BEARING CAPACITY ENHANCEMENT FOR PILING APPLICATIONS

BEARING CAPACITY ENHANCEMENT FOR PILING APPLICATIONS

The present invention relates to a tool for enhancing the bearing capacity of a pile, in particular but not exclusively the bearing capacity of a rotary- bored pile and/or a pile formed by a continuous flight auger or other piling auger.

It is well-known in the construction industry to enhance the bearing capacity ^~of^~ a pile by using an under-reamer to enlarge the diameter of a portion of the shaft in which the pile is to be formed. By enlarging the diameter of this portion, the end bearing capacity is increased, and accordingly it is possible to reduce the length of the pile, since less shaft friction is required to bear a given load. This is described, for example, in U.K. patent no. 2 222 621 granted to the present applicant. However, this technique requires the use of a special tool which must be lowered into the bore hole after the bore hole has been formed, which increases installation time and may risk disturbing the walls of the bore hole, thereby reducing the integrity of the completed pile. It is known, for example from the present Applicant's International patent application

WO 98/13554 or from GB 1 391 110 and ALT 586947, to provide a continuous flight auger with an extendible/retractable element which is controllable so as to be extended beyond the circumference of the auger flight or to be retracted to a position within the circumference of the auger flight. These disclosures describe a procedure in which such an auger is first driven with the element retracted so as to penetrate the ground to a certain depth. The element is then extended and the auger rotated (either in reverse or in the same direction) while being withdrawn from the ground. As the auger is being withdrawn, concrete or grout is concomitantly supplied to the tip of the auger stem so as to form an underground pile-type structure corresponding generally to the volume occupied by the auger at its maximum depth. By extending the extensible/retractable element as the auger is rotated and withdrawn and while concrete is supplied, a pile having a helical projection along its length may be formed. The helical projection provides enhanced skin friction and consequently enhanced bearing capacity for the pile. These prior art references teach that soil disturbance during penetration should be kept to a minimum, which is why it is specified that the element is retracted upon penetration and only extended during withdrawal. However, the extendible/retractable elements disclosed in the aforementioned prior art references are difficult to operate reliably, particularly in view of the harsh conditions generally encountered at the tip of the auger. Firstly, since auger piling generally involves some compression during penetration of the soil surrounding the auger, it can be difficult to ensure that the element is properly extended prior to withdrawal of the auger. Secondly, further difficulties may be faced when attempting to retract the element, due to the presence of soil between the extended element and its retracted position within the diameter of the auger flight .

According to a first aspect of the present invention, there is provided a piling auger for use in forming cast-in-situ piles in a region of ground, the auger being provided with at least one flight having a pitch and at least one element which is fixedly mounted so as to project beyond the circumference of the at least one flight of the auger, wherein the at least one element is shaped and positioned so that, when the auger is rotated in a given direction and allowed to penetrate the ground, a first spatial volume is swept out by the element, and when the auger is withdrawn from the ground while being rotated in the said direction, a second spatial volume is swept out by the element, characterised in that the second volume is greater than the first volume.

According to a second aspect of the present invention, there is provided a method of installing a cast-in-situ pile using an auger provided with at least one flight having a pitch and^: at least one element which is fixedly mounted so as to project beyond the circumference of the at least one flight of the auger, wherein the auger is first rotated in a given direction and allowed to penetrate the ground to a predetermined depth and then withdrawn from the ground with continued rotation in the given direction while concrete or grout is pumped or otherwise supplied to the tip of the auger, characterised in that the at least one element is shaped and positioned so that, when the auger penetrates the ground, a first spatial volume is swept out by the element, and when the auger is withdrawn from the ground a second spatial volume is swept out by the element, the second volume being greater than the first volume . Advantageously, the piling auger is a continuous flight auger, although other rotary piling augers may be employed where appropriate.

As the auger is first rotated into the ground to a given depth so as to form a generally cylindrical bore hole, the element acts so as to score a generally helical groove or the like in the wall of the bore hole, although it should be appreciated that since this groove is unsupported, the presence of a rotating auger in the ground will tend to cause the groove to collapse or to fill with excavated soil. The pitch and/or width of the notional helical groove is determined by the rate of penetration and rotation of the auger, as well as by the size and configuration of the element. For example, if the element is in the form of a generally linear projection having a major axis which is angled so as to be substantially parallel to its path of travel during penetration, then the element will tend to score a groove of substantially the same cross- section as the normal cross-section of the element. This is because the surface area presented by the element to the soil surrounding-- the bore hole during penetration approaches or reaches a minimum, due to the element being disposed at an angle corresponding substantially to its path of travel during penetration, and the resulting disturbance to the soil surrounding the wall of the bore hole will be relatively small. In this way, disturbance to the soil surrounding the auger is kept relatively small during penetration, which is desirable with regard to the structural integrity of the resulting pile. In some applications, the element may be angled so as to be substantially parallel to the pitch of the at least one auger flight .

However, upon withdrawal of the auger from the ground, with continued rotation in the same direction as during penetration, the element will present a significantly larger surface area to the soil surrounding the bore hole without any need for the element to be adjusted or reconfigured, since the path of travel of the element during withdrawal is at a different angle to that of the path of travel during penetration, and the element will tend to present itself obliquely to the soil rather than following the line of its major axis. Accordingly, the element will sweep out a greater volume of soil as the auger is withdrawn with rotation than when the auger is rotated into the ground. As the auger is withdrawn, concrete or grout is concomitantly supplied to the end region of the auger so as to form an underground pile. It is to be noted that during withdrawal of the auger and concomitant supply of concrete or grout, the soil displaced by the element is immediately replaced by concrete or grout, which may be pumped or otherwise supplied under pressure. The supply of concrete or grout serves to prevent collapse of the notional resultant substantially helical groove formed upon withdrawal of the auger, this groove having the opposite sense or handedness to-- the notional groove formed during penetration.

The pile which is formed using the method and apparatus of the present invention will have a generally cylindrical form having a substantially helical projection disposed circumferentially thereabout. It is to be appreciated, however, that due to unpredictable ground conditions, the pile may not be truly cylindrical, and may incorporate bulges and other irregularities. The projection serves to increase the apparent diameter of the pile, and thereby is effective in increasing end bearing capacity as well as skin friction. Additionally, since the helical projection achieved by way of the present invention serves to key the pile into relatively undisturbed ground, additional skin friction is attained. Moreover, by virtue of the displacement of soil caused by the element, the density of the soil surrounding the bore hole may be increased, thereby helping further to enhance ultimate skin friction. It is generally preferred that the element is located as close to the tip of the auger as possible, so that the resulting pile is provided with the helical projection along substantially its entire length. The flights of some commercially-available augers are thicker near the tip of the auger than elsewhere, and an appropriate location for the element would be on the circumference of the flight in this region.

When using an auger with a toothed boring head, it may be advantageous to mount or incorporate the element on the external tooth. The tooth itself is designed to be replaceably fitted to the boring head, and in the event of damage to the protruding element caused as a result of hard digging or underground obstructions, it can be a relatively quick and simple procedure to replace the tooth and/or the entire boring head. Where a double-start auger-- (i.e. an auger with two flights) is used, it is envisaged that a projecting element may be provided on each flight, thereby resulting in a cast-in-situ pile having two concentric helical projections. 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:

FIGURE 1 shows the lower end of an auger having a flight which is fitted with a projecting element in accordance with the present invention;

FIGURE 2 is a schematic representation of the different surface areas presented to the soil by the element during penetration and withdrawal of the auger; FIGURE 3 shows a cross-section of a groove formed by the element during withdrawal of the auger;

FIGURE 4 shows a toothed auger having a projecting element fitted onto the tooth of the auger;

FIGURE 5 is a plan view of the tooth-mounted element of Figure 4;

FIGURE 6 shows in schematic form the area of soil affected by the element during penetration and withdrawal of the auger; and

FIGURE 7 shows the lower part of a cast-in-situ pile formed by an embodiment of the present invention.

Referring now to Figure 1, there is shown a lower part of a continuous flight auger 1 having a stem 2 , a flight 3 and a tip 4. An element 5 is fixedly mounted on the circumference of the flight 3 at its lower end. The element 5 is fixed in such a way that it permanently extends beyond the circumference of the flight 3, and is not retractable.

Figure 2 shows in schematic form the element 5 of Figure 1 in more detail. The particular element 5 shown in Figure 2 has a generally trapezoidal shape having a length 1 of 100mm, a- major width w_maj of 20mm, a minor width w_min of 10mm and a height h of 25mm, although it will be apparent that various other dimensions and shapes may be used. The element 5 is shown disposed at an angle to the horizontal (approximately 6° in the present example) , which corresponds substantially to the angle at which the element 5 will sweep out a minimum volume when the auger 1 is rotated into the ground at a rate of 10 revolutions per metre penetration. In other words, the element 5 is disposed at an angle to the horizontal which corresponds substantially to the angle of the path of travel 6 of the element during penetration. This angle may be substantially the same as the angle of the pitch of the flight 3 relative to the perpendicular to the longitudinal axis of the auger stem 2, at least in applications where it is envisaged that the auger 1 will be rotated into the ground at a rate according to the pitch of the flight 3. Alternatively, where it is appropriate for the auger 1 to be over- or under-rotated into the ground, the angle at which the element 5 is disposed to the horizontal will need to be adjusted accordingly.

It may be seen that at the appropriate angle of disposition, the cross-sectional area of the volume swept out during penetration of the auger 1 will correspond substantially to the cross-sectional area of the element 5 itself .

Upon subsequent withdrawal of the auger 1 with rotation in the same direction as that used during penetration, and at a rate of approximately 5 revolutions per metre withdrawal, for example, the path of travel 7 of the element 5 during withdrawal is angled at an angle β to the horizontal in the opposite vertical direction to that of the path of travel 6 during penetration (approximately 8° in the present example) . Consequently, the _:element 5 sweeps out a significantly larger volume upon withdrawal than upon penetration. In the example shown in Figure 2, the major width w'_maj of the resulting cross-section of the volume swept out by the element during withdrawal is 38mm and the minor width w'_min is 26mm, as compared to 20mm and 10mm during penetration; in both cases, the height h of the cross-section is still 25mm. The cross-section of the volume swept out by the element 5 during withdrawal is shown in Figure 3. During withdrawal of the auger 1, concrete or grout is pumped or otherwise supplied to the tip 4 so as to fill the notional spatial volume occupied by the auger 1 and the notional volume swept out by the element 5. In this manner, a projection of grout or concrete (not shown) is formed outside the cylindrical envelope of the auger 1, which projection will describe a helix about the notionally cylindrical body of the resulting pile (not shown) having a pitch of 5 revolutions per metre (where the auger 1 is withdrawn at this rate, as in the present example) .

For a pile formed with an auger having an envelope diameter of 450mm (i.e. the diameter of the circumference of the flight 3) , the aforementioned helical projection will provide the characteristics of a pile having a notional straight-sided diameter of

500mm but using significantly less concrete or grout. This increase in apparent diameter results in a pile having enhanced load bearing capacity by virtue of increases in the skin friction and the end bearing capacity. With reference now to Figures 4 and 5, there is shown the lower end region of an auger 10 having a flight 11 and a tooth 12 mounted detachably at the start of the flight 11. The tooth 12 is positioned and angled so as to help the flight 11 bite into the soil upon penetration. The tooth 12-- is located within the envelope defined by the flight 11 of the auger 10 so as to be substantially flush therewith. An element 13 of similar configuration to that of the aforementioned element 5 is welded or otherwise attached to the tooth 12 so as to project outside the envelope defined by the flight 11. The element 13 is positioned on the tooth 12 so that the major axis of the element 13 is substantially parallel to the path of travel of the element 13 upon penetration of the auger 10 into the ground, which in this example is about 6° from the perpendicular to the longitudinal axis of the auger 10.

The auger 10 of Figures 4 and 5 is used in the same way as the auger 1 of Figures 1 to 3 and as hereinbefore described. However, since the element 13 is mounted on a detachably-mounted tooth 12, any damage to the element 13 caused by hard digging or by underground obstructions can be easily overcome simply by replacing the tooth 12 and providing a new element 13. This can be significantly less time-consuming than seeking to refashion or repair a permanently-mounted element such as may be provided in the embodiment of Figures 1 to 3.

Figure 6 shows in schematic form the parts of the interior wall 15 of a bore hole formed by the auger 10 of Figures 4 and 5. The interior wall 15 is shown as a flattened projection of a cylinder in the plane of the sheet, side A of the projection adjoining side B when the projection is imagined as a cylinder. The thinner groove 16 in the wall 15 of the bore hole is formed by the element 13 during penetration, and the substantially thicker groove 17 is formed during withdrawal, owing to the larger area presented by the element 13 during withdrawal. Upon withdrawal, it is expected that the thinner groove 16 will close up, although it is not considered to be critical if this does not occur. _^{■ ' '}-

As the auger 10 is withdrawn with concomitant supply of concrete or grout, a cast-in-situ pile 17 with a substantially helical projection 18 is formed in the ground, a bottom part of the pile 17 being shown in Figure 7. The projection 18 provides additional load bearing by increasing skin friction and bearing capacity, thereby making the pile 17 behave as if it were a pile of greater diameter without the use of the substantial amount of additional concrete or grout that would otherwise be required.