EP0903442A1

EP0903442A1 - Tension member with yielding means

Info

Publication number: EP0903442A1
Application number: EP98203198A
Authority: EP
Inventors: Petrus Johannes Cornelius Maria De Kort; Peter Antonie Waalwijk
Original assignee: Ballast Nedam Funderingstechnieken BV
Current assignee: Ballast Nedam Funderingstechnieken BV
Priority date: 1997-09-19
Filing date: 1998-09-21
Publication date: 1999-03-24
Also published as: NL1007078C2

Abstract

The invention relates to an elongate member (1) for placing under strain of tension, in particular for an at least partly removable anchor, which is provided with means for causing yielding thereof close to a given location (3) when a determined tensile force is exceeded.

The tension member can herein be formed by a number of tension elements (4,5) mutually connected by coiling, while the yielding means are formed by a weakened part (6) arranged in one of the tension elements, for instance an interruption (6) close to the given yielding location.

Means (9) can be arranged round the tension member close to the yielding location for holding the tension elements.

The invention also relates to a method for forming such an elongate tension member.

Finally, the invention relates to an anchor provided with such a tension member.

Description

The invention relates to an elongate member for placing under strain of tension, in particular for an at least partly removable anchor, which tension member is formed by a number of tension elements mutually connected over at least a part of their length and which is provided with means for causing the tension member to yield close to a given location when a determined tensile force is exceeded, which yielding means are formed by a weakened part arranged in at least one of the tension elements close to the given location, in addition to means arranged round the tension member at least close to the yielding location for holding the tension elements. Such an elongate tension member is known from EP-A-0 244 353 and, when an end thereof is embedded in a grout plug, forms a grouted anchor which can be applied to anchor particular constructions in a load-bearing layer in the ground.
Particularly when grouted anchors are used to anchor temporary constructions, such as for instance sheet-piling round a pit excavation, it is often required that these anchors be removed when work is finished in order to prevent problems with anchors which have been left behind. Such problems occur for instance when pile-driving has to be carried out at a later stage in the part of the ground where the anchor is situated, wherein the abandoned anchor can then form an obstruction.
A problem herein is that the anchors are by their nature not simple to remove, since they are after all designed to be fixed firmly in the ground. The requirement of easy release is therefore very much at odds with the purpose for which an anchor is used. It is therefore proposed not to remove the entire anchor but only the tension member.
It is known for this purpose to embody the tension member as a tension rod which is provided on its bottom end with screw thread. This tension rod is then connected via a threaded sleeve to a second, considerably shorter tension rod which is likewise provided with screw thread and which is embedded almost completely in the grout plug. Round the long, non-embedded part of the tension rod is then also arranged a flexible plastic tube which is filled with a lubricant so that the tension rod can be rotated in this tube, whereby its outer end can be screwed out of the threaded sleeve. In this manner the larger part of the tension rod can be unscrewed from the grout plug and pulled out of the ground through the encasing tube so that only the grout plug, the second tension rod protruding therefrom over a few decimetres and the encasing tube still remain behind in the ground.
This known construction has the drawback that it is comparatively costly, because compared with for instance a cable a tension rod is already expensive and the necessary additional bearing sleeve also entails costs. Assembly and later removal of the anchor with the screw connection is moreover complicated and time-consuming.
A tension member has already been described in EP-A-0 244 353 which consists of a bundle of tension elements, of which at least the central element is severed to form a determined yielding location. The tension member is weakened at the position of the yielding location by severing of one or more tension elements, whereby it will yield when a predetermined load is exceeded. The tension member can thus be pulled loose of the grout plug in simple manner by exerting a force thereon greater than the design load. Choosing the yielding location in the proximity of the top part of the grout plug ensures that only this plug remains behind in the ground.
In the known tension member a sleeve is arranged round the yielding location, therefore at the position of the severed tension element, which sleeve is pressed round the tension member. The tension member is strengthened at the position of the yielding location by this pressed sleeve.
The invention has for its object to provide an improved tension member of the above described type. This is achieved according to the invention in that the holding means are arranged shifted relative to the yielding location in the longitudinal direction of the tension member. Because, in contrast to the known tension member, the holding means are not therefore arranged round the yielding location, the holding means can already be arranged prior to forming of the yielding location, whereby manufacture of the tension member is simplified. Furthermore, the tension member is thus not strengthened by the holding means at the position of the yielding location, which would anyway not be consistent with the intended purpose of the yielding location.
The holding means are preferably arranged free of tension round the tension member. In this manner the holding means do not contribute in any way toward unintended strengthening of the tension member but have the sole function of holding the tension elements together. The tension elements which yield first when the yielding load is applied are hereby held in place and uncoiling of these elements is prevented, which is important in ensuring that the tension member will indeed yield at the chosen location. It is pointed out here that the tensile force between the central wire and the wires trained round the central wire is only transmitted by means of friction.
When the yielding location is chosen in the grout plug by which the anchor is fixed in the ground, the holding means can be formed by this grout plug. No separate provisions are then required, while furthermore there are no longer any parts at all protruding outside the grout plug which are left behind in the ground.
The invention also relates to a method for forming an elongate member for placing under strain of tension, in particular for an at least partly removable anchor, which tension member is formed by mutually connecting a number of tension elements over at least a part of their length and which is provided with means for causing yielding thereof close to a given location when a determined tensile force is exceeded by arranging a weakened part forming the yielding means close to the given yielding location in at least one of the tension elements, wherein means are arranged round the tension member at least close to the yielding location for holding the tension elements. Such a method is characterized according to the invention in that the holding means are arranged shifted relative to the yielding location in the longitudinal direction of the tension member.
The weakened part is herein preferably formed in a tension element which is enclosed by a number of non-weakened tension elements which are coiled therearound.
A simple method of providing the tension member with yielding means consists of a commercially available tension member consisting of coiled tension elements being spread apart up to a position close to the yielding location, the weakened part being formed in at least one central tension element and the tension member being reconstructed by re-coiling the non-weakened tension elements round the weakened tension element. The tension member or tension cable herein only has to be uncoiled and later coiled again over a small length, i.e. to the position above the expected top part of the grout plug.
The holding means, which can be formed by a sleeve, can advantageously be arranged before the tension elements are uncoiled, whereby they can also form a boundary to prevent the tension elements being uncoiled too far.
When the tension member is embedded in a grout plug for fixing the anchor in the ground and the yielding location is chosen in this grout plug, the holding means can simply be formed by the grout plug itself.
Finally, the invention further relates to an anchor in which a tension member as described above is applied.
The invention will now be elucidated on the basis of a number of embodiments, wherein reference is made to the annexed drawing, in which:
figure 1 shows a partly broken-away perspective view of a tension member according to a first embodiment of the invention,
figure 2 shows a cross-section along the line II-II in figure 1,
figure 3 shows a cross-section corresponding with figure 2 through an alternative embodiment of the tension member,
figures 4 and 5 show schematic views of an anchor during use and after removal of the tension member, and
figures 6, 7 and 8 show the different steps of the manufacture of the tension member.
A tension member 1 for an anchor 2 which can be partially removed from the ground 8 (figure 4) is provided with means for causing tension member 1 to yield close to a given location 3 when a determined tensile force is exceeded. These yielding means can be formed by a local weakening of tension member 1 in the proximity of the chosen yielding location 3. In the shown embodiment tension member 1 is formed by a number of tension elements or wires 4, 5 (figure 1) mutually connected over their whole length. The six outer wires 4 are herein coiled round the central tension element or core wire 5. The yielding means are formed by an interruption 6 in core wire 5 (figure 2) which can be severed or cut through.
Because wires 4, 5 are mutually connected, loads on tension member 1 are in principle borne by all wires 4, 5 together. The connection between wires 4, 5 is herein formed by the fact that when a tensile load is exerted on tension cable 1 the wires 4 coiled round core wire 5 tend to straighten and move toward the imaginary axis of their spiral shape. A normal force is herein applied to core wire 5 which is thereby clamped. Tensile stresses are thus transmitted between the different wires 4, 5, whereby the tensile load is transferred from core wire 5 at the position of the interruption to outer wires 4 and beyond the interruption a part of the tensile force is transferred back to core wire 5. Only in the direct vicinity of interruption 6 does core wire 5 therefore bear the load to a lesser extent or not at all, so that tension member 1 is there slightly weakened. Tension member 1 is herein dimensioned such that it can withstand the loads to be anticipated during normal use. Only when a determined excessive load is exerted on tension member 1 will it yield in the proximity of the interruption 6 in core wire 5. By arranging this interruption 6 slightly above or precisely in the expected upper part of grout plug 7, tension member 1 can be removed almost completely from the ground after pulling apart thereof, so that only grout plug 7 remains behind in the ground (figure 5).
In order to ensure that tension member 1 does indeed yield at the chosen location 3, it is of great importance that wires 4, 5 are held together at the position of yielding location 3 until they have all yielded. When the outer wires yield they tend to spring outward and uncoil from core wire 5. If this happens the tension member 1 is weakened everywhere the yielded wires have detached from core wire 5, whereby there is the danger of tension member 1 yielding at a location wholly other than the chosen location 3, and too large a part of anchor 2 thus remaining behind in the ground 8. Provided to hold wires 4, 5 together are holding means, in the shown embodiment in the form of a sleeve 9 of plastic or metal. Other holding means such as for instance a small grout plug, can of course also be envisaged.
These holding means, which are arranged free of tension round wires 4, 5, serve only to hold the wires together and do not therefore contribute per se to the tensile strength of tension member 1. The holding means are not therefore arranged round interruption 6 but are on the contrary shifted in longitudinal direction.
For heavier anchors the tension member 1 can have a larger cross-section than discussed above, with a larger number of tension elements or wires coiled together. An additional layer with outer wires 10 can for instance be formed round the wires 4 coiled round core wire 5 (figure 3). In this case it may be necessary to weaken a plurality of wires to prevent the yielding load becoming so high that it is practically no longer possible to achieve. Some of the intermediate wires 4 can for instance also be interrupted in addition to core wire 5.
The starting point for manufacture of tension member 1 can be a coiled tension cable which is easily obtainable on the market. Sleeve 9, which can already play a part during production, can be arranged round this cable 1 at the desired yielding location 3. Cable 1 is then uncoiled from the end 11 thereof intended for embedding in grout plug 7 (figure 6). This uncoiling is continued until the desired yielding location 3 is reached. The sleeve 9 pre-arranged round cable 1 herein prevents wires 4 being uncoiled beyond the desired yielding location 3. Core wire 5 is then severed or cut through at the desired yielding location 3 (figure 7). The severed part 12 is then held against the remaining part 13 of core wire 5 and outer, wires 4 are coiled therearound again, whereafter the end 11 of cable 1 is then tied off or otherwise fixed (figure 8). Re-coiling of cable 1 is relatively simple since the outer wires 4 will retain their spiral shape during the uncoiling.
The invention thus enables forming of a tension member 1 for an anchor 3 with relatively little effort and at low cost which after use can be removed in simple manner by exerting a predetermined tensile force thereon.

Claims

Elongate member for placing under strain of tension, in particular for an at least partly removable anchor, which tension member is formed by a number of tension elements mutually connected over at least a part of their length and which is provided with means for causing the tension member to yield close to a given location when a determined tensile force is exceeded, which yielding means are formed by a weakened part close to the given location arranged in at least one of the tension elements, in addition to means arranged round the tension member at least close to the yielding location for holding the tension elements, characterized in that the holding means are arranged shifted relative to the yielding location in the longitudinal direction of the tension member.
Tension member as claimed in claim 1, characterized in that the holding means are arranged free of tension round the tension member.
Tension member as claimed in claim 1 or 2, characterized in that the holding means take the form of a sleeve.
Tension member as claimed in claim 1 or 2, characterized in that the holding means are formed by a grout plug fixing the anchor in the ground.
Method for forming an elongate member for placing under strain of tension, in particular for an at least partly removable anchor, which tension member is formed by mutually connecting a number of tension elements over at least a part of their length and which is provided with means for causing yielding thereof close to a given location when a determined tensile force is exceeded by arranging a weakened part forming the yielding means close to the given yielding location in at least one of the tension elements, wherein means are arranged round the tension member at least close to the yielding location for holding the tension elements, characterized in that the holding means are arranged shifted relative to the yielding location in the longitudinal direction of the tension member.
Method as claimed in claim 5, characterized in that the weakened part is formed in a tension element which is enclosed by a number of non-weakened tension elements and the non-weakened tension elements are coiled round the or each tension element with weakened part.
Method as claimed in claim 6, characterized in that a tension member consisting of coiled tension elements is spread apart up to a position close to the yielding location, the weakened part is formed in at least one central tension element and the tension member is reconstructed by re-coiling the non-weakened tension elements round the weakened tension element.
Method as claimed in claim 7, characterized in that the holding means are arranged round the tension member before it is spread apart.
Method as claimed in claim 7, characterized in that the tension member is embedded in a grout plug for fixing the anchor in the ground and the holding means are formed by the grout plug.
Anchor provided with at least one tension member as claimed in any of the claims 1-4.