GB2085950A - Driven composite piles - Google Patents

Abstract

A driven composite pile has a tubular steel casing (1) and a pile extension (5), preferably a rolled steel section, which is seated in a gravel or dry concrete plug (2) at the lower end of the casing. The pile is especially suitable for use in ground where a layer of potentially unstable soft subsoil overlies harder material, the pile extension serving to key the pile in the harder material. Driving the pile includes the steps of driving the casing (1) until it meets the harder material (4), driving the pile extension (5) through the plug (2) to a required depth, and then forming a concrete core (6) in the casing. A portion of the extension (2) may be left above the plug (2) and a reinforcing structure (7) embedded in the core (6) to provide increased tensile strength. <IMAGE>

Description

SPECIFICATION Driven composite pile This invention relates to a driven pile for supporting buildings and other structures, and to a method of driving such a pile.

It is known to drive a cased pile by pitching a tubular steel casing vertically on the ground and driving the casing into the ground with a drop hammer acting on a dry gravel or dry concrete plug in the lower end portion of the tube. The initial impacts of the hammer compact the plug and force it against the inner surface of the tube to form a secure base upon which the hammer can be dropped to drive the casing to a required depth. The pile is completed by filling the casing with concrete. The casing is a permanent part of the pile.

This known pile is limited to use in relatively soft subsoils, due to its solid circular cross section. On a site where a soft subsoil is underlaid by a relatively hard subsoil it may be found that the soft subsoil is potentially unstable or that there is insufficient friction between the outer surface of the casing and the subsoil to provide a required resistance to uplift.

According to this invention an upright driven pile comprises an elongate outer casing, an internal plug in a lower end portion of the casing, and an elongate pile extension having a small cross-sectional area than the casing, the extension being seated in the plug and extending below the lower end of the casing. The plug is preferably formed of gravel or concrete.

According to another aspect of the invention, there is provided a method of driving a pile, comprising forming a plug in a lower end portion of a pile casing, driving the casing into the ground in an upright position to a predetermined depth, and driving a pile extension of relatively small cross section through the plug so that it extends beyond the lower end of the casing. The casing may then be filled with concrete to provide a base for a structure to be supported on the pile.

The casing is preferably a steel casing which is driven by an internal drop hammer acting on the plug in the base of the pile.

When the pile has been driven to a required depth, which may be for example where the toe of the pile meets a layer of hard subsoil, the hammer is removed and a steel H-section or other rolled section is pitched in the casing and then driven into the plug and on into the ground below the toe of the casing. Due to its small cross-sectional area, it is able to penetrate the hard subsoil, and may be driven down until the top of the section is flush with the top of the plug, the major part of the section extending below the casing to form a relatively stable footing for the pile as a whole.

Where the pile is required to withstand uplift as well as compressive forces, it is preferable to cease driving the pile extension before its upper end is flush with the top of the plug so that a substantial length of the extension projects above the plug inside the casing. The projecting portion then enables a tensile connection to be formed between the pile extension and a column of concrete cast in-situ inside the casing. To give this concrete core extra tensile strength, a steel bar reinforcing structure is pitched inside the casing, overlapping the projecting end of the extension and extending up to the top of the pile.

The pile extension is not necessarily a rolled steel H-section. It may instead comprise two channel sections laid back to back, or it may be a T-section, angle section, or any section fabricated from these. In a light pile, a steel rod or rods may be used for the extension.

The invention will now be described by way of example with reference to the drawing, which is a series of diagrams illustrating the procedure for driving a pile and showing two different embodiments.

Referring to the stages labelled (a) to (d) in the drawing, a pile in accordance with the invention may be driven as follows.

Firstly, one end portion of a casing, which may be 1 50 to 600mm in diameter, is filled with dry gravel, a light gauge metal cap being used to hold the gravel in the casing until pile driving begins. The casing is then positioned upright at the required location for the pile, and the base of the casing is driven into the ground in known manner using a drop hammer acting inside the casing on the gravel plug. The length of the plug is approximately three times the casing diameter. This stage of the procedure, in which the casing is driven into soft subsoil, is shown at (a) in the drawing. The casing is indicated by the reference numeral 1, the gravel plug by 2, and the drop hammer by 3. It is difficult to drive the casing beyond the point at which the toe of the pile meets a layer of hard subsoil 4 as shown at (b).To penetrate the hard subsoil 4, a rolled steel H-section 5 is pitched inside the casing 1 and driven through the compacted gravel plug 3 into the hard subsoil, as shown at (c), to form a pile extension. The H-section extension 5 is able to penetrate the plug 2 and the hard subsoil 4 since it has a relatively small cross-sectional area. This is combined with a relatively high outer surface are which provides good frictional uplift resistance. As shown at (d) the H-section extension 5 may be driven downwardly until its top end is flush with the upper surface of the plug 2 to achieve a required bearing capacity. A concrete core 6 is then formed in-situ inside the casing 1, with steel starter bars embedded in the top portion to provide a key for a concrete foundation above the pile.The completed pile is founded in the firm hard subsoil by means of the extension 5 and is therefore less susceptible to instability of the soft subsoil than for example the known cased pile referred to above.

In a variation on the procedure described above the H-section extension 5 may be pitched in the casing 1 before the toe of the casing reaches hard subsoil, the combination of the casing and the extension thus being driven down to the required set. The extension 5 penetrates the plug 2 as the casing begins to meet the resistance of the hard subsoil.

When the pile is to be subjected to uplift forces, the tensile strength of the pile can be improved by leaving a length of the pile extension 5 projecting above the plug 2 as shown at (e). This provides a key for a tensile connection when the concrete core 6 is formed inside the casing 1 and around the extension 5. In this case the core 6 is reinforced by a steel reinforcing structure 7 having a plurality of longitudinal bars and helical or ring bars. The reinforcement structure 7 overlaps the extension 5 and extends up to the top of the pile.

In addition to the advantages already referred to, a pile in accordance with the invention has the advantage that it can be driven into relatively hard stata without causing undue vibration, and thus minimising disturbance to existing structures such as an existing building which is being underpinned.

Claims (14)

1. An upright driven pile comprising an elongate outer casing, an internal plug in a lower end portion of the casing, and an elongate pile extension having a smaller cross sectional area than the casing, the extension being seated in the plug and extending below the lower end of the casing.

2. A pile according to claim 1, wherein the plug is formed of gravel or concrete, and surrounds a concrete core extending from the plug substantially to the top of the casing.

3. A pile according to claim 2, wherein the pile extension comprises a rolled steel section.

4. A pile according to claim 3 wherein the rolled steel section is of H-section.

5. A pile according to any of claims 2 to 4, wherein an upper portion of the pile extension extends above the plug inside the concrete core.

6. A pile according to claim 5, including reinforcing elements embedded in tne concrete core, which elements overlap the upper portion of the pile extension and extend upwardly relative thereto.

7. A method of driving a pile, comprising forming a plug in a lower end portion of a pile casing, driving the casing into the ground in an upright position to a predetermined depth, and driving a pile extension of relatively small cross section through the plug so that it extends beyond the lower end of the casing.

8. A method according to claim 7 in which the plug is formed of gravel or dry concrete and in which a concrete core is cast inside the casing after the pile extension has been driven to a required depth.

9. A method according to claim 8 in which the pile extension is driven to a required depth such that an upper portion of the pile extension extends above the plug inside the concrete core.

10. A method according to claim 9 in which, prior to forming the concrete core, a reinforcing structure is. positioned inside the casing with its lower end overlapping the upper portion of the pile extension.

11. A method according to any of claims 7 to 10 in which the pile extension is driven to a depth such that its top end is level with the top end of the plug, and in which the combination of the pile casing and the extension is then driven to a required depth.

1 2. A method according to any of claims 7 to 10 for providing a pile in ground having a layer of relatively soft subsoil overlying harder material, in which the casing is driven into the subsoil until its lower end is substantially at the level of the upper boundary of the harder material, and in which the pile extension is then driven through the plug into the harder material.

1 3. An upright driven pile constructed and arranged substantially as herein described and shown in the accompanying drawing.

14. A method of driving a pile substantially as herein described with reference to the accompanying drawing.