GB2362672A

GB2362672A - Conical foundation pile having a screw thread for urging the pile into the ground

Info

Publication number: GB2362672A
Application number: GB0012367A
Authority: GB
Inventors: Frederick Thomas Alan Coakley; Ian Alick Last Brixey; Keith Stuart Miller; Gary Andrew Long
Original assignee: M40 TRAINS Ltd; Laing Rail Ltd
Current assignee: M40 TRAINS Ltd; Arriva Trains Holdings Ltd
Priority date: 2000-05-22
Filing date: 2000-05-22
Publication date: 2001-11-28
Also published as: WO2001090489A1; AU5860601A; EP1287207A1; GB0012367D0

Abstract

A foundation pile member (1) is in the form of a body of revolution tapering downwardly between a broad upper end (2) and a narrower lower end (3). The periphery (4) of the foundation is provided with a ridge (7), preferably in the form of a screw thread (6), which urges it into the ground when rotated and subjected to a downward force. The foundation may be formed of concrete, plastics or wood and may further have a covering of friction-reducing material. A recess (12) or projection may be provided at the upper end (2) of the foundation for engagement by a rotating drive.

Description

2362672 1 M&C Folio: GBP82034 FOUNDATION Document #: 512701 This invention

relates to foundations for supporting structural loads, and in particular relates to a foundation member which may be installed quickly and efficiently on a building site.

In construction projects it is often necessary to construct foundations in a very short time period because of time restrictions in gaining access to the required work areas or the need to quickly assemble structures on site. This precludes the use of conventional techniques involving the use of in situ concrete, which requires several days to gain sufficient strength to be load supporting. The alternative is a pre- formed system, of which many are available, but their method of installation normally involves driving or vibrating the system into the ground. The resulting noise and vibration can often be unacceptable in built-up areas, greatly limiting their use. A further problem is that on many sites the upper layer of subsoil is in a loose or soft state and conventional foundations need to extend to considerable depth to achieve adequate bearing.

The present invention provides a foundation member in the form of a body of revolution which tapers downwardly and the periphery of which has one or more spirally extending ridges.

The invention also provides a method of installing a foundation, including placing the lower end of the foundation member in the ground and rotating the foundation member while it is subjected to a downward force so that it is urged downwards by the interaction of the spirally extending ridge or ridges and the subsoil.

As the foundation member is installed in the ground, the surrounding subsoil is displaced and compacted, thereby enhancing its capacity.

2 The invention will be described further by way of example only with reference to the accompanying drawings. in which:

Figure 1 is a plan view of a preferred embodiment of a foundation member; Figure 2 is a side view of the foundation member; Figure 3 is an enlarged section on line Ill-III in Figure 2; Figure 4 is an axial section through a foundation member similar to that shown in Figures 1 to 3, showing a pilot hole; Figure 5 is a view, similar to Figure 4, of another embodiment of the foundation member; and Figure 6 is a side view of another embodiment of the foundation member.

The foundation member 1 illustrated in Figures 1 to 3 is in the form of an inverted truncated cone whose broader upper end 2 may, for example, be about 600 mm in diameter, whose narrower lower end 3 may, for example, be 75 mm in diameter, and whose height H may, for example, be about 1.2 m. The angle (x between the downwardly tapering periphery 4 of the foundation member 1 and its vertical axis 5 may, for example, be about 12'. The foundation member is preferably made of pre-cast reinforced concrete and may, for example, weigh about half a tonne.

The periphery 4 of the foundation member 1 is provided with a single continuous screw thread 6 which extends upwards from the lower end 3 of the foundation member and terminates near the upper end 2. The screw thread 6 is constituted by a spirally extending ridge 7 which is integral with the foundation member 1. The ridge 7 has a cross-section (Figure 3) which is substantially mirror-symmetrical with respect to a line 8 perpendicular to the vertical axis 5.

3 The ridge has an upwardly facing upper flank 9, a rounded crest 10, and a downwardly facing lower flank 11. The pitch P of the screw thread 6 may, for example, be about 1/7th of the height H of the foundation member 1 (and is constant over the whole extent of the screw thread).

The upper end of the foundation member 1 has a recess 12 which is, for example, square in plan and may have chamfered corners 13.

To install a foundation using the foundation member 1, the lower end 3 of the foundation member is initially placed in the ground, preferably after a pilot hole 14 (Figure 4) of smaller diameter than the upper end 2 has been drilled in the ground. The depth of the pilot hole 14 may be approximately equal to the height H of the foundation member or may be much shorter, e.g. about half the height, depending on the state of the subsoil. A drive bar is then placed in the recess 12 and is rotated, e.g. using a similar drilling rig to that used to form the pilot hole 14. As the foundation member 1 is rotated, it is subjected to a downward force due to its own weight and an additional downward force applied by the drilling rig. The spirally extending ridge 7 bites into the subsoil and urges the foundation member 1 downwards so that it is screwed into the ground. Rotation is continued until the upper end 2 is substantially flush with or below the ground level. In the illustrated embodiment, this will occur after about 7 turns of the foundation member. The dimensions of the recess 12 are such that there remains a sufficient thickness of material around it to prevent tensile cracking during installation, and the recess 12 is sufficiently deep to allow the rotating drive bar to maintain the foundation member 1 in a vertical position during installation. The process of installation causes little or no noise or vibration.

4 Various modifications may be made within the scope of the invention. Depending on the ground conditions, the periphery 4 may be covered with a slip material, e.g. bitumen, in order to reduce friction and thereby reduce the torque required during installation. Although pre-cast reinforced concrete is the preferred material for the foundation member, it may be made of any other suitable material with inherent tensile strength, such as plastics material or wood, for example. It will be appreciated that the choice of material depends on various factors, such as weight, durability, and cost. The shape of the recess 12 may be varied to suit the cross-section of the drive bar. Alternatively, the recess may be replaced by an upward projection, which may be integral with or cast into the material of the foundation member and which can engage with a drive socket.

The lower end of the foundation member may taper to a rounded-off point or may be truncated. In Figure 2, for example, the lower end is slightly more truncated than in Figure 4. A more severely truncated foundation member is described below (Fig. 5).

The periphery 4 may have a substantially constant taper, as shown, or may have a varying taper depending on the condition of the subsoil. For example, the angle of taper may increase or decrease, gradually or stepwise, from the upper end portion to the lower end portion. The angle cc of taper which is selected will depend on the compressibility of the subsoil. In weaker, more compressible subsoil a wider taper (at most 201, preferably at most 15') may be used to displace the subsoil more and to increase its compaction as the foundation member is screwed into the ground, preferably after forming a relatively small pilot hole. In stronger, less compressible subsoil, a narrower taper (at least 2', preferably at least 5') may be used, the lower end of the foundation member being truncated; this allows easier installation in a larger pilot hole 14. The truncated end surface of the cone acts in end bearing in addition to the wedging effect of the tapering periphery.

The substantially symmetrical form of the ridge 7 assists in allowing the foundation member to be removed from the ground by a reversed screwing action. If this is not required, the upper flank 9 may be made more nearly horizontal and the lower flank 11 may slope downwards more steeply. The cross-sectional size and shape of the screw thread 6 and its pitch P are chosen to provide efficient screwing in of the foundation member and to ensure that the shear surface 15 for vertical loading occurs within the subsoil (not at the interface at the periphery 4) and are dependent on the angle cc of taper and the type of subsoil. The pitch P may, for example, be from one-fifth to one twentieth-of the height H. The screw thread 6 in the embodiment illustrated is a single start screw thread, but, depending on the required pitch, a double start or multiple start screw thread may be used. It may be sufficient for the screw thread to extend only about half way up the foundation member. Although the screw thread 6 has been illustrated as constituted by a single continuous ridge 7, there may be situations in which it would be preferable to interrupt the screw thread 6 periodically so that, in effect, a series of ridges extending spirally would be provided.

Although the ridge 7 has been shown as being formed integrally with the body of the foundation member 1, the ridge could be partially embedded in or fixed to the body of the foundation member.

The foundation member I shown in Figure 5 is suited to stronger subsoils than the foundation member shown in Figures I to 3 or Figure 4. It will be noted that the angle a is smaller (for example, about 6'), the foundation member I is more truncated, and the pilot hole 14 is relatively large.

The foundation member 1 shown in Figure 6 is similar to that shown in Figure 1 except that the ridge 7 is not mirror-symmetrical. Instead, the lower flank of the ridge 7 is coincident with the tapering periphery 4 of the foundation member and the upper flank 9 is more nearly horizontal (at least adjacent the peripheral surface immediately above it).

6

Claims

1. A foundation member in the form of a body of revolution having a vertical axis, the foundation member tapering downwardly between a broader upper end portion and a narrower lower end portion, the periphery of the foundation member having at least one spirally extending ridge which urges the foundation member downwards when its lower end portion is placed in the ground and it is rotated in one direction about its vertical axis while subjected to a downward force.

2. A foundation member as claimed in claim 1, in which the at least one ridge constitutes at least one screw thread extending upwards from the lower end portion of the foundation member.

3. A foundation member as claimed in claim 2, in which the at least one screw thread extends at least half way up the foundation member.

4. A foundation member as claimed in claim 2 or 3, in which the pitch of the at least one screw thread is at most one-fifth of the height of the foundation member.

5. A foundation member as claimed in any of claims 2 to 4, in which the pitch of the at least one screw thread is at least one-twentieth of the height of the foundation member.

6. A foundation member as claimed in any preceding claim, in which the at least one ridge has a cross-section which is substantially mirrorsymmetrical with respect to a line perpendicular to the vertical axis.

7. A foundation member as claimed in any of claims 1 to 5, in which the at least one ridge has a lower flank coincident with a tapering peripheral surface of the foundation member.

7

8. A foundation member as claimed in any preceding claim, in which the at least one ridge has a rounded crest.

9. A foundation member as claimed in any preceding claim, in which the foundation member tapers substantially conically.

10. A foundation member as claimed in any preceding claim, in which the angle between the tapering periphery of the foundation member and the vertical axis is at least 20 and at most 200.

11. A foundation member as claimed in any preceding claim, in which the foundation member is made from reinforced concrete, plastics material, or wood.

12. A foundation member as claimed in any preceding claim, in which the periphery of the foundation member has a covering of friction-reducing material.

13. A foundation member as claimed in any preceding claim, in which the upper end portion of the foundation member is provided with a recess or projection for engagement by a rotating drive.

14. A method of installing a foundation, using a foundation member according to any preceding claim, including placing the lower end portion of the foundation member in the ground and rotating the foundation member about its vertical axis while it is subjected to downward force so that it is urged downwards by the interaction of the at least one spirally extending ridge and the subsoil while the tapering periphery of the foundation member displaces and compacts the subsoil.

8

15. A method as claimed in claim 14, in which the downward force to which the foundation member is subjected exceeds the force of gravity on the foundation member.

16. A method as claimed in claim 14 or 15, including forming a pilot hole in the ground before placing the lower end portion of the foundation member in the ground.

17. A foundation member substantially as described with reference to, and as shown in, Figures 1 to 3, Figure 4, Figure 5, or Figure 6 of the accompanying drawings.

18. A method of installing a foundation, substantially as described with reference to the accompanying drawings.