GB2520723A

GB2520723A - A screw pile and a method of pile driving a screw pile

Info

Publication number: GB2520723A
Application number: GB1321051.3A
Authority: GB
Inventors: Trevor C Burden; Rajagopal Jeyachandhiran
Original assignee: Francis and Lewis International Ltd
Current assignee: Francis and Lewis International Ltd
Priority date: 2013-11-29
Filing date: 2013-11-29
Publication date: 2015-06-03
Also published as: GB201321051D0

Abstract

A screw pile, comprising a hollow shaft 12, a first end (fig 1, 14) and a ground engaging second end 16, the screw pile arranged to be driven into the ground by rotation of the shaft, the shaft having an internal thread 20 formed along at least a portion of its internal wall. The internal thread may be located close to the ground engaging end 16 and is comprised of a protruding wall perpendicular to the axis of the shaft 24 and extending helically along the interior wall. The height of the protruding wall 24 may decrease towards the leading edge (fig 3, 26). The thread may extend for one or less rotations along the internal wall. The screw pile may also have a secondary internal thread and a number of external helical threads 18. The ground engaging end 16 may be provided with serrations, these could be in the form of blades or teeth.

Description

A SCREw PIIE AND A METHOD OF PILE DRIVING A SCREw PIIE

FIELD OF THE INVENTION

This invention relates to ground piles and in particular screw piles.

BACKGROUND TO THE INVENTION

Helical scrcw piles provide a fast and efficient method of installing low cost foundations for a large range of applications.

Conventional helical screw piles consist of a sled shall onto which is welded a series of low-pitched circular steci platcs. Thc shafts arc scrcwcd into thc ground with hydraulic or electrically powered drilling equipment. The screw piles then become the foundation upon which structures may be built.

As the load-bearing capacity required by a particular foundation increases, the diameter of hclical scrcw piles uscd also typically increascs. This Icads to an incrcasc in disturbance of the ground surrounding pile shafts, particularly due to the interaction of the screw plates with thc surrounding earth. This is detrimental to the strength of thc foundation and in particular a pilc's resistancc to lateral thads. Additionally, as thc pilc diameter incrcascs thc chance of hitting obstructions in thc pile path as thc pilc is dnvcn into thc ground is incrcascd.

SuIMARY OF THE INVENTION According to a first aspect of the invention, there is provided a screw pile, comprising a hollow shaft, a first end and a ground engaging second end, the screw pile arranged to be driven into the ground by rotation of the shaft, the shaft having an internal thread formed along at least a portion of its internal wall.

The presence of an internal thread formed along the internal wall of the shaft provides several advantages over known screw piles. Firstly. as the pile is driven into the ground the internal thread disrupts the soil entering the shaft thereby reducing the resistance of the plug of soil which extends up inside of the shaft during insertion of the pile into the ground. Secondly, the presence of the internal thread means that external threads, present in prior art screw piles, are not required. As such, the overall diameter of the pile can be reduced such that.

disturbance of the zone of soil or ground surrounding the pile shaft is greatly reduced. This increases the overall support that the surrounding earth provides to the pile and thus increases the capacity of the pile for resisting lateral loads and moments applied thereto. Additionally, by reducing the overall diameter of the pile. the chances of hitting obstrucilons in the pile path during insertion of the pile into the ground is reduced.

A leading edge of the internal thread may be located proximate to the ground engaging second end of the shalt, such that the thread prelerably engages with the ground as the pile is placed in position to he installed.

In some embodiments, the internal thread may comprise a protruding wall directed towards the centre of the shaft and extending helically along at least a portion of the length of the internal wall.

The height of (he protruding wall preferably decreases towards the leading edge of the internal thread, thereby reducing resistance of the leading edge of the internal thread upon rotation ol the pile during instaflation.

In some embodiments, the ratio between the diameter of the shaft and the pitch ol the internal thread is preferably between 2 and 3.

The internal thread may extend for one or fewer complete revolutions of the shaft.

Alternatively, the internal thread may extend for more than one complete revolution of the shaft..

The hollow shaft may further comprise an external thread extending along at least a portion of an external wall of the shaft. By providing an external thread in combination with the internal thread, because of the increased overall area of thread in contact with the soil (internal and external), the longitudinal driving force provided by the thread during rotation of the pile is increased and the resistance of the pile to axial forces, e.g. compression loads, is also increased.

Where Lw external thread is provided, a leading edge of the external thread may be located further away from the ground engaging second end of the shaft than the leading edge of the internal thread. Thus, the leading edge of the internal thread may engage with the ground before (hat of the external thread when the pile is driven into (he ground. The ratio between the diameter of the shaft and the pitch of the external thread may also between 2 and 3. As with the internal thread, the external thread may extend for one or fewer revolutions of the shaft or for more than one complete revolutions of the shaft.

In some embodiments, the shaft may comprises one or more secondary internal threads formed along at least a portion of the internal wall of the shaft and separate from each other and the internal thread. Such secondary internal threads may provide additional downward force to the pile and reduce resistance to the plug of soil forced into the shaft as the pile is driven into the ground.

In order to reduce resistance to entry of the ground engaging second end of the shaft into the ground, the ground engaging second end may comprise one or more serrations formed in a bottom edge of the second end. Each of the scrrations may indude a cutting edge, such that as the pile is rotated, the cutting edge(s) may slice into the ground, cutting a path for the remainder of the pile.

According to a second aspect of the invention, there is provided a method of driving a screw pile as described above into the ground, the method comprising engaging the ground engaging second end of the screw pile with the ground and applying torque to the first end of (he screw pile.

The method may further comprise applying force to the first end of the shaft in a direction parallel to the longitudinal axis of the shaft.

BRIEF DEscRIPTION OF THE DRAwINGs

Embodiments of the present invention will now he described, by non-limiting example only, with reference to the accompanying drawings, in which: Figure 1 is a graphical illustration of a screw pile according to an embodiment of the present invention; Figure 2 is a cross section of the screw pile shown in Figure 1 taken across section A-A of Figure 1: Figure 3 is a plan view of an internal thread of the screw pile shown in Figure 1; and Figure 4 is a side view of the internal thread of the screw pile shown in Figure 1.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Figure 1 is an illustration of a screw pile 10 in accordance with an embodiment of the present invention. The screw pile 10 comprises a hollow shaft 12 with a first end 14 and a ground engaging second end 16 opposite to the first end 14. The shaft 12 may be manufactured from any suitable material and preferably galvanised steel or the like so as to prevent degradation due to environmental conditions, whilst providing adequate strength both to withstand the stress of installation and to support structures built thereon.

The first end 14 of the shaft 12 may include known engaging means (not shown) operable to engage with the machinery, such as a pile driver, to impart. both torque and downward force on the screw pile 10 to drive the pfle 10 into the ground.

Figure 2 shows a cross section of the ground engaging second end 16 of the screw pile 10 taken at section A-A of Figure 1. As with known screw piles, the screw pile 10 comprises an external thread 18 extending helically along a portion of the length of the shaft 12. However, in contrast to known screw piles. as an addition or alternative to the external thread 18 the shaft 12 comprises an internal screw thread 20 situated proximate to the second end 16 of the screw pile 10, exlending helically along a porlion ol the length of the inlerior wall ol the hollow shaft 12.

The presence of an internal thread 20 running along the interior of the shaft 12 provides several advantages over known screw piles. Firstly. as the pile 10 is driven into the ground the internal thread 20 disrupts the soil entering the shaft cavity thereby reducing the resistance of the plug of soil which extends up inside of the pile shaft 12 during insertion of the pile 10 into the ground. Secondly, the presence of the internal thread 20 nieans that the diameter of the external thread 18 can be reduced or indeed the external thread 18 can be omitted altogether. As such, disturbance to the zone of soil or earth surrounding the pile shaft 12 is greatly reduced, thereby increasing the overall support that the surrounding earth provides to the pile and thus increasing the capacity of the pile 10 for resisting lateral loads and moments applied thereto. Thirdly, by reducing the overall diameter of the pile 10, the chances of hitting obstructions in the pile path during insertion of the pile 10 into the ground is reduced. Whilst the presence of the internal thread 20 means that the diameter of the external thread 18 may be reduced or the external thread 18 omitted altogether, when the internal thread 20 is provided in combination with an external thread 18, because of the increased overall area of thread in contact with the soil (internal and external), the longitudinal driving force provided by the thread during rotation of the pile 10 is increased and the resistance of the pile to axial forces, e.g. compression loads, is also increased.

As previously mentioned, whilst in the illustrated embodiment the screw pile 10 is provided with an external thread 18 in addition to the internal thread 20, the external thread 18 is not essential and may he omitted.

As shown in Figures 1 and 2, the internal thread 20 may be formed of an internally protruding wall 24 directed towards the centre of the shaft and extending helically along at least a portion of the length of the internal wall of the shaft 12. Similarly. the external thread 18 may comprise an outwardly protruding wall 22 directed away froni the centre of the shaft 12, running around the outside of the shaft 12 and extending helically along the shaft 12. In some embodiments, the external 1 8 and/or internal 20 threads may he provided by pitched circular steel plates welded onto the exterior and interior respectively of the shaft 12 as shown in Figure 2.

It will he appreciated that in other embodiments, both the external and internal threads 18, 20 may be provided in any suitable form or dimensions (e.g. thread angle, pitch. lead, minor and/or major diameter) provided that the threads 18, 20 supply both the required axial strength to the pile 10 for a particular application and the requisite driving force needed to pull the pile 10 into the ground during installation.

Figure 3 shows a plan view of the internal thread 20 of the screw pile 10. As shown, the thread 20 preferably extends around the internal wall of the shaft 12 for slightly less than one complete revolution of the shaft 12 (3600). However in other embodiments the internal thread 20 may extend around (he shaft for more than one revolution depending on requirements as regards torque and hearing capacity of the pfle 10.

In order to reduce the resistance of the leading edge 26 of the internal thread as the shaft is rotationally driven into the ground, the minor diameter of the thread, that is die distance between the internal edge of the thread and the rotational axis of the shaft 12, preferably increases towards the leading edge 26 of the thread 20.

Additiona'ly or allernatively, the second end 16 of the shaft 12 may he profiled so as to reduce the resistance to entry into (he ground of the ground engaging second end 16 of the shaft 12. For example, one or more scrrations 27a, 27h may he formed in the second end 16 of the shaft 12 to form a selTated edge 28 extending around the second end 16. Preferably, the serration(s) 27a, 27b extend continuously around the circumference of the second end 16.

Whilst Figures 1 and 2 show the shaft 12 having two serrations 27a, 27b, in other embodiments the second end 16 may comprise a single serration or more than two selTations.

Serrations may take any foim or shape known in the art. In some embodiments the teeth or cutting edges of the serrations, generally designated 32. may he profiled so as to form blades.

For example. each cutting edge 30 may be sharpened. The serrations 27a. 27h are preferably located equidistant from one another around the serrated edge 28. For instance, where the serrated edge 28 comprises two selTations 27a, 27b. the cutting edges 32 may be situated directly opposite each other around the circumference of the second end 16.

Referring now to Figure 4. a side view of the internal thread 20 is shown, illustrating the pitch 34 of the internal thread 20 relative to its diameter 36. In a preferred embodiment, the ratio between the major diameter 36 and the pitch 34 of the internal thread 20 is between 2 and 3. For instance, if the shaft 12 has a diameter of 600 mm, the pitch 34 of the internal thread is preferably in the region of 200/300 mm and more preferably 250 mm. It may also be preferable to match the pitch of the internal thread 20 with the pitch of the external thread 18 (where present). By doing so. the downward force provided by the internal and external threads 20, 18 when driving the pile 10 into the ground will also he substantially matched.

Whilst in embodiments described above, the shaft is provided with only one internal thread 20, in other embodiments the shaft 12 may be provided with two or more separate internal threads located along the length of the shaft 12 each providing the advantages discussed in itlalion to the single internal thread 20 described above.

To install thc pile 10, hydraulic or clcctric ally powcttd drilling equipment such as a pilc driver, may engage with the engaging means at the first end of the shaft 12 and rotate the shaft 12. As the shaft 12 rotates, the internal and optional external thread(s) may engage with the ground pulling the pile 10 down and into the ground. Optional downward force may he applied, either manually or by a pile driver, so as to aid engagement of the thread(s) with the ground.

Claims

CLAIMS1. A screw pile, comprising a hollow shaft, a first end and a ground engaging second end, the screw pile arranged to he driven into the ground by rotation of the shalt, the shaft having an internal thread formed along at least a portion of its internal wall.
2. A screw pile as claimed in claim 1, wherein a leading edge of the internal thread is located proximate to the gronnd engaging second end of the shaft.
3. A screw pile as claimed in claims 2, wherein the internal thread comprises a protruding wall directed towards the centre of the shaft and extending helically along at least a portion of the length of the internal wall.
4. A screw pile as claimed in claim 3. wherein the height of the protruding wall decreases towards the leading edge of the internal thread.
5. A screw pile as claimed in any preceding claim, wherein the ratio between the diameter of the shaft and the pitch of the internal thread is between 2 and 3.
6. A screw pile as claimed in any preceding claim, wherein the internal thread extends for one or fewer complete rotations of the thread.
7. A screw pile as claimed in any preceding claim, wherein the hollow shaft further comprises an external thread extending along at least a portion of an external wall of the shaft.
S. A screw pile as claimed in any of claims 2 to 7, wherein a leading edge of the external thread is located further away from the ground engaging second end of the shaft than the leading edge of the internal thread.
9. A screw pile as claimed in claims 7 or 8, wherein the ratio between the diameter of the shaft and the pitch of the external thread is between 2 and 3.S
10. A screw pile as claimed in any of claims 7 to 9, wherein the external thread extends for one or fewer rotations of the thread.
11. A screw pile as claimed in any preceding claim, further comprising one or more secondary internal threads formed along at least a portion of the interna' wall of the shaft and separate from each other and the internal thread.
12. A screw pile as claimed in any preceding claim, further comprising one or more secondary external threads formed along at least a portion of the external wall of the shaft and separate from each other and (he external thread.
13. A screw pile as claimed in any preceding claim, wherein the ground engaging second end comprises one or more serrations formed in a bottom edge of the second end.
14. A method of driving a screw pile according to any of claims 2 to 13 into the ground, the method comprising engaging the ground engaging second end of the screw pile with the ground and applying torque to the first end of the screw pile.
15. A method as claimed in claim 14, further comprising applying force to the first end of the shaft in a direction parallel to the longitudinal axis of the shaft.