GB2360047A

GB2360047A - A removable mechanical anchor with a non-stick coating

Info

Publication number: GB2360047A
Application number: GB0100942A
Authority: GB
Inventors: Peter Gilbert Shotton
Original assignee: Cementation Foundations Skanska Ltd
Current assignee: Cementation Skanska Ltd
Priority date: 2000-03-10
Filing date: 2001-01-12
Publication date: 2001-09-12
Anticipated expiration: 2021-01-12
Also published as: GB0100942D0; GB2360047B

Abstract

A removable anchor has a fixed length formed from a rigid threaded bar having a non-stick surface. The bar may be solid or hollow and can have a left-hand machine thread. The fixed length can be coupled to a free length by couplers having a thread opposite to that on the fixed length. The free length can be formed from at least one steel strand.

Description

1 Grouted Mechanical Anchors 2360047 The present invention relates to

grouted mechanical anchors, Le. anchors which are grouted into a substrate. The term mechanical anchor is used here to describe a variety of fixings including ground anchors or anchorages, tie- backs, soil nails, rock bolts, and even access fixings. All of these fixtures rely on the transfer of stress (normally tensile stress) into the substrate.

Grouted mechanical anchors can be used to provide lateral support to a vertical or sub-vertical soil-retaining wall. Examples of such use are retaining walls for basement construction, quays, docks, and the like. They call also be used vertically, for example to prevent uplift of a raft or even a whole structure.

Another example of the use of vertical anchors is to provide reaction for compressive loading onto e.g. a test pile.

The process of constructing an anchor involves initially drilling a hole at a predetermined inclination into the soil. A casing may be installed where necessary, usually extending for only part of the depth of the hole. The hole may be dry or flooded with water or drilling fluid. The anchor is then inserted into and fixed in the hole and grouted in position, or alternatively, grout may first be placed in the hole and the anchor then pushed to the required depth into the grout.

Typically a grouted mechanical anchor comprises three elements: a fixed anchorage length, a free length, and a stressing head. Ill certain applications the free length may be very short, or even non-existent.

2 The fixed length is the remote section of the anchor at the base of the hole; this section transmits the load into the soil, and is normally grouted in by means of a grout which is initially fluid and then sets solid. The stressing head is at the exposed face of the structure which is being anchored, and is obviously accessible. The free length is that section which connects the stressing head to the fixed length. The free length may be constructed of several strands, each strand comprising several individual high tensile steel wires, usually arranged in the form of a wire rope. Alternatively, the free length inay comprise one or more solid bars. The free length is fitted with a plastic sheath or similar, to 10 ensure that no load is transferred to the soil or rock.

Several types of fixed grouted mechanical anchor lengths are in common use.

With one type, the end of a length of strand is cast in resin or grout, in a corrugated sheath. This pre-fabricated fixed length is then placed into the hole. The hole may already be partially or wholly filled with a cementitious grout, with the fixed length of the anchor being pushed into the grout; alternatively the anchor may be placed first, and the grout then introduced by means of a tremmie tube. As a variation of this type, the end length of strand may be bonded directly into the soil by grouting in situ without pre-fabrication. This type of anchor transfers the load from the strand via the bond to either the resin or the grout.

With another type of anchor, the strand is fonned. as a loop around an expandable metal 'balloon'. The assembly is pushed into the anchor hole, and grout is then pumped into balloon under pressure. This action causes the balloon to expand and apply pressure to the wall of the anchor hole. Depending on the nature of the strata and the pressure applied to the grout, the soil around the balloon may be pushed outwards. This type of anchor transfers 3 the load from the strand by mechanical means; the balloon engages with the ground and the strand is looped around the expanded balloon.

With yet another type of anchor the strand is formed as a loop fitted around a small steel half-wheel. The steel element is embedded in grout which transfers the load to the soil or rock.

Grouted mechanical anchors may be designed for permanent or temporary use. An example of temporary use occurs in the case of basement construction. In this situation, grouted mechanical anchors may be used to provide lateral support to the perimeter wall during bulk excavation of the basement; then at a later stage, after the basement slabs have been installed and the concrete has gained sufficient strength, the anchors may be de-stressed, and the lateral loads taken by the slabs. In cases like this the anchors are termed temporary anchors.

In some instances it may be acceptable to leave the complete anchor in situ, after de-stressing. However, anchors used in this manner (i.e. essentially horizontally) inevitably extend beyond the confines of the perimeter wall, into areas often owned by others. Since these anchors would present constructioll problems if and when the adjacent site is developed, it is often a condition of the party wall agreement that the anchors be removed after destressing.

With a removable anchor, what is removed is the free anchor length.

The fixed length, which can be typically up to 10 m long but may exceed 20 in, and comprises both grout and steel elements, is not removed. All of the types of anchor mentioned above may be made removable.

The common method of removing the free length is by applying a tensile load. With the first type of anchor mentioned above, this may be achieved by 4 providing a strand with an in-built weakened zone, positioned close to the fixed length; the strand will break when the load applied exceeds a certain amount. With both the expandable 'balloon' type and the halfwheel type, a tensile force is applied to one end of the strand which is looped around the balloon or 5 half wheel, pulling the strand around the balloon and then back up the hole.

In both cases, the method of removing the free length can be inherently dangerous. Where the strand is broken, the energy stored in the high tensile strand can be released as an explosive force when the strand breaks. Where the strand is pulled around the balloon, a considerable force may be required, because the strand is relatively stiff, and the radius of the loop around the balloon or half wheel is relatively small.

An alternative method of separating the free length from the fixed length has been to use an explosive charge. The free length is then removed by applying a tensile load to the strand.

The free length recovered using any of the systems mentioned above cannot be re-used with any confidence.

The general objective of the present invention is to provide a grouted mechanical anchor which can be removed more easily.

Accordingly the present invention provides a removable gn-outed mechanical anchor wherein the fixed length comprises a rigid threaded bar with a non-stick surface finish. The bar may be solid or hollow.

In its preferred form, both the free and fixed lengths of the anchor are removable, and in addition, both the free and fixed lengths can be reused.

A removable grouted mechanical anchor embodying the invention and the method of using it will now be described, by way of example, with reference to the drawings, in which:

Fig. 1 shows the fixed anchor end; Fig. 2 shows the sheath seal portion; Fig. 3 shows the anchor head; and Figs. 4A and 4B show alternative methods of connecting the free length to the fixed length.

Referring to Fig. 1, the fixed anchor length comprises a steel bar 10, which may be either solid or hollow. This bar has a left-handed machine thread., which may be a 'rope' thread or a square-cut thread; if the latter, the edges of the thread are chamfered with a small radius. The fixed length is coated with a non-stick material such as PTFE. The PTFE may be painted or sintered on, or may be applied as a tape wrapped round the bar, or may be in the form of a threaded sleeve placed over the bar. The objective is to minimize any bonding between the fixed length and grout.

The free length 11 may be a solid or hollow bar, or even a number of 20 wire strands. The free length is connected to the fixed length by means of a threaded nipple or coupler 12; the thread of the coupler has the opposite orientation to the thread of the fixed length, i.e. it is a right-hand thread. The free length is surrounded by a sheath 13 (Fig. 2), preferably of a plastics material, which is sealed at the junction 14 with the fixed length. The fixed length has one or more spacers 15 to ensure that grout will completely surround the fixed length.

The fixed and free lengths may be joined in any convenient way, e.g. by means of a clevis fitting (Fig. 4A) or a square or hex fitting (Fig. 4B).

In both cases, the two parts of the fitting are moved axially together as 6 indicated by the arrows 30 and a locking pin 32 is then inserted transversely through holes in the two engaging ends as indicated by the arrow 3 1.

For installation, the anchor assembly may be pushed into the grout-filled hole; alternatively, the anchor assembly may be placed first into the hole, and grout then introduced by means of a grout injection tube 16. (If the fixed length has a central hole, the grout may be introduced through that hole.) The anchor terminates at an anchor head (Fig. 3) comprising an anchor head plate 20 having a grout injection hole 21 and a grout vent hole 22, and attached to the free length by a nut 23. The bar or tube used for the free length may be extended at the anchor head, using couplers, etc. so that a hollow stressing jack can be used in a conventional manner. The loads applied to the free length will be locked off using nuts & washers. Similarly, where strands are used for the free length, they will pass through a conventional stressing head, hollow jack assembly, etc., and the loads will be locked off to the anchor head by means of collets or wedges.

When the anchor has performed its temporary function and is no longer required, it will be removed. The anchor will first be de-stressed. The end of the free length will then be rotated; this action effectively unscrews the fixed length from the grout mass. The anchor will then be removed ftom the hole. The sheath which surrounds the free length remains in position.

It should be noted that, depending on the nature of the strata and the loads to be carried, more than one anchor may be placed in a single hole. In this situation, the fixed lengths will be positioned sequentially and not overlap each other.

7

Claims

Claims

A removable grouted mechanical anchor wherein the fixed length comprises a rigid threaded bar with a non-stick surface finish.
2 An anchor according to claim 1 wherein the threaded bar is solid.
3 An anchor according to claim 1 wherein the threaded bar is hollow.
4 An mechanical anchor according to any previous claim wherein the fixed length has a machine thread.
5 An anchor according to either previous claim wherein the fixed length is coated with a non-stick material.
6 An anchor according to any previous claim wherein the fixed length has a left-hand thread.
7 An anchor according to any previous claim wherein the free length is coupled to the fixed length and the sections (if more than one) of the fixed length are coupled together by couplers having a thread opposite to the thread of the fixed length.

8 An anchor according to any previous claim wherein the free length comprises at least one steel strand.

9 A grouted mechanical anchor substantially as herein described.
8 Any novel and inventive feature or combination of features specifically disclosed herein within the meaning of Article 414 of the International Convention (Paris Convention).
9 A grouted mechanical anchor substantially as herein described.

1 1 k

9 Amendments to the claims have been filed as follows 1 A removable grouted mechanical anchor wherein the fixed length comprises a rigid threaded bar with a non-stick surface finish whereby the anchor is removable by unscrewing..

2 An anchor according to claim 1 wherein the threaded bar is solid.

3 An anchor according to claim 1 wherein the threaded bar is hollow.

4 An anchor according to any previous claim wherein the fixed length has a machine thread.

An anchor according to either previous claim wherein the fixed length is coated with a non-stick material.

6 An anchor according to any previous claim wherein the fixed length has 20 a left-hand thread.

An anchor according to any previous claim wherein the free length is coupled to the fixed length and the sections (if more than one) of the fixed length are coupled together by couplers having a thread opposite to the thread 25 of the fixed length.

8 An anchor according to any previous claim wherein the ftee length comprises at least one steel strand.