GB2127882A

GB2127882A - In situ pile construction

Info

Publication number: GB2127882A
Application number: GB08227980A
Authority: GB
Inventors: Roger Alfred Bullivant
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-09-30
Filing date: 1982-09-30
Publication date: 1984-04-18

Abstract

A method of constructing a pile in situ comprises forming an elongated hole in the subsoil using a suitable self propelled or top driven hole forming device, and as the device is removed from the hole after it has reached a predetermined depth pumping cementitious material under pressure into the hole, the retracting device forming a plug in the hole to prevent the escape of cementitious material up the hole.

Description

SPECIFICATION In situ pile construction This invention relates to the construction of in situ piles.

My British Patent Application No. 7916657 (Serial No. 2 048 999 A), the disclosure of which is incorporated herein by reference, discloses a method of constructing a pile in situ comprising: forming a hole in the subsoil by allowing an elongate self-propelled soil displacing mole to descend through the subsoil; withdrawing the mole; filling the hole with concrete; allowing such mole to descend within the filled hole before the concrete sets to produce a pile footing and to displace concrete into the hole wall; withdrawing such mole; and refilling the hole with concrete and allowing the concrete to set to form an in situ pile with an integral footing. My British Patent Applications Nos. 8023589, 8002198 and 8101226 disclose other methods of forming in situ piles also involving the use of an elongate self-propelled soil displacing mole to form the hole.Those methods have been proved in practice to show a number of advantages. Problems may however arise in certain subsoils. For example loose, gravelly or wet subsoils may collapse into and fill the hole as the mole is extracted, before the concrete can be poured. Lining of a hole to form a cased pile as disclosed in No.

8023589 is expensive and the cased pile does not exhibit the desired degree of skin friction to a surrounding loose or watery soil.

Clay subsoils are also liable to inward collapse. Cased piles have the same defect as with loose or watery soils in that the desired degree of skin friction is not exhibited. It will be appreciated that needle piles are intended to give support to a building structure as much by skin friction along their length as by end loading.

British Patent No. 1,183,824 discloses the formation of piles by drilling a pile hole with a special Archimedian screw drill having a central tube. When the desired depth is attained the drill is retracted while concrete or the like is supplied through the central tube to fill the hole. A disadvantage of this method is that it is a soil removal method. This leads to problems of spoil removal and is also disadvantageous in loose or watery soils where it may be necessary to penetrate very deeply to find a solid substratum for pile end support.

According to the present invention there is provided a method of constructing a pile in situ comprising forming an elongated hole in the subsoil by allowing a hole forming device to descend through the subsoil, the device being provided with a conduit closed at the forward end during descent, opening said forward end and supplying concrete through said conduit while withdrawing the device from the hole and allowing the concrete to set to form an in situ pile.

In certain instances after an initial withdrawal sequence the device is caused to move again in a downwards direction to compact the concrete in the bottom of the previously formed pile and the surround ground.

The device is preferably an elongate self propelled soil displacing mole but may also be a top driven tube or rod, or a soil displacing member of the type which includes, mounted therein, vibratory means to cause the member to vibrate in directions substantially transverse to the direction of penetration such that the member vibrates through the subsoil under its own self-weight.

The concrete is preferably pumped under pressure through said conduit. The walls of a hole liable to collapse are thereby prevented from collapse because the length of hole beneath the mole may be maintained fully charged with concrete under pressure throughout withdrawal of the mole. Moreover it is found that not only do the walls not collapse, but the concrete flows and enters into any interstices, fissures and cavities at the foot of and in the walls of the hole. A pile of exceptional load-bearing capacity may thereby be produced because the consequent rough and irregular walls of the pile greatly enhance the skin friction with the surrounding compacted subsoil.

In loose or watery subsoils the concrete can flow and spread to form a pile of effective diameter substantially greater than the nominal hole diameter, and having an irregular outline efficiently locked into a locally displaced and compacted subsoil. In certain cases it may be possible to provide a desired load bearing capacity without the necessity of forming a pile hole deep down to a hard substratum. In a clay subsoil the concrete also penetrates any interstices, fissures and cavities in the subsoil. Moreover the mole is a tight fit in the hole, particularly when at its lowest point when the concrete pumping is commenced. The concrete may therefore be pumped at a significant pressure, the mole itself serving as a seal during its withdrawal.

The mole may be provided with an integral conduit extending therethrough from the forward to the rear end. Alternatively a standard mole may be employed, the conduit being secured to the outer surface thereof. In that case the conduit is a rigid, e.g. steel, tube of diameter suitably in the range 1 to 4 centimetres. The tear end of the conduit may be connected to a concrete pump at the earth's surface by any convenient tubing. The mole may be provided with more than one such conduit.

The forward end of the conduit may be closed during descent by means of a lost bolt or stud, for example a stud having a stem located within the forward end of the conduit and a head seated over the face of the forward end. Alternatively a simple flap valve may be employed. The purpose is simply to prevent the conduit from becoming clogged with subsoil during descent of the mole.

In those instances where the mole is redriven after an initial complete or partial withdrawal, means are provided for closing the open end of the conduit so that concrete does not re-enter it.

The term concrete is used herein in its broadest sense to include mortar, cement, grout, micro-concrete and other settable materials effective for forming piles. Steel reinforcement rods may be run down the hole after withdrawal of the mole and before the concrete has set.

The mole is also preferably provided with a swaging ring located near its rearward end. A suitable ring would be a simple steel collar secured to and surrounding the rear end of the mole. In suitable subsoils the swaging ring effectively localizes the sealing pressure or tightness of fit of the mole to the wall of the hole. Moreover, since the hole is thereby formed slightly larger in diameter than the mole, except for the lowest length, the mole is slightly easier to extract after it has been withdrawn from the lowest length where formation of a good dense pile footing is important. It will be appreciated that the presence of the concrete under pressure beneath the mole in the lowest length of the hole assists in breaking any vacuum in the initial stage of extracting the mole, particularly in a clay subsoil. The mole is usually withdrawn by reversing its action, as described in No.

7916657, and also hoisting it out with a draw wire attached to a simple tripod lifting rig.

The method described above is of considerable advantage in a system for forming a pile supported concrete slab similar to that described in our copending application 7925405. In such a system a slab is cast on the ground and either when casting the slab or soon thereafter holes are formed in the slab. The method described above can then be carried out, through said holes, to form piles for supporting the slab.

In a further modification rather than suppiying concrete under pressure to the forward end of the mole compressed air is supplied through the conduit. This not only releases the vacuum created at the forward end of the mole as it is being withdrawn but also prevents the ingress of ground water into the hole, such ingress often leading to collapse of the hole. The mole can be withdrawn to the top of the hole while compressed air is constantly supplied through the conduit. When the mole reaches the top of the hole, but before it is withdrawn so that it acts as a plug for the hole concrete can be poured into the hole either through the conduit by means of a suitable valve therein or through an additional conduit.

In certain soil conditions the mole can be completely removed provided concrete is poured into the hole soon after removal and before the hole begins to collapse.

In order instances it may be desirable to replace compressed air with a bentonite slurry. In such an instance after removal of the mole or when the mole is at the top of the hole a concrete pipe is inserted into the hole, terminating near its bottom, and concrete is supplied through the pipe, the bentonite being displaced out of the top of the hole.

It will be clear that the above method can be substituted for the method described in the last paragraph of page 2 and thereafter.

Claims

1. A method of constructing a pile in situ comprising forming an elongated hole in the subsoil by allowing a hole forming device to descend through the subsoil, the device being provided with a conduit closed at the forward end during descent, opening said forward end and supplying concrete through said conduit while withdrawing the device from the hole and allowing the concrete to set to form an in situ pile.

2. A method as claimed in claim 1, in which the hole forming device is an elongate self propelled soil displacing mole.

3. A method as claimed in claim 1, in which the hole forming device is a top driven tube or rod.

4. A method as claimed in claim 1, in which the hole forming device is a soil displacing member of the type which includes, mounted therein, vibratory means to cause the member to vibrate in directions substantially transverse to the direction of penetration such that the member vibrates through the subsoil under its own self-weight.

5. A method as claimed in any one of the preceding claims, in which the concrete is pumped under pressure through said conduit.

6. A method as claimed in any one of the preceding claims, in which the hole forming device is provided with an integral conduit extending therethrough from the forward to the rear end.

7. A method as claimed in any one of claims 1 to 5, in which the hole forming device has a conduit for concrete secured to the outer surface thereof.

8. A method as claimed in claim 6 or claim 7, in which the rear end of the conduit is connected to a concrete pump at the earth's surface by tubing.

9. A method as claimed in claim 6, 7 or 8, in which the mole is provided with more than one conduit.

1 0. A method as claimed in any one of claims 6 to 10, in which the forward end of the or each conduit is closed during descent by means of a lost bolt or stud.

11. A method as claimed in any one of claims 6 to 10, in which the forward end of the or each conduit is closed by a replaceable closure member operable from the surface.

1 2. A method as claimed in any one of claims 1, 2 and 4 to 12, in which the hole forming device is provided with a swaging ring located near its rearward end.

1 3. A method as claimed in any one of the preceding claims, in which after an initial partial withdrawal of the hole forming device it is again caused to move down the hole to compact the concrete therein.

1 4. A method as claimed in any one of the preceding claims, in which the pile is constructed through a hole preformed through a concrete slab cast on the ground and the pile is united to the slab.

1 5. A method of constructing a pile as claimed in claim 1, substantially as hereinbefore described.

1 6. A method of constructing a pile in situ comprising forming an elongated hole in the subsoil by allowing a hole forming device to descend through the subsoil, the device being provided with a conduit closed at the forward end during descent, opening said forward end and supplying a medium under pressure through said conduit while withdrawing the device from the hole or to the top thereof and replacing the medium with concrete which is allowed to set to form an in situ pile.

17. A method as claimed in claim 16, in which the pressure medium is compressed air.

1 8. A method as claimed in claim 16, in which the pressure medium is bentonite slurry.

19. A method as claimed in claim 16, substantially as hereinbefore described.