GB2345715A - Composite auger piling with enlarged base footing - Google Patents

Abstract

A method of forming a cast-in-situ pile having a shaft and a base of greater cross sectional area than the shaft by using an auger is disclosed. The auger is first bored into the ground until its tip reaches a given depth. The auger is then lifted by a predetermined distance while concrete or grout is supplied to the tip so as to form the base of the pile and then backscrewed out of the ground while continuing to supply concrete or grout to the tip so as to form a continous shaft of smaller cross-sectional area than the base of the pile. The shaft may alternatively be formed by lowering a pre-cast element throught the hollow stem of the auger into the subterranean mass of concrete or grout prior to backscrewing, or by way of conventional driven or auger piling techniques after the auger has been backscrewed out of the ground.

Description

COMPOSITE AUGER PILING The present invention relates to auger piling techniques, and in particular to a method of forming cast-in-situ piles with enlarged bases and enhanced end bearing capacities.

Auger piling, also known as auger cast piling, is a well-known method of forming underground structures such as piles or the like. An auger is rotated into the ground to a given depth, and then withdrawn. As the auger is withdrawn, soil trapped on the flight of the auger is lifted from the ground while concrete or grout is pumped to the tip of the auger through its stem or along a separate pipe. The concrete or grout fills the notional void which would be left in the ground due to the withdrawal of the auger, and thereby forms an underground cast-in-situ pile upon setting.

Auger piling techniques are described in more detail in the present Applicant's U. K. patent application no. 9515652.7, the disclosure of which is incorporated into the present application by reference.

The optimum pile geometry in ground conditions typified by soft soils, such as clay, overlying granular materials or other good founding strata is one which takes advantage of the potential bearing capacity of a good founding stratum by ensuring that the base of the pile rests in or on the stratum. Under these conditions, the surface area of the pile base in the founding stratum is often the major component which determines the load bearing capacity of the pile, and an increase in the pile base diameter can produce a substantial enhancement of the end bearing capacity.

Underreaming piling tools are well-known for bored cast-in-situ piles, but these can cause significant disturbance to the soil surrounding the resulting pile, which is generally undesirable. Moreover, conventional underreaming is unobtainable in cohesionless soils because the bore walls and the roof of the underream become unstable when undercut.

Furthermore, it is often the case that good founding strata are of limited thickness, and the bearing capacity that could be achieved by founding in such strata may be equated to an equivalent greater depth into the softer underlying soils to achieve the same ultimate pile capacity.

It is known to enhance end bearing capacity in driven cast-in-situ piling methods such as the Franki (TM) enlarged base technique. Here, a hollow steel tube fitted with a sacrificial tip is driven to depth, into a good founding stratum. The tube is then filled or partially filled with a dry concrete mix and a hammer is lowered down the tube so as to drive out the volume of dry concrete. The tube is then filled with wet concrete to the required level and the hollow steel tube is extracted. The enlarged base thereby produced has the advantages of enhanced end bearing capacity and a degree of compaction of the soil surrounding the base. However, often for reasons of noise nuisance, driven piles are not generally desirable in many areas.

Furthermore, the actual diameter of the enlarged base of such a driven cast-in-situ pile is not precisely known, which leads to uncertainties in design and forecasting of pile behaviour.

According to a first aspect of the present invention, there is provided a method of forming a cast-in-situ pile having a shaft and a base of greater cross-sectional area than the shaft by using an auger having a stem, a flight and a tip, the method comprising the steps of: i) boring the auger into the ground until the tip reaches a given depth; ii) lifting the auger by a predetermined distance while supplying concrete or grout to the tip so as to form a subterranean mass of concrete or grout as the base of the pile; and iii) backscrewing the auger out of the ground while continuing to supply concrete or grout to the tip so as to form a continuous shaft of smaller crosssectional area than the base of the pile.

During the lifting stage of step ii), the auger may continue to be rotated in a forwards direction, or rotation may be stopped. As the auger is being backscrewed out of the ground in step iii), soil on the flight of the auger will be left in the ground upon extraction of the auger, leaving a notional void substantially of the diameter of the auger stem which is concomitantly filled with concrete or grout as the auger is withdrawn.

In this way, a pile with an enlarged base section and a relatively narrow shaft can be formed, which provides good support with the use of less concrete or grout than would be needed to form a conventional straight-sided pile of similar load-bearing capacity.

Furthermore, less spoil is generated, since this is left in the ground from the auger flight during backscrewing, and the time taken to install a pile is much reduced. The shaft of the pile will have a diameter corresponding substantially to the outer diameter of the auger stem, whereas the base of the pile will have a diameter corresponding substantially to the diameter of the auger flight.

By controlling the rate of supply of concrete or grout, for example by way of a pump and an electromagnetic flowmeter, and by controlling and monitoring the auger penetration, lift and rotation rates, for example by way of an electronic computer, it is possible to form piles with predictable configurations and characteristics. With some piling rigs it may be found necessary to control the lift and rotation electronically.

For example, if the auger is backscrewed out of the ground at a rate determined by the pitch of the flight, then the concrete or grout will fill the volume previously occupied by the auger flight so as to form a pile shaft with a helical concrete projection.

Alternatively, if the auger is over-rotated by a predetermined amount during backscrewing, then the soil on the flight of the auger will be pushed into the ground, thereby closing the gap left by the flight of the auger and achieving a pile with an enlarged base section and a substantially cylindrical shaft of a diameter corresponding generally to the outer diameter of the auger stem, thereby effecting an even greater efficiency of concrete or grout supply.

The auger stem may be modified to include a mechanism which increases the apparent diameter of the auger stem only during backscrewing, thereby allowing the main shaft of the resulting cast-in-situ pile to have predetermined diameters not restricted to the diameter of the auger stem itself. One mechanism for offering a larger cross-section of the auger stem consists of a hinged gate or the like which, when the auger is counter-rotated or backscrewed, is forced into an extended position by the flow of material back down the auger flight or flights. Furthermore, it is envisaged that the mechanism may be adjustable during backscrewing, so as, for example, to form a pile with an enlarged top as well as an enlarged base.

Alternatively, an enlarged top may be formed by using a different tool subsequent to the installation of the shaft of the pile.

In addition, a grout pipe may be introduced and placed at the bottom of the base of the pile.

Injection of grout at the base of the pile will tend to pre-load the end bearing in a different manner to that achieved with conventional single-diameter piles, since there will also be equivalent end bearing resistance from above the base of the pile. Conventional base grouting is often achieved by attaching a grout pipe to a reinforcing cage or bars which is or are lowered into the concrete forming the pile shaft before this has set, such that the end of the grout pipe is positioned near or under the concrete. Once the concrete forming the shaft of the pile has set, grout is injected through the pipe under pressure so as to tend to push apart the underside of the pile shaft and the soil below. In this way, a loading force is applied which can result in subsequent pile behaviour appearing to be stiffer.

According to a second aspect of the present invention, there is provided a method of forming a cast-in-situ pile having a shaft and a base of greater cross-sectional area than the shaft by using an auger having a hollow stem, a flight and a tip, the method comprising the steps of: i) boring the auger into the ground until the tip reaches a given depth; ii) lifting the auger by a predetermined distance while supplying concrete or grout to the tip so as to form a subterranean mass of concrete or grout as the base of the pile; iii) lowering a pre-cast element of smaller crosssectional area than the base of the pile through the hollow stem of the auger into the subterranean mass of concrete or grout; and iv) backscrewing the auger out of the ground.

According to a third aspect of the present invention, there is provided a method of forming a cast-in-situ pile having a shaft and a base of greater cross-sectional area than the shaft by using an auger having a stem, a flight and a tip, the method comprising the steps of: i) boring the auger into the ground until the tip reaches a given depth; ii) lifting the auger by a predetermined distance while supplying concrete or grout to the tip so as to form a subterranean, substantially cylindrical mass of concrete or grout as the base of the pile; iii) backscrewing the auger out of the ground; and iv) forming the shaft of the pile by way of conventional driven or auger piling techniques.

Where conventional auger piling techniques are used to form the shaft of the pile, a second auger of smaller diameter than the initial auger may be bored to depth so as to reach the base mass and then lifted while supplying concrete or grout to its tip so as to form the shaft of the pile.

Where conventional driven techniques are used to form the shaft of the pile, it is advantageous to employ a driving tip. The driving tip may have a pointed configuration or a functionally-equivalent geometry so as to seek to reduce the amount of soil trapped underneath the tip during driving. In this way, relatively little soil is captured at the tip during driving, thereby enabling a good connection to be made to the concrete or grout of the base, which will generally still be unset and hence workable.

Alternatively, the subterranean mass of concrete or grout forming the base of the pile may be allowed to set prior to forming the shaft, in which case it is important to ensure that there is adequate top-to-end contact between the set base and the subsequently formed shaft. In this alternative, the use of a driving tip may not be advantageous, since the required connection and load transfer between base and shaft is not so readily achieved.

In general, the use of a continuous flight auger (CFA) is preferred in the methods of the present invention, but in some applications an auger with a discontinuous flight or flights may be utilised, provided that there is at least one circumference of flight at or near the tip of the auger so as to form the base of the pile.

For a better understanding of the present invention, and to show how it may be carried into effect, reference shall now be made, by way of example, to the accompanying drawings, in which: FIGURES 1 to 3 show the installation of a pile with an enlarged base and a narrower shaft according to the first aspect of the present invention; FIGURES 4 to 7 show the installation of a pile according to the first aspect of the present invention with the additional feature of an extendible element provided on the auger stem; FIGURES 8 to 10 show the installation of a pile with an enlarged base and a narrower shaft according to the second aspect of the present invention; FIGURES 11 to 14 show the installation of a pile with an enlarged base and a narrower shaft according to the third aspect of the present invention, where the shaft of the pile is formed by driven piling techniques; FIGURES 15 to 19 show the installation of a pile with an enlarged base and a narrower shaft according to the third aspect of the present invention, where the shaft of the pile is formed by auger piling techniques; and FIGURES 20 to 24 show the use of a piling rig to install a pile with an enlarged base and a narrower shaft according to the first aspect of the present invention.

Referring first to Figures 1 to 3, a continuous flight auger 1 having a stem 2, a flight 3 and a tip 4 is rotated into the ground 5 to a given depth. As shown in Figure 2, the auger 1 is then withdrawn by a predetermined amount with continued forward rotation while concrete or grout is pumped through the tip 4 of the auger 1 through its stem 2 so as to form a subterranean mass of concrete or grout which constitutes an enlarged base 6. The auger 1 is then backscrewed out of the ground 5 while concrete or grout continues to be pumped through the stem 2 of the auger 1, thereby forming a pile comprising the enlarged base 6 and a shaft 7 of relatively smaller cross-sectional area, as shown in Figure 3. The shaft 7 of the pile of Figure 3 is surrounded by a helical concrete or grout projection 8 which results from concrete or grout filling the space originally occupied by the flight 3 of the auger 1 when the auger 1 is backscrewed and withdrawn at a rate corresponding to the pitch of the auger flight 3. Alternatively, the auger 1 may be over-rotated during backscrewing so as to close up the space originally occupied by the flight 3 and thereby to result in a pile shaft 7 with no helical projection 8 (not shown).

In an alternative technique, shown in Figures 4 to 7 (Figures 4 and 5 corresponding to Figures 1 and 2), the auger 1 may be rotated to depth and partially withdrawn, while concrete or grout is pumped, so as to form an enlarged base 6 as before. An extendible element 9 provided on the auger stem 2 is forced into an extended position upon backscrewing of the auger 1 as shown in Figure 6, thereby increasing the apparent diameter of the stem 2 of the auger 1 during backscrewing. Concrete or grout is pumped to the tip 4 of the auger 1 during backscrewing so as to form a shaft 7'of greater diameter than the stem 2 of the auger 1. The extendible element 9 may be controlled so that it is moved into the extended position only as the tip 4 of the auger 1 nears the surface of the ground 5 so as to form a pile with an enlarged head portion (not shown).

With reference now to Figures 8 to 10, the auger 1 is rotated into the ground 5 and then partially withdrawn, with concomitant supply of concrete or grout to the auger tip 4 so as to form an enlarged base 6, as before. A precast element 11 of smaller crosssectional area than the enlarged base 6 is then lowered through the stem 2 of the auger 1 until it reaches the base 6, and the auger 1 is backscrewed out of the ground 5, without supplying concrete or grout, so as to form a pile comprising the enlarged base 6 and the precast element 11.

Figures 11 to 14 show the use of an auger 1 to form an enlarged pile base 6 as before (Figures 11 and 12). The auger 1 is then backscrewed out of the ground 5 without concrete or grout being supplied, and a precast pile 12 of smaller cross-sectional area than the enlarged base 6 is driven into the ground until it reaches the enlarged base 6, thereby forming the desired pile configuration as shown in Figure 14. The precast pile 12 is shown as including a reinforcing element 13, but it is to be noted that the reinforcing element 13 is optional.

Figures 15 to 19 show the use of an auger 1 to form an enlarged pile base 6 as before (Figures 15 and 16). The auger 1 is then backscrewed out of the ground 5 without concrete or grout being supplied, and a second continuous flight auger 14 having a flight diameter less than that of the auger 1 is rotated to depth, as shown in Figure 18. The second auger 14 is then withdrawn while concrete or grout is supplied through its stem 15 so as to form a pile shaft 16, as seen best in Figure 19.

Figures 20 to 24 show the use of a piling rig 17 to install a pile in ground consisting of a layer of topsoil 18 overlying a stratum of soft soil 19, such as clay, which in turn overlies a stratum of a granular or other good founding material 20. The piling rig 17 includes an auger 1 as hereinbefore described which is first rotated to depth as shown in Figure 20 until its tip 4 is located within the founding stratum 20. An amount of spoil 21 is brought to the surface of the ground 5 as soil is carried up the flight 3 of the auger 1, as shown in Figure 21. The auger 1 is then partially withdrawn, with continued forward rotation, while concrete or grout is supplied to the tip 4 of the auger 1 through its stem 2, thereby forming an enlarged concrete or grout base portion 6 at least partially located within the founding stratum 20. The auger 1 is then backscrewed out of the ground 5 with over-rotation and concomitant supply of concrete or grout so as to form a straight-sided pile shaft 7 of smaller crosssectional area than the enlarged base 6. At least a portion of the spoil 21 generated during penetration of the auger 1 is returned to the ground 5 during backscrewing, which firstly reduces the amount of spoil 21 which subsequently needs to be disposed of, and which secondly can serve to compact the soil around the base 6 and shaft 7 of the pile so as to result in improved stiffness and load-bearing capacity.

Claims (10)

CLAIMS:

1. A method of forming a cast-in-situ pile having a shaft and a base of greater cross-sectional area than the shaft by using an auger having a stem, a flight and a tip, the method comprising the steps of: i) boring the auger into the ground until the tip reaches a given depth; ii) lifting the auger by a predetermined distance while supplying concrete or grout to the tip so as to form a subterranean mass of concrete or grout as the base of the pile; and iii) backscrewing the auger out of the ground while continuing to supply concrete or grout to the tip so as to form a continuous shaft of smaller crosssectional area than the base of the pile.

2. A method according to claim 1, wherein the auger is backscrewed out of the ground at a rate according to the pitch of the auger flight.

3. A method according to claim 1, wherein the auger is over-rotated during backscrewing, so as to push soil on the flight of the auger into the ground.

4. A method according to claim 1,2 or 3, wherein the apparent diameter of the auger stem is increased during backscrewing by way of an extendible element which is arranged to project from the auger stem as the auger is backscrewed out of the ground.

5. A method according to claim 4, wherein the element is controlled so as to project by variable amounts during backscrewing, thereby allowing the diameter of the shaft of the pile to be varied along its length.

6. A method according to claim 5, wherein the element is controlled so as to enlarge the diameter of the shaft of the pile near the surface of the ground, thereby forming a pile with an enlarged head.

7. A method of forming a cast-in-situ pile having a shaft and a base of greater cross-sectional area than the shaft by using an auger having a hollow stem, a flight and a tip, the method comprising the steps of: i) boring the auger into the ground until the tip reaches a given depth; ii) lifting the auger by a predetermined distance while supplying concrete or grout to the tip so as to form a subterranean mass of concrete or grout as the base of the pile; iii) lowering a pre-cast element of smaller crosssectional area than the base of the pile through the hollow stem of the auger into the subterranean mass of concrete or grout; and iv) backscrewing the auger out of the ground.

8. A method of forming a cast-in-situ pile having a shaft and a base of greater cross-sectional area than the shaft by using an auger having a stem, a flight and a tip, the method comprising the steps of: i) boring the auger into the ground until the tip reaches a given depth; ii) lifting the auger by a predetermined distance while supplying concrete or grout to the tip so as to form a subterranean, substantially cylindrical mass of concrete or grout as the base of the pile; iii) backscrewing the auger out of the ground; and iv) forming the shaft of the pile by way of conventional driven or auger piling techniques.

9, A method according to any one of the preceding claims, wherein the auger is a continuous flight auger.

10. A method of forming a cast-in-situ pile having a shaft and a base of greater cross-sectional area than the shaft, substantially as hereinbefore described with reference to the accompanying drawings.