GB2064625A

GB2064625A - Method of Hole Forming for Miniature Piles

Info

Publication number: GB2064625A
Application number: GB8036987A
Authority: GB
Original assignee: Harrison & Co Soils & Foundati; PILING TECH Ltd
Current assignee: Harrison & Co Soils & Foundati; PILING TECH Ltd
Priority date: 1979-12-11
Filing date: 1980-11-18
Publication date: 1981-06-17

Abstract

A method of forming a hole of not more than 300 mm transverse dimension in readiness for production of a miniature pile filling the hole, which comprises driving a mandrel (10) into the ground by the action of a vibrating hammer (11), the mandrel being an integral structure of stepped sections (10A, 10B, 10C) of reducing diameter from the upper section (10A) to the lowermost section (10C). A cylindrical hole may be formed using a shoe attached to the lowermost section, the shoe having a transverse dimension greater than the maximum transverse dimension of the mandrel. <IMAGE>

Description

SPECIFICATION Improvements in Methods of Hole Forming for Miniature Piles This invention relates to a method of hole forming as the initial step in miniature piling, which as used herein means the formation of miniature piles of not more than 300 mm transverse dimension.

Miniature piles providing foundations for relatively light loads are conventionally cast in situ by driving a mandrel into the ground to form a hole and filling the formed hole with concrete reinforced by steel rods. Hitherto, usual practice has employed a cylindrical or more commonly a smoothly tapered mandrel, preferably terminating in a pointed leading end, to form the initial hole in which the pile is to be cast.

It is an object of the present invention to provide an improved method of hole forming as the initial step in miniature piling.

According to the invention, there is provided a method of forming a hole in the soil in readiness for formation of a miniature pile filling the hole, which comprises the step of driving into the ground by means of a vibrating hammer a mandrel which has a plurality of sections one above the other, said mandrel sections being of differing transverse dimensions providing a stepped reduction from any one section to the section immediately below it.

Thus, it has been found that, when driving a mandrel into the ground by vibration, a stepped mandrel provides a more stable hole, both in rigid and in soft soils, than a smoothly tapered mandrel. In addition, when the hole is filled with concrete, a resultant stepped miniature pile structure can be formed which is more homogeneous and thereby stronger than a smoothly tapered pile.

Sometimes, however, it is preferred to produce a straight walled miniature pile. In this instance, the lower end of the stepped mandrel may carry a leading member of transverse dimension exceeding the maximum dimension of said mandrel. In this case also, although the resulting hole and eventual pile are straight walled, the use of a stepped mandrel has been found advantageous for transmitting the vibrating action of the hammer to the leading member, which latter may be pointed to lead the way into the ground. Additionally, wet concrete mix can be poured into the hole behind the leading member, either directly or through a hollowly formed mandrel, as penetration proceeds. The steps on the mandrel are then materially advantageous in assisting compaction of the wet concrete mix as hole formation proceeds.This in turn assists in the avoidance of voids when having reached a desired depth of penetration the vibrating mandrel is subsequently withdrawn, leaving the leading member in the ground, since the mix is more readily enabled to flow into the space vacated by the mandrel as withdrawal proceeds. In this method, infilling with further concrete mix is continued during said withdrawal of the vibrating mandrel.

Practical methods in accordance with the invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows a simple method of hole forming using a stepped mandrel; and Figure 2 shows a practical method of miniature pile formation using the stepped mandrel.

In Figure 1, a steel mandrel 10 is shown being driven into the ground by the action of a vibrating hammer (hydraulic vibrator) 11.

The mandrel 10 has a plurality of integrally formed sections 1 0A, 1 OB, 1 OC of successively reduced transverse dimension from the upper section 1 OA to the lowermost section 1 0C. The mandrel 10 may, of course, have more than three sections depending on the depth of hole required, usually of the order of six to ten metres. In a typical case, the three sections 1 0A, 1 OB, 1 0C illustrated may have respective transverse dimensions of 200 mm, 150 mm and 100 mm.

The term "transverse dimension" is employed because the mandrel 10 may have a cylindrical or a polygonal transverse cross-section. In addition, the mandrel 10 may either be solid, or hollowly formed for a purpose later mentioned in connection with Figure 2. Preferably, the lowermost section 1 OC of the mandrel 10 carries a pointed end 1 OD which leads the way into the soil.

The simple method of hole forming shown in Figure 1 results in a hole having stable walls, and is a suitable technique for both rigid and soft soils.

More especially, as compared with conventional practice employing a smoothly tapered mandrel, this improved result is due to the fact that the stepped mandrel results in improved dynamic vibrating action and efficiency in creation of the hole. This improved dynamic action is in part retained even when a cylindrical hole is to be formed, as will now be described with respect to the method of miniature pile formation illustrated in Figure 2. In connection with this method now to be described, attention is also drawn to our copending patent application No. 7942673.

In Figure 2, the reference 10 again denotes a rigid mandrel having sections of reducing diameter from its upper end (e.g. 200 mm transverse dimension) to its lower end (e.g. 100 mm transverse dimension). The mandrel 10, which may be of say six to ten metres length depending on the depth of pile structure required, is being driven into the ground by a vibrating hammer in the form of an hydraulic vibrator 11.

However, in the method of Figure 2, the lower end of the mandrel 10 is preceded by a steel plate or as shown by a pyramidal or conical shaped member 12 of steel or concrete which has a maximum transverse dimension not exceeding 300 mm but greater than the maximum transverse dimension of the mandrel, e.g. 200 mm or 250 mm, which is appreciably greater than the transverse dimension (100 mm) of the lower section of the mandrel. The point 13 of the leading member 12 is directed downwardly to lead the way into the soil, while the upper face of this member 12 may carry a flange or flanges 14 to maintain its lateral position with respect to the mandrel 10.

When the mandrel 10 has made an initial penetration into the soil, say of 200 mm depth, a wet concrete mix is poured into the annular space 15 around the mandrel 10 behind the leading member 12. This infilling with wet concrete mix is continued as the mandrel 10 is driven deeper into the ground by the vibrating action of the vibrator 11, until the required depth of pile structure has been attained. As penetration by the mandrel 10 proceeds, compaction of this wet concrete mix is assisted by the steps on the mandrel between its sections of reducing diameter.

The mandrel 10 is then relatively slowly withdrawn out of the soil, while the action of the vibrator 11 is continued. The steel leading member 12 is left behind at the bottom of the hole in the soil. As the slow withdrawal of the mandrel 10 proceeds, the space vacated by the mandrel is continuously filled in with wet concrete mix. In stating this, it is to be appreciated that the space being filled at this time is not simply a central hole in the previously poured in concrete, because this concrete remains wet and, assisted by the compaction which has previously taken place, to a certain extent tends to flow inwardly and downwardly to fill in the central hole left by the retracting mandrel, while leaving a sufficient outside lining against the soil to reduce risk that the hole will collapse.When withdrawal of the mandrel 10 is complete, a small depression is usually left in the top of the pile structure to be filled in with concrete. The concrete pile structure may then be finished by driving reinforcing rods downwardly into the concrete before it sets.

The effect of the method is to maintain a constantly repeated tamping action on the concrete as it is continuously poured in during penetration and withdrawal of the mandrel 10, due to the vibrating action of the vibrator 11 acting on the upper end of said mandrel throughout the process. This tamping action is materally assisted by the use of a stepped mandrel, as compared to the use of a straight or smoothly tapered mandrel.The result is a dense homogeneous pile structure of high strength, which can be formed in situ in a soft or water bearing soil without the use of a steel lining, and which in the case of more rigid soil does not require a double insertion of the mandrel to form a dry concrete lining and does not exhibit the inherent structural weakness of a pile having an outer concrete lining formed separately from and at a different time to the central concrete core.

It will be appreciated that various modifications of the above-described practical methods are possible within the scope of the invention. For example, the use of a pointed end on the lowermost section of the mandrel is not essential whilst, in the method of Figure 2, instead of pouring the concrete mix directly into the hole, the stepped mandrel may be hollowly formed so that the mix can be fed down through the mandrel, during its vibration, in order to infill the hole. It should also be made clear that Figure 2 not only serves to illustrate a complete method of pile formation, but also a modified method of forming a hole in readiness for production of a straight walled miniature pile, which method is modified from that described with reference to Figure 1 by the provision of the leading member on the lowermost section of the stepped mandrel.

Having formed the straight walled hole by this modified method, the formation of the pile can, if desired, be carried out as a later, separate step.

Claims

1. A method of forming holes in the soil in readiness for formation of a miniature pile filling the hole, which comprises the step of driving into the ground by means of a vibrating hammer a mandrel which has a plurality of sections one above the other, said mandrel sections being of differing transverse dimensions providing a stepped reduction from any one section to the section immediately below it.

2. A method according to claim 1, wherein the mandrel has a polygonal transverse cross-section.

3. A method according to claim 1 or claim 2, wherein the stepped sections of the mandrel form an integral structure.

4. A method according to claim 1 or claim 2 or claim 3, wherein the lowermost section of the mandrel has a pointed leading end,

5. A method according to any of claims 1 to 4, for the formation of a cylindrical hole, wherein the mandrel has a leading member, of transverse dimension exceeding the maximum transverse dimension of said mandrel, at the leading end of the lowermost section.

6. A method of forming a miniature pile which comprises forming a hole by the method of any of claims 1 to 5, and infilling the completed hole with concrete.

7. A method of forming a miniature pile which comprises forming a hole by the method of claim 5, infilling the hole with wet concrete mix as penetration by the vibrating mandrel proceeds, and withdrawing the vibrating mandrel after the required depth of penetration has been reached, leaving the leading member in the ground beneath the concrete.

8. A method according to claim 7, wherein infilling with wet concrete mix is continued as the vibrating mandrel is withdrawn.

9. A miniature pile formed by the method of any of claims 6 to 8.

10. A method of forming a hole substantially as hereinbefore described with reference to the accompanying drawings.