GB2297772A - Building founation structures and method of making them - Google Patents

Abstract

A foundation structure comprises piles 1, preferably of reinforced concrete, and cast- in-situ annular concrete collars 7 each surrounding a respective pile. Precast concrete beams 8 rest each with its ends on a respective two of the said collars and there are stitched joints between the ends of the beams and the respective piles. Such a structure is made by first installing the piles; second forming annular recesses around them; third casting in-situ concrete in the annular recesses to form the collars, each with its upper surface at a prearranged design level; fourth after the in-situ concrete has hardened placing precast concrete beams to rest each with its ends on two of the collars; and fifth forming the stitched joints. In this way, the height and level of the beams is determined by the levelling of the in-situ concrete of the collars and is not affected by any imprecision in setting the piles or cutting their tops.

Description

Building Foundation Structurea and Method of making Them This invention relates to building foundations of the kind in which beams for the support of walls or like structures are supported on piles of concrete, steel or other suitable material. In known structures of this kind (see for example GB2186009A, GB2264321A, GB2274666A), the beams are normally supported, from the time they are initally placed, upon piles or on pile caps fitted to the tops of the piles, and the height and levelling of the beams is uncertain because of the limited accuracy with which the piles can be vertically positioned or cut to the required level. It is an object of the invention to overcome this limitation.

In accordance with one aspect of the invention, a foundation structure comprises a plurality of piles; a plurality of cast-in-situ annular concrete collars each surrounding a respective one of the piles; a plurality of precast concrete beams each with its ends resting on a respective two of the said collars; and stitched joints between the ends of the beams and the respective piles.

The invention includes buildings including such a foundation structure.

In accordance with another aspect of the invention, a method of making a building foundation comprises: first installing a plurality of piles; second forming annular recesses around the piles; third casting in-situ concrete in the annular recesses to form collars each with its upper surface at a prearranged level; fourth after the in-situ concrete has hardened sufficiently placing precast concrete beams to rest each with its ends on two of the collars; and fifth forming stitched joints between each pile and the beam or beams supported on its collar.

By a stitched joint is meant one in which reinforcing metal is exposed in each of the reinforced concrete members to be connected and is embedded in a common body of in-situ concrete (with or without bridging the reinforcing metal).

The piles may be of steel (say of H-section or in the form of a circular or non-circular tube) but reinforced concrete piles are preferred. These may be either precast or cast-in-situ, but precast piles are preferred. In either case, it will normally be necessary to cut back the top of each pile to expose reinforcing metal for stitching, and this is preferably done after the collars have been formed and hardened but before the beams are placed; since this cutting back does not affect the height of the beams it is not a precision operation - a tolerance of a few centimetres may be allowed. The beams will normally be cast with an appropriate portion of the reinforcing metal exposed at each end to avoid site work and minimise risk of damage.

The recesses round the piles may be of any convenient shape and may be formed by any appropriate excavation technique, but we prefer when site conditions permit to form them by using a suitable metal former driven into the ground (for instance by using a piling rig); depending on site conditions it may be appropriate to leave the former in place to guard against collapse of the recesses before the in-situ concrete is in place and set or it may be desirable to remove it for re-use. In a preferred alternative, the former is reusable and has on at least its sides, and preferably also on its base, a lining comprising a wire mesh preferably sandwiched between layers of or otherwise embedded in plastics or other sheet material. Paper or cardboard may be a viable alternative sheet material for use in dry soil or as one or some of the layers of a sheet having two or more layers.

The wire mesh is preferably of steel wires, and welded mesh is preferred though mesh with twisted joints ("chicken wire" structure, for example) may be suitable in some cases.

It will readily be appreciated that the mesh must be strong enough to survive the driving process without serious damage and to give the support necessary to retain the soil until the in situ concrete can be poured.

On grounds of cost and environmental considerations, the preferred plastics sheet material is polythene (also called polyethylene).

Welded steel mesh embedded in polythene is commercially available to the construction industry under the Trade Mark PECAFIL and is used for disposable shuttering above ground or in excavations; but it may be necessary to reinforce the commercial material by welding in a few additional or larger wires or by folding, for example at the top edge, to make it strong enough for use in this invention.

The former, and consequently the collar, may be of a simple flat-bottomed design or it may be of a tapered shape, for example an inverted pyramid or cone, in each case with an open centre for passage of the pile.

The collars may be cast and levelled by entirely conventional technique, and may be reinforced or not as economics and site conditions dictate. The collars provide only interim support for the beams, and in some site conditions they may have sufficient load-bearing capacity merely through resting on the ground; where the nature of the site makes it inappropriate to rely on this, the part of the pile to be embedded in the collar may be roughened or cut to form interlocking formations to ensure transfer of load from the collar to the pile. A concrete pile may for example be roughened by scabbling or a steel pile by cutting holes in it and/or welding projections to it.

In making the stitched joints, the exposed reinforcing metal of the pile and the beam or beams may be connected by welding or by tie wires, or additional metal components may be placed to overlap with the two zones of exposed metal without any metallic connection, or in most cases poured concrete alone may be used to make the stitched joints. In most cases appropriate shuttering will be required to complete a mould for the poured concrete.

The beams may support any appropriate superstructure; in many cases some of them will directly support cavity walls forming the exterior of a domestic or other relatively small building, and in such cases the beam is conveniently stepped (higher to the inside of the building). Steps may also be used to support suspended floors, and in such cases ventilation openings may be provided through the lower part of the beams; or the beams may serve as shuttering to enclose floor slabs resting on the ground (preferably through a compressible layer, say of expanded polystyrene, to accomodate possible "heave* of the ground (and incidentally to offer some thermal insulation). Beams that are to support only internal load-bearing partition walls may be of rectangular cross-section.

The invention will be further described, by way of example, with reference to the accompanying drawings in which Figure 1 is a cross-section through a portion of one form of foundation structure in accordance with the invention; Figure 2 is a diagrammatic cross-section of one design of former for creating a recess in the exercise of this invention in the form illustrated by Figure 1; Figure 3 is a diagrammatic cross-section of another design of former showing a recess being formed; and Figure4 is a cross-section, corresponding to Figure 1, of a part of a foundation structure made using the former of Figure 3.

In making the foundation structure shown in part in figure 1, a pile 1 of reinforced concrete is first placed in the ground. In most site conditions, we prefer it to be a driven precast pile and for the types of building to which the invention is mainly appropriate, a pile precast in a single section without joints may be sufficient. In some site conditions a segmental precast pile or a pile cast in situ in a hole formed by use of a driven steel casing and/or an auger may be preferred.

A fabricated steel former 2 (Figure 2) is now positioned around the projecting top of the pile 1. The former is square in plan and is made up of a lower pyramidal part 3 and an upper parallel-sided part 4; an upward extension S facilitates handling. The former 2 is driven in to the ground, say by impact or vibration from a pile-driving rig, to form a recess 6 which is filled with concrete to form a collar 7 (Figure 1). Where the ground is insufficiently stable for the recess to be self-supporting after the former 2 has been removed, shuttering or other forms of support may be immediately inserted; in extreme cases, it may be desirable for the former 2 to be fitted with a suitable liner which remains in the ground to give support after the former has been withdrawn and until the concrete is poured.

The surface of the pile exposed on removal of the former is cleaned and roughened to promote adhesion; optionally small portions of the concrete can be cut away, for instance by chiselling, drilling or sawing, to provide for interlock either by penetration of the poured concrete into the opening(s) so formed or by the insertion of a member or members, preferably of plastics or other non-corroding material, which project into the recess into which the concrete is to be poured and so form a key bond. The collar 7 may be reinforced by appropriate metal inserts. In all cases, the concrete poured to form the collar 7 needs to be levelled to the appropriate design height - in simple cases the same for all the collars.

Once the concrete of the collar 7 has hardened sufficiently, the concrete of the projecting pile top is cut back nearly flush with it and the reinforcing metal so exposed is re-shaped (and/or cut) if necessary to ensure at least that it will be fully embedded when the stitching concrete is later placed and preferably so that it will contribute usefully to the stength of the stitched joint.

Precast concrete beams 8 are now laid with the ends of each resting on the collars 7 surrounding different piles.

Reinforcing metal 9 of the beams is exposed at the ends, and may overhang the piles themselves. Additional reinforcing metal may be added if desired to bridge between the reinforcing metal of the beams and that of the piles, with or without wire tying or welding it to either or both of them.

Straight and/or angled shuttering, as appropriate, is clamped to the ends of the beams and/or propped externally to complete an enclosure for stitching concrete, which is next poured to bond the beams soundly to each other and to the pile and so provide a solidly integrated foundation beam structure supported by the piles but at a height determined solely by the levelling of the collars.

In the example shown, the beam 8 is stepped, the lower step 10 supporting the outer ply 11 of a brick cavity wall while the upper step 12 supports both the inner ply 13 of the wall and a suspended floor 14. The beam may have passages 15 cast into it to provide ventilation between the underfloor void 16 and wall cavity 17.

Figure 3 shows an alternative design of former 21 which is an open-topped flat-bottomed box of sheet steel with suitable internal supports (shown by way of example as angle supports 22 along the edges) being driven into the soil 23 by a conventional pile-driving rig acting through an annular thrust member (not shown) on the base 24 of the former 21.

The centre of the base has an opening to admit the top of a pre-installed pile 25. Preferably the former 21 is lined externally with PECAFIL sheeting 26 comprising a welded mesh of steel wires of 4.Omm diameter on 75mm centres in one direction and of 5.5mm diameter on 150mm centres in the perpendicular direction embedded in low-density polythene with an average thickness corresponding to about 2.64kg/m2.

Two pieces of this sheeting are used, each bent to a simple channel shape and covering the base - apart from the area of the opening in it - and a pair of opposite sides of the former (so that there are two thicknesses over the base and one thickness over each side of the former); we have not found it necessary to fold the PECAFIL sheet round the vertical edges of the former or to seam the vertical corners in any way. To strengthen the PECAFIL sheet, an additional steel wire 27 of 12mm diameter is welded along the top edge of each side (this wire could be extended round the corners, if preferred). Once the former is driven to the required depth, it is withdrawn leaving the PECAFIL sheets 26 in situ; the top of pile 25 can then be trimmed as required, additional shuttering added above ground if needed, and in situ concrete 28 (Figure 4) poured and levelled to form a collar at the design height; remaining steps are exactly as before.

Claims (49)

1 A foundation structure comprising a plurality of piles; a plurality of cast-in-situ annular concrete collars each surrounding a respective one of the piles; a plurality of precast concrete beams each with its ends resting on a respective two of the said collars; and stitched joints between the ends of the beams and the respective piles.

2 A foundation structure as claimed in claim 1 in which the piles are of reinforced concrete.

3 A foundation structure as claimed in claim 2 in which the piles are precast.

4 A foundation structure as claimed in any one of claims 1-3 in which the collar is flat-bottomed.

5 A foundation structure as claimed in any one of claims 1-3 in which the collar is tapered.

6 A foundation structure as claimed in any one of claims 1-3 in which the collar is in the form of an inverted pyramid surrounding the pile.

7 A foundation structure as claimed in any one of claims 1-3 in which the collar is in the form of an inverted cone surrounding the pile.

8 A foundation structure as claimed in any one of claims 1-7 in which the collars are reinforced.

9 A foundation structure as claimed in any one of claims 1-8 in which there are interlocking formations to ensure transfer of load from the collar to the pile.

10 A foundation structure as claimed any one of claims 1-9 in which the stitched joints include welded joints or tie wires between exposed metal of the pile and of the beam or beams.

11 A foundation structure as claimed in any one of claims 1-10 in which the stitched joints include additional metal components placed to overlap with exposed metal.

12 A foundation structure as claimed in any one of claims 1-11 in which the collar is enclosed by a lining comprising a wire mesh.

13 A method as claimed in claim 12 in which the wire mesh is of steel wires.

14 A method as claimed in claim 12 or claim 13 in which the wire mesh is welded.

15 A method as claimed in any one of claims 12-14 in which the wire mesh of the lining is embedded in sheet material.

16 A method as claimed in claim 16 in which the sheet material is a plastics material.

17 A method as claimed in claim 16 in which the plastics material is polythene.

18 A method as claimed in any one of claims 12-15 in which the wire mesh of the lining is embedded in plastics sheet material and extends to the base of the former.

19 A method as claimed in claim 18 in which the plastics material is polythene.

20 A building characterised by a foundation structure as claimed in any one of claims 1-11 or 18-19.

21 A building characterised by a foundation structure as claimed in any one of claims 12-17.

22 A building as claimed in claim 20 or claim 21 in which at least one beam of the foundation structure directly supports a cavity wall.

23 A bulding as claimed in claim 22 in which the said the beam is stepped, being higher to the inside of the building.

24 A building as claimed in any one of claims 20-23 in which at least some of the beams are stepped and directly support a suspended floor.

25 A building as claimed in claim 24 in which ventilation passages are provided through the lower part of the beams.

26 A building as claimed in any one of claims 20-25 in which at least some of the beams enclose floor slabs resting on the ground.

27 A method of making a building foundation comprising: first installing a plurality of piles; second forming annular recesses around the piles; third casting in-situ concrete in the annular recesses to form collars each with its upper surface at a prearranged level; fourth after the in-situ concrete has sufficiently hardened placing precast concrete beams to rest each with its ends on two of the collars; and fifth forming stitched joints between each pile and the beam or beams supported on its collar.

28 A method as claimed in claim 27 in which the piles are of reinforced concrete.

29 A method as claimed in claim 28 comprising using precast reinforced concrete piles.

30 A method as claimed in claim 28 or claim 29 comprising cutting back the top of each pile to expose reinforcing metal for stitching.

31 A method as claimed in claim 28 or claim 29 comprising cutting back the top of each pile to expose reinforcing metal for stitching after the collars have been formed and hardened but before the beams are placed.

32 A method as claimed in any one of claims 27-31 comprising using beams precast with a portion of the reinforcing metal exposed at each end.

33 A method as claimed in any one of claims 27-32 comprising forming the recesses round the piles using a suitable metal former driven into the ground.

34 A method as claimed in claim 33 in which the metal former has on at least its sides a lining comprising a wire mesh and in which the lining is left in place in the soil and the collar cast in situ in the lining.

35 A method as claimed in claim 34 in which the wire mesh is of steel wires.

36 A method as claimed in claim 34 or claim 35 in which the wire mesh is welded.

37 A method as claimed in any one of claims 34-36 in which the wire mesh is embedded in sheet material.

38 A method as claimed in claim 37 in which the sheet material is a plastics material.

39 A method as claimed in claim 38 in which the plastics material is polythene.

40 A method as claimed in any one of claims 34-36 in which the lining is of wire mesh embedded in plastics material and extends to the base of the former.

41 A method as claimed in any one of claims 27-40 comprising roughening or cutting the part of the pile to be embedded in the collar.

42 A method as claimed in any one of claims 27-41 in which exposed metal of the pile and the beam or beams are connected in the stitched joints by welding or by tie wires.

43 A method as claimed in any one of claims 27-42 in which additional metal components are placed in the stitched joints to overlap with exposed metal of the pile and at least one of the beams, without any metallic connection.

44 A foundation structure substantially as described with reference to Figures 1 and 2.

45 A foundation structure substantially as described with reference to Figures 3 and 4.

46 A building substantially as described with reference to Figures 1 and 2

47 A building substantially as described with reference to Figures 3 and 4.

48 A method of making a foundation structure substantially as described with reference to Figures 1 and 2.

49 A method of making a foundation structure substantially as described with reference to Figures 3 and 4.