GB2360809A

GB2360809A - A screw pile

Info

Publication number: GB2360809A
Application number: GB0104330A
Authority: GB
Inventors: Peter Dunn; Duncan Clift Mcgregor
Original assignee: SCREW FAST FOUNDATIONS Ltd
Current assignee: SCREW FAST FOUNDATIONS Ltd
Priority date: 2000-03-28
Filing date: 2001-02-22
Publication date: 2001-10-03
Anticipated expiration: 2021-02-22
Also published as: GB2360809B; GB0104330D0

Abstract

A screw pile has a shaft (10) and at least one helix (12) acting as a screw thread. The helix (12) has a plough formation (20) on the trailing edge which may point upwardly or downwardly and serve to radially compress soil as the pile penetrates the ground. Such an arrangement is said to improve resistance to tension and compression loads.

Description

2360809 P09841 A SCREWPILE AND METHOD OF INSTALLATION THIS INVENTION

concerns screw piles adapted to penetrate the ground to serve as an anchor to support tension or compression loads. Such anchors are used, for example, to secure telecommunication masts and the like.

It is known to provide a screw pile comprising a shaft having, at longitudinally spaced locations, a series of helices acting as screw threads to cause the pile to penetrate the ground upon the application of torsion to the shaft.

Typically, torsion is applied to the head of a shaft and the helices screw into the ground without the need to exert a large downwards force. The applied torque is typically known to be proportional to the pull-out force which the anchor is required to resist. After the pile is screwed into the ground upwards tension is applied to the shaft whereupon there will be some upwards movement until the helices have "bedded-in" or re- compacted the soil above the helices, which has been disturbed by the downward passage of the helices. Once the bedding-in has been achieved further upwards movement is prevented or at least greatly reduced.

Such devices can be used also to resist compression loads. These employ the same principle whereby, after penetration, a downwards linear force is applied to the shaft -2to cause the helices to bed-in after which further downwards movement is effectively prevented. The amount of bedding-in movement in a downwards direction is far less than that in an upwards direction since, beneath the first or lowermost helices, little or no disturbance of the soil has occurred.

Where a ground anchor is required to resist both tension and compression such as might occur in an alternating and varying manner as a result of windage against a mast anchored to the ground by such means, it is necessary, in some cases, to provide two anchors one of which resists an upward pull and the other resists a downwards force.

An object of the present invention is to provide a screw pile which is selfbedding as it penetrates the ground whereby little or no linear bedding movement is required or available.

According to the present invention, there is provided a screw pile comprising a shaft having at least one helix acting as a screw thread to cause the pile to penetrate the ground upon rotation of the shaft; characterised in that the helix has a plough formation to cause the surrounding ground to be compressed radially outwardly as the helix penetrates the ground.

An enlarged diameter shaft portion may extend from the helix in a direction opposite to the direction of penetration.

The pile may have a series of helices at longitudinally spaced positions along the shaft, and each may have a plough formation with an enlarged diameter shaft portion extending from the helix in the direction of penetration.

Each helix may be of greater diameter than the immediately preceding helix lying in the order of penetration.

The helices may be spaced at intervals of approximately three times the helix diameter.

The plough formation may be provided in an inner region of the trailing edge of a helix and comprising a portion thereof bent upwardly and outwardly with respect to the helix and the axis of the shaft respectively.

The plough formation may be provided in an inner region of the trailing edge of a helix and comprising a portion thereof bent downwardly and outwardly with respect to the helix and the axis of the shaft respectively.

A supply of grout may be provided and introduced into the bore created by the pile as it descends.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:- Fig. 1 illustrates the lower region of a screw pile made in accordance with the invention; Fig. 2 is an end view; Fig. 3 is an end view of a modification; and Fig. 4 is a schematic view of a further modification, in use.

In conventional screw piles a shaft carries at longitudinally spaced locations a series of helices which may be of equal diameter or may vary in diameter from end-to-end of the pile. Each helix takes approximately one turn around the shaft and thus may be formed initially as a disc which is cut radially and twisted into a helix and welded to the outer surface of the shaft. The helices are provided at spaced intervals equal to approximately three times the diameter of each helix. The helices are spaced in this way such that areas of pressure applied to the surrounding ground from the helices in use, do not overlap. If a continuous helix were used, rather like a self-tapping screw, considerably more torque would be required to install it with very little improvement in its pull- out resistance.

Although a continuous helix would leave no voids or loose ground in its wake unlike the spaced helix system and would therefore not require bedding in, nevertheless the installation force required is excessive and undesirable in most applications.

Therefore, the present invention is concerned with avoiding the considerable movement required in a conventional screw pile to re-compact the loosened soil but without a large increase in the application torque required for installation.

Referring now to the drawings, there is provided a central shaft generally indicated at 10 and usually of tubular form. In the present application this consists of a leading tube 11 having an angled end to provide a rearning action in the ground. A first helix 12 is situated at and attached to the tube 10 at the junction between the leading tube 11 and a second stage, larger diameter tube 13. At the upper end of the tube 13 is a further helix 14 with a slightly greater diameter than that of the helix 12, and above this is a further tube section 15 of greater diameter than the tube section 13. Thus, for example, the helix 14 may be in the region of 30Omm in diameter and the helix 12 some 250mm in diameter. Three or more such helices each with an enlarged tube section above it may constitute the screw pile.

The direction of rotation of the pile when penetrating the ground is' illustrated by arrow 16, this being clockwise for a right-hand thread.

Referring to Fig. 2, it will be seen that each disc-shaped helix is cut in a curved, generally radial, line 17. In the inner region of the trailing edge of each helix there is a plough formation 20 created by bending downwardly and outwardly, the inner trailing edge region of the helix. Thus when the pile is screwed into the ground the plough formation 20 of each helix serves, much as a ploughshare, to force the surrounding soil radially outwardly towards the outer edge of the helix thus re- compacting the soil as the helix -6penetrates the ground. In this way, the soil is compressed beneath each helix and has no tendency to bed-in or give when a compression load is subsequently applied to the pile.

In some cases the plough formation 20 should, instead, be bent upwardly and outwardly, particularly for very dense ground materials.

The void or disturbed soil consequently formed above each helix close to the shaft is taken up and occupied by the larger diameter shaft above each helix. Thus, there is little or no bedding in of the helices when a load is placed in tension on the pile.

It will be seen therefore, that as the pile is installed, it becomes resistant to tension and compression loads thus reducing or substantially removing any capability of the pile to bed-in subsequent to installation.

Because each plough formation is located close to the shaft wall, its resistance to the applied torque during installation is minimal whilst its compacting action is substantial.

Referring now to Fig. 3, in an alternative arrangement a straight radial cut 18 provides the helical trailing edge to which a plough formation is added by way of a small triangular plate 19 welded to the edge.

Referring now to Fig. 4 in an alternative arrangement the shaft 10 may be of uniform diameter throughout its length and joined in sections, for example, at 30.

In the previous embodiments, the enlarged diameter shaft above each helix served to occupy the entire void created by the helix by its downward passage. In the present example however, the void created by the plough formation 20 is filled by a grouting material as the screw pile penetrates the ground. For this purpose, an open reservoir 31 is provided at the top of the bore site and is kept filled with grout 32 which thus follows the screw pile as it descends. Indeed, the helices assist in drawing the grout downwardly along the bore but being incomplete discs there is no tendency for voids to occur above the helices.

If required, each helix may be of greater or different form from the preceding helix, depending upon the soil parameters.

Once the grout is set, an extremely rigid and secure screw pile is created which, however, cannot be removed other than by excavation and so this method would be adopted in cases where subsequent removal of the screw pile is not required.

Several advantages accrue from this modified form of the invention. For example, the smaller diameter shaft or shaft sections can be used for the full length of the pile. Secondly, the grout serves to lubricate the rotational interface as the helices descend so that reduced installation torques are permitted for the same bearing capacity.

The grout also serves as a protective barrier against corrosion. The shafts which are usually of steel will be typically galvanised but the grout will serve to reduce scratching of the galvanised surfaces as well as offering extra corrosion protection.

The frictional resistance provided by the grout/soil interface enhances the load bearing capacity of the pile.

Claims

1. A screw pile comprising a shaft having at least one helix acting as a screw thread to cause the pile to penetrate the ground upon rotation of the shaft; characterised in that the helix has a plough formation to cause the surrounding ground to be compressed radially outwardly as the helix penetrates the ground.

2. A screw pile according to Claim 1, wherein an enlarged diameter shaft portion extends from the helix in a direction opposite to the direction of penetration.

3. A screw pile according to Claim 1 or Claim 2, having a series of helices at longitudinally spaced positions along the shaft, each helix having a plough formation and an enlarged diameter shaft portion extending from the helix in a direction opposite to the direction of penetration.

4. A screw pile according to Claim 3, wherein each helix is of greater diameter than the immediately preceding helix lying in the order of penetration.

5. A screw pile according to Claim 3, wherein the helices are spaced along the shaft at intervals of approximately three times the helix diameter.

6. A screw pile according to any preceding claim, wherein the plough formation is provided in an inner region of the trailing edge of the helix in the direction of rotation of the shaft.

7. A screw pile according to Claim 6, wherein the plough formation comprises a portion of the trailing edge of the helix bent upwardly and outwardly with respect to the helix and the axis of the shaft respectively.

8. A screw pile according to Claim 6, wherein the plough formation comprises a portion of the trailing edge of the helix bent downwardly and outwardly with respect to the helix and the axis of the shaft respectively.

9. A screw pile according to Claim 6, wherein the plough formation comprises a plate welded to said inner region of the trailing edge of the helix.

10. A screw pile according to Claim 1 or Claim 2, having a series of helices at longitudinally spaced positions along the shaft, and the shaft is of uniform diameter throughout its length and joined in lengthwise sections.

11. A method of installing a screw pile according to any of Claims 1, 2, 4, 5, 6, 7, 8, 9 or 10, comprising the steps of causing the pile to penetrate the ground by rotation of said shaft and introducing a grouting material around the shaft as the screw pile penetrates the ground whereby a void created behind the or each helix by the plough formation thereon as the screw pile penetrates the ground is filled by the grouting material.

12. A method according to Claim 11, including the step o. f providing a grout filled reservoir at ground level to enable the grout to follow the screw pile as it penetrates the ground.