GB2357113A

GB2357113A - Remedial reinforcement of foundations and walls

Info

Publication number: GB2357113A
Application number: GB9929010A
Authority: GB
Inventors: Victor Handley; Michael Frank Ellis
Original assignee: ELLE VAN
Current assignee: ELLE VAN
Priority date: 1999-12-09
Filing date: 1999-12-09
Publication date: 2001-06-13
Anticipated expiration: 2019-12-09
Also published as: GB2357113B; GB9929010D0

Abstract

A method is described of strengthening foundations or walls (1) of a structure, which includes creating a bore in the structure, passing a reinforcing wire (5) through the bore and tensioning the wire (5), thereby placing the structure under compression. Preferably tension is applied to the wire (5) by tensioning jacks, whereby the wire is locked against end plates (6) by wedged anchorages (7). The bore may have a diameter of 25 mm, the wire (5) 15 mm and the tensioning force applied to the structure can be in the range of 50-100 kN.

Description

2357113 VANE1296.UK Title: Post-Tensioniniz of Structures This invention

relates to a method for strengthening the support or foundations of structures and to structures strengthened thereby. The method is applicable particularly to the repair of existing structures having damaged foundations. ' Buildings and similar structures are conveniently erected on supports such as foundations or footings laid at the bottom of a trench, or individual spot bases or piles, the supports supporting the walls of the structure, including the substructure walls disposed beneath ground level.

For foundations to be sound they need to be strong, particularly if movement in the substrate is likely to occur. Hitherto strength has been achieved by making foundations large in size and/or reinforcing them as, for example, in the case ofreinforced concrete beams. Sometimes, when subsidence occurs, foundations yield, whether by reason of weakness on their part or because subsidence is greater than could reasonably be anticipated. In such cases, remedial work is necessary to prevent further damage to the structure and such work conveniently takes the form of underpinning which is labourious and hence costly.

There has now been devised a method for the repair or strengthening of structures having damaged or inadequate foundations, which method overcomes or substantially mitigates disadvantages associated with the prior art.

According to the invention, there is provided a method of strengthening the support or foundations of a structure which includes the steps of a) creating a bore extending the length of that portion of the structure which is to be strengthened; b) passing a tendon through said bore, and VANE1296.UK 2 C) tensioning said tendon, thereby placing said portion of the structure under compression.

When used herein, the "tendon" includes cables, wire, rods and the like. Preferably, the tendons are tensioned from one or both ends of the structure by tensioning jacks and are locked against end plates by wedged anchorages. The tendons are preferably unbonded within grease packed plastic sheaths, but they may be disposed with clearance within the tubes and after installation bonded to the tubes by injection into the tubes of concrete or other bonding materials.

Most commonly, the portions of the structure into which the tendon is introduced will be in the substructure, ie within the region extending from ground level to the top of the foundations. When the tendon has been tensioned it places the structure under a compressive force which strengthens the structure and lessens the risk of further damage by movement of the substrate.

The tensioning force applied to the structure will vary, depending on the nature and length of the structure, but is typically of the order of 50-100kN.

The bore is most conveniently cut through the structure using a diamond tipped cutter. In a typical case, the bore may have a diameter of 25mm to enable a tendon of typical diameter of 15mm to be threaded through the bore.

The method according to the invention is advantageous primarily in that it strengthens the structure, yet is much less labourious and costly than conventional underpinning. The method is therefore particularly suited to the repair and strengthening of a structure which has been damaged causing cracking which does not warrant the expense of conventional underpinning.

It is also particularly advantageous in dealing with partial settlement or heave to a structure.

The invention will now be described in greater detail, by way of example only, with reference to the accompanying drawings, in which VANE1296.UK 3 Figure 1 is a schematic view of a brick built wall showing evidence of subsidence and to which the method according to the invention can be applied; Figure 2 shows a first stage in the can-ying out of the method according to the invention; Figure 3 shows a further stage in the method of the invention; Figure 4 shows a further stage in the carrying out of the method of the invention; and Figure 5 shows the wall ofFigure 1 after completion ofthe method according to the invention.

Referring first to Figure 1, a wall (1) may, for example, form one face of a residential dwelling. The wall (1) is constructed in conventional manner in brickwork, on a concrete footing or foundation (2) set below ground level. The wall (1) is of cavity wall construction.

Movement of the substrate has resulted in damage to the wall, as evidenced by the crack (3) which has propagated through the brickwork and mortar j oints of the wall (1). The method of the invention may be carried out by way of repair and to strengthen the structure.

In a first stage in carrying out the method, soil around the ends of the structure is excavated (see Figure 2). Investigations may need to be carried out to determine the positions of any utility supplies (eg gas pipes, electric mains cables etc) so that any such supplies are not damaged in subsequent stages of the method.

A bore (4) is then cut through the brickwork (see Figure 3) at a level between the foundation (2) and ground level. A diamond-tipped cutter may be used for this purpose. Where the wall (3) is made at a level at which a cavity exists between the inner and outer leaves of the cavity wall (1), it is necessary for the bore to be formed only in the brickwork at each end of the wall (1). Normally, however, the cavity in a wall (1) of such construction would, below ground level, be filled with concrete, in which case the bore must be cut through the full length of the wall (1).

VANE1296.UK 4 A tensioning cable or tendon (5) is threaded through the bore (4). The tendon is typically a multi-strand (eg seven strand) high strength tensile steel cable, loosely surrounded by a flexible plastic sheath and packed with grease for ease of movement therein. Prior to or after threading of the tendon (5) into the bore (3), the plastic sheath is stripped from the end portions of the tendon so that bare portions protrude from the ends of the bore (3) in readiness for tensioning.

The tendon (5) is then tensioned using tensioning jacks, the tension being transmitted to the wall (1) by means of end plates (6) and comb grips (7). Tensioning by such means is well known in other structural applications and hence will not be further described. The tensioned assembly is shown in Figure 4.

Any excess length of protruding tendon (5) is cut off and the excavations at the ends of the wall (1) are filled in. Finally, minor structural damage can be made good. The tension of the tendon (5) acts to strengthen the wall (1). The risk of ffirther damage to the wall (1) from further movement of the substrate is thereby greatly lessened. Thus, the structure is strengthened and substantially repaired.

It will be appreciated that many modifications may be made to the method described above without departing from the scope of the invention. Thus for example, more than one tendon may be installed. A plurality of tendons may be installed across the same wall below ground level. Alternatively, a similar approach may be taken at first floor and/or roof level, either instead of or in addition to tensioning below ground level.

VANE1296.UK 5

Claims

1. A method of strengthening the support or foundations of a structure which includes the steps of a) creating a bore extending the length of that portion of the structure which is to be strengthened; b) passing a tendon through said bore, and C) tensioning said tendon, thereby placing said portion of the structure under compression.

2. A method as claimed in Claim 1, wherein the tendon is tensioned from one or both ends of the structure by tensioning jacks and locked against end plates by wedged anchorages.

3. A method as claimed in Claim I or Claim 2, wherein the tendon is unbonded within a grease packed plastic sheath.

4. A method as claimed in any preceding claim, wherein the portion of the structure into which the tendon is introduced is in the substructure, ie within the region extending from ground level to the top of the foundations.

5. A method as claimed in any preceding claim, wherein the tensioning force applied to the structure is of the order of 50-IOOkN.

6. A method as claimed in any preceding claim, wherein the bore is cut through the structure using a diamond tipped cutter.

7. A method as claimed in any preceding claim, wherein the bore has a diameter of VANE1296.UK 6 25mm and the tendon a diameter of 15mm.

8. A method of strengthening the support or foundations of a structure, substantially as hereinbefore described and as illustrated in the accompanying drawings.