GB2178460A

GB2178460A - Minimising heave effects on a pile

Info

Publication number: GB2178460A
Application number: GB08519053A
Authority: GB
Inventors: Robert Michael Sloma
Original assignee: Individual
Current assignee: Individual
Priority date: 1985-07-29
Filing date: 1985-07-29
Publication date: 1987-02-11
Also published as: GB2178460B; GB8519053D0

Abstract

To minimise or relieve the effects of heave (i.e.the lift effect) upon a pile by the surrounding ground material (e.g. soil, clay or the like), a pile bore (1) has its upper region (2) of enlarged width dimension (D) and first and second surfaces (12,21) are provided one within the other in mutual spaced relation internally of said enlarged width upper region such that said first surface (12) is for engagement by the encompassing ground material and said second surface (21) is for encompassing and engagement of the pile. The surfaces may be formed as shown on spaced tubular members (10,20) or by a tubular body of foamed or cellular plastics material having surfaces defined by a skin or membrane like portion of the body. <IMAGE>

Description

SPECIFICATION Improvements in or relating to piling This invention relates to pulling.

A problem of increasing concern associated with piling is so-called "heave", this being the lift effect upon a pile by the ground material around the pile. Such "heave" tendencies are generally of greatest concern where the ground material is at least mainly of clay and the site has been cleared of trees, shrubs and other flora. It is thought to be due in such circumstances to the tendency of the desiccated clay in the uppermost regions of the ground material to regain its natural moisture content and, in so doing, to expand in volume thereby raising the ground level. It has been found virtually impossible to predict accurately the heave forces likely to be encountered in any practical situation.For example, in one particular case one "expert" put the expected heave force (to be resisted by the pile) at a value of 40kN/m2 and another "expert" in the same case put it at a value of 1 00kN/m2. With regard to the effect on a pile of the heave forces arising in the ground material, much depends upon the estimated c-value, i.e. cohesion value, indicative of the cohesive force likely to arise between the ground material and the pile. Accordingly, in attempting to ensure an adequate factor of safety in pile construction, it has become customary to require substantially high tensile strength for the piles. This has necessitated incorporating in concrete piles, steel tensioning wires in greater number and/or thickness than warranted if potential heave forces could be accurately predicted or even discounted totally.The precautionary use of excess steel in the piles dramatically increases the cost of piling.

It is therefore considered desirable to provide a method and apparatus for piling which can overcome the aforementioned and/or other difficulties and disadvantages of piling procedures prevalent heretofore.

According to one aspect of this invention there is provided a piling method wherein the ground material (e.g. soil, clay or the like) is provided with a pile bore having its upper region of enlarged width dimension, and first and second surfaces are provided one within the other in mutual spaced relation internally of said enlarged width upper region such that said first surface is for engagement by the encompassing ground material and said second surface is for encompassing and engagement of the pile.

According to another aspect of this invention there is provided heave relieving means for piles, comprising a first surface for engagement by encompassing ground material, and a second surface spaced from, and disposed internally of, the first surface and for engagement of the pile.

In use, the first surface (at least initially) holds the ground material away from the pile (and from its encompassing second surface) to prevent the application of heave forces from the ground material onto the pile. However, if the first surface is subsequently crushed laterally inwards by the heave forces in the ground material, the space or void between the first and second surfaces can accomodate or permit a take up of the expanding ground material so that any heave forces eventually applied to the second surface and therefrom to the pile are likely to be substantially reduced with respect to the heave forces initially applied by the ground material on the first surface prior to its being crushed.

Preferably said first surface is in continuous surrounding relation with respect to the second surface and such as to be engageable all round by said ground material. Advantageously, said first surface is the outer surface of a tubular wall. Preferably said first surface is cylindrical.

Preferably said second surface is for continuous surrounding relation with respect to the pile and for engagement thereof all round its perimeter (in the upper region of the pile bore). Advantageously, said second surface is the inner surface of a tubular wall.

Preferably said first surface is cylindrical.

In one preferred construction the heave relieving means comprises two cylindrical tubular members one within the other and held fast with one another in mutual spaced relation by one or more annular spacer rings. Preferably two such rings are provided, one at each end of the composite, two-member construction.

Advantageously, the two tubular members are of differing lengths. Preferably, the inner extends beyond the outer at at least one end, the or each of the end rings being shaped to provide a tapering transition between the outer to the inner tubular member.

Advantageously the two tubular members have some or all of their surfaces of low coefficient of friction.

They may be tubes of cardboard - with or without a slip-inducing coating (e.g. of plastics material) on some or all of their surfaces - or be tubes of plastics material. The or each spacer ring may be of expanded polystyrene.

In an alternative construction, the heave relieving means comprises a tubular body of foamed or cellular plastics material, the body having cylindrical inner and outer surfaces each defined by a "skin" or membranelike portion of the foamed or cellular body, said skins serving as the aforesaid first and second surfaces, the voids of the foamed or cellular structure between the skins serving to space the two surfaces apart.

By way of non-limiting example, embodiments of the present invention will now be described with reference to the accompanying drawings of which: Figure 1 is a schematic axial cross-sectional view of a structure embodying one aspect of this invention in an intermediate stage of a method according to this invention, and Figure 2 is a similar view to that of Figure 1 but illustrating a modification thereof.

The present invention is considered applicable both to short bored piles and to long bored piles. Generally speaking, short bored piles have a pile length of from 2.5 to 4.5m and provide a load carrying capacity ranging from 40 to 95kN for a 250mm diameter pile and from 65 to 1 45kN for a 350mm diameter pile.

Long bored piles generally have a pile length exceeding 4.5m, preferably of from 4.5 to 1 sum, and provide a substantially greater load carrying capacity.

Each of the illustrated embodiments is for a long bored pile of length L of between 6 to 12m, preferably 1 Om, and diameter d of between 150 to 450mm, preferably 300mm. The pile bore 1 is drilled to the required depth L with an auger of the appropriate diameter d, and the uppermost region 2 of the bore 1 is coaxially augured to an enlarged diameter D and to a depth h below ground level of between 0.5 to 4m, preferably 1.5 to 2.5m.

A structure 5 providing heave relief means is inserted into the uppermost region 2 of the pile bore.

The structure 5 comprises outer and inner tubes 10,20 of cardboard coated with a plastics material. The plastics material coating has a low coefficient of friction and is applied to the interior and exterior faces 11, 12 of outer tube 10 and to the interior and exterior faces 21, 22 of inner tube 20. The two tubes 10, 20 are held in mutually spaced relation at their opposite ends by upper and lower end rings 15, 16 of expanded polystyrene. The rings 15, 6 are each of generally frusto-conical shape such as to provide a tapering connection or transition between the outer tube 10 and the longer inner tube 20 projecting longitudinally from each end of outer tube 10.The end rings 15, 16 are each adhesively attached to surfaces 11, 22 of the outer and inner tubes 10, 20 so as to interconnect the two tubes and retain them as coaxial parts of the self-supporting structure 5.

The outer surface 12 of tube 10 is of substantially the diameter D, the diameter of enlarged upper bore region 2. The inner surface 21 of tube 20 is of substantially the diameter d, the diameter of the lower main portion of bore 1. The space S between the two surfaces 11, 22 (which defines the thickness of the annular void between tubes 10, 20) may be in the range 10 to 175mm, preferably in the range 25 to 50mm.

It will be appreciated that due to the tapered nature of lower end ring 16, the structure 5 can be readily inserted into the top of the bore's lower region and be located coaxially therewith. Concrete to form the pile is introduced into the bore by pouring through the top of structure 5. Either prior to or during the introduction of the concrete, steel reinforcing "wires" or rods 8 are provided in the bore to extend just in an upper portion of of the stepped bore 1, e.g. for just the top 6m so as to extend both within inner tube 20 and below it in the narrower section of the bore 1. Preferably, for this specific embodiment, just four high tension steel rods 8 are employed, each of 1 6mm diameter (i.e. four Y1 6 wires).

It is considered that the location and form of structure 5 will, in use, function as follows. Initially, the tube 10 of structure 5 holds the ground material away from the pile (and from the inner tube 20 encompassing it) to prevent the application of heave forces from the ground material onto the concrete pile.

However, if the heave forces become too great and subsequently crush the tube 10 laterally inwards, the space or void S between the mutually facing surfaces 11, 22 of tubes 10, 20 accomodates or permits a take up of the expanding ground material such as to dissipate most (if not all) of these heave forces and such that any heave forces eventually applied to the inner tube 20, and therefrom to the concrete pile, are likely to be substantially reduced. Furthermore, due to the low-friction coating of plastics material applied to all the surfaces of tubes 10 and 20, the lift force component of the heave forces tends to induce slip initially between the ground material and the outer surface 12 of tube 10.Increasing heave forces will result in a crushing of tube 10 and, due to the lift force component, a shearing of the end rings 15, 16 and/or of their bond to one or other of the tubes 10,20.

Thereafter, said lift force component will induce continued slip between the ground material and the outer surface 12 of tube 10, will initiate slip between the coated facing surfaces 11, 22 of the two tubes 10 and 20, and also slip between the coated surface 21 of tube 20 and the outer surface of the concrete pile. It will be appreciated that due to such slip between several surfaces, the ground material may undergo physical "heave" but not actually transmit any significant lift forces arising from such "heave" onto the concrete pile. Accordingly the amount of any tensile reinforcement required may be reduced.In this regard it should be noted that if a conventional piling procedure had been adopted in which the bore 1 was not enlarged at its upper region and no structure 5 employed, then the steel reinforcing wires or rods would have to be increased both in number and diameter, e.g. six Y20 or Y25 wires, and they would have had to extend the full length L of the concrete pile.

The modified structure 25 illustrated in Figure 2 is similar to structure 5 of Figure 1 in that it comprises outer and inner tubes 30, 40 interconnected by upper and lower end rings 35, 36. The difference between the two embodiments lies principally in that the two tubes 30, 40 are of substantially similar length with inner tube 40 projecting longitudinally of the bottom of outer tube 30 and the latter projecting logitudinally beyond the top of inner tube 40. The bottom end ring 36 is similar to end ring 16, but the top end ring 35 is somewhat funnel-shaped and tapers inwardly as it extends downwardly. This shape facilitates pouring in of the concrete for the pile and allows the top of the concrete to flare outwardly to provide an enlarged surface area for supporting a conventional ground beam.

The present invention is not limited to the abovedescribed and illustrated embodiments, and (in addition to the aforementioned) various other modifications may be made thereto. For example, the self-supporting outer tube may be replaced by a thin-walled element of sheet material which, when subjected to pressure from the ground material, would tend still to retain its cylindrical shape and to function as a slipping membrane against the inner tube (if eventually deflected to such an extent). In another exemplary modification the heave relieving means comprises a tubular body of foamed or cellular plastics material, the body having cylindrical inner and outer surfaces each defined by a "skin" or membrane-like portion of the foamed or cellular body, the voids of the foamed or cellular structure between the skins serving to space the two skins apart.

Amendments to the claims have been filed, and have the following effect: (a) Claims 1, 20, 38 above have been textually amended as follows: (c) Claim 38 above has been re-numbered as 39.

1. A piling method wherein the ground material (e.g. soil, clay or the like) is provided with a pile bore having its upper region of enlarged width dimension, and first and second surfaces are inserted in said enlarged width upper region, said first surface being for engagement by encompassing ground material and being readily deformable by heave of the ground material to accomodate heave induced strain, and said second surface being disposed within and in spaced relation to the said first surface and for encompassing engagement of the pile.

20. Heave relieving means for a pile, comprising a first surface for engagement by encompassing ground material, said first surface being readily deformable by heave of the ground material thereby to accomodate heave induced strain, and a second surface disposed within and in spaced relation to the first surface and for encompassing engagement of the pile.

38. Heave relieving means for a pile, wherein the upper region of the pile is surrounded by a sleeve spaced therefrom, the sleeve's exterior surface being engaged by encompassing ground material, and wherein the sleeve is readily deformable by heave of the of the ground material thereby to accomodate heave induced strain.

Claims

1. A piling method wherein the ground material (e.g. soil, clay or the like) is provided with a pile bore having its upper region of enlarged width dimension, and first and second surfaces are provided one within the other in mutual spaced relation internally of said enlarged width upper region such that said first surface is for engagement by the encompassing ground material and said second surface is for encompassing and engagement of the pile.

2. A piling method according to Claim 1, wherein said first surface is in continuous surrounding relation with respect to the second surface and such as to be engageable all round by said ground material.

3. A piling method according to Claim 1 or Claim 2, wherein said first surface is the outer surface of a tubular wall.

4. A piling method according to any preceding Claim, wherein said first surface is cylindrical.

5. A piling method according to any preceding Claim, wherein said second surface is for continuous surrounding relation with respect to the pile and for engagement thereof all round its perimeter (in the uppper region of the pile bore).

6. A piling method according to any preceding Claim, wherein said second surface is the inner surface of a tubular wall.

7. A piling method according to any preceding Claim, wherein said second surface is cylindrical.

8. A piling method according to any preceding Claim, wherein the first and second surfaces are defined by heave relieving means comprising two cylindrical tubular members one within the other and held fast with one another in mutual spaced relation by one or more annular spacer rings.

9. A piling method according to Claim 8, wherein two such rings are provided, one at each end of the composite, two-member construction.

10. A piling method according to Claim 8 or Claim 9, wherein the two tubular members are of differing lengths.

11. A piling method according to Claim 10, wherein the inner extends beyond the outer at at least one end, the or each of the end rings being shaped to provide a tapering transition between the outer to the inner tubular member.

12. A piling method according to any one of Claims 8 to 11, wherein the two tubular members have some or all of their surfaces of low coefficient of friction.

13. A piling method according to any one of Claims 8 to 11, wherein the two tubular members comprise tubes of cardboard.

14. A piling method according to any one of Claims 8 to 13, wherein the two tubular members are provided with a slip-inducing coating on some or all of their surfaces.

15. A piling method according to Claim 14, wherein said coating is of plastics material.

16. A piling method according to any one of Claims 8 to 12, wherein the two tubular members comprise tubes of plastics material.

17. A piling method according to any one of Claims 8 to 16, wherein the or each spacer ring is of expanded polystyrene.

18. A piling method according to any one of Claims 1 to 7, wherein the first and second surfaces are each defined by a "skin" or membrane-like portion of a tubular body of foamed or cellular plastics material, the first and second surfaces being respectively the inner and outer surfaces of the tubular body, the voids of the foamed or cellular structure between the skins serving to space the two surfaces apart.

19. A piling method according to Claim 1 and substantially as herein described with reference to the accompanying drawings.

20. Heave relieving means for piles, comprising a first surface for engagement by encompassing ground material, and a second surface spaced from, and disposed internally of, the first surface and for engagement of the pile.

21. Heave relieving means according to Claim 20, wherein said first surface is in continuous surrounding relation with respect to the second surface and such as to be engageable all round by said ground material.

22. Heave relieving means according to Claim 20 or Claim 21, wherein said first surface is the outer surface of a tubular wall.

23. Heave relieving means according to any one of Claims 20 to 22, wherein said first surface is cylindrical.

24. Heave relieving means according to any one of preceding Claims 20 to 23, wherein said second surface is for continuous surrounding relation with respect to the pile and for engagement thereof all round its perimeter (in the uppper region of the pile bore).

25. Heave relieving means according to any one of preceding Claims 20 to 24, wherein said second surface is the inner surface of a tubular wall.

26. Heave relieving means according to any one of preceding Claims 20 to 25, wherein said second surface is cylindrical.

27. Heave relieving means according to any one of preceding Claims 20 to 26, comprising two cylindrical tubular members one within the other and held fast with one another in mutual spaced relation by one or more annular spacer rings, the two tubular members defining said first and second surfaces.

28. Heave relieving means according to Claim 27, wherein two such rings are provided, one at each end of the composite, two-member construction.

29. Heave relieving means according to Claim 27 or Claim 28, wherein the two tubular members are of differing lengths.

30. Heave relieving means according to Claim 29, wherein the inner tubular member extends beyond the outer tubular member at at least one end, the or each of the end rings being shaped to provide a tapering transition between the outer to the inner tubular member.

31. Heave relieving means according to any one of Claims 27 to 30, wherein the two tubular members have some or all of their surfaces of low coefficient of friction.

32. Heave relieving means according to any one of Claims 27 to 31, wherein the two tubular members comprise tubes of plastics material.

33. Heave relieving means according to any one of Claims 27 to 30, wherein the two tubular members comprise tubes of cardboard.

34. Heave relieving means according to any one of Claims 27 to 31, wherein the two tubular members are provided with a slip-inducing coating on some or all of their surfaces.

35. Heave relieving means according to Claim 34, wherein said coating is of plastics material.

36. Heave relieving means according to any one of Claims 27 to 35, wherein the or each spacer ring is of expanded polystyrene.

37. Heave relieving means according to any one of Claims 20 to 25, comprising a tubular body of foamed or cellular plastics material, the body having cyliindri cai inner and outer surfaces each defined by a "skin" or membrane-like portion of the foamed or cellular tubular body, said skins serving as the aforesaid first and second surfaces, the voids of the foamed or cellular structure between the skins serving to space the two surfaces apart.

38. Heave relieving means substantially as herein described with reference to and/or as illustrated in the accompanying drawings.