WO1992000425A2

WO1992000425A2 - Improved methods for providing foundations for building structures

Info

Publication number: WO1992000425A2
Application number: PCT/GB1991/001011
Authority: WO
Inventors: Roger Alfred Bullivant
Original assignee: Roxbury Limited
Priority date: 1990-06-23
Filing date: 1991-06-21
Publication date: 1992-01-09
Also published as: GB2259315A; GB9223376D0; WO1992000425A3; GB9014040D0; GB2259315B; AU8058091A

Abstract

A method of forming a building's foundation comprises providing in the ground in which the foundation is to be formed from the level of that ground (10) a trench (12) the depth of which is generally equal to the depth of a foundation beam to be subsequently formed in the trench and prior to forming the beam providing piles (14) extending from the trench bottom at spaced intervals therealong, the beam being formed by placing the reinforcement (14, 18) in the trench (12), connecting the reinforcement (16, 18) with the piles (14) and thereafter filling the trench (12) substantially to its top with concrete (22) which, on setting forms a beam integral with the piles (14). It is preferable that the trench sides converge downwardly.

Description

IMPROVED METHODS FOR PROVIDING FOUNDATIONS FOR BUILDING STRUCTURES

The present invention concerns an improved method for providing a foundation for a building structure.

The most commonly employed method of providing a foundation for a building structure is to excavate a flat bottomed and vertical sided trench in the bottom of which is formed a foundation or footing by pouring concrete therein.

Whereas this method is relatively simple it has a number of drawbacks. For example, it employs a relatively large volume of concrete. As a result of the use of a trench and the situation of the footing substantially below ground level it requires excavation of soil considerably in excess of the volume of concrete and this often gives rise to difficulties if water is present as the trench must be free of water when the concrete is poured. The construction of the trench depends to a large extent on the condition of the ground in which it is excavated. For example, if the ground is poor the trench walls will collapse unless shuttering is employed, this being time consuming and expensive. If the ground in which the trench is provided is subjected to heave eg., it is clay, special provisions must be employed. Perhaps the greatest disadvantage is the difficulties which have to be overcome when foundation has to be provided in undulating ground, that is, where the foundation or footing extends over an area of ground of different levels. In these circumstances if the ground is not made artificially level prior to digging the trench, a stepped trench must be dug, that is, a trench comprising a plurality of horizontal sections each stepped relative to its neighbouring section to take account of the rise and fall in the ground level.

The quality of foundations formed by the above traditional methods depends upon the excavation reaching what can be termed "good ground" that is stable ground conditions. This is not often possible to guarantee.

A further disadvantage of existing foundation forming techniques is that as the foundation is being formed it is subject to inspection by a Local Authority Building Inspector. This is often inconvenient and can cause unnecessary delays to the overall working schedule.

An object of the present invention is to obviate or mitigate these disadvantages.

According to the present invention there is provided a method of forming a foundation for a building structure comprising providing from below the level of the ground in which the foundation is to be formed a trench which is of substantially the same depth as the depth of a foundation beam to be formed therein, forming piles at spaced intervals in said trench leading from the base of the trench, placing reinforcement in said trench and filling the trench with a settable material to form said foundation beam with the tops of said pile cast therein.

Preferably the sides of the trench converge downwardly.

Preferably the cross-section of the trench is substantially triangular. Alternatively the cross-section of the trench is substantially trapezoidal.

Preferably the reinforcement is a steel assembly of substantially V-shaped cross-section. The tops of the piles may be connected to the reinforcing steel prior to the pouring of the settable material. The steel assembly preferably includes two longitudinally extending reinforcing members and a plurality of mutually spaced link members. Preferably the limbs of the V-shaped link members diverge upwardly in their in use position. Each limb may be provided at its end with an outwardly directed extension. Each extension is preferably provided with an end cap for locating the link against the trench side. At least one spacer member is preferably provided for the reinforcement assembly to maintain a correct spacing between the base of the assembly and the trench base. Preferably each of two upper longitudinally extending reinforcing members is fixed to a limb at its junction with its extension. Preferably the fixing is by means of welding. Preferably the lowermost longitudinally extending reinforcing member is not positioned on the assembly until after the remaining link members and longitudinal members have been positioned in the trench. Additional link members may be provided for the steel assembly and are positioned after the assembly without the bottom longitudinal member, has been placed in the trench.

Preferably prior to the introduction of the reinforcing assembly the trench is lined with an impervious sheet member. The impervious member may be polythene. Alternatively, prior to the placement of the reinforcing assembly the surfaces of the trench are lined with a compressable material so as to enable the foundation to withstand movement of the ground therearound. The compressable material may be polystyrene or compressed paper, as a proprietory clayboard product. In ground which is not sufficiently stable a more rigid lining material may be employed.

Preferably a low friction coat is applied to at least the upper end of the pile to permit movement of the ground relative to the pile.

Preferably an inflatable tube is inserted into a pre-formed hole in the ground in which a pile may be cast in situ, and inflated to maintain the hole open until the pile is formed by pouring a settable material into the hole.

Alternatively steel as precased piles may be driven or augered piled used.

Preferably the settable material is concrete.

Preferably the depth of the trench is between 12" and 24" (300 mm to 600 mm).

Preferably when the trench is formed in sloping ground its depth is increased such that the upper surface of the foundation beam cast therein comprises a plurality of mutually stepped horizontal sections, the depth of a step being an integral of 3" (77 mm) for brickwork, and as otherwise required for blockwork and the minimum depth of the beam not reduced.

Alternatively, when the trench is formed on sloping ground, the trench conforms to the contours of the slope so that the upper surface of the foundation beam cast therein also confirms to the countours of the slope, a second layer of foundation is then formed on top of the foundation beam comprising a plurality of mutually stepped horizontal sections, the depth of each step being an integral of 3" (77 mm). For brickwork and as otherwise required for blockwork.

Further alternatively, when a foundation is to be formed in sloping ground, the ground is levelled by conventional techniques and the trench is formed from the level of the excavated ground.

Further according to the present invention there is provided a trench forming machine comprising a chassis having ground engaging means and a vibratory assembly mounted thereon, a trench forming wheel having a cross-section corresponding to the cross-section of the trench to be formed being rotatably mounted on said vibrating assembly, means being provided to select amplitude, frequency and applied direction of vibratory force such that the wheel is caused to penetrate the ground on which it rests to form a trench.

Preferably by arranging for the direction of the vibrating force to have a forward component of motion in the direction of the trench to be formed the trench forming assembly can be self propelled. Alternati ely, it may be provided with means for moving it along the trench,

Preferably means are provided for raising or lowering the trench forming wheel relative to the chassis.

Still further according to the present invention there is provided a trench forming machine including a former having a cross-section corresponding to the cross-section of the trench, driving means for forcing the former into the ground to form a section of trench and means for moving said former and replacing it with a pile forming means whereby said driving means may drive said pile.

Preferably said driving means comprises an impact assing said former/pile into the ground.

An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawing in which: Fig. 1 shows a diagrammatic cross-section through a foundation for a building structure;

Fig. 2 shows a diagrammatic cross-sectional elevation of a section of foundation formed in sloping ground;

Fig. 3 shows a diagrammatic cross-sectional elevation similar to Fig. 2 but illustrating a modification;

Fig. 4 shows a diagrammatic elevation of a trench forming apparatus;

Fig. 5 shows a diagrammatic cross-section of the apparatus of Fig. 4, and

Fig. 6 shows a diagrammatic elevation of a further trench forming apparatus.

A foundation for a building structure is formed using an excavator which, from ground level 10, excavates a substantially triangular cross-section trench 12, of depth 15" (380 mm) and opening with 15" (380 mm) with a V-shaped bucket the dimensions of which correspond exactly to the dimensions of the trench. That is the bucket requires no overcutter as normally called for in excavator buckets. This ability of the bucket to cut a correctly shaped and sized trench is due, in part, to the "wedge" action provided by the bucket shape and provides a trench which itself performs the task normally performed by additional shuttering. Figs. 2 and 3 show that the trench follows the inclination of the ground in which it is formed and it will be realised that this trench forming operation, as it is at ground level excavates the minimum amount of soil and can be performed with the minimum of difficulty. Alternatively, the trench may be formed by rolling or hammering the ground with suitable forming machinery to be described later. These latter trench forming techniques have the advantage of providing a trench with sides of increased density and therefore stability. In view of the shallowness of the trench it is unlikely that it will readily fill with surface water unless the soil is particularly water laden or the weather conditions during the formation operation are particularly wet .

A plurality of piles 14 are driven at approximately 10 feet (3 metres) intervals, the pile heads 15 extending into the trench. Any pile driving technique suitable to the circumstances may be employed, for example, hammering, vibrating or augering, and the depth to which the piles are driven is predetermined in accordance with the load bearing qualities required and the soil conditions into which they are driven. A low friction slip coating 14 may be provided about at least the upper third of each pile when the ground in which the foundations are being formed is succeptible to movement or heave. This low t friction coating 14 allows the ground to move about the pile without significantly affecting the position or stability of the pile. It is possible for the piles to be driven prior to the excavation of the trench but this is likely to make the trench constructions marginally more complicated as a result of the protrusion of the pile heads into the trench.

As well as driving pre-formed piles, it is possible to use piles formed in situ. The holes in which these piles are to be formed are produced by driving a tube into the ground by any convenient method as described above. The tube is then removed to leave the hole in which the pile is to be formed. It is often found that there is a substantial interval between the formation of the hole and the insertion of the pile forming material, often concrete, therein. To prevent the hole collapsing during this interval an inflatable tube is inserted into the hole and inflated to a pressure sufficient to maintain the hole open. When the pile hole is to be filled, the tube is deflated and removed from the hole and the concrete is poured.

Prior to or after the pile placing operation the trench is lined by a polythene sheet which extends beyond the upper edges of the trench. In ground which is subjected to heave the polythene sheet lining may be substituted by any suitable compressible material for example, compressed paper or polystyrene. In loose ground the polythene sheet may be replaced by a more rigid lining to support the surrounding ground.

This lining will take up any slight movement of the ground substantially about the trench, thereby helping prevent the foundations from moving and possibly cracking.

On completion of the piling operation and trench lining a generally V-shaped cross-section reinforced steel assembly 16 comprising three longitudinally extending reinforcing rods 18 and surrounding transverse link members 20 is positioned within the trench spaced from the trench walls and bottom, optionally with the pile heads 15 connected to the reinforcement. Concrete 22 is then poured into the trench around the reinforcement following normal concrete pouring techniques with the upper surface 24 of the concrete being at a near ground level 10 when the ground is level.

The reinforced steel assembly is of particular significance. It comprises three longitudinal members 18, two being positioned in use near to the surface 24 of the concrete and one 18 being positioned, in use, close to the base of the trench.

In accordance with normal reinforcement construction techniques the members are held in position by transversely extending link members 20 but contrary to normal construction techniques the link members 20 do not form a continuous loop but are of substantially 'V' shaped configuration with the open end of the V uppermost and its side limbs 17 diverging upwardly. The upper ends of the side limbs 17 have outwardly directed extension 19 which, in use, are substantially horizontal. Plastics as concrete material end spacer caps 21 are fitted to these extensions .

Normal reinforcement construction techniques call for the fabrication of the reinforcing assembly on site by bending straight reinforcing bars into shape to form a link. Each link is then attached to the longitudinal members normally by wiring. This is a time-consuming technique which is not particularly suitable to the present invention, especially when it is applied on undulating ground as a reinforcement assembly produced by conventional techniques exhibits a considerable rigidity which would prevent it from complying substantially with the contour of the trench base. Additionally, in view of the inherent inaccuracies in in situ bar fixing, a normally constructed reinforcement assembly may not conform exactly to the profile of the trench so that the more important longitudinal members would not be correctly positioned at their optimum operational location.

The open top nature of the reinforcing assembly 16 of the present invention obviates these disadvantages, not only because it lends itself to prefabrication in a factory environment rather than in situ but also because it provides on site a relatively flexible assembly which will conform substantially to the profile of the base of the trench and which also, owing to its open top nature can be readily manipulated on site to achieve optimum positioning of the reinforcing bars.

The reinforcing assembly is manufactured in standard or occasionally special lengths in an orientation which is inverted compared with that shown in the drawing by bending links 20 on a former from precut lengths of reinforcing bar or automatically from a coil and placing them at predetermined spaced intervals on an assembly jig. A pair of longitudinal reinforcing members 18 of predetermined length is then laid on the links, each at the junction between a limb and its extension 19, and permenantly fixed thereto by, for example, spot welding. Reinforcing assemblies of this nature can be made up in a plurality of different lengths and once assembled one can be nested one within another for transportation, occupying the minimum storage and transportation space. Bundles of different length assemblies are delivered to site and on site it is a simple matter to select an assembly of suitable length for the conditions in which it is to operate and place it in the trench after having positioned in the trench bottom suitable spacer blocks 23 of plastics or other suitable material. The spacer blocks 23, together with the end caps 21 thus ensure the correct positioning of the longitudinal members 18 relative to the trench walls.

The design of the foundation beam of the present invention normally calls for the placement of additional link members 20 at and near the piles 14 and this can readily be achieved on site by utilising additional link members which have not been preconnected to longitudinal members 18, simply placing them in the trench at the desired spacing over the prepositioned longitudinal members 18. The final stage in the construction of the reinforcing assembly is the lowering of the bottom longitudinal reinforcing member 18 to the base of the reinforcing links 20. The entire reinforcing system is then in position and concrete can be poured thereon in the normal manner.

It will be realised that the reinforcement construction system described in the immediately preceding paragraphs offers considerable opportunity for cost reduction in production time as well as the structural and utilization flexibility called for by the invention. Clearly, if design considerations dictate, it is a simple matter to change the diameter of the bottom longitudinal reinforcing member 18 . A suitable diameter bar can be chosen from stock and simply lowered onto the prepositioned assembly.

In inclined or undulating ground, as shown in Fig. 2, the upper surface 24 of the concrete is formed with a plurality of 3" (80 mm) deep steps 26 by the use of simple transverse shuttering boards, the viscosity of the concrete being such that with steps of this depth a continuous stepped beam can be cast without employing shuttering of greater depth than the steps, that is, shuttering to the depth of the trench. Alternatively, in inclined or undulating ground as shown in Fig. 3 the concrete 22 is poured into the trench around the reinforcement such that the upper surface 24 of the concrete conforms substantially with contours of the ground. A second layer of concrete 25 is then poured onto the layer 24 and is formed with a plurality of 3" (80 mm) deep steps 26 as described above.

Further, it is to be appreciated that steps of a depth of integrals of 3" (80 mm) may be formed with the employment of suitable shuttering. This is a convenient step depth when bricks have to be laid on the foundation. If alternative building elements have to be laid, for example concrete blocks, the dimension of the step is varied appropriately.

In an alternative method for use on sloping ground the ground is first levelled in the normal manner and the trench is formed from the new ground level.

In a further modification a more conventional reinforcing system can be utilized. In this system the reinforcing assembly is constructed in situ and the links 20 take the normal closed triangular form. The disadvantages of such a system have been described above but in certain instances it may be applicable. It has been mentioned earlier that in addition to forming the trench by soil removal using an excavator provided with a V shaped bucket it is possible to form the trench by rolling or hammering. Figs. 4 and 5 show diagrammatically an elevation and end view of a suitable apparatus.

The apparatus comprises a chasis 40 which may be mounted on wheels or crawler track 42. The chasis supports a vibrating assembly 44 which in turn rotatably mounts the trench forming wheel 46 which, has a substantially V shaped cross-section. Steering means, not illustrated, may be provided to control the direction of movement of the wheel assembly.

In operation once the assembly has been positioned at the starting point of the trench to be dug the vibrator is set in motion, the amplitude, frequency and direction of the vibration being so chosen that the wheel penetrates into the ground. The direction of applied force may be further chosen such that the vibration also causes movement of the wheel assembly in the direction of the trench to be dug. Alternatively, the wheel assembly may be provided with separate means for moving it in the desired direction. The assembly includes height adjustment means so that trenches of different depths can be formed .

An alternative trench forming machine is illustrated diagrammatically in Fig. 6 and comprises a wheeled or tracked vehicle 50 having at one end an assembly 52 to removably position a former 54 in the path of an impact hammer 56 which is also mounted on the vehicle. The former 54 has a cross-section corresponding to the cross-section of the trench, that is triangular, and can be hammered into the ground to form a short length of trench. It is then lifted out of the trench and moved along to a position above the next section of trench to be formed with the former just overlapping the previously formed section. The operation is then repeated.

After a number of trench section forming operations the former mounting assembly 52 is actuated to remove the former 54 from its operating position below the hammer 56 and to bring a pile driving assembly 58 into this position. A pile 60 can then be driven into the bottom of the trench 62 by the same machine.

The present invention provides a reinforced concrete foundation beam the strength of which equals a conventional rectangular cross-section non-reinforced beam which utilises only 60?ό of the concrete of the standard beam and which, even with its reinforcing steel, costs no more than a standard non-reinforced cross-section beam. As the trench 12 in which the beam is formed is formed at ground level it is easy to form and does not suffer from water filling problems. In addition, as the foundation formation is systemized it need not be inspected by Local Authority Inspectors at all stages throughout its construction. The inclined sides of the trench can readily accommodate ground which is subjected to heave, all that is necessary being for the pile to be constructed, according to known techniques, for ground which is subjected to heave, and the sides of the trench to be lined with a compressible material, for example, plastics material sheets or clayboard as described above.

In a modified arrangement the substantially triangular cross-section of the beam is replaced by a more trapezoidal cross-section, that is a beam having downwardly converging sides but a substantially horizontal base. In extreme conditions a rectangular cross-section beam can be provided. As the trench is not of considerable depth and normally has downwardly converging sides it is self-supporting and readily constructable even in bad ground.

In a further modification no trench need be formed but more rigid lining material is used to form shuttering for a triangular section cast in situ reinforced concrete beam which is formed at ground level. The shuttering may be permanent and sacrificial or removable and re-usable. After the beam has been cast the ground it encloses or which surrounds it may be in filled.

Claims

C l aims

1. A method of forming a foundation for a building structure characterised in that it comprises providing from the level of the ground in which the foundation is to be formed a trench which is of substantially the same depth as the depth of a foundation beam to be formed therein, forming piles at spaced intervals in said trench leading from the base of the trench, placing reinforcement in said trench and filling the trench with a settable material to form said foundation beam with the tops of said pile cast therein.

2. A method as claimed in claim 1, characterised in that the sides of the trench are formed such that they converge downwardly.

3. A method as claimed in claim 1 or claim 2, characterised in that the trench is formed such that its cross-section is substantially triangular.

4. A method as claimed in claim 1 or claim 2, characterised in that the trench is formed such that its cross-section is substantially trapezoidal.

5. A method as claimed in any one of the preceding claims, characterised in that the reinforcement is a steel assembly of substantially V-shaped cross-section.

6. A method as claimed in claim 5, characterised in that the tops of the piles are connected to the reinforcing steel prior to the pouring of the settable material.

7. A method as claimed in claim 5 or claim 6, characterised in that the steel assembly includes two longitudinally extending reinforcing members and a plurality of mutually spaced link members.

8. A method as claimed in claim 5, 6 or 7, characterised in that the limbs of the V-shaped link members diverge upwardly in their in use position.

9. A method as claimed in claim 8, characterised in that each limb is provided at its end with an outwardly directed extension.

10. A method as claimed in claim 9, characterised in that each extension is provided with an end cap for locating the link against the trench side.

11. A method as claimed in any one of claims 5 to 10, characterised in that at least one spacer member is provided for the reinforcement assembly to maintain a correct spacing between the base of the assembly and the trench base.

12. A method as claimed in any one of claims 7 to 11, characterised in that each of two upper longitudinally extending reinforcing members is fixed to a limb at its junction with its extensions.

13. A method as claimed in claim 13, characterised in that the fixing is by means of welding.

14. A method as claimed in any one of claims 5 to 13, characterised in that the lowermost longitudinally extending reinforcing member is not positioned on the assembly until after the remaining rib members and longitudinal members have been positioned in the trench.

15. A method as claimed in claim 14, characterised in that additional link members may be provided for the steel assembly and are positioned after the assembly without the bottom longitudinal member, has been placed in the trench.

16. A method as claimed in any one of the preceding claims, characterised in that prior to the introduction of the reinforcement the trench is lined with an impervious sheet member.

17. A method as claimed in claim 16, characterised in that the impervious member is polythene.

18. A method as claimed in any one of claims 1 to 15, characterised in that prior to the placement of the reinforcement the surfaces of the trench are lined a compressible material so as to enable the foundation to withstand movement of the ground therearound.

19. A method as claimed in claim 18, characterised in that the compressible material is polystyrene, clayboard, or compressed paper.

20. A method as claimed in any one of the preceding claims, characterised in that a low friction coat or sleeve is applied to at least the upper end of the pile to permit movement of the ground relative to the pile.

21. A method as claimed in any one of the preceding claims, characterised in that an inflatable tube is inserted into a pre-formed hole in the ground in which a pile may be cast in situ, and inflated to maintain the hole open until the pile is formed by pouring a settable material into the hole.

22. A method as claimed in any one of claims 1 to 20, characterised in that steel, pre-cast or augered piles are used.

23. A method as claimed in any one of the preceding claims, characterised in that the settable material is concrete .

24. A method as claimed in any one of the preceding claims, characterised in that the depth of the trench is between 300 mm and 600 mm.

25. A method as claimed in any one of the preceding claims, characterised in that when the trench is formed in sloping ground its depth is increased such that the upper surface of the foundation beam cast therein comprises a plurality of mutually stepped horizontal sections with the minimum depth of the beam not reduced below said predetermined depth.

26. A method as claimed in any one of claims 1 to 24, characterised in that when the trench is formed on sloping ground, the trench conforms to the contours of the slope so that the upper surface of the foundation beams cast therein also conforms to the contours of the slope, a second layer of foundation is then formed on top of the foundation beam comprising a plurality of mutually stepped horizontal sections.

27. A method as claimed in any one of claims 1 to 24, characterised in that when a foundation is to be formed in sloping ground the ground is levelled by conventional techniques and the trench is formed from the level of the excavated ground.

28. A trench forming machine characterised in that it comprises a chassis (40) having ground engaging means (42) and a vibratory assembly (44) mounted thereon, a trench forming wheel (46) having a cross-section corresponding to the cross-section of the trench to be formed being rotatably mounted on said vibrating assembly, means being provided to select amplitude, frequency and applied direction of vibratory force such that the wheel is caused to penetrate the ground on which it rests to form a trench.

29. A machine as claimed in claim 28, characterised in that by arranging for the direction of the vibrating force to have a forward component of motion in the direction of the trench to be formed the trench forming assembly can be self propelled.

30. A machine as claimed in claim 28 or 29, characterised in that means are provided for raising or lowering the trench forming wheel relative to the chassis.

31. A trench forming machine characterised in that it includes a former having a cross-section corresponding to the cross-section of the trench, driving means for forcing the former into the ground to form a section of trench and means for moving said former and replacing it with a pile or pile forming means whereby said driving means may drive said pile.

32. A machine as claimed in claim 31, characterised in that said driving means comprises an impact assembly for hammering said former/pile into the ground.