GB2283266A - Heave relief apparatus for piling - Google Patents

Abstract

Pilebore-insertable heave relief apparatus for the mouth-adjacent region of a pile bore, the apparatus constituting in situ casting apparatus and comprising a self-supporting hollow structure 12 of relatively rigid material having its perimeter encompassed by a generally tubular element 14 of relatively flexible material to provide in use for relative slip between the said structure and said element. The self-supporting hollow structure of relatively rigid material comprises a sheet rolled to extend over an arc of at least 180 DEG preferably at least 365 DEG . In a piling method the in situ casting/heave relief apparatus is located in a mouth-adjacent region of a preformed pile bore in ground material, and a suitable settable composition (e.g. concrete or the like) is poured into the bore via the rolled inner sheet of said self-supporting hollow structure 12 such that the main body of the pile is cast by the bore and a mouth-adjacent region of the pile is cast by the said rolled inner sheet, the composition pressure tending to unroll, at least partially, the said inner sheet into substantially intimate slipping contact of its encompassing flexible element 14 and urge the latter towards intimate contact of the bore surface around it. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO PILING DESCRIPTION A problem of increasing concern associated with piling is so-called "heave", this being the lift effect upon a pile by the ground material (e.g. soil, clay, or the like) 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 substantially 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 100kN/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, and also in increasing the depth of the pile to provide sufficient anchorage for the steel wires. The precautionary use of excess steel in, and increased depth of, the piles dramatically increases the cost of piling.

One known idea, which might conceivably be considered an appropriate solution to heave-associated problems is that proposed - for a wholly different purpose - in US Patent No. 4721418. This suggests wrapping a sheet-like formation as a jacket around an already-constructed steel pile so as to shield the pile from direct contact with backfill earth and maintain a slip plane between the pile and the pile jacket thereby permitting the pile jacket to move downwardly when the backfill material settles. However, this proposal is not concerned with pile heave relief, where the earth is liable to move the pile upwards, and is only applicable to a finished or completed pile (e.g. a precast concrete pile 'for a prefabricated steel pile), to the perimeter of which access can still be gained.

Wholly different considerations apply where - as is more often the case - access cannot be gained to the perimeter of the pile and different kinds of heave relief measures have to be devised that, instead, require installation of the heave relief apparatus into the borehole prior to pouring the pile concrete into the pile bore. Examples of such different, pre-pile-formation, heave relief measures are suggested in UK Patent Publications Nos. 2178460, 2195688, 2241525 and 2261009. This last-mentioned publication discloses heave relief apparatus for the mouthadjacent region of a pile bore, the apparatus including a generally tubular device and another tube - in encompassing relation to the first-mentioned tube - having the form of a sleeve of flexible sheet material that, prior to apparatus use, serves as a wrapping for the first-mentioned tube.As disclosed, the generally tubular device comprises an extruded plastics material tube composed of two substantially co-axial skins integral with a plurality of longitudinally extending membranes that interconnect the two skins and define a plurality of longitudinally extending cavities between them.

In order to provide this particular heave relief apparatus in a range of sizes to suit different bore diameters, it is necessary for the extruded tube to be produced in a corresponding range of different sizes, and this demands a considerable manufacturing set-up expense in providing the molds and tooling required for each nominal tube diameter in the range.

One solution for larger diameter piles (e.g. greater than 240mm diameter) is to use rigid cardboard sleeves. However these have a wall thickness of about 6 to 12mm, are very stiff (so that they cannot readily adapt to any taper or out-of-cylindricality of the borehole) and are heavy whereby they are unwieldy and difficult to manoeuvre into position.

Furthermore these cardboard tubes rarely fit neatly into a pile bore and, if they are too loose, they can too easily fall down the pile bore instead of remaining in a mouthadjacent position. The usual environment provides a bore that tapers somewhat inwardly as it extends downwardly so that if one avoids too tight a fit of the lower end of the rigid tube within the borehole (to ensure easy and nondamaging insertion), the upper end of the rigid tube will be widely spaced from the encompassing perimeter of the borehole and the resultant pile will lack lateral stability at its upper end. This lack of lateral stability considerably affects the design of the pile and significantly increases the reinforcement necessary particularly where the pile is to be subjected to bending forces.The lack of lateral pile stability, i.e. the gap between the rigid tube and the wall of the pilebore, also increases the risk of impact damage upon the pile's upper end.

It is clearly desirable to provide a piling method and heave relief apparatus that can be used as a casting device for casting the mouth-adjacent region of a pile, and which can avoid the need for separate extrusion molds for different size tube diameters. Desirably the apparatus may also be adaptable to borehole dimensions that vary from the nominal (i.e. borehole formation tolerances), be light in weight and easy to manoeuvre and use.

According to a first aspect of the present invention there is provided pilebore-insertable heave relief apparatus for the mouth-adjacent region of a pile bore, the apparatus comprising a self-supporting hollow structure of relatively rigid material having its perimeter encompassed by a generally tubular element of relatively flexible material to provide in use for relative slip between the said structure and said element, characterised in that said self-supporting hollow structure of relatively rigid material comprises a sheet rolled to extend over an arc of at least 1800, preferably of the order of 360 (and advantageously of spiral form extending marginally greater, e.g. by at least 5 , than 36O).

Advantageously the pilebore-insertable heave relief apparatus constitutes an in situ casting apparatus for the mouth-adjacent region of the pile.

Preferably the generally tubular element is of flexible plastics material. One suitable example is polythene, e.g.

0.1 to 0.5mm thick. Preferably the self-supporting hollow structure comprises a sheet of relatively rigid plastics material. One suitable example is polypropylene, e.g. of twin-walled construction. The sheet of rigid plastics material may be lmm to 5mm thick.

In one preferred embodiment the rolled self-supporting sheet has an inherent resiliency tending to unroll it and return it towards its initial flat form, and is restrained against such return by the encompassing flexible element.

According to another aspect of this invention there is provided a piling method comprising the steps of: forming a pile bore in ground material; locating within a mouth-adjacent region of the pile bore heave relief apparatus according to said first aspect aspect of the invention, and pouring a suitable settable composition (e.g. concrete or the like) into the bore via the rolled inner sheet of said self-supporting hollow structure such that the main body of the pile is cast by the bore and a mouth-adjacent region of the pile is cast by the said rolled inner sheet, the composition pressure tending to unroll (partially) the said inner sheet into substantially intimate slipping contact of its encompassing flexible element and urge the latter towards intimate contact of the bore surface around it.

By way of example, one embodiment of the present invention will now be described with reference to the accompanying drawings of which Figure 1 is a schematic perspective view of a casting device and heave relief apparatus according to the present invention, Figure 2 is a general longitudinal cross-sectioned view of the casting device and heave relief apparatus of Fig 1 when in situ in a pile borehole in ground material, Figure 3A is a transverse section along the line III-III of Fig 2 prior to forming of a pile, and Figure 3B is a transverse section along the line III-III of Fig 2 after forming of a pile.

The illustrated heave relief apparatus/casting device 10 comprises an inner, self-supporting, polypropylene sheet 12 rolled up from an initially flat, rectangular, off-cut into marginally more than a single turn, the generally tubular construction thus formed being encompassed by an outer tubular element or sleeve 14 of heavy gauge polyethylene.

For use in a pile bore 15 of nominally 350mm diameter, the sheet 12 of device 10 is about lmm thick and has an arcuate length of approximately 1200mm so as to enable sheet 12 to extend for about 100mm more than the single 360 turn of nominal 350mm diameter (i.e. an added arcuate extent of about 30").

The encompassing tubular element or sleeve 14 may be of any suitable wall thickness (e.g. 0.1 to 0.5mum) sufficient to provide sleeve 14 with a strength capable of restraining the inner sheet 12 (a) against bursting through, and (b) against unrolling (e.g. to less than 1800 arcuate extent) as - due to its inherent resiliency - it tends to return towards its initial flat form.

The relatively thin-walled flexible material of sleeve 14 may be resiliently expansible outwardly - in a generally radial direction - by the unrolling tendency of the selfsupporting sheet 12, the expanding and restraining forces in respectively the inner sheet 12 and the outer sleeve 14 generally counter-balancing one another.

The unrolling tendency of sheet 12 is such that the encompassing flexible sleeve 14 thereby adopts a taut state that is.substantially rigid - or functionally so - which permits the composite sheet-plus-element structure 10 to be manhandled and generally manouevred on site and inserted (as a generally rigid, self-supporting entity) into the mouth of pile bore 15 before the pile concrete is poured therein.

In use, pile bore 15 is drilled in the ground material 20 and any necessary reinforcing rods or cage 16 then inserted and located centrally of pile bore 15 (see Figs 2 and 3A).

Thereafter, the apparatus/device 10 is inserted into the mouth of pile bore 15 to encompass the rods or cage 16 (if provided). Optionally the apparatus/device 10 is initially centred in position by wedges 18, although these may be subsequently removed, e.g. as soon as the concrete or other settable composition that is poured into the pile bore 15 (through the apparatus/device 10) reaches the bottom of the apparatus/device 10 and can then support it. The apparatus/device 10 becomes properly located in position in the pilebore by the pressure of the wet concrete or other settable composition 22 poured through the apparatus/device 10 into the borehole 15 so as to be cast (mainly) by that borehole and, adjacent the borehole mouth, by the inner sheet 12 of the device 10.The inner sheet 12 tends to unroll due to the pressure of the wet concrete within it (see Fig 3B), and consequently urges the flexible tubular element A4 into a generally-fitting engagement of the pilebore 15 as the element 14 deforms at many discrete locations into the interstices of the ground material 20 forming the surface of the pilebore 15.

Nevertheless, once the concrete has set and cured, the two components (i.e. sheet 12 and tubular element 14) of the device 10 permit a predetermined slip effect of one layer acting against the other to minimise frictional heave forces being applied from the bore surface 15 to the pile (these forces being mainly manifested in the upper, mouthadjacent region of the bore), and this is achieved with very low cost materials.

Furthermore, since the apparatus/device 10 is inserted in the bore hole 15 prior to pouring in the concrete (or other settable composition), the 'burst' pressures are of little or no concern. If the concrete within rolled inner sheet 12 causes the sheet 12 to unroll somewhat and such that it expands the outer tubular element 14, the latter will merely expand outwardly to the limits of the borehole 15 perhaps a maximum distance of (say) 10 to 20mm. This in itself is advantageous as the upper region of the finished pile will then be less likely to crack due to impact damage and/or lack of side restraint (as has sometimes occurred with single-layer rigid pile sleeves of the prior art).

Moreover, the ability of the sheet 12 to stretch - under the high pressure of the concrete and/or of the heave forces encountered - allows the sheet 12 to conform closely to the ground material forming the bore surface 15, and mold itself into the interstices of such ground material.

This stretching also aids heave force accomodation by the apparatus or device 10.

In effect the flexible outer sleeve 14 - which, on its own, is of a collapsible and unstable nature - combines with the rolled inner sheet 12 - which, on its own, is likewise of an unstable nature - to form a generally rigid structure 10 having its effective stability provided by the tendency of the inner sheet to unroll and return to its initial flat form, and by the generally counter-balancing resistance to this unrolling that is provided by the generally tubular element 14. This thus allows the production of a heave relief and in situ casting sleeve for a pile of large nominal diameter and yet with the sleeve having a very thin wall thickness to minimise the amount of potential movement of the sleeve in the pilebore and thus render the pile as tight to the bore as possible - whilst still permitting heave-induced slip between the sleeve and the pile.

Other sizes, dimensions and materials may be utilised in providing modifications to the above-described embodiments of the invention, and such modifications will be readily apparent to those skilled in this art. Furthermore the invention is not to be deemed limited to the particular embodiments hereinbefore described which may be varied in construction and detail (within the scope of the patent monopoly hereby sought). For example, it is considered that the heave relief device may comprise more than two mutually slidable, generally tubular elements to provide enhanced heave relief by slippage across more than one interface. In another example the inner sheet 14 may be of twin-walled polypropylene, e.g. up to 5mm thick. Other examples will be apparent to those skilled in this art.

Claims (14)

1. Pilebore-insertable heave relief apparatus for the mouth-adjacent region of a pile bore, the apparatus comprising a self-supporting hollow structure of relatively rigid material having its perimeter encompassed by a generally tubular element of relatively flexible material to provide in use for relative slip between the said structure and said element, characterised in that said self-supporting hollow structure of relatively rigid material comprises a sheet rolled to extend over an arc of at least 1800.

2. Apparatus according to Claim 1 and constituting in situ casting apparatus for the mouth-adjacent region of the pile.

3. Apparatus according to Claim 1 or Claim 2, wherein said sheet is rolled to extend over an arc of the order of 3600.

4. Apparatus according to any preceding Claim, wherein said sheet is of spiral form extending marginally greater' than 360".

5. Apparatus according to any preceding Claim, wherein said sheet is of spiral form extending at least 5" greater than 360".

6. Apparatus according to any preceding Claim, wherein the generally tubular element is of flexible plastics material

7. Apparatus according to Claim 6 wherein said flexible plastics material is polythene.

8. Apparatus according to Claim 6 or Claim 7, wherein said .flexible plastics material is in the range 0.1 to 0,5mm thick.

9. Apparatus according to any preceding Claim, wherein the self-supporting hollow structure comprises a sheet of relatively rigid plastics material.

10. Apparatus according to Claim 9, wherein said relatively rigid plastics material is polypropylene.

11. Apparatus according to Claim 9 or Claim 10, wherein said relatively rigid plastics material is of the order of lmm thick.

12. Apparatus according to Claim 9 or Claim 10, wherein the self-supporting hollow structure comprises a twinwalled sheet of relatively rigid plastics material between lmm and 5mm thick.

13. Apparatus according to any preceding Claim, wherein the rolled self-supporting sheet has an inherent resiliency tending to unroll it and return it towards its initial flat form, and is restrained against such return by the encompassing flexible element.

14. A piling method comprising the steps of: (a) forming a pile bore in ground material; (b) locating within a mouth-adjacent region of the pile bore apparatus according to any preceding Claim; and (c) pouring a suitable settable composition (e.g. concrete or the like) into the bore via the rolled inner sheet of said self-supporting hollow structure such that the main body of the pile is cast by the bore and a mouth adjacent region of the pile is cast by the said rolled inner sheet, the composition pressure tending to unroll, at least partially, the said inner sheet into substantially intimate slipping contact of its encompassing flexible element and urge the latter towards intimate contact of the bore surface around it.