BEARING FOUNDATION MEMBER

Abstract: The periphery of a downwardly tapering bearing foundation member (1), e.g. made of reinforced concrete, has a spirally extending ridge (7) which projects from the tapering peripheral surface (4) and which is small in radial and axial extent compared with the axial spacing (P) of the turns of the spirally extending ridge. The lower end of the foundation member (1) is placed in the ground and it is rotated while subjected to a downward force, preferably greater than the force of gravity. The spirally extending ridge (7) bites into the subsoil and urges the foundation member (1) downwards while the tapering periphery (4) displaces and compacts the subsoil.

Description: BEARING FOUNDATION MEMBER This invention relates to foundations for supporting structural loads, and in particular relates to a bearing 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 friction pile, of which many are available, but their method of installation normally involves driving or vibrating the friction pile into the ground. The resulting noise and vibration can often be unacceptable in built-up areas, greatly limiting their use. To avoid this problem it has been proposed to provide friction piles with screwthreads. 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 bearing foundation member comprising a body of revolution having a vertical axis, the body having a peripheral surface tapering downwardly from a broader upper end to a narrower lower end, the foundation member having at least one spirally extending ridge which projects from the tapering peripheral surface and which is small in radial and axial extent compared with the axial spacing of the turns of the at least one spirally extending ridge, the foundation member being urged downwards when the lower end of the body is placed in the ground, the body is rotated in one direction about its vertical axis while subjected to a downward force, and the at least one spirally extending ridge bites into the subsoil. The invention also provides a method of installing a bearing foundation, including placing the lower end of the body of the foundation member in the ground and rotating the body 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 peripheral surface of the foundation member displaces and compacts the subsoil. The displacement and compaction of the surrounding subsoil enhances its bearing capacity, enabling the foundation member to be supported by the near-surface soils, obviating the need to penetrate to more competent strata at greater depth as in the case of conventional friction piles. 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 bearing foundation member; Figure 2 is a side view of the foundation member; Figure 3 is an enlarged section on line III-III in Figure 2; and Figure 4 is an axial section through a bearing foundation member similar to that shown in Figures 1 to 3, showing a pilot hole. The bearing foundation member 1 illustrated in Figures 1 to 3 is in the form of an inverted slightly-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 (i.e. about twice the maximum diameter). The angle α 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 spirally extending ridge 7 which extends upwards from the lower end 3 of the foundation member and terminates near the upper end 2. The spirally extending ridge 7 is integral with the foundation member 1 and has a cross-section (Figure 3) which is substantially mirror-symmetrical. The ridge 7 has an upwardly facing upper flank 9, a rounded crest 10, and a downwardly facing lower flank 11. The pitch P of the spirally extending ridge 7 may, for example, be about l/7th of the height H of the foundation member 1 (and is constant over the whole extent of the screw thread). The ridge 7 is small in both radial and axial extent compared with the pitch P (the axial spacing of the turns) and does not substantially project beyond the upper end 2 when viewed from above, as can be seen from the drawings. 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 bearing 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. 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. The ridge 7 is free of friction-reducing material. 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. 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 α of taper which is selected will depend on the compressibility of the subsoil. In weaker, more compressible subsoil a wider taper (at most 20°, 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 5°, preferably at least 10°) may be used, the lower end of the foundation member being truncated. The truncated end surface of the cone acts in end bearing in addition to the wedging effect of the tapering periphery. The ratio of height to largest diameter of the foundation member varies with the taper: for more compressible subsoil the ratio could be as low as one; for less compressible subsoil it could be as high as five. 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 spirally extending ridge 7 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 α of taper and the type of subsoil. The pitch P may, for example, be from one-fifth to one-twentieth of the height H. A single spirally extending ridge 7 is used in the embodiment illustrated, but, depending on the required pitch, two or more such ridges may be used. It may be sufficient for the ridge to extend only about halfway up the foundation member. Although the ridge 7 has been illustrated as a single continuous ridge, there may be situations in which it would be preferable to interrupt the ridge 7 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.

Claims: Claims 1. A bearing foundation member comprising a body of revolution having a vertical axis, the body having a peripheral surface tapering downwardly from a broader upper end to a narrower lower end, the foundation member having at least one spirally extending ridge which projects from the tapering peripheral surface and which is small in radial and axial extent compared with the axial spacing of the turns of the at least one spirally extending ridge, the foundation member being urged downwards when the lower end of the body is placed in the ground, the body is rotated in one direction about its vertical axis while subjected to a downward force, and the at least one spirally extending ridge bites into the subsoil. 2. A bearing foundation member as claimed in claim 1, in which the at least one ridge extends spirally upwards from the lower end portion of the body, preferably at least halfway up the body. 3. A bearing foundation member as claimed in claim 1 or 2, in which the pitch of the at least one spirally extending ridge is at most one-fifth of the height of the body. 4. A bearing foundation member as claimed in any of claims 1 to 3, in which the pitch of the at least one spirally extending ridge is at least one-twentieth of the height of the body. 5. A bearing foundation member as claimed in any preceding claim, in which the ratio of the height of the body to the diameter of its upper end is in the range from 1 to 5, preferably about 2. 6. A bearing foundation member as claimed in any preceding claim, in which the at least one ridge has a cross-section which is substantially mirror-symmetrical. 7. A bearing foundation member as claimed in any preceding claim, in which the at least one ridge has a rounded crest. 8. A bearing foundation member as claimed in any preceding claim, in which the peripheral surface tapers substantially conically. 9. A bearing foundation member as claimed in any preceding claim, in which the angle between the tapering peripheral surface and the vertical axis is at least 5° and at most 20°. 10. A bearing foundation member as claimed in any preceding claim, in which the at least one ridge lies substantially within the circumference of the upper end of the body when viewed from above. 11. A bearing foundation member as claimed in any preceding claim, in which the body is made from reinforced concrete, plastics material, or wood. 12. A bearing foundation member as claimed in any preceding claim, in which the tapering peripheral surface has a covering of friction-reducing material. 13. A bearing foundation member as claimed in any preceding claim, in which the upper end of the body is provided with a recess or projection for engagement by a rotating drive. 14. A method of installing a bearing foundation, using a bearing foundation member according to any preceding claim, including placing the lower end of the body of the foundation member in the ground and rotating the body 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 peripheral surface of the foundation member displaces and compacts the subsoil. 15. A method as claimed in claim 14, in which the downward force to which the body 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 of the body in the ground.

Inventors: BRIXEY IAN ALICK LAST, COAKLEY FREDERICK THOMAS ALAN, LONG GARY ANDREW, MILLER KEITH STUART, BRIXEY, IAN, ALICK, LAST, COAKLEY, FREDERICK, THOMAS, ALAN, LONG, GARY, ANDREW, MILLER, KEITH, STUART
Applicants: BRIXEY IAN ALICK LAST, COAKLEY FREDERICK THOMAS ALAN, LAING RAIL LTD, LONG GARY ANDREW, M40 TRAINS LTD, MILLER KEITH STUART, BRIXEY, IAN, ALICK, LAST, COAKLEY, FREDERICK, THOMAS, ALAN, LAING RAIL LIMITED, LONG, GARY, ANDREW, M40 TRAINS LIMITED, MILLER, KEITH, STUART
Application year: 2001
Main classification: E02D5
IPC codes: E02D5, E02D7