GB2210919A

GB2210919A - Foundation piles

Info

Publication number: GB2210919A
Application number: GB8723766A
Authority: GB
Inventors: Roy Melvyn Rawden
Original assignee: CENTRAL PLANT INVESTMENTS LIMI
Current assignee: CENTRAL PLANT INVESTMENTS LIMI
Priority date: 1987-10-09
Filing date: 1987-10-09
Publication date: 1989-06-21
Also published as: GB8723766D0

Abstract

A continuous flight auger-injection piling method employs a hollow-stem auger 10 provided with a pressure-sensing arrangement 12 at its lower end. An electrical pressure signal is supplied by the sensing arrangement 12 to electrical slip rings 28 contacted by bushes 29 at the rotary driving head 13 of the piling machine. From there, the electrical signal is supplied to a control arrangement whereby at least one of the auger extraction rate or the concrete pumping rate may be controlled dependent upon the sensed pressure in the injected concrete at the lower end region of the auger. <IMAGE>

Description

FOUNDATION PILES This invention relates to methods of forming foundation piles, and also to apparatus for performing the methods of this invention.

When constructing a building on ground which may not support normal foundations, it is a common practice to form foundation piles in the ground, and then construct the building on those piles. For many years, a pile-driving technique has been employed., in effect hammering a pre-formed hollow steel pipe into the ground to a suitable depth, whereafter the pipe is filled with concrete or other suitable settable material to form the pile. After a number of such piles have been formed, the construction work may commence, picking up on the exposed tops of the piles.

More recently, there has been developed a method for forming foundation piles known as a "Continuous flight auger-injected" piling. In this method, a longflighted auger having a hollow stem is rotated into the ground, down to the required pile depth. Then, as the auger is extracted from the ground, concrete (or a similar constructional settable material such as a grout) is pumped into the upper end of the hollow stem of the auger, so as to leave through the open lower end of the auger stem and fill the void left by the rising auger. When the lower end of the auger has been raised substantially to ground level, the concrete pumping is halted, leaving a plug of concrete in the bore in the ground formed by the auger. If required, reinforcements such as steel bars may then be inserted into the concrete plug.

It will be appreciated that, as compared to conventional pile driving, the continuous flight augerinjected piling method is relatively fast and hence economical to perform; it can moreover be used in a wide variety of soil conditions. There is the further advantage that environmental disturbance - and particularly noise and vibration - are kept to a minimum, especially as compared to conventional piledriving techniques. Despite these advantages, experience has shown that foundation piles produced by the continuous flight auger-injected piling method often are defective, because it is difficult properly to match the rate of extraction of the auger to the rate of pumping of the concrete which is to fill the void left as the auger is withdrawn.

A long flighted hollow stem auger of the kind used with the continuous flight auger-injected piling method normally will be bored into the ground by means of a socalled continuous flight auger machine, which includes a winch to extract the auger, following boring to the required pile depth. The rate of extraction of the auger is controlled by the operator of the continuous flight auger machine, who also indirectly has to control the pumping of the concrete into the bore formed by the auger, during the extraction thereof. When the piling method was first used, the operator had to attempt to control the auger extraction using only his skill and experience - which often was very limited, so leading to a defective pile.As a result, there have been various proposals for assisting the driver to control the pumping of the concrete and/or the rate of extraction of the auger, in an attempt reliably to produce betterformed piles. These proposals have included supplying the operator with information indicative of the instantaneous depth of penetration of the auger, the number of rotations performed by the auger, the speed of rotation of the auger, the quantity of concrete used, and the rate of flow of concrete during the pumping operation. There have moreover been proposals to measure the pressure of the concrete pumped along the duct leading to the auger itself, in an attempt to control the pressure of the concrete leaving the lower end of the auger and filling the bore formed thereby.

None of the above proposals have met with any great success, for various reasons. For example, relating the pumped quantity of concrete or the rate of flow thereof to the rate of extraction of the auger is unsatisfactory, for no proper account can be taken of ground voids, and the strength and permeability of the soil. Attempts at measuring the concrete pressure in the duct leading to the auger are inaccurate unless the exact flow and frictional characteristics of the concrete in the duct and auger stem are known, but in any event it is found that if it is possible to register a significant pressure in the duct leading to the auger, the pressure at the lower end of the auger will normally be so high that the concrete usage becomes most excessive. As a result, the operator will normally ignore the indicated pressure and instead prefer to economise on the concrete usage.

It is a principal aim of the present invention to improve the above-described continuous flight augerinjecting piling method, to allow better control over the pile formation during the auger extraction.

According to the present invention, there is provided a method of forming a foundation pile, comprising boring into the ground with a hollow-stemmed auger and then extracting the auger whilst pumping a settable material into the upper end of the hollow stem of the auger so that the material runs into the formed bore, the pressure in the settable material being sensed at a point below the upper end of the auger during the auger extraction step and at least one of the rate of auger extraction and the rate of feeding of the settable material being controlled dependent upon the sensed pressure.

Though the piling method of this invention would normally be used with concrete as the settable material, the method equally could be used with other settable constructional materials, such as a suitable form of grout. However, in the following references will exclusively be made to concrete, though the term should be construed broadly so as to encompass such other settable constructional materials as may be employed in a continuous flight auger-injected piling method.

Most preferably, the method of this invention is performed by sensing the pressure in the concrete at or closely adjacent the lower end of the auger, whereby the sensed pressure may give an accurate indication of the filling of the bore as the auger extraction proceeds.

However, adequate operation may instead be obtained by sensing the pressure within the auger stem, at some point above the lower end of the auger but below the rotary drive head associated therewith.

In performing the method of this invention, the sensed instantaneous pressure of the concrete may be compared with a predetermined minimum pressure value which may itself be related to the instantaneous depth of penetration of the auger. If then the instantaneous pressure rises relative to the predetermined minimum value, the auger extraction rate may be increased, whereas if the instantaneous pressure falls relative to the predetermined minimum value, then the rate of extraction may be decreased. Should a void in the ground conditions be encountered, or should the permeability of the ground locally increase, then the instantaneous pressure will fall and so the auger extraction rate may be reduced until the voids are filled.Instead of controlling the extraction rate, the concrete pumping rate may be controlled; should the pressure rise, then the concrete pumping rate may be reduced, and vice versa. A combination of these two may be employed, if required.

When performing the method of this invention, it will be appreciated that the means to sense the pressure will ordinarily rotate with the auger, and the arrangement must therefore allow the sensed pressure measurement to be transferred to a non-rotating part of the piling machine, in order that the sensed pressure measurement may be used to control one of the rate of pumping of the concrete or the rate of auger extraction.

One preferred form of pressure sensor means produces an electrical output signal indicative of the sensed pressure, the output signal being fed along a cable to a signal transfer point adjacent the upper end of the auger, whereat the signal is transferred to a nonrotating part of the apparatus. For example, electrical slip rings could be employed for this purpose to transfer the signal; alternatively some form of noncontact signal transfer means could be provided using for instance induced signal arrangements. An alternative is to provide a pressure sensor which supplies an electrical signal to a telemetry arrangement, whereby the pressure signal may be transmitted to a suitable receiver on a non-rotating part of the piling machine.Yet another possibility is to sense the pressure with a sensor providing an hydraulic pressure signal, which hydraulic signal is transferred through a suitable form of rotatable pressure coupling from the upper end of the auger to an adjacent non-rotating part of the machine.

In a most preferred method of the invention, the pressure in the concrete is sensed within the lower end portion of the hollow auger stem. Conveniently, an electrical output signal is produced dependent upon the sensed pressure and which is transferred by a protected cable external of the hollow stem to the upper end of the auger, a suitable slip ring arrangement being provided to transfer that electrical signal to a stationary part of the machine, for further processing by a control unit on a fixed part of the piling machine.

According to a second aspect of the present invention, there is provided an auger for use in forming a continuous flight auger-injected pile, which auger comprises a hollow stem continuous flight auger provided with means to sense pressure adjacent the lower end or within the hollow stem of the auger, which pressure sensing means provides an output signal, and means to transfer the output signal from the auger to a control unit separate from the auger and arranged to control operation of a pile forming process using the auger.

Preferably, the pressure sensing means is provided within the hollow stem of the auger adjacent the lower end thereof, to sense the pressure of concrete delivered to the lower end of the auger from the upper end of the hollow stem. Alternatively, a suitable pressure sensor may be suspended from the lower end of the auger by means of a flexible tension element, so as to be able to measure pressure immediately below the lower end of the auger, in concrete already run into the bore upon extraction of the auger. In either case, the pressure sensing means may provide an electrical output signal which is fed along a cable external of the hollow stem to the upper end of the auger whereat the signal is transferred to a control unit.Alternatively a telemetry arrangement may be provided to transfer the output signal of the pressure sensing means at or adjacent the lower end of the auger to a control unit on the auger-driving machine.

Yet another possibility is for the auger to have an hydraulic pressure-sensing means, located at or adjacent the lower end of the auger and providing an hydraulic pressure signal which is transferred at or adjacent the upper end of the auger to a suitable transducer mounted on a stationary part of the augerdriving machine, by means of a rotary joint.

According to another aspect of the present invention, there is provided a continuous flight augerinjection piling machine comprising in combination an auger of this invention as described above and a continuous flight auger-driving machine able to rotate the auger to cause the auger to bore into the ground, an extraction arrangement to raise the auger out of the ground following the boring, and means to supply concrete to the upper end of the auger during the extraction thereof.

The continuous flight auger-injection piling machine of this invention should have a control unit which receives the output signal from the pressure sensing means of the auger. The control unit may then process that output signal either to provide an indication of the sensed pressure to the operator of the machine, or actually to control the operation of the machine. For example, the control unit may serve to control the operation of the extraction arrangement which raises the auger following boring, dependent upon the sensed pressure. Alternatively, the control unit may control the pumping of concrete to the upper end of the auger, during the extraction step. In this way, piling machine of this invention may operate largely automatically, without the operator having directly to control the extraction of the auger and/or the pumping of the concrete during the extraction step.

By way of example only, a continuous flight augerinjection piling method of this invention will now be described in greater detail, reference being made to the accompanying drawings, in which: Figure 1 is a diagrammatic view showing the piling method of this invention, part-way through a piling operation; and Figure 2 is an enlarged view of the lower end of a continuous flight auger arranged in accordance with the present invention, for use in a piling method of the invention.

Referring initially to Figure 1, there is shown a continuous flight auger 10 having a hollow stem 11 provided with a pressure sensing arrangement 12 at the lower end of the hollow stem 11; this is described in greater detail below with reference to Figure 2. The auger 10 is supported by a continuous flight augerinjection piling machine (not shown) which is provided with suitable means to cause the rotation of the auger 10, so causing the auger to bore into the ground. The upper end of the auger 10 is supported by a rotary drive head 1 3 which forms a part of a continuous flight augerinjection piling machine, the drive head 13 being slidably mounted on the mast 14 of the piling machine.

A winch arrangement including a pulley 1 5 and a cable 1 6 is provided to haul the drive head 13 up the mast 14, and so extract from the ground an auger which has been driven into the ground previously, upon the rotation thereof.

A concrete pump is used for pumping concrete from a hopper into a duct 1 7 leading to a so-called swan-neck 18, provided at its lower end with the rotary joint connecting the swan-neck 1 8 to the upper end of the hollow stem 11 of the auger 10.

Figure 2 shows on a greater scale the pressure sensing arrangement provided at the lower end of the hollow stem 11 of the auger 10. A tube 20 is attached to the lower end of the hollow stem 11, the tube 20 having a greater internal diameter than that of the stem 11, and fitted into the tube 20 is a resilient tubular membrane 21 whereby a sealed chamber 22 is formed between the outer wall of the membrane 21 and the inner wall of the tube 20. The chamber 22 is filled with a liquid such as a water/glycol mixture.

A pressure sensing transducer 23 is mounted externally of the tube 20, to sense the liquid pressure within the chamber 22 through an orifice 24. The transducer 23 provides an electrical output signal dependent upon the sensed pressure, which signal appears on wires 25. The transducer 23 is located within a protective housing 26, and a protective tube 27 for the wires 25 extends from the housing 26 to the upper end of the auger 10, the tube 27 running parallel to the auger axis, adjacent the outer wall of the stem 11 through an aligned series of holes formed in the flight of the auger.

Adjacent the upper end of the auger, the wires 25 are connected to a suitable number of slip rings 28, which slip rings are contacted by brushes 29 mounted on the rotary drive head 13 of the piling machine. The brushes are connected by further wires (not shown) to a control unit (also not shown) forming a part of the piling machine.

In operation, the machine first causes the auger 10 to bore into the ground, upon the rotation thereof, in a manner well known in the art. During this step, a disposable cover plate (not shown) should be present over the open lower end of the hollow stem 11 of the auger 10. Once the auger has been bored into the ground to the required depth, concrete pumping is commenced, to cause concrete to flow along the duct 17, through the swan-neck 18 and into the hollow stem 11 of the auger.

During this, the auger is gradually lifted out of the ground by the cable 16, hauled by the winch arrangement forming a part of the piling machine. The cover plate will be lost off the end of the stem at the start of this extraction operation.

As the concrete runs down the hollow stem into the void formed by lifting the auger, the pressure sensing transducer 23 will record the concrete pressure immediately adjacent but within the lower end of the hollow stem 11. The output signal obtained from the transducer is transferred to the control unit, whereat the signal is processed to give control of the rate of lifting of the auger. This should be done in such a manner that should the sensed concrete pressure rise, then the auger will be lifted more quickly, whereas should the sensed concrete pressure fall, then the auger will be lifted more slowly. Empirical tests may be performed to produce an envelope of appropriate concrete pressures having regard to the depth of penetration of the auger, and then the rate of lifting of the auger should be performed to keep the sensed pressure within that empirical range.

Once the auger has fully been lifted clear of the concrete-filled bore hole, reinforcing rods may be pressed into the concrete before it sets, to give sufficient strength to the pile.

Claims

CLAIMS.

1. A method of forming a foundation pile, comprising boring into the ground with a hollow-stemmed auger and then extracting the auger whilst pumping a settable material into the upper end of the hollow stem of the auger so that the material runs into the formed bore, the pressure in the settable material being sensed at a point below the upper end of the auger during the auger extraction step and at least one of the rate of auger extraction and the rate of feeding of the settable material being controlled dependent upon the sensed pressure.

2. A method according to claim 1, in which the settable material comprises unset concrete.

3. A method according to claim 1 or claim 2, in which the pressure in the concrete is sensed at or closely adjacent the lower end of the auger.

4. A method according to claim 1 or claim 2, in which the pressure is sensed within the auger stem, at some point above the lower end of the auger but below the rotary drive head associated therewith.

5. A method according to any of the preceding claims, in which the sensed instantaneous pressure of the concrete is compared with a predetermined minimum pressure value related to the instantaneous depth of penetration of the auger.

6. A method according to any of the preceding claims, in which an electrical output signal indicative of the sensed pressure is produced, the output signal being fed along a cable to a signal transfer point adjacent the upper end of the auger, whereat the signal is transferred to a non-rotating part of the apparatus.

7. A method according to claim 6, in which electrical slip rings are employed at the transfer point to transfer the signal.

8. A method according to claim 6, in which noncontacting signal transfer means using induced signal arrangements are provided at the transfer point.

9. A method according to any of claims 1 to 6, in which an electrical signal indicative of the sensed pressure is supplied to a telemetry arrangement, whereby the pressure signal may be transmitted to a suitable receiver on a non-rotating part of the apparatus.

10. A method according to any of claims 1 to 6, in which an hydraulic pressure signal indicative of the sensed pressure is produced, which hydraulic signal is transferred through a rotatable pressure coupling from the upper end of the auger to an adjacent non-rotating part of the apparatus.

11. A method of forming a foundation pile according to claim 1 and substantially as hereinbefore described.

12. An auger for use in forming a continuous flight auger-injected pile, which auger comprises a hollow stem continuous flight auger provided with means to sense pressure adjacent the lower end or within the hollow stem of the auger, which pressure sensing means provides an output signal, and means to transfer the output signal from the auger to a control unit separate from the auger and arranged to control operation of a pile forming process using the auger.

13. An auger according to claim 12, wherein the pressure sensing means is provided within the hollow stem of the auger adjacent the lower end thereof, to sense the pressure of concrete delivered to the lower end of the auger from the upper end of the hollow stem.

14. An auger according to claim 12, wherein a pressure sensor is suspended from the lower end of the auger by means of a flexible tension element, so as to be able to measure pressure immediately below the lower end of the auger.

15. An auger according to any of claims 12 to 14, wherein the pressure sensing means provides an electrical output signal which is fed along a cable external of the hollow stem to the upper end of the auger whereat the signal is transferred to said control unit.

16. An auger according to any of claims 12 to 14, wherein the auger has an hydraulic pressure-sensing means located at or adjacent the lower end of the auger and providing an hydraulic pressure signal which is transferred at or adjacent the upper end of the auger to a suitable transducer mounted on a stationary part of the auger-driving machine, by means of a rotary joint.

17. An auger according to claim 12 and substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

18. A continuous flight auger-injection piling machine comprising in combination an auger according to any of claims 12 to 17 and a continuous flight auger-driving machine configured to rotate the auger to cause the auger to bore into the ground, an extraction arrangement to raise the auger out of the ground following the boring, and means to supply concrete to the upper end of the auger during the extraction thereof.

19. A machine according to claim 18, wherein there is provided a control unit which receives the output signal from the pressure sensing means of the auger, which control unit processes that output signal either to provide an indication of the sensed pressure to the operator of the machine, or to control the operation of the machine.

20. A continuous flight auger-injection piling machine substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.