking point - Google Patents

Abstract

A predetermined heat-actuated breaking point in a tension member (1) of a pressure-grouted anchor is surrounded by a annular sleeve (6) which is filled with an aluminothermic reaction mixture in which is embedded an igniting system (16) which can be triggered electrically. The annular sleeve (6) comprises an outer tube (7) with an inner tube (10) respectively accommodating an individual element (1') of the tension member (1). The cavity (15) formed between the outer tube (7) and inner tube (10) and filled with an aluminothermic reaction mixture is closed by end covers (8 and 9 respectively) which are disposed at distances a and b respectively from the ends of the outer tube (7). The cavities thus formed at the ends are filled by plugs (20, 21) consisting of a refractory material which sets, for example concrete. In the case of an inclined anchor, the incandescent molten reaction mixture is thereby prevented from escaping from the outer tube (7) prematurely. <IMAGE>

Description

SPECIFICATION Device for separating the tension member of a prestressed pressure-grouted anchor by forming a predetermined breaking point The invention relates to a device for separating the tension member a prestressed pressure-grouted anchorbyforming a predetermined breaking point at a given location on its length, in particularatthe transition from the anchoring length to the free steel length, comprising an annular sleeve surrounding the tension member in the vicinity of the predetermined breaking pointto be formed and filled with an aluminothermic reaction mixture in which is embedded an igniting system which can be triggered electrically.

A pressure-grouted anchor comprises a tension member which is inserted in a borehole and in the depth of the borehole, by injecting material which sets, such as cement mortarorthe like, is bonded to the borehole walls and thus to the surrounding ground. Overthe remainder ofthetension member, which may comprise one or more elements consisting of steel rods, wires orstrands, a pressure-grouted body thus formed is connected in force-locking mannerwith the structural memberto be anchored.

The length ofthetension member, overwhich itis embedded in the pressure-grouted body, is designated anchoring length Lv and the remaining portion, which is freely extensible for the purpose of prestressing, is designated the free steel length Lfst.

Pressure-grouted anchors can beusedforthe permanent anchoring of structures in the ground but they can also be used temporarily, for example forthe rearward anchoring of a trench or excavation wall. If the tem porarily used pressure-grouted anchor extends into the adjacent plot of land, as a rule it has to be removed upon completion ofthe building opera tionsforwhich itwasused.

For the removal of a pressure-grouted anchor a separation facility for the tension member is generally provided atthe transition from the anchoring length Lv to the free steel length Lfst, thereby enabling the free portion ofthetension memberto bewithdrawnfrom the borehole and possibly recovered. It is usually readily possible to remove the pressure-grouted body itself, which seldom is of a greater length than about4 to 8m, when during excavation operations in the adjacent plotflattening work is carried out, e.g. using bulldozers.

The most significant of the various possibilities for severing the tension member of a pressure-grouted anchoristhe application of heatso asto reduce the steel tensile strength. Firstly, by heat application it is possible to reduce the tensile strength of tension members of any design. Secondly, the means necessaryfor heat generation can be installed together with the tension member, without substantially enlarging the borehole diameter, and generally can be kept in working order even over a relatively protracted period oftime. Finally, if the predetermined breaking point is provided by reducing strength as a result of heat action, the tension member can be utilised with its full cross-sectionall thetime rt is infuse.

To generate the heat required for reducing the tensile strength of the tension member it is known to heat the tension member in the vicinity of the predetermined breaking point by means of an exothermic reaction, e.g. an aluminothermic mixture, to a temperature at which its tensile strength is decreased to such an extent that it can be readily separated and withdrawn from the borehole (DE-PS 24 33 244). In this case the aluminothermic mixture is disposed in an annular sleeve which surrounds but is spaced from thetension member in vicinity of the predetermined breaking point to be formed and which has an ignitor.

Since pressure-grouted anchors often lie in ground water it is not only necessary to protectthe igniting system, priorto ignition,fromthe ingress of moisture but also during deflagration, in the event of damage to the annular sleeve, to preventwaterfrom coming into contact with the burning aluminothermic mixture and thereby cooling it or extinguishing it before the necessary heat is generated forforming the predetermined breaking point. Another problem isthat, upon deflagration, this mixture decreases substantially in voluneand, in any case with inclined anchors, runs together atthe respective lower end ofthe annular sleeve.The dangerthereby arises that a portion ofthe incandescent molten mixture will flow downwards intothegroundthroughtheannularspace between thetension memberand the annularsleeve and the remainder ofthe mixture left inside the sleeve will be insufficientto begin to meltthetension memberto such an extent that it could be separated with certainty atthe predetermined breaking point.

The invention is based on the object of developing a device of the type defined atthe beginning in such a way that, paricula rly in the case of pressure-grouted anchors in ground water, severing ofthe anchor tension member is reliably ensured at the desired predetermined breaking point.

According to the invention, for a device ofthe type defined atthe beginning, this object is achieved in that the annular sleeve comprises an outertube with at least one innertube accommodating the tension member or an individual element of the tension member, and in that the cavity formed between the outertube and inner tube is closed by end covers which are disposed at a distance from the ends of the outertube, the open cavities thus formed at the ends being filled by plugs consisting of a refractory material which sets, e.g. concrete.

Preferably, the igniting system consists of a material which is enclosed in an elongate cartridge, which ignites as abruptly as possible and burns rapidly at high temperature, and in which there is embedded an igniting pellet of pyrotechnic material which is in direct contact with an ignition lead through which an electric current can pass. The material ofthe igniting system enclosed in the cartridge is preferably a mixture of copper and thermite powders.

The outer tube is preferably joined at its exterior end to an extension tube which has a fluid-tight connec tiontoa sheathingtubesurroundingthetension member in the vicinity of the free steel length. The outertube andlorthe innertube or inner tubes preferably consist of metal; likewise, the extension tube consists of metal and is welded to the outertube.

The sheathing tube surrounding the tension member in the vicinity ofthe free steel length preferably consists of plastics material.

The ignition lead is advantageously disposed in the vicinityofthefree steel length in a sheathing tube which extends as far as into the plug atthe exterior end of the outer tube. Over a part of its length, the ignition lead can be embedded directly in the material ofthe plug.

The advantage ofthe invention lies in that the plugs consisting of refractory material which sets, for example concrete, and provided additionallyto the end covers ofthe annular sleeve make it possible to preclude substantially any melting through of the covers and, even if the covers start to melt, prevent the incandescent molten mixture from flowing downwards. Even if a partofthe mixture penetrates into the relatively narrow annular space between the tension member or a part ofthe tension member and the inner tube surrounding it, the length ofthe plug has the resultthatthematerial rapidly cools and thus clogs this annularspace.

As a result ofthe special design ofthe igniting system, in particularthe choice of a material which ignites as abruptly as possible and burns rapidly at a high temperature, e.g. a copper/thermite mixture, the aluminothermic mixture enclosed in the annular sleeve is caused to ignite rapidly and burn quickly.

After abrupt ignition,thecopperlthermite mixture of the igniter burns at about 1,900 C. Since theignition temperature,forexample ofthermite, is at approximately 1 .000 C, rapid combustion ofthethermite is thus ensured and afterthe burning ofthe innertubes ground water is prevented from penetrating int the incendiary composition. The igniter consisting of a copper/thermite mixture is advantageously accommodated in an elongate cartridge which extends over an as great as possible portion ofthe length of the annular sleeve, this also resulting in raped ignition ofthe aluminothermic mixture.

The invention will be explained in moredetail below on the basis of one example of embodiment illustrated in the drawing, wherein Figure 1 shows a longitudinal section through a pressure-grouted anchor; Figure 2 shows, on an enlarged scale, a longitudinal section through an annularsleevefilled with aluminothermic mixture; Figure 3 shows a cross-section along the line Ill-Ill in Figure 2, and Figure 4 shows a cross-section along the line IV-IV in Figure 2.

Figure 1 shows a longitudinal section through a pressure-grouted anchorwith a steel tension member 1 ,for example a steel wire strand,which is inserted in a borehole2.Apressure-grouted body3 is formed in the lower portion ofthe borehole 2 by injection of material which sets, for example cement mortar. The tension member 1 is anchored in the pressure-grouted body 3 over its anchoring length Lv. Overthe remaining port on ofitstotal length, namely the portion ofthe free steel length Lfst, the tension member 1 is freely extensible and atthe exterior end, e.g. for securing a trench wall 4, it can be anchored by means of an anchoring 5 which does not form part of the subject-matter ofthe invention.Overthe portion of the free steel length Lfst the tension member 1 is protected alainstcorrosion by a sheathing, e.g. a plastics tube; the borehole may also be filled in this zone.

In the vicinity ofthe transition from the anchoring length Lv to the free steel length Lfst a device 6 is embedded in the pressure-grouted body 3, by means of which a predetermined breaking point can be provided by heat action on the tension member 1 The tension member 1 can be severed atthis predetermined breaking point either by applying an external tensile force or by maintaining the anchoring force, so that its section extending over the portion ofthe free steel length Lfst can be withdrawn from the borehole 2.

One example of embodimentforsuch a device for forming a predetermined breaking point is illustrated in longitudinal section and cross-section in Figures 2 to4.

In the embodiment illustrated, the device 6 is in the form of an annular sleeve comprising a steel outer tube 7 which is closed by two end covers 8 and 9 respectively. The covers 8 and 9 are situated at a respective distance a and bfrom the ends ofthe outer tube 7. In the embodiment illustrated, the tension member 1 is of multiple design and comprises three individual parts 1'. Accordingly, three innertubes 10 are disposed inside the outer tube 7, which pass through the covers 8 and 9 in corresponding bores and in each ofwhich extends one of the individual elements 1'. The inner tubes 10 are somewhat longer than the outertube 7 and extend beyondthe latter art both ends.

In orderto be able to provide a seal forthe sheathing tube 11 surrounding the tension member 1 in the vicinity of the free steel length Lfst, which sheathing tube consists of plastics material, e.g. polyethylene, an extension tube 12 is flitted into the outertube 7 at its exterior end and is joined to the outer tube 7 by welding beads 13. In its portion protruding from the outertube 7 the extensiontube 12 is provided with baffles 14, e.g. annularly appiied welding beads, so that in this zone a tight seating can be provided by externally applied clamping means or pressing rings.

The cavity 15 formed between the inner wall ofthe outertube 7 and the outer walls of the inner tubes loins filled with an aluminothermic mixture, e.g.thermite powder. In this mixture is embedded an elongate cartridge 16 containing a copper/thermite mixture. An igniting pellet 17, as used for example in explosive charges, is inserted in the copper/thermite mixture. An ignition lead 18 is connected to this igniting pellet 17 and is directed to the exteriorthrough a corresponding bore in the cover 9 and for mechanical protection is placed in a plastics sheathing tube 19.

The cavities formed by the covers 8 and 9 being set back relative to the end faces ofthe outertube 7 are filled with concrete plugs 20 and 21 respectively.

Accordingly, the plug 21 at the exterior end ofthe device 6, which is situated inside the extension tube 12, protrudes beyond the end face ofthe outertube 7.

In the plug 21 there is also embedded the sheathing tube 1 9for protecting the ignition lead 18 which, in orderto achievethe most effective seal possible against liquid penetrating from the exterior, is embedded for a short distance directly in the concrete of the plug 21.

The annular sleeve 6 can be inserted in the borehole 2 simultaneously with the tension member 1; in the radial sense it occupies only slightly more space than the tension member itself but contains from the outset all the necessary means provided for the subsequent reaction and formation of a predetermined breaking point The igniting pellet 17 is ignited by applying an electric voltage to the ingition lead 18andthe copper/thermite mixture contained in the cartridge 16 is caused to react. This mixture ignites abruptly and burns at very high temperature. Thethermite powderfilling the remainder of the cavity 15 reacts rapidly and the resulting heating ofthestrands 1' eithercausesthestrandsto meltortosuffersuch a reduction in strength that a predetermined breaking point is reliably formed. Thuswhateverthe angle of inclination of the anchor, the incandescent molten mixture is prevented from escaping from the tube 7 by the plug 20 or plug 21. Furthermore, the plugs constitute a reliable seal against the ingress of ground waterforthe period oftime until the igniting system is actuated.

Claims (11)

1. Adeviceforseparating thetension memberofa prestressed pressure-grouted anchor by forming a predetermined breaking point at a given location on its length, in particular at the transition from the anchoring length to the free steel length, comprising an annular sleeve surrounding the tension member in the vicinity ofthe predetermined breaking pointto be formed andfilledwith an aluminothermic reaction mixture in which is embedded an igniting system,the annular sleeve comprising an outertube with at least one innertube accommodating the tension member oran individual elementofthetension member, and the cavity formed between the outertube and inner tube being closed by end covers which are disposed inwardly from the ends of the outertube, the cavities thus formed atthe ends being filled by plugs consisting of a cast refractory material.

2. A device according to Claim 1, wherein the igniting system consists of a material which is enclosed in an elongate cartridge, which ignites as abruptly as possible and burns rapidly at high temperature, and in which there is embedded an igniting pellet of pyrotechnic material which is in direct contactwith an ignition lead through which an electric current can pass.

3. A device according to Claim 2, wherein the material of the igniting system enclosed in the cartridge is preferably a mixture of copper powder and thermite powder.

4. A device according to any of Claims 1 to 3, wherein the outertube is preferablyjoined at its exterior end to an extension tube which has a fluidtight connection to a sheathing tube surrounding the tension member in the vicinity of the free steel length.

5. A device according to any of Claims 1 to 4, wherein the outertube and/orthe innertube orthe innertubes are of metal.

6. Adevice according to Claim 5,wherein the extension tubeis of metal and is welded to the outer tube.

7. A device according to Claim or 6, wherein the sheathing tube surrounding the tension member in the vicinity ofthe free steel length consists of plastics material.

8. A device according to any of Claims 1 to 7, wherein the ignition lead is disposed in the vicinity of the free steel length in a sheathing tube which extends as far as into the plug at the exterior end of the outer tube.

9. A device according to Claim 8, wherein over a part of its length the ignition lead is embedded directly in the material ofthe plug.

10. A device as claimed in any ofthe preceding Claims, wherein the cast refractory material is concrete.

11. Atension-member separating device substantially as described with reference to and as illustrated in the accompanying drawings.