GB2064627A

GB2064627A - Method of Piling

Info

Publication number: GB2064627A
Application number: GB7942673A
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: 1979-12-11
Publication date: 1981-06-17
Also published as: GB2064627B

Abstract

A method of forming a miniature pile of not more than 300 mm transverse dimension, wherein a mandrel (10) preceded at its lower end by a leading member (12) is driven into the ground by a vibrating hammer (11) while wet concrete (15) is poured into the annular space around the mandrel behind the leading member as penetration proceeds, and the mandrel is subsequently withdrawn, leaving the leading member in the ground beneath the concrete while infilling is completed during said withdrawal of the mandrel. <IMAGE>

Description

SPECIFICATION Improvements in Methods of Piling This invention relates to a method of piling, and in particular to a piling method applicable to 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. Thus, in one known method, dry concrete is placed in the initial hole formed by the mandrel, and said mandrel, driven by an hydraulic vibrator, is used to compact the dry concrete to form a lining to the hole. The lined hole is then filled with concrete around suitable reinforcement rods to complete the pile structure.

It is a disadvantage of this method that, prior to the formation of the lining, the initial hole must stand open. If the soil is soft or water bearing, a steel casing must be initially driven into the ground, thereby to provide a steel lining in substitution for the dry concrete lining used in more rigid soils.

It is an object of the present invention to provide an improved method of miniature piling which is applicable both to rigid and soft soils.

According to the invention, there is provided a method of forming a miniature pile (of not more than 300 mm transverse dimension) in situ in the ground, which comprises driving into the ground a mandrel preceded at its bottom end by a leading member which has a transverse dimension not exceeding 300 mm but greater than the transverse dimension of the mandrel; pouring concrete into the annular space around the mandrel behind the leading member as said mandrel is being driven into the ground: withdrawing the mandrel when the required pile depth has been attained, leaving the leading member in the ground beneath the concrete; and completing the pile by filling in the space created during withdrawal of the mandrel.

Preferably, the mandrel is driven into the ground by a vibrating hammer (hydraulic vibrator) and the mandrel is also vibrated during its withdrawal, further concrete being poured into the space created by withdrawal of the mandrel as the withdrawal of said mandrel is taking place.

Less desirably, the space vacated by the mandrel may be filled in after the mandrel has been fully retracted.

Conveniently, the preferred method employs a tapered mandrel having a larger transverse dimension at its upper end than at its lower end preceded by the leading member. The mandrel may be uniformly tapered, or have successive sections of reducing transverse diameter.

Independently of the cross-sectional shape of the mandrel, which may be circular or polygonal, the leading member may have a circular or polygonal shape, and may take the form either of a plate or more preferably of a cone or pyramid tapering to a point which leads the way into the soil.

A practical method in accordance with the invention is illustrated in the accompanying drawing.

In the drawing, the reference 10 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.

The lower end of the mandrel 10 is preceded by a pyramidal or conical shaped steel member 12, which has a maximum transverse dimension not exceeding 300 mm but greater than the maximum transverse dimension of the mandrel, e.g. 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 1 5 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.

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 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 ensure that the hole will not 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. 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 method are possible within the scope of the invention. For example, the use of a mandrel having a taper (either uniform or stepped) is not essential; it is necessary only that the mandrel should have a transverse dimension sufficiently less than that of the leading member (itself not more than 300 mm) to enable concrete infilling to be carried out while the mandrel is being driven into the soil. Furthermore, it may in some cases be possible to drive in the mandrel by means other than a vibrating hammer, although the latter is generally to be preferred.

Claims

1. A method of forming a miniature pile, of not more than 300 mm transverse dimension, in situ in the ground, which comprises driving into the ground a mandrel preceded at its bottom end by a leading member which has a transverse dimension not exceeding 300 mm but greater than the transverse dimension of the mandrel; pouring concrete into the annular space around the mandrel behind the leading member as said mandrel is being driven into the ground; withdrawing the mandrel when the required pile depth has been attained, leaving the leading member in the ground beneath the concrete; and completing the pile by filling in the space created during withdrawal of the mandrel.

2. A method according to claim 1, wherein the mandrel is driven into the ground by a vibrating hammer.

3. A method according to claim 2, wherein the mandrel is also vibrated during its withdrawal, further concrete being poured into the space created by withdrawal of the mandrel as the withdrawal of said mandrel is taking place.

4. A method according to any of claims 1 to 3, employing a tapered mandrel having a larger transverse dimension at its upper end than at its lower end preceded by the leading member.

5. A method according to claim 4, wherein the mandrel tapers from a maximum dimension at its upper end to a minimum dimension at its lower end, said maximum mandrel dimension being less than the transverse dimension of the leading member.

6. A method according to any of claims 1 to 5, wherein the leading member takes the form of a steel plate or a cone or pyramid of steel or concrete tapering to a point which leads the way into the soil.

7. A method according to claim 3 or any of claims 4 to 6 when appendant to claim 3, wherein, following an initial penetration of the ground by the mandrel, wet concrete mix is infilled into the annular space behind the leading member as the mandrel is driven further into the ground to its maximum depth of penetration, and the mandrel is then withdrawn at a lesser speed than the speed of penetration while infilling of the hole is continued to replace wet concrete mix which flows into the space vacated by the mandrel.

8. A method of forming a miniature pile substantially as hereinbefore described with reference o the accompanying drawing.