GB2091313A

GB2091313A - Methods of underpinning

Info

Publication number: GB2091313A
Application number: GB8101226A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-01-15
Filing date: 1981-01-15
Publication date: 1982-07-28
Also published as: GB2091313B

Abstract

A method of supporting a pre- existing building structure comprises drilling a hole from at or above ground level down through the pre-existing building structure to the subsoil, said hole extending at a small angle to the vertical, extending said hole down through the subsoil vertically or substantially at said angle by soil displacement, and forming a pile in situ in said hole, the head of the pile being bonded to said structure.

Description

SPECIFICATION Methods of underpinning This invention relates to methods of underpinning building structures.

Conventional structure underpinning is effected by digging down below the level of existing footings, which may have subsided, forming new and massive concrete footings beneath the existing footings, and working thus in short sections around the building. After allowing the new concrete time to gain strength, jacks can be arranged to act between the old and new concrete footings to raise any subsided portions of the building.

That procedure is difficult, time-consuming and labour intensive in view of the need to dig down well below existing footing level in each section around the building.

My Patent Application No. 80021 99 discloses a method of supporting a pre-existing wall comprising: forming a plurality of pairs of piles in situ adjacent to the wall; forming pile cap members each extending across the top of the piles of a said pair to support said wall; creating a generally horizontal line of weakness along the wall above the cap members; inserting a plurality of hydraulic expansion devices in openings formed in said line of weakness; lifting the wall above said line by means of said devices; and retaining said wall in said lifted position to permit removal of said devices.

That method avoids the above-noted disadvantages of conventional underpinning and has the major advantages of being much less time-consuming and also avoiding the necessity for disturbing or undermining the existing footings.

The method particularly described in Application No. 8002199 comprises the step of forming a concrete ring beam in a shallow trench around the building, the ring beam having integral arms extending into apertures in the wall to form said pile cap members. This procedure is therefore still fairly complicated, particularly in certain situations.

According to the present invention there is provided a method of supporting a pre-existing building structure comprising drilling a hole down through the pre-existing structure to the subsoil, said hole extending substantially vertically or at a small angle to the vertical, extending said hole down through the subsoil vertically or substantially at said angle by soil displacement, and forming a pile in situ in said hole, the head of the pile being bonded to said structure.

The hole down through the pre-existing structure is preferably drilled from a point substantially at or above existing ground level.

The building art is conservative and its natural reaction is that it would be very difficult and tedious to try to form a hole vertically or at a small angle down through a brick, stone or concrete wall and its concrete footings and/or other underlying solid structure. However experiments have shown that it is not at all difficult when appropriate known equipment is utilized from other arts, for example from the borehole drilling art. Thus a diamond core drill and a rotary percussion drill have each been tried and can provide the required hole without undue difficulty and in a short time period.

In practice the angle of the hole is selected to be as small as practical for a given situation or vertically if the form of the existing structure permits. Moreover, for a strip foundation, calculations or a test dig can be carried out to reveal the position of the centre line of the strip foundation. The hole should then lie at an angle to pass substantially down through that centre line so that the resultant pile gives a balanced support to the foundation. Angles of up to 100 or even greater may be used in particular situations.

The extension of the hole down through the subsoil may be effected by a number of methods depending upon ease of access and headroom, and the nature of the subsoil. For example the hole may be extended by means of a self-propelled soil displacing mole as disclosed in my Patent Application No. 7916657, and may be lined as disclosed in my Patent Application No. 8023589.

Alternatively the hole may be formed by means of top-driven steel tubes as disclosed in my Patent Application No. 8040772. The disclosure in my British Patent Applications Nos. 791 6657; 8023589 and 8040772 are incorporated herein by reference, and reference should be made to those specifications for further details of these methods.

A further method of forming the hole is to use a closed ended tube, e.g. of the type described in the method of my Application No. 8040772, but to drive the tube into the ground by hammering on the closed end of the tube from within the tube. In a further method solid piles or pile sections may be hammered into the ground thereby displacing soil during their insertion into the ground.

When a hole is first formed in the ground, an in situ pile may be formed by inserting steel reinforcing rods in the hole and filling it with concrete. The concrete will normally inherently bond itself, as it sets, to the existing building structure around the upper end of the hole. The binding strength may be improved by roughening or under-cutting the hole or by other methods known in the concrete art.

Where a wall structure is to be supported by a method according to the invention, a succession of said piles may be formed along the wall, adjacent piles being formed in holes commenced from opposite sides of the wall and thus lying at opposite inclinations to the vertical. By this means the inherent lateral thrust loading on the wall, due to the inclination of any one pile, is counterbalanced by its oppositely inclined neighbour. A corner wall structure is inherently much more stiff and thus, where that is more convenient, a number of adjacent piles may all be formed from one side or the other.

A slab or other laterally extensive structure may also be supported by a number of piles not necessarily in counterbalanced opposite inclination to one another. For example, where a house foundation comprises a foundation slab covering the entire ground floor area and including an integral step-down, thickened edge portion around its periphery which serves as a foundation for the exterior house walls, then it may be possible to underpin the structure by working entirely from outside the house. In that case all the piles formed along any one side of the slab would be inclined in the same sense, but the lateral forces in the slab as a whole would be balanced by the row of oppositely inclined piles formed at the opposite side of the house.

It will be seen that a particular practical advantage of the invention is that it makes it possible to underpin a building structure without any significant time-consuming and labour intensive digging or excavation work.

Claims

1. A method of supporting a pre-existing building structure comprising drilling a hole down through the pre-existing building structure to the subsoil, said hole extending generally vertically, extending said hole down through the subsoil generally vertically by a soil displacement method, providing a pile in said hole, and bonding said pile to said structure.

2. A method according to claim 1 wherein said drilled hole and said extended hole extend at a small angle to the vertical.

3. A method according to claim 2 wherein said angle is less than 100.

4. A method according to any one of claims 1 to 3 wherein said hole down through the preexisting building structure is drilled from a point substantially at or above ground level.

5. A method according to any one of claims 1 to 4 wherein said hole down through the preexisting building structure is drilled by means of a diamond drill.

6. A method according to any one of claims 1 to 5 for supporting a building structure having a strip foundation, wherein said hole is drilled to pass down substantially through the centre line of said strip foundation.

7. A method according to any one of claims 1 to 6 wherein said step of extending said hole is effected by means of a self-propelled soil displacing mole.

8. A method according to any one of claims 1 to 6 wherein said step of extending said hole is effected by means of top-driven steel tubes.

9. A method according to any one of claims 1 to 8 wherein said step of providing a pile is effected by forming a concrete pile in situ in said hole.

10. A building structure supported by a method according to any one of claims 1 to 9.