GB2356659A

GB2356659A - Pile formation

Info

Publication number: GB2356659A
Application number: GB9927241A
Authority: GB
Inventors: Peter Gilbert Shotton; Ron Payne
Original assignee: Cementation Foundations Skanska Ltd; Kvaerner Cementation Foundations Ltd
Current assignee: Cementation Skanska Ltd
Priority date: 1999-11-18
Filing date: 1999-11-18
Publication date: 2001-05-30
Anticipated expiration: 2019-11-18
Also published as: WO2001036753A1; AU1405901A; GB9927241D0; GB2356659B

Description

2356659 Pile Forming The present invention relates to forming piles using

continuous flight auger (CFA) methods, and more specifically to forming piles wherein the finished concrete is at some distance below the surrounding ground level.

A standard technique for forming a CFA pile is to screw the auger to the desired founding depth. The auger is fitted with a blade or flight which spirals io helically around a hollow shaft, for the full length of the auger. The boring head of the auger may be fitted with two cutting edges, and with a second flight (known as 'double start', 'double flight'). The hollow shaft is used to pump concrete to the base of the auger, immediately filling the pile section while the auger is being withdrawn (so that the bore is never empty). The term "concrete" is used herein for any cementitious mixture containing also aggregate, water, and admixtures; such mixtures include mortar and concrete in the narrow sense.

A positive pressure and an excess concrete flow rate are maintained as the auger is steadily withdrawn, to ensure that the pile section is completely filled with concrete conventionally up to ground level, i.e. the level at which the piling plant is placed during construction of the piles. If required, after the auger has been fully withdrawn, reinforcing elements can be inserted into the concrete before it sets.

There are certain situations where the design requires that the head of the piles terminate at a reduced level. These situations particularly include projects where the general level of the site has to be reduced at a later date, e.g. to form a basement. Currently, with CFA, this is conventionally achieved by casting the concrete to ground level and then breaking down the excess height of piles during the excavation of the substructure. This requirement impedes the excavation of the I substructure and the breaking-down is costly. Additionally there is a risk that the construction plant carrying out the excavation may damage some of the piles; it is not unknown for a pile to be damaged or cracked below the final reduced level.

The objective of the'Present invention is to provide an improved techtnique for constructing piles for such situations.

According to one aspect, the invention provides a method of forming a pile with a finished concrete casting level (the pile top level) which is below ground io level, comprising:

a) inserting a casing extending down to at least the intended pile top level b) drilling a bore with a continuous flight auger thiough the casing and continuing drilling until the desired pile bottom level is reached; c) withdrawing the auger and simultaneously injecting concrete until the pile top level is reached; and d) filling the bore above the pile top level with soil and withdrawing the casing.

The invention thus provides a technique which permits the construction of continuous flight auger piles with a finished concrete casting level which is below ground.

The casing may be inserted either before drilling by the continuous flight auger (e.g. by a vibrator or even a rotary rig) or simultaneously with the continuous flight auger, If no reinforcement is required, step (d) may be performed by reversing the rotation of the auger as it is withdrawn through the casing.

2 If reinforcement is required, then between steps (c) and (d), the bore within the casing may be emptied and reinforcement introduced into the concrete below the pile top level. To achieve this, the auger may include closure means at its lower end with those closure means being actuated to withdraw the soil carried by the auger as the auger is withdrawn through the casing. Alternatively, the concrete injection may continue during at least the first part of the withdrawal of the auger through the casing, with the concrete in the casing then being removed. Optionally, the removal of the concrete from within the casing may be performed by the auger.

io The casing is preferably inserted to slightly below the pile top level, to ensure that no soil enters the casing above the pile top level.

In the preferred forms of the invention, in order to construct a CFA pile with a cast concrete level which is below ground, a suitable length of casing is first inserted into the soil. The length of casing will be such that it extends to below the (future) head level of the concrete. The casing may be installed as a separate operation or concurrent with the flight auger boring the pile. The concrete is introduced in the conventional manner, i.e. under pressure, through the hollow stein and to the base of the CFA.

In one preferred form, where there is no requirement for any reinforcement to be placed in the pile, when the base of the auger reaches the required (low) concrete casting level during withdrawal, no further concrete is pumped through the auger. The auger is then rotated counter-clockwise and at the same time is withdrawn completely from the pile. This operation allows the soil which is on the flight to be discharged, thus avoiding any empty hole which would otherwise be left when the auger is completely withdrawn from the ground. The casing can be removed either together with the auger, or as a separate operation.

3 It is often desirable to install reinforcement in the pile.

In one preferred form, which allows reinforcement to be installed, the base of the auger is fitted with a mechanism which acts as a plug to prevent loss of soil from the auger when it is withdrawn through a casing or even the soil. One form of plug may be one or more plates, preferably steel, which can be rotated to form a closure at the base of the auger. During the concreting phase, when the base of the auger is at the required (low) concrete casting level, no further concrete is pumped io through the auger. The closure plug is activated, and the auger completely withdrawn. At this point in time the casing remains in position. The reinforcement is placed within the casing and pushed down to the required level, into the concrete. The casing may then be removed, either by means of a separate lifting appliance, or by the CFA rig. The potential void created by removal of the casing may be filled before or after the casing is withdrawn.

In another preferred form, which allows reinforcement to be installed, the pile forming technique and sequence is generally as described above, but tile placement of concrete through the hollow stem continues until the auger has been completely removed. At this point in time the casing remains in position. Note that during the process of withdrawal, all soil, etc. is removed from the auger flight as it is being withdrawn. The auger is then used, with or without any attachment at the base, to remove the concrete from within the casing, down to the low casting level required. (In a modification of this form, the concrete remaining in the casing is removed by a separate tool.) The reinforcement is placed within the casing and pushed down to the required level, into the concrete. The casing may then be removed, either by means of a separate lifting appliance, or by the CFA rig. The potential void created by removal of the casing may be filled before or after the casing is withdrawn.

4 The invention also provides an auger with closure means at its lower end. The closure means may comprise a plate rotatable through a restricted angle about the vertical axis of the auger, or one or more flaps hinged to the leading edge of the 5 flight of the auger. The flaps may be bottom-hinged or top-hinged.

Various methods of forming piles in accordance with the invention and pile forming machinery and augers therefore will now be described, by way of example, with reference to the drawings, in which:

Fig. I is a general view of the machinery; Fig. 2 describes the sequence of operations according to one aspect of the invention; Fig. 3 describes the sequence of operations according to another aspect of the invention; Figs. 4A and 4B show an auger with closure means at its lower end; and Figs. 5A and 5B show another auger with closure means at its lower end.

Fig. I shows pile forming machinery 10 consisting of a crawler unit 11 carrying a mast or column 12. The mast 12 has mounted on it an auger driving unit 13 which carries an auger 14. The driving unit 13 includes a motor for rotating the auger 14, and can also move itself up and down the mast. The auger passes through a guide collar 18 mounted on the bottom end of the rnast 12. The auger 14 consists of a hollow central shaft 15 around which a continuous blade or flight 16 is attached, and may be formed from a number of sections bolted together. The mast 12 carries concrete supply tubing 17 at its top end, coupled to the top end of the auger shaft by flexible tubing (not shown).

Conventionally, during construction of a pile, the unit 13 rotates the auger clockwise and is lowered as the auger is screwed into the ground. When the auger has reached the desired depth, the unit 13 pulls the auger up out of the hole (with or without rotation). While the auger is being withdrawn from the pile, a pump (not shown) pumps concrete down the hollow shaft 15 to fill the section vacated by the auger, thus forming the pile.

Fig. 2 shows one method of forming a pile with a finished concrete level below ground level. A casing 20 is sunk in the ground 21, either as a separate operation or by means of the CFA rig (Fig. 2A). The auger is then screwed into the ground to the desired founding level (Fig. 213). Concrete is then pumped through the hollow stem of the auger into the bottom of the bore 22 while the auger is gradually withdrawn (Fig. 2Q. When the concrete has risen to a short distance above the level of the bottom or toe of the casing, pumping is terminated. The closure mechanism at the bottom of the auger is actuated, and the auger is completely withdrawn (Fig. 2D). If required, a reinforcing cage 23 is lowered into the casing and then pushed down to the required level in the concrete (Fig. 2E). Backfill 24 is placed in the void of the casing 20 (Fig. 2F) and the casing is then removed either by the CFA rig or as a separate operation (Fig. 2G).

Fig. 3 shows a modified method of forming a pile with a finished concrete level below ground level. The method follows that of the Fig. 2 method except that the in. ection of concrete as the auger is withdrawn from the bore is continued until the auger is fully withdrawn, so that the casing 20 is filled with concrete (Fig. 3C), and the CFA auger or another suitable too] is then used to remove concrete from within the casing, down to the required level (Fig. 3D).

Figs. 4A and 4B show a method of achieving a seal at the base of the auger to retain the soil above the base of the auger. The auger has a baseplate 30 6 mounted on the vertical axis (i.e. the central bore 15) of the auger for rotation through a limited angle about that axis. When the auger is in the boring or drilling position, as shown in Fig. 4A, the clockwise rotation of the auger caries the baseplate to the position shown where the flight 16 of the auger is open so that soil is carried up on the flight as the soil is dislodged. When the auger is rotated in the opposite direction, the baseplate turns about the central bore 15 to close the entrance to the auger flight, as shown.

Figs. 5A and 5B show a second form of auger with a closure member. The i o closure member consists of one or more flaps 31 which are hinged on the top of the leading edge of the auger flight 16. The flaps may alternatively be hinged on the underside of the flight above that shown. During boring with clockwise rotation, the flat lies generally flat against the auger flight and causes essentially no obstruction to soil being gathered onto the auger flight. During anti-clockwise rotation, however, any movement of soil back down the auger flight will engage with the flap 31 and cause it to rise to the position shown in Fig. 4B, effectively closing off the auger flight. Similarly, for top-hinged flaps, any movement of soil will cause them to descend, effectively closing off the auger flight.

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Claims

Claims

1 A method of forming a pile with a finished concrete casting level (the pile top level) which is below ground level, comprising:

a) inserting a casing extending down at least to the intended pile top level is reached; b) drilling a bore with a continuous flight auger through the casing and 10 continuing drilling until the desired pile bottom level is reached; c) withdrawing the auger and simultaneously injecting concrete until the pile top level is reached; and d) filling the bore above the pile top level with soil and withdrawing the casing. 15 2 A method according to claim 1, wherein the casing is inserted to slightly below the pile top level.

3 A method according to claim I wherein step (d) is performed by reversing the rotation of the auger as it is withdrawn through the casing.

4 A method according to claim I wherein, between steps (c) and (d), the bore within the casing is emptied and reinforcement is introduced into the concrete below the pile top level.

A method according to claim 4 wherein the auger includes closure means at its lower end and those closure means are actuated to withdraw the soil carried by the auger as the auger is withdrawn through the casing.

8 6 A method according to claim 4 wherein concrete injection continues during withdrawal of the auger through the casing and the concrete in the casing is then removed.

7 A method according to claim 6 wherein the removal of the concrete from within the casing is performed by the auger.

8 A method of forming a pile with a finished concrete casting level (the pile top level) which is below ground level substantially as herein described with io reference to the drawings.

9 A pile with a finished concrete casting level (the pile top level) which is below ground level formed by the method of any previous claim.

10 An auger with closure means at its lower end.

I I An auger according to claim 10 wherein the closure means comprise a plate rotatable through a restricted angle about the vertical axis of the auger.

12 An auger according to claim 10 wherein the closure means comprise a flap hinged to the leading edge of the flight of the auger.

13 An auger substantially as herein described with reference to Figs. 4A and 4B or 5A and 5B.

14 Any novel and inventive feature or combination of features specifically disclosed herein within the meaning of Article 4H of the International Convention (Paris Convention).

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