GB1603425A

GB1603425A - Piles

Info

Publication number: GB1603425A
Application number: GB1668977A
Authority: GB
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1978-05-30
Filing date: 1978-05-30
Publication date: 1981-11-25

Description

(54) IMPROVEMENTS IN OR RELATING TO PILES (71) We, BICC LIMITED, a British Com pany, of 21 Bloomsbury Street, London WC1B 3QN, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement :- This invention relates to piles for supporting buildings or other structures and is particularly concerned with a method of casting concrete piles in situ, piles piles by that method, and structures founded on such piles.

In one known method, a metal tube is first driven into the ground by hammering, vibratory driving, or in any other suitable manner. The tube may be closed at its lower end by a detachable shoe, in which case ground material (hereinafter called"soil") is displaced sideways and compacted during driving; or it may be open, in which case soil is excavated from the inside of the tube after driving. In either case concrete is then introduced into the tube (together with any reinforcing members that may be required) and the tube is then withdrawn to leave a concrete pile in contact with the soil.

In some cases the tube has been vibrated at a small amplitude during withdrawal to reduce the frictional drag of the soil on the tube and ! or to produce pressure suaves in the soil and/or the concrete in an attempt to produce better contact and possibly interpenetration between the concrete and the soil.

In accordance with the present invention, a method of casting a concrete pile i/! si. u comprises : driving a metal tube into the ground; filling the hollow interior of the tube to a substantial depth with a wet concrete mixture ; vibrating the tube longitudin- ally to produce in it reciprocations with an amplitude of at least 8 mm to liquify the wet concrete mixture or to maintain it in a liquidified state and so to establish or maintain a substantial hydrostatic pressure at the lower end of the tube; and withdrawing the tube whilst the concrete is in a liquified state.

Preferably the tube is filled to substantially its full depth with the wet concrete mixture so that the hydrostatic pressure is as high as possible; the surface of the concrete may be initially considerably higher than ground level if desired.

The tube may be vibrated during the filling process, but this is not essential and if it is done the amplitude does not necessarily exceed 8 mm. In some cases it may be essential to continue vibrating the tube with the amplitude specified during at least a substantial part of the withdrawal of the tube, or for the whole of the withdrawal, but this depends on the rate of withdrawal in relation to the rate at which the concrete would change from its liquified condition.

Amplitudes of 9-12-9 mm have been found satisfactory; it is believed that larger amplitudes could be used but no advantage is expected to arise from doing so. Vibration frequencies up to at least 25 Hz can be used, and frequencies in the range 20-25 Hz are preferred.

Any convenient means for vibrating the metal tube at suitable high frequencies and high amplitudes may be employed, e. g. a generator of variable frequency so that the generator can be tuned to the resonant frequency of the metal tube, but it is preferred to employ a hydraulic vibrator manufactured and sold by Precedes Techniques de Construction as type 20H4. This vibrator will subject the metal tube to vibration having a frequency of approximately 25 Hz and will produce in the tube in the direction of its length reciprocations having an amplitude of approximately 10 mm.

When a pile is formed by the method of the invention, the pressure exerted by the liquified concrete so formed against the soil in which it is embedded results in a thrust against the soil which can be as much as four times that of the existing soil and actually produces a prestressing of the soil which is not obtained by a concrete pile of similar size formed by the known method of forming a pile in-situ as hereinbefore described.

The invention will be further described, by way of example, with reference to the accompanying drawings in which figures 1-4 show successive steps of one method in accordance with the invention and figures 5-9 successive steps of another.

In the first method, a steel tube 1 (figure 1) is fitted with a detachable shoe 2 and a driving cap 3 and is driven to the required depth in the soil 4 by a hammer 5 using piling leaders 6 in the usual way. After removal of the cap 3, reinforcement 7 (figure 2) is positioned and wet concrete 8 pumped into the tube 1 to fill it substantially com- pletely. A high amplitude vibrator 9 (figure 3) is then secured to the top of the tube and operated to create longitudinal vibrations with an amplitude of about 10 mm in the tube.

When this has liquified the concrete the tube 1, still vibrating, is slowly withdrawn, and the concrete flows into intimate contact with the soil 4. Additional concrete can be poured as the tube is withdrawn if this is necessary to compensate for the increase in diameter of the concrete body. On setting of the concrete, a reinforced concrete pile 10 of excellent load-bearing capacity results.

The method of figutes 5-9 is identical but for the fact that the shoe 2 (figures 1-4) is omitted and the consequent body of soil I I inside the tube (figure 5) is removed in a separate step by means of an auger or other excavator 12 (figure 6). This method is preferred when it is desired to minimise interference with the existing structure of the soil.

Claims

WHAT WE CLAIM IS :- 1. A method of casting a concrete pile in situ comprising : driving a metal tube into the ground; filling the hollow interior of the tube to a substantial depth with a wet concrete mixture; vibrating the tube longitudinally to produce in it reciprocations with an amplitude of at least 8 mm to liquify the wet concrete mixture or to maintain it in a liquified state and so to establish or maintain a substantial hydrostatic pressure at the lower end of the tube; and withdrawing the tube whilst the concrete is in a liquified state.
2. A method of casting a concrete pile in situ comprising: driving a metal tube closed at its lower end by a detachable shoe into the ground; filling the hollow interior of the tube to a substantial depth with a wet concrete mixture; vibrating the tube longitudinally to produce in it reciprocations with an amplitude of at least 8 mm to liquify the wet concrete mixture or to maintain it in a liquified state and so to establish or maintain a substantial hydrostatic pressure at the lower end of the tube; and withdrawing the tube whilst the concrete is in a liquified state.
3. A method of casting a concrete pile in situ comprising: driving a metal tube open at its lower end into the ground; excavating soil from inside the tube; filling the hollow interior of the tube to a substantial depth with a wet concrete mixture; vibrating the tube longitudinally to produce in it reciprocations with an amplitude of at least 8 mm to liquify the wet concrete mixture or to maintain it in a liquified state and so to establish or maintain a substantial hydrostatic pressure at the lower end of the tube; and withdrawing the tube whilst the concrete is in a liquified state.
4. A method as claimed in any one of claims 1-3 in which the tube is filled to substantially its full depth with the wet concrete mixture.
5. A method as claimed in any one of claims 1 in which the tube is vibrated during filling.
6. A method as claimed in any one of claims 1-5 in which the tube is vibrated at the amplitude specified during at least a substantial part of the withdrawal of the tube.
7. A method as claimed in any one of claims 1-6 in which the amplitude of reciprocation is 9-12-9 mm.
8. A method as claimed in any one claims I-7 in which the frequency of vibration is up to 25 Hz.
9. A method as claimed in any one of claims 1-7 in which the frequency of vibration is 20-25 Hz.
10. A method of casting a concrete pile in situ substantially as described with reference to figures 1-4.
11. A method of casting a concrete pile in sitii substantially as described with reference to figures 5-9.
12. A concrete pile cast in situ by the method claimed in any one of claims 1-11.
13. A building or other structure founded on at least one pile cast in situ by the method claimed in any one of claims 1-1 I.