GB2075575A

GB2075575A - Method of strengthening foundations of structures

Info

Publication number: GB2075575A
Application number: GB8110558A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-05-06
Filing date: 1981-04-03
Publication date: 1981-11-18
Also published as: GB2075575B

Abstract

Foundations of structures are repaired by digging a trench around the perimeter, the substructure walls are cleaned, tension cables are placed in the trench spaced from the bottom of the trench and from the substructure walls, the cables being sheathed (e.g. in a grease packed plastics sheath), concrete is poured about the cables after shuttering has been placed in position thereby forming a surrounding beam section 22 that contact the substructure walls, and when the concrete has set the cables are tensioned and their end portions locked against ends of the beam section, thus placing the beam section under compression. <IMAGE>

Description

SPECIFICATION Method of and means for strengthening the support or foundations of structures and structures stengthened thereby This invention relates to a method of and means for strengthening the support or foundations of structures and structures strengthened thereby. It is applicable particularly through not exclusively to the repair of existing structures having damaged foundations. Buildings and like 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 of course 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 the foundations large in size and/or reinforcing them as e.g. in the case of reinforced 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 laborious and hence costly.

It is with these problems that the invention is concerned, viz the repair or strengthening of structures having damaged or inadequate foundations, and also the provision of strengthened foundations for structures yet to be built.

According to a first aspect, the invention provides a method of strengthening the support or foundations of or for a structure which includes the steps of excavating a trench adjoining the structure perimeter, supporting at least one tensioning tendon in the trench spaced from the bottom of the trench, the tendon being sheathed, casting concrete in situ to form a beam about the tendon, the beam contacting the substructure walls or foundations and forming a ring or hoop, tensioning the tendon when the concrete has set, locking end portions of the tendon thereby placing the beam under compression, the beam embracing and bearing against the said walls or foundations. When used herein, the term "tendon" includes cables, wire, rods and the like. Preferably at least one end portion of the tendon is locked against an end of the beam.In the case of a structure having rectilinear sides, a straight beam section is preferably formed at each straight side, and the beam sections are interconnected at the perimeter corners to form a jointed ring or hoop.

Such a hoop, if comprised of highly stressed beam sections, is of a strength and rigidity, very high in relation to the cross section of the beam sections, aided particularly by the closed system of forces acting within it. Preferably one or more sides of the beam sections are deliminated by formwork or shuttering. Preferably, the tendons are tensioned from one or both ends of each beam section by tensioning jacks and 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 their tubes by injection into the tubes of concrete or other bonding substance.

The portions of the walls contacted by the beam will generally be in the substructure, i.e.

within the region extending from ground level to the top of the foundations. When the tendons have been tensioned, the hoop not only embraces and bears horizontally against the substructure walls, inducing in them a compressive force, but also forms a strong tie powerfully resisting movement of the substructure. In the case of a new structure yet to be built, the hoop greatly strengthens the new foundations. And in the case of an existing structure damaged by movement of the foundations, the hoop greatly lessens the risk of further damage.

Preferably, in the repair or strengthening of existing structures having damaged or inadequate foundations, the substructure walls or foundations in the region where they are to be contacted by the beam, are treated to make for improved bonding with the beam, as e.g. by removal of earth and cleaning or e.g. by surface scoring.

Preferably formwork or shuttering delimits the undersurface of the beam and the sides remote from the structure walls. Part of the substructure walls at each corner of the structure perimeter may be broken away or omitted and the vacant space cast with concrete united with the concrete of the beam; this should assist in the transmission of forces from the beam to the substructure walls, thereby improving the grip on the latter. A cross beam may be installed, extending between intermediate sections of the ring or hoop, the cross beam contacting and bearing against a substructure wall or foundation of or for a dividing wall.

According to a second of its aspects, the invention provides means for strengthening the support or foundations of or for a structure, comprising a beam of concrete cast in a trench adjoining the structure perimeter, a sheathed tensioning tendon within the beam, the beam contacting the substructure walls or foundations and forming a ring or hoop, locking means locking the tendon under tension thereby placing the beam under compression, the beam embracing and bearing against the substructure walls or foundations. Preferably four beam sections are interconnected at their ends to form a rectilinear jointed hoop.

According to a third of its aspects, the invention provides a structure having its foundations strengthened or its substructure walls supported as described in relation to the first and second aspects of the invention set forth above.

A preferred form of the invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is the ground plan of a pair of semi detached houses, Figs. 2(a) and 2(b) show the foundation and adjoining parts of the external walls of the structure, in section, respectively taken at a higher and at a lower ground level, Fig. 3 depicts a ring beam, in section, on an enlarged scale, Fig. 4 shows a perimeter corner to an enlarged scale, and Fig. 5 corresponds to Fig. 2(a) but in relation to a party wall.

The structure shown in the drawings consists of a pair of brick built semi-detached houses. Its perimeter in the nature of things is rectangular, its external walls 10 being cavity walls and standing on foundations or footings 12 and 14, and its dividing wall 16 on footings 18. Movement of the substrate has resulted in damage to the foundations and cracking of the walls (not shown).

The structure will be assumed to have been erected on sloping ground 20, the ground by the right hand end wall (as seen in Fig. 1) and consequently its footings 12 being at the same level as at the dividing wall and the level by the left end wall and its footings 14 being lower.

By way of repair and to strengthen the structure a jointed hoop or ring beam generally designated 22 encircies the external walls of the structure. To install it, a level trench 24 indicated by chain link (Fig. 2(a)) is excavated along the structure perimeter adjacent the outer walls, extending down as far as the level of the highest footings 12 (i.e. not down to the level of the lower footings 14 of Fig. 2(b)). The brick surface 26 along the part of the substructure (i.e. of the walls between the footings and the ground) to be contacted by the beam 22 is cleared of soil and cleaned. Shuttering or formwork (not shown) is installed in each straight section of the trench to define the free outer side of the beam and, where appropriate, to define the bottom of the beam.And, where appropriate, protective flexible webbing 28 is placed on the footings 12 to prevent direct contact between beam and footings. Conventional steel reinforcement bars 30 encircled at spaced locations by steel tie wires 32 are positioned in the trench. And a tensioning cable or tendon 34 is positioned by tie wires in the trench, spaced from the bottom of the trench and spaced from the brick wall and from the shuttering, so as to be in the centre of the beam when cast.

The tendon is multi-strand (e.g. 7-strand) high strength tensile steel cable, loosely surrounded by a flexible plastics sheath and packed with grease for ease of movement therein. Typically, such tendon is able to withstand a strain of e.g 10-1 5 tons or e.g. 20-40 tons. Prior to installation, the plastics sheath will have been stripped from the end portions of the tendon and the bared portions will protrude from the ends of the straight sections of the beam in readiness for tensioning.

Concrete is then cast in situ in the formwork and allowed to set. The adjoining brickwork having been cleaned, the concrete forms a bond with it.

When the concrete has set, the tendons are tensioned from both ends as schematically indicated at 36 (Fig. 4) by the use of tensioning jacks, the tension being transmitted to the beam sections by way of end plates 38 and cone grips 40; post-tensioning in this way of cast concrete beams is well known (for other structural applications) and hence will not be further described. Thereafter the trench is filled-in and the soil tamped down.

Each section of the ring beam itself constitutes a strong and rigid beam. The cross-over of the tendons at the perimeter corners causes the ring beam to form a unitary structure, strongly resistant to deformation. Moreover the tension of the tendons tend to contract the ring beam about the substructure which is thereby powerfully embraced. The risk of further damage to the structure upon further movement of the substrate is greatly lessened. The structure with its damaged foundations, has been strengthened and substantially repaired.

Turning to Fig. 5, it will be seen that a cross beam 42 is disposed along one side of the dividing wall 16, at the same horizontal level as the ring beam, and extends between intermediate sections of the ring beam. Its construction is self evident.

Manifestly, its lends further strength and rigidity to the ring beam and indeed may be regarded as forming part of a pair of adjoining ring beams, contributing to the strength of both, in addition of course to strengthening the dividing wall substructure.

A modification is indicated in the top right corner of Fig. 1. Part of the brickwork at the corner of the building, at the intended level of the ring beam, is broken away to form a void. When the beam is cast, this void is filled with additional concrete unitary with that which forms the adjoining sections of the beam. The additional concrete block 44 so formed participates in the transmission of forces from the adjoining parts of the ring beam to the brickwork thereby helping to impart compressive forces to the walls in the direction of their length.

It will be appreciated that many modifications may be made to the construction described above without departing from the scope of the invention.

Thus for example, each section of beam may have more than one tendon (e.g. 2 or 4). Additional beam sections may be provided to sub-divide the ring beam into a greater number of subsidiary ring beams. After tensioning, the tendons could be bonded to the sheaths containing them, by the injection of concrete, although in that case the tendons would have to be housed in tubes of a size to permit this.

And of course the invention is applicable to the installation of new structures in which case the beam would enbrace substructure walls or vertically extending surfaces of foundations.

Consequently they would be of increased strength and rigidity enabling economy in the size of the footings or other foundations to be achieved for a given desired strength.

Claims

1. A method of strengthening the support or foundations of or for a structure which includes the steps of excavating a trench adjoining the structure perimeter, supporting at least one tensioning tendon in the trench spaced from the bottom of the trench, the tendon being sheathed, casting concrete in situ to form a beam about the tendon, the beam contacting the substructure walls or foundations and forming a ring or hoop, tensioning the tendon when the concrete has set, locking end portions of the tendon thereby placing the beam under compression, the beam embracing and bearing against the said walls or foundations.

2. A method according to claim 1 wherein at least one end portion of the tendon is locked against an end of the beam.

3. A method according to claim 1 or claim 2 wherein the structure has rectilinear sides, a straight beam section is formed at each straight side, and the beam sections are interconnected at the perimeter corners to form a jointed ring or hoop.

4. A method according to claim 3 wherein one or more sides of the beam sections are delimited by formwork or shuttering.

5. A method according to any one of claims 2 to 4 wherein the tendons are tensioned from one or both ends of each beam section by tensioning jacks and locked against end plates by wedged anchorages.

6. A method according to any one of the preceding claims wherein the tendons are unbonded within grease packed plastics sheaths.

7. A method according to any one of claims 1 to 5 wherein the tendon(s) are disposed with clearance within tubes and after installation bonded to their tubes by injection into the tubes of concrete or other bonding substance.

8. A method according to any one of the preceding claims wherein the substructure walls or foundations, in the region where they are to be contacted by the beam, are treated to make for improved bonding with the beam.

9. A method according to any one of the preceding claims wherein part of the substructure walls at each corner of the structure perimeter is broken away or omitted, and the vacant space cast with concrete united with the concrete of the beam.

10. A method according to any one of the preceding claims which includes the further steps of excavating an additional trench alongside the location of a dividing wall of the structure, forming a cross beam in that trench about a tensioning tendon in that trench, the cross beam extending between intermediate sections of the ring or hoop, the cross beam contacting and bearing against a substructure wall or foundation of or for the dividing wall.

11. Means for strengthening the support or foundations of or for a structure, comprising a beam of concrete cast in a trench adjoining the structure perimeter, a sheathed tensioning tendon within the beam, the beam contacting the substructure walls or foundations and forming a ring or hoop, locking means locking end portions of the tendon under tension thereby placing the beam under compression, the beam embracing and bearing against the substructure walls or foundations.

12. Means according to claim 11 wherein at least one end portion of the tendon is locked against an end of the beam.

13. Means according to claim 11 or claim 12 wherein four beam sections are interconnected at their ends to form a rectilinear jointed hoop.

14. A method of strengthening the support or foundations of or for a structure, the method being substantially as herein described with reference to the accompanying drawings.

1 5. A structure having its foundations strengthened or its substructure walls supported by the method or by the means claimed in any one of the preceding claims.