WO1994017252A1

WO1994017252A1 - Improvements in or relating to foundations

Info

Publication number: WO1994017252A1
Application number: PCT/GB1994/000116
Authority: WO
Inventors: Roger Alfred Bullivant
Original assignee: Roxbury Ltd
Current assignee: Roxbury Ltd
Priority date: 1993-01-30
Filing date: 1994-01-21
Publication date: 1994-08-04
Anticipated expiration: 1995-07-30
Also published as: GB2289494B; GB2289494A; GB9301891D0; AU5863594A; GB9514216D0

Abstract

A foundation pier (28) for a building structure is formed by making a downhardly converging hole in the ground, for example by hammering or vibrating a conical mandrel (10, 12, 14) into the ground, and filling the hole with concrete (24) to form a pier having a slenderness or aspect ratio of length to thickness of no greater than 10. Precast beams (30) are placed between adjacent piers (28) upon which the structure is built and a floor slab is cast in situ utilising the beams (30) as lateral shuttering.

Description

Improvements in or relating to Foundations

The present invention concerns improvements in or relating to foundations, especially but not exclusively vertical foundations in ground, the condition of which can be termed good to marginal.

Current civil engineering comprehension is that a heavy load can be borne on ground if it is made

sufficiently competent by spreading the load over an area which is dictated by the load bearing capacity of the soil making up the ground, that is, the stronger the soil the smaller the area of required foundation.

Utilising this comprehension current foundations take the form of pads or strips spreading the applied load over the ground. The concept of using piles in good ground has not been contemplated and is not considered to be viable or good engineering practice.

This current comprehension dictates that in poor ground which is not sufficiently strong to support pad or strip foundations, piles are employed, being driven into the ground to reach down to lower firmer strata and/or providing fπctional support thus increasing load bearing capacity, not only as a result of the firmer strata acting indirectly as a foundation pad but also as a result of the fπctional effects of the ground on the shaft of the pile, these effects increasing the load bearing characteristics of the pile. Current piles generally have a length which is much in excess of their diameter or thickness. For example, a square cross-section pile having a thickness of 250 mm often has a length of 10 metres. The ratio of length to thickness is expressed as the slenderness aspect ratio, the example quoted above giving an aspect ratio of 40. The provision of foundation pads, strip foundations and piles can all be relatively expensive and are often dependent upon the conditions of

installation, for example, ground which has a high water content is unsuitable for the strip foundation system because the trench formed to receive the foundation often fills with water before the foundation can be laid or pumping or other measures have to be used to deal with this problem at an increased cost.

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

According to the present invention there is provided a foundation pier comprising a member with a vertical or generally vertical axis and an aspect ratio of no greater than 10, having downwardly converging sides formed from a cementitious material cast in situ in a preformed hole in the ground.

Preferably the member is an inverted conical f r u s t um .

Preferably the sides of the cone are inclined at 2.5° to the vertical.

The member may be triangular, rectangular, pol y gona l or elliptical in cross-section.

Preferably the member incorporates reinforcement. Typically the member has an upper diameter of 0.2 to 1 metres and a length between 2 and 10 metres.

Preferably the member incorporates holding down means for a building structure to be supported on the pier. Preferably the holding down means may be fixing bolts. Preferably the fixing bolts are attached to the reinforcement within the member.

Further according to the present invention there is provided a method of forming a foundation pier comprising forcing a downwardly converging mandrel into the ground to a predetermined depth to form a hole of corresponding shape having an aspect ratio of no greater than 10, removing the mandrel and filling the hole with a settable material.

The hole may be filled simultaneously with or after removal of the mandrel.

Preferably prior to filling the hole with settable material reinforcement means are fitted in the hole. The reinforcement means may incorporate holding down means for a building structure to be supported on the pier.

Preferably the mandrel is forced into the ground by hammering. Alternatively it may be vibrated into the ground.

Preferably the mandrel is guided such that its longitudinal axis remains substantially vertical as it is driven.

Preferably the mandrel is extended in length by fitting a hollow extension of external dimensions smaller than the upper end of the mandrel to its upper end.

Another aspect of the present invention provides a building structure supported on a plurality of primary foundation piers of the type described in any of the preceding ten paragraphs, the building structure

including also a floor slab cast in situ within a surrounding assembly of beams supported on said piers which arranged at spaced intervals around the periphery of the structure.

Preferably additional piers may be provided below the floor slab.

Preferably the floor slab and primary piers are capable of mutual vertical displacement.

Preferably the floor slab is tied against

horizontal movement relative to the primary piers.

Preferably a connection for the floor slab to each pier comprises one or more vertical tubes attached to the pier and mounting, for sliding movement along the axis of the tube, a bar connected to reinforcement of the floor slab.

Preferably the floor slab is a dual spanning floor slab.

Preferably the floor slab is supported during its formation on a layer of insulating material, for example by the said beams, which ground is made up to the required level prior to laying the layer. Preferably holding down means for the building structure are formed in the primary foundation piers and are capable of resisting horizontal and turning moment loading. They may comprise mutually spaced holding down assemblies, each of which is mounted in a single pier.

Preferably where the pier has a rectangular, polygonal or elliptical cross-section the longer transverse axis is arranged generally at right angles to the horizontal load or tilting moment to be resisted.

An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:-

Fig. 1 shows a mandrel for forming a pier;

Fig. 2 shows a pier formed in situ;

Fig. 3 shows a plan view of a section of the perimeter of a building supported on a pier;

Fig. 4 shows a diagrammatic view of means for connecting a floor slab to a pier;

Fig. 5 shows a plan view of an alternative pier; Fig. 6 shows an alternative mandrel assembly; and

Figs. 7 and 8 show alternative assemblies for guiding the mandrel.

The mandrel shown in Fig. 1 is intended to be hammered into good to reasonable ground by a pile driving hammer impacting upon the upper impact member 10 of the mandrel. The mandrel comprises a- mild steel conical sleeve 12 mounted to a steel driving tube 14 connected to the pad 10 and to the sleeve 12 by a plurality of ribs 16 which, at their upper ends, may include apertures 18 for mounting handling shackles (not shown). Typically the mandrel is between 2 and 6 metres long and it has a slenderness or aspect ratio which is not greater than 10. The aspect ratio is obtained by dividing the length of the mandrel by its diameter. The angle of inclination of the walls forming the sleeve 12 to the vertical is between 1° and 3° and preferably around 2 ½ °. The sleeve is provided with a conical tip 20 which, in certain instances, may be sacrificial, that is, it may be left down the hole formed by the mandrel. Normally it is fixed to the end of the mandrel and removed from the hole on removal of the mandrel.

It is desirable that the longitudinal axes of the mandrel is maintained vertical during the mandrel driving operation and Figs. 7 and 8 illustrate two guide assemblies for achieving this, especially during the initial driving steps.

The guide assembly shown in Fig. 7 comprises a removable clamp 70 temporarily attached to the mandrel approximately midway along the pile driver mast 72 by a slider assembly 74 and moves vertically down the mast during the initial driving of the mandrel. The clamp is released when the mandrel is embedded sufficiently in the ground to ensure that further driving will be generally vertical.

In the modification illustrated in Fig. 8 the mandrel is provided with two diametrically opposed guide fins 76 which on driving slide through a slot 78

provided in a guide 80 fixed to the mast 72.

After the mandrel has been hammered to a depth which may either be predetermined or which may

correspond to a specified downward resistance measured by equating the hammer force applied to the penetration per blow, it is removed and, bearing in mind that the ground into which it has been hammered is good to reasonable, this results in a hole being left in the ground, the shape of which corresponds to that of the mandrel. As can be observed from Fig. 2 a reinforcing cage 22, which has been designed in accordance with the loading conditions to be supported, is then placed in the hole which is immediately filled with concrete 24 which can be prepared on site. Fixing bolts 26, which may be fixed to the reinforcing cage or positioned in the hole prior to the pouring of the concrete 24, project upwardly from the concrete to provide hold-down means for stanchions, beams, etc. of the building structure to be supported on the foundation pier.

If desired the concrete can be poured as the mandrel is removed. It is supplied to the interior of the ascending mandrel and enters the hole through the hole in the base left by the sacrificial tip. The reinforcement is placed into the poured concrete before it sets.

In a modification the mandrel can be vibrated into the ground but this is in some instances undesirable because the vibration tends to liquify certain soils as opposed to the compaction of the soil achieved by the downwardly converging mandrel when it is hammered into the ground. The hole may collapse after vibratory driving. Additionally the reactive force on the mandrel cannot readily be perceived in a vibratory insertion method. It will be realised therefore that the

consolidating force of the downwardly converging mandrel can enhance the load bearing capacity of the ground to make it sufficiently good to support the foundation pier.

The foundation pier thus relies on an operating concept which is revolutionary different from existing concepts. With the pier the load is supported

vertically. In other words it provides a vertical foundation. With strip foundations, pad foundations and normal piling techniques the load is spread over a horizontaly extended area. The increased diameter of the foundation pier, especially at its upper end, means that fixing means can be arranged at mutual spacing which are so great that one pier is capable of providing reaction forces to horizontal and tilting moment loads thereon. With the conventional piling technique two or more piles driven alongside each other are required.

Fig. 9 illustrates a situation where the

foundation pier is utilised in ground where the top layer T does not provide good load bearing

characteristics, such characteristics being achieved only by the lower layer L. If a conical mandrel was utilised and driven down until a sufficient reaction obtained the upper diameter of the hole thus formed would be large due to the upwardly diverging nature of the former, so that a large volume of concrete would be required to fill the hole when the mandrel was removed.

The modified mandrel shown in Fig. 9 obviates this problem. The mandrel shown is similar to that

illustrated in Fig. 1 but has a tubular extension 90 at its upper end, the length of the extension being greater than the depth of the layer T and the diameter less than the diameter of the mandrel at its top. A bond breaking band 92 is provided at the top of the mandrel and has a diameter slightly greater than the diameter of the top of the mandrel.

In a further modification the extension 90 could converge upwardly.

The use of the bond breaker 92 and/or the tapering extension means that there is relatively little contact between the ground and the extension 90 over the layer T and the loading on the pier is picked up over the layer L.

The poor ground conditions over the layer T often results in soil falling into the hole formed by the mandrel on its removal from the ground. This can be avoided by filling the hole with concrete through the tip of the mandrel as it is removed. Alternatively, and bearing in mind the ground over the length L is unlikely to collapse, means can be provided at the top of the mandrel to collect soil falling into the hole from the layer T and to collect it for removal on the top of the mandrel.

In constructing a building a plurality of

foundation piers are arranged generally around the periphery of the building at predetermined spaced intervals. When the foundation piers 28 (only one of which is shown in Fig. 3) have been formed subsequent construction of the building to be supported thereon commences with the laying prefabricated beams 30, between each pier. Additionally each pier normally supports a stanchion 32 or portal frame of the building, the stanchion or frame being held down by the fixing bolts 26. Peripheral walls can be built on the beams 30 and roofs, etc. supported on the portal frames.

The floor of the building is cast in situ and it is desirable that after casting mutual vertical movement between the floor and the pier can be allowed. A

preferred form of floor is a cast in situ, dual-spanning concrete floor which is formed within shuttering

provided by the surrounding beams 30 and on a

polystyrene slab laid in the ground to isolate the floor from the ground. Clearly appropriate reinforcement 34 (Fig. 4) will be placed on the polystyrene slab prior to the pouring of the floor concrete. The slab is laid directly on the ground and if the level thereof is not as desired then it is made up by earth excavated from the site or imported material.

It is desirable that the floor slab is isolated against vertical displacement relative to the pier but fixed against horizontal displacement. To this end, as can be seen in Figs. 3 and 4 transversley extending reinforcing members 36 are cast into the top of the pier 28 and are connected to a pair of vertically extending metal sleeves 38. The reinforcement 34 of the floor slab is bent such that it has a vertically extending section 40 passing through the tube 38, obviously, this being placed before the floor slab is poured.

Preferably a metal isolation sheet 42 covers that surface o f the pier 28 against which the floor is cast. It will be realised therefore that when the floor is cast it is isolated from the pier 28 by the sheet 42 and consequently movement in a vertical direction between the two can take place while the connection of the reinforcement 34 with the tubes 30 prevents relative horizontal movement. Fig. 3 shows a modified pier which has a

non-circular cross-section with the longer axis o f the rectangle lying parallel to the axis of the building's outer wall. A pier of this shape is designed to resist horizontal or tilting loads on the pier acting in a direction generally perpendicular to the axis of the beams 30. In further modifications the pier may have a triangular, rectangular, elliptical or any other suitable cross-section.

Fig. 5 shows a plan view of a modified pier arranged to support a stanchion 32 at an intermediate location within the building structure. In this instance the pier is of the conical configuration shown in Fig. 2 and has a substantially square surrounding steel sleeve 42' to isolate it from the floor slab, each corner of the sheet including tubes 38 through which the floor slab reinforcing bars 34 can slide.

Various modifications can be made without

departing from the scope of the invention. For example, certain circumstances may allow a material other than concrete to be poured into the hole formed by the mandrel to form the pier. For instance consolidated stone may be employed or alternative settable mixtures.

It will be realised that the method of construction described above is flexible and if, for example, on a site area where the ground condition varies additional support is required, a pile can be driven through the base of the hole formed by the mandrel prior to the formation of the foundation pier. As described above, a conical cross-section pier is generally the preferred form but eliptical, rectangular, square polygonal and other piers may be employed, provided that the aspect ratio is less than 10 and that the pier converges in the downward direction.

The methods of forming the hole with the mandrel 10 vary. It is preferable as indicated in the

specification above, that the mandrel is hammered into the ground but vibratory driving techniques may be employed. To remove a mandrel when it has been driven to the desired depth may be achieved by ensuring that the impact force of the hammer is converted to an upward direction to relieve the stiction between the mandrel sleeve and the surrounding ground.

Fig. 6 shows an alternative mandrel assembly which has been designed with a view to ensuring that when the mandrel is driven into the ground its axis remains substantially vertical.

The assembly comprises a guide tube 50 having lugs 52 for mounting it to a vehicle. A mandrel 54 which is similar to the mandrel described with reference to Fig. 1 has a cylindrical portion 56 at its upper end which is slidably mounted and thereby guided within the tube 50. The assembly is designed so that the centre of gravity of the driving mass 58, conveniently of lead or any other dense material, is located at a considerable distance below the upper end of the mandrel. With this in mind the mass 58 is located in the mandrel and has an external profile, in the case shown in the drawing, a step profile, which is complimentary to the internal profile of the mandrel. The mandrel is provided with a striking surface 60 and a hammering mass 58 is connected to an hydraulic piston and cylinder device 62 which allows it to be raised and then dropped against the striking surface 60 at an impact rate of, for example, 60 blows per minute. As the hammering mechanism does not form part of the present invention it will not be described. The piston and cylinder device 62 is

suspended on a rope 64.

Latch means (not shown) are provided to connect the mandrel 54 to the sleeve 50 when the mandrel has been driven to a predetermined depth. By operating the rams 66 located at the base of the sleeve the mandrel assembly can be lifted thereby extracting the mandrel from the hole that it has formed. In tests where the method of the present invention has been compared with standard strip or pad foundation a time saving in excess of a factor of 10 can be

achieved by utilising the inventive method. The savings in material costs may also be considerable.

Claims

C l a i ms :

1. A foundation pier comprising a member with a vertical or generally vertical axis and an aspect ratio of no greater than 10, having downwardly converging sides formed from a cementitious material cast in situ in a preformed hole in the ground.

2. A foundation pier as claimed in Claim 1, in which the member is an inverted conical frustum.

3. A foundation pier as claimed in Claim 2, in which the sides of the cone are inclined at 2.5° to the vertical.

4. A foundation pier as claimed in Claim 1 or Claim 2 or Claim 3 in which the member is triangular,

rectangular, polygonal or elliptical in cross-section.

5. A foundation pier as claimed in any one of the preceding claims, in which the member incorporates reinforcement.

6. A foundation pier as claimed in any one of the preceding claims, in which the member has an upper diameter of 0.2 to 1 metres and a length between 2 and 1 0 me t r e s .

7. A foundation pier as claimed in any one of the preceding claims, in which the member incorporates holding down means for a building structure to be supported on the pier.

8. A foundation pier as claimed in which the holding down means may be fixing bolts.

9. A foundation pier as claimed in Claim 8, in which the fixing bolts are attached to the reinforcement within the member.

10. A method of forming a foundation pier comprising forcing a downwardly converging mandrel into the ground to a predetermined depth to form a hole of corresponding shape having an aspect ratio of no greater than 10, removing the mandrel and filling the hole with a

settable material.

11. A method as claimed in Claim 10, in which the hole is filled simultaneously with or after removal of the mandrel.

12. A method as claimed in Claim 10 or Claim 11, in which prior to filling the hole with settable material reinforcement means are fitted in the hole.

13. A method as claimed in Claim 12, in which the reinforcement means incorporate holding down means for a building structure to be supported on the pier.

14. A method as claimed in any one o f Claim 10 to 13, in which the mandrel is forced into the ground

by hammering.

15. A method as claimed in any one of Claims 10 to 13, in which the mandrel is forced into the ground by applying a vibratory force thereto.

16. A method as claimed in any one of Claims 10 to 15, in which the mandrel is guided such that its

longitudinal axis remains substantially vertical as it is driven.

17. A method as claimed in any one of claims 10 to 16, in which the mandrel is extended in length by fitting a hollow extension of external dimensions smaller than the upper end of the mandrel to the said upper end of the mandrel.

18. A building structure supported on a plurality of primary foundation piers of the type described in any of claims 1 to 9 the building structure including also a floor slab cast in situ within a surrounding assembly of beams supported on said piers which arranged at spaced intervals around the periphery of the structure.

19. A building structure as claimed in Claim 18, in which additional piers may be provided below the floor slab.

20. A building structure as claimed in Claim 18 or Claim 19, in which the floor slab and primary piers are capable of mutual vertical displacement.

21. A building structure as claimed in Claim 20, in which the floor slab is tied against horizontal movement relative to the primary piers.

22. A building structure as claimed in Claim 21, in which a connection for the floor slab to each pier comprises one or more vertical tubes attached to

the pier and mounting, for sliding movement along the axis of the tube, a bar connected to reinforcement of the floor slab.

23. A building structure as claimed in any one of Claims 19 to 22, in which the floor slab is a dual spanning floor slab.

24. A building structure as claimed in any one of Claims 19 to 23, in which the floor slab is supported during its formation on a layer of insulating material, for example polystyrene.

25. A building structure as claimed in Claim 24, in which the insulating layer is surrounded by the said beams, which ground is made up to the required level prior to laying the layer.

26. A building structure as claimed in any one of claims 19 to 25, in which holding down means for the building structure are formed in the primary foundation piers and are capable of resisting horizontal and turning moment loading.

27. A building structure as claimed in Claim 26 in which the holding down means comprise mutually spaced holding down assemblies, each of which is mounted in a single pier.

28. A building structure as claimed in any one of the preceding claims, in which where the pier has a

rectangular, polygonal or elliptical cross-section the longer transverse axis is arranged generally at right angles to the horizontal load or tilting moment to be resisted.