Method of and device for severing a tension member in a prestressed grouted anchor

A prestressed grouted anchor has a portion of an axially extending prestressed tension member grouted in a receiving material while the remaining portion of the tension member extends from the grouted portion. The remaining portion is separated from the grouted portion at a selected breaking point by reducing the tensile strength of the tension member as the result of exposing it to the action of heat, such as developed in an exothermic chemical reaction. If the tension member is prestressed and the resulting elastic elongation is maintained during the heating action with the elongation being greater than the elongation required under the heating action for effecting the severing of the tension member, then with progressive heat the tensile strength of the tension member is reduced and the elastic elongation progressively changes to plastic elongation with the tension member finally severing automatically at the selected breaking point.

SUMMARY OF THE INVENTION 
The present invention is directed to a method of removing the portion of a 
tension member in a prestressed grouted anchor which extends from the 
grouted portion. The removal is effected by severing the two portions of 
the tension member, particularly at the location of the junction between 
the grouted portion and the free portion of the tension member. A selected 
breaking point is formed by reducing the tensile strength of the 
prestressed tension member by applying heat to the member, especially in 
an exothermic chemical reaction. Further, the invention relates to a 
device for providing the heating action at the desired breaking point. 
A grouted anchor includes a tension member which is introduced into a 
borehole in a receiving material and at the base of the borehole the 
tension member is grouted in using a hardenable or settable material, such 
as a cement grout or the like. A grouted anchor is secured to a structural 
member to be anchored by means of the remaining portion of the tension 
member extending from the grouted portion. The tension member may be 
formed as one or more elements made up of steel rods, wires or cables. The 
length of the tension member embedded in the grout is called the anchored 
length L.sub.v, the remaining portion of the member, which is freely 
expandible under prestress, is called the free length L.sub.fst. Such 
grouted anchors may be used as permanent anchors, that is, serving to 
permanently anchor a structure, or they may be used temporarily, such as 
for anchoring the wall of an excavation. If such a temporary grouted 
anchor extends into an adjacent lot, as a rule, it must be removed after 
the completion of the construction operation in which it is used. 
To remove a grouted anchor, usually a breaking point or severing location 
is provided at the transition between the anchored length L.sub.v and the 
free length L.sub.fst so that the free length can be pulled out of the 
borehole and recovered, if it is desired to do so. The grouted portion, 
which is usually in the range of 4 to 8 m, is easily removed when surface 
earth moving is carried out in the adjacent lot, such as with bulldozers. 
There are various possibilities for breaking or severing the tension member 
in a grouted anchor and the most important is the use of heat to reduce 
the tensile strength of the steel in the tension member, because the means 
for generating the heat can be installed along with the tension member 
without appreciably increasing the diameter of the borehole. Further, the 
heating means can be maintained operational for a relatively long period 
of time. Moreover, if the desired breaking point is produced by a 
reduction in strength caused by heat, the full transverse cross-section of 
the tension member can be used during its entire period of utilization. 
In producing the heat required for reducing the tensile strength of the 
tension member, it has been known to heat the tension member by an 
electric heating element, and also to generate the heat by induction using 
a coil encircling the tension member at the desired breaking point. 
Another known method involves heating the tension member in the region of 
the breaking point by means of an exothermic chemical reaction, such as 
obtained from an aluminothermic mixture with the tension member being 
heated to a temperature at which its tensile strength is reduced so that 
it can be easily severed and pulled out of the borehole. 
During the installation of grouted anchors, in particular for stressing the 
tension members, prestressing jacks are available particularly for anchors 
of high load capacity and such equipment is very costly to handle. In the 
removal of the anchors it is important not to require any equipment or at 
least any heavy equipment. 
Therefore, it is the primary object of the present invention to provide the 
separation of the tension member in an automatic operation using the 
action of heat on the member. 
In accordance with the present invention, the tension member is prestressed 
so that its elastic elongation is maintained during the heating operation 
and is greater than the elongation required to achieve the separation of 
the tension member under the action of the heat. 
As the elastic elongation of the tension member depends mainly on its free 
length, it is important to match the magnitude of the prestressing force 
to the free length so that during the heating operation the tension member 
severs automatically. Accordingly, it is possible to replace and/or 
increase the elongation due to the prestressing force at least while the 
heating operation is in effect by the action of external and axially 
acting forces. Such additional forces can be attained by spring elements 
acting on the tension member. 
Further, it is also possible to reduce the elongation required for severing 
the tension member under the action of heat by applying other external 
forces. Preferably such forces are obtained by applying forces acting at 
the desired breaking point transversely of the axial direction of the 
tension member, or by subjecting the tension member to dynamic stresses. 
In addition, it is possible to reduce the elongation required for severing 
the tension member under the action of heat by controlling the heat 
supply. Such control can be achieved by regulating the quantity of heat 
supplied per unit of time and/or by regulating the duration of the heating 
action or determining the dimensions of the area at which heat is applied, 
particularly the axial length of the area over which heat is applied, and 
determining these various factors based on the free length L.sub.fst. 
In accordance with the present invention, the elastic elongation of the 
tension member due to prestressing is greater than the elongation required 
under the action of heat for achieving the severing or breaking of the 
tension member, with the progressive heating of the tension member its 
tensile strength is reduced so that the elastic elongation gradually 
transforms into a plastic elongation with the tension member separating at 
the selected breaking point. As tests have shown, the separation of the 
portions of the tension member does not take place suddenly, but rather as 
a result of the gradual reduction of the prestressing force by increased 
elongation. After the tension member breaks, the free portion can be 
removed out of the borehole by applying only limited force. 
In the invention, the heating action developed in an aluminothermic 
reaction has proved to be especially advantageous. The materials required 
for such a reaction, a mixture of aluminum shot and iron oxide powder, can 
be easily ignited by an ignition rod accommodated relatively easily in a 
ring sleeve assembly laterally enclosing the tension member. With such a 
ring sleeve assembly no appreciable increase in the borehole diameter is 
needed. With the appropriate composition of the mixture, the amount of 
heat can be determined quantitatively by the reaction velocity and 
reaction temperature, and by means of the external dimensions of the 
sleeve assembly, the amount of heat can be determined qualitatively, for 
example, by using a large diameter and a short length the location of the 
area of heat development can be limited, leading to a reduction in the 
elongation required for severing the tension member. 
Accordingly, the invention includes a device for establishing the desired 
breaking point. The device is made up of a ring sleeve assembly laterally 
enclosing a portion of the tension member in the region of the selected 
breaking point. The ring sleeve assembly is filled with an aluminothermic 
reaction mixture with an electrically actuatable ignition system embedded 
in the mixture. The ring sleeve assembly includes a relatively thin-walled 
inner sleeve of a readily burnable and/or heat-conducting material and a 
thicker-walled outer sleeve formed of a heat insulating material. 
The inner sleeve may be formed of a plastics material, that is, a material 
which has sufficient strength but at the same time burns rapidly due to 
the heating action, whereby the aluminothermic mixture is quickly placed 
in direct contact with the tension member. Alternatively, the inner sleeve 
may be formed of metal, for instance, steel sheet, which has the advantage 
that the heat from the aluminothermic reaction is transferred quickly to 
the tension member and is maintained over a relatively long period of 
time. 
Appropriately, the outer sleeve is formed of a plastics material, a ceramic 
or the like so that the heat developed in the reaction is not transferred 
prematurely or excessively to the outside. 
Preferably, the ignition system consists of a stable rod-shaped 
pryrotechnic mixture which is in direct contact with a power line through 
which current can be supplied. In one arrangement, the rod-shaped mixture 
is encircled spirally by the power line. 
A pressure relief line may be connected to the ring sleeve assembly which 
extends into a portion of the borehole free of the grout. Preferably, the 
pressure relief line is connected to the ring sleeve assembly by a one-way 
valve. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of this disclosure. For a better understanding of the invention, its 
operating advantages and specific objects attained by its use, reference 
should be had to the accompanying drawings and descriptive matter in which 
there are illustrated and described preferred embodiments of the 
invention.

DETAILED DESCRIPTION OF THE INVENTION 
In FIG. 1 an axially extending section is provided through a grouted anchor 
including a steel tension member 1, such as a steel cable, inserted into a 
borehole 2 formed in a receiving material. A body of grout 3 is injected 
into the inner portion of the borehole 2 and provides an anchor for the 
inner end of the tension member 1. The body of grout 3 is a hardenable 
material, such as a cement grout. Within the body of grout 3, the tension 
member extends for an anchored length L.sub.v. Outwardly from the body of 
grout 3 the remaining portion of the tension member is free and has a free 
length L.sub.fst so that the member 1 is freely expandable and is anchored 
at the entrance end of the borehole for supporting an excavation wall 4 by 
a device 5, however, such a device does not form a part of the invention. 
In the transition region of or at the junction between the anchored length 
L.sub.v and the free length L.sub.fst of the tension member, a device 6 is 
embedded in the body of grout 3 for providing heat to the tension member 1 
so that a selected breaking point can be established where the tension 
member is severable whereby its portion extending for the free length 
L.sub.fst can be pulled out of the borehole 2. 
An embodiment of the device 6 for forming a selected breaking point is 
shown in axially extending and transverse sections in FIGS. 2 and 3. 
The device 6 is a ring sleeve assembly made up of an inner sleeve 7 and an 
outer sleeve 8 with the outer sleeve having opposite end walls 9, 10 
extending transversely of the axial direction of the tension member 1. As 
illustrated in FIG. 2, the inner sleeve 7 is slightly longer than the 
outer sleeve 8 so that the inner sleeve projects outwardly from each of 
the end walls 9, 10. Further, the inner sleeve is formed of a thin, 
readily burnable material or of a thin sheet metal. In comparison to the 
inner sleeve 7, the outer sleeve 8 has thicker walls and is formed of a 
heat-insulating material, such as a plastics material or a ceramic 
material. 
While the inner sleeve 7 fits closely around the tension member 1, the 
outer sleeve 8 is spaced radially outwardly from the inner sleeve so that 
an annular space is formed between the two sleeves. A mixture 11 capable 
of an exothermic chemical reaction is filled into the space between the 
sleeves. A particularly suitable aluminothermic mixture of aluminum shot 
and iron oxide powder can be used as the mixture 11. For ignition of the 
mixture 11, an ignition system is provided including a rod-shaped igniter 
12 and an ignition line 13 of electically conductive materials which 
extend spirally around the igniter. The ignition line 13 is, in turn, 
connected to a power cable 14. The conducting cross-section of the 
ignition line 13 is dimensioned relative to the cable 14 so that it 
becomes incandescent as soon as current is passed through it and 
immediately fires the igniter 12. The igniter is a shaped and form-stable 
pyrotechnic mixture known per se, which burns at a high temperature and 
contains oxygen-yielding compounds whereby for combustion no atmospheric 
oxygen is required. The igniter 12 serves to initiate the exothermic 
chemical reaction in the aluminothermic mixture 11. 
Depending on the composition of the aluminothermic mixture, pressure may 
build up during the reaction. As a rule, such pressure is absorbed by the 
hardened body of grout 3 in which the ring sleeve assembly 6 is embedded. 
The body 3 also affords corrosion protection. If undesirable effects on 
the environment are to be prevented, the pressure may be reduced by 
inserting a one-way valve 15 into the outer sleeve 8 of the ring sleeve 
assembly, note FIG. 4. The valve 15 is connected to a pressure relief line 
16 extending through the body of grout 3 to the portion of the borehole 2 
which is free of the grout, note FIG. 5. In this arrangement, any 
excessive pressures developed during the reaction can be released without 
any danger. 
An essential feature of the invention is that the ring sleeve assembly 6 
can be placed into the borehole 2 along with the tension member of the 
grouted anchor, since the ring sleeve assembly requires little in the way 
of additional space radially outwardly from the tension member. 
Accordingly, with the insertion of the tension member into the borehole 
all the measures required for the subsequent heat reaction and the 
formation of the selected breaking point are taken care of. By applying 
electric power to the cable 14, the igniter starts to burn causing the 
mixture to react. Depending on the composition of the mixture and the 
length and diameter of the ring sleeve assembly, in a relatively short 
time the selected breaking point is formed while maintaining the tension 
on the tension member 1 with the automatic severing of the tension member 
in the region of the ring sleeve assembly. When the free length L.sub.fst 
is separated from the anchored portion of the tension member, it can be 
easily pulled out of the borehole 2. As pointed out above, the tension 
member is prestressed and its elastic elongation is maintained during the 
heating action so that the elastic elongation is greater than the 
elongation required under the heating action to effect the severing of the 
tension member into its separate portions. As the heat from the exothermic 
reaction progresses, the tensile strength of the tension member is reduced 
with the elastic elongation progressively changing over to plastic 
elongation until the tension member finally separates automatically at the 
selected breaking point. 
As shown in FIG. 6, a spring 17 can be placed on the tension member 1 at 
the surface of the receiving material for replacing and/or increasing the 
elongation due to the pre-stressing force at least while the heating 
operation is in effect by the action of external and axially acting 
forces. 
In FIGS. 7 and FIG. 8 an explosive charge 18 is placed within the device 6 
so that it detonates at a specific temperature and applies external forces 
acting transversely of the axial direction. 
While specific embodiments of the invention have been shown and described 
in detail to illustrate the application of the inventive principles, it 
will be understood that the invention may be embodied otherwise without 
departing from such principles.