GB2365037A

GB2365037A - Cased rotary hole forming tool

Info

Publication number: GB2365037A
Application number: GB0017901A
Authority: GB
Inventors: Peter Gilbert Shotton
Original assignee: Cementation Foundations Skanska Ltd; Kvaerner Cementation Foundations Ltd
Current assignee: Cementation Skanska Ltd
Priority date: 2000-07-20
Filing date: 2000-07-20
Publication date: 2002-02-13
Anticipated expiration: 2020-07-20
Also published as: GB2365037B; GB0017901D0

Abstract

The present invention relates to a method and apparatus for forming a hole in the ground particularly in areas of restricted headroom. A hole-forming tool 1 comprising an auger 6 mounted inside a longitudinal casing 5 is described in which the tool is provided with a means 7, 8, 9 to exert a downward crowd force to the top of the auger. The tool is also configured such that the auger may rotate independently of the casing. The present invention allows a hole to be formed in a number of incremental stages and excavated soil is retained within the casing and can be discharged by back screwing the auger.

Description

<Desc/Clms Page number 1> Cased Rotary Hole Forming Tool The present invention relates to the formation of holes in the ground, and in particular, but not exclusively, to the formation of holes in areas in which the workable height above the ground is restricted to 5 metres or less.

It is often necessary to build underground structures, such as foundation elements, in areas where the workable height is restricted. The restriction may be the result of a physical restriction (such as a basement slab, bridge deck, etc.), high voltage overhead power lines, falling radius (e.g. adjacent to rail tracks, roads, viaducts, etc.).

The construction of such structures will inevitably involve the excavation-of a hole in the .ground which has a depth of considerably larger magnitude than the restricted workable height. When piles are required in areas with restricted workable height, they can be constructed using tripod rigs, minipile rigs, driven precast, and even CFA rigs. Alternatively, telescoping kelly bars comprising a plurality of sections are known.

In conditions of restricted workable height, a11 of the aforementioned methods have disadvantages in that they impose limitations on the size/depth, and thereby load-carrying capacity of the resultant pile. Additionally the physical effort required of the labour force is very demanding, and therefore output is relatively slow and the unit cost high. Rotary boring methods which utilise a telescoping kelly bar are less labour intensive but can be difficult to control and do not offer an effective crowd force. These methods have therefore been found to be both time-consuming and ineffective.

The present invention seeks to alleviate some of

4 the aforementioned problems and advantageously provides a means of excavating holes in the ground in areas of restricted workable height.

According to the present invention there is provided a hole forming tool comprising an auger mounted inside a longitudinal casing, characterised in that (i) said tool further comprises a means to provide a downward crowd force to the top of the auger, and (ii) said auger is arranged such that it may rotate independently of said casing.

The present invention advantageously allows a hole to be formed in the ground by a number of incremental stages. The excavation of the hole is therefore progressive and allows a pile to be formed using a relatively short tool. The excavated soil is retained within the casing and, after the tool has been withdrawn from the hole, can be discharged by back screwing the auger. The tool is advantageously provided with a means to clamp the casing during the discharge of the spoil.

Once the excavation process has been completed, the hole may be filled with a hardening mixture such as concrete or grout. This mixture can be placed by tremie pipe or by concrete pump line.

In addition to the rotational motion of the auger, penetration of the tool into the ground is effected by means of a so called "crowd force". This force acts on the top of the auger in the direction of penetration and in many piling operations is achieved by means of a kelly bar which can be forced downwards by applying a load to the head of the kelly bar.

An important aspect of the present invention is that the tool itself has an "in-built" mechanism for providing a downward crowd force to the top of the rotary auger. Preferably, one or a number of hydraulic thrust plates are arranged inside the tool

casing so as to exert a pressure against the inside surface of the bore. Each of the thrust plates may, for example, be coupled by means of a hinged lever to one or more crowd rams positioned above the rotary auger._ After the thrust plates have been forced against the surface of the bore, the crowd force exerted by the ram then provides an axial force on to the auger. In this way, the cased auger can be pushed downwards to assist penetration of the ground. Preferred embodiments of the present invention utilise two sets of thrust plates/crowd rams which are advantageously arranged on opposite sides of the casing.

Alternatively the thrust plates can be forced outwards by means of rams which are independent of the vertical crowd rams.

In order for the crowd facility to function satisfactorily, it is necessary to prevent the casing from rotating with the motion of the auger. This can be achieved in a number of ways.

For example, the provision of a square or rectangular casing is particularly useful since this shape will serve to suppress the tendency of the casing to rotate with the motion of the auger. However, the profile of the casing can be any shape although square, rectangular or circular profiles are the most common. Clearly the profile of the resultant pile depends upon the configuration of the casing and, as such, the conditions of the process are likely to influence the choice of casing used. A substantial part of the load bearing capacity of the pile is carried in the friction generated at the interface between the pile and the surrounding ground. Consequently, a square profile, which has a greater perimeter than say, a circular profile, will have a greater load bearing capacity for an equivalent volume of concrete.

In order to prevent the rotation of a hole forming tool which is provided with a circular or oval casing, the outer surface of the casing is advantageously provided with one or more anti-rotation fin (s) which extend(s) beyond the profile of the tool. The anti- rotation fin(s) may run along the entire length of the casing or just along a part thereof and serve to suppress the tendency of the circular/oval casing to rotate with the motion of the auger.

The rotational motion and the crowd facility of the auger can be powered either hydraulically or electrically. Preferably, the tool is provided with, for example, a hydraulic system which causes *a hydraulic motor (positioned inside the casing) to rotate the auger and/or operate the pressurised thrust plates.

The configuration of the auger can be chosen depending on the boring conditions. For example, an auger having one or a number of continuous helical flight(s) may be provided wherein the pitch of the flight, the number of flights and the rotational speed can be chosen as necessary. Furthermore, the present invention can be utilised in direct boring applications as well as in conjunction with reverse circulation drilling equipment.

The casing may be constructed from steel, plastics or any other suitably rigid material According to a second aspect of the present invention there is provided a method of forming a hole in the ground comprising the steps of: i) applying to the ground a hole forming tool comprising an auger mounted inside a .longitudinal casing, wherein said tool also comprises means to provide a downward crowd force, ii) causing the tool to penetrate the ground to a predetermined depth;

iii) withdrawing the tool from the hole and then discharging excavated soil retained inside said casing; iv) repeating steps i) to iii) above until the required depth has been reached, For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: Figures 1A and 1B show a hole forming tool of the present invention in which the casing has a square profile; Figures 2A and 2B show a hole forming tool of the present invention in which the casing has a circular profile; Figure 3 shows an example of a clamp and crowd device which is used to provide the downward crowd force in accordance with the present invention; Figure 4 shows an example of a suitable rig configuration for use with a hole forming tool of the present invention.

Figure 1A shows, from above, a tool 1 of the present invention having a longitudinal casing 5 with a square shaped profile. A multi-part hoisting system 3 is also provided on the top of the tool allowing it to be lifted by way of a rope suspension.

Figure 1B shows a cross section through the length of the tool. The rotary auger 6, having one or more continuous helical flights, is mounted inside the casing 5 and the drawing shows a pair of hydraulically powered thrust plates 7, which are arranged such that they may exert a pressure against the inside surface of the bore. The hydraulic thrust plates 7 are arranged so as to be adjacent to a detachable bracket 4 which is initially flush with the outer surface of the casing 5. When the thrust plates are operated, the brackets are caused to. move outwards against the inner surface of

the bone.

Each of the thrust plates is also coupled to a crowd ram 8, which contacts a plate 9 attached to the top of the auger such that it may exert an axial downward force thereon. This force is braced against the sides of the bore by means of the hydraulically operated thrust plates, thereby providing the tool with an "in-built" mechanism for supplying the downward force necessary for the tool to penetrate the ground.

A motor 10, connected to hydraulic motor hoses 11, is also arranged inside the tool casing and serves to power both the rotational motion of the auger and the pressurised thrust plates 7. Although the embodiments described involve the use of hydraulic power to supply the motor and crowd mechanism, an electrically powered tool is equally feasible.

Figures 2A and 2B show another embodiment of the present invention in which the profile of the casing is substantially circular. In order to simplify the illustration, the clamp-crowd facility is not shown.

In order to prevent the rotation of the casing during rotary boring, the tool is provided with a pair of stabiliser fins 14 on the outside surface of the casing. These fins extend beyond the envelope of the tool into the surrounding soil, such that, in use, they suppress the tendency of the circular casing to rotate with the auger 15.

Figure 3, shows an example of a clamp-crowd device for providing the downward crowd force according to the present invention. The device comprises a thrust plate 20 which is either hydraulically or electrically powered such that it exerts a horizontal force against the inside surface of the bore. The thrust plate is coupled to a longitudinal crowd ram 21, by means of a lever 22 hingeably connected to the thrust plate 20 at one end, and to the crowd ram 21 at the other. This

configuration allows the crowd ram to move up and down relative to the thrust plates forced against the sides of the bore.

Figure 4, shows an example of a suitable rig arrangement for use with a hole forming tool 30 of the present invention. The tool is suspended by means of ropes 31 connected to the top of the tool by way of a multi-part hoisting system. The length of the ropes 31 determine the maximum depth to which the tool can penetrate the ground. The rotational motion of the tool is powered by a motor 35 connected to hydraulic hoses 32 wound around a hose winch drum 33. The rig may need to be adapted to account for the restricted headroom. In particular, the height of the operator cab 34 may need to be reduced depending on the headroom available. Alternatively it is pbssible to operate the rig by means of remote control, in which case no operator cable is required. Similarly the power source, which can be electrical or hydraulic, can be located remotely.

Claims

CLAIMS: 1. A hole forming tool comprising an auger mounted inside a longitudinal casing, characterised in that (i) said tool further comprises a means to provide a downward crowd force to the top of the auger, and (ii) said auger is arranged such that it may rotate independently of said casing.
2. A hole forming tool according to claim 1, wherein the tip of the auger protrudes from the longitudinal casing.
3. A hole forming tool as claimed in claim 1 or 2, wherein the means for providing said crowd force comprises at least one crowd ram which exerts a downward force on the top of said auger, wherein the force exerted by said crowd ram reacts against the side of the hole by means of a thrust plate which may be pressurised or operated by a separate ram.
4. A hole forming tool as claimed in claim 2, wherein said pressurised thrust plate (s) is or are hydraulically operated.
5. A hole forming tool as claimed in any preceding claim, wherein a hydraulic motor is arranged inside the casing of the tool which supplies the power necessary to rotate the auger and/or to provide the downward crowd force.
6. A hole forming tool according to any one of claims 3, 4 or 5, wherein said tool is provided with a pair of said thrust plates which are positioned on opposite sides of said casing.
7. A hole forming tool according to any preceding claim, wherein said casing has a square or rectangular profile.
8. A hole forming tool according to any one of claims 1 to 6, wherein said casing has a substantially circular profile.
9. A hole forming tool according to claim 8,

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wherein the outer surface of said casing is further provided with at least one stabiliser fin which serves, in use, to suppress the rotational motion of said casing.
10. A method of forming a hole in the ground comprising the steps of: i) applying to the ground a hole forming tool comprising an auger mounted inside a longitudinal casing, wherein said tool also comprises means to provide a downward crowd force to the top of the auger; ii) causing the tool to penetrate the ground to a predetermined depth; iii) discharging excavated soil retained inside said casing; iv) repeating steps i) to iii) above until the required depth has been reached, " wherein said auger is arranged such that, in use, it may rotate independently of said casing.
11. A method of forming a hole in the ground according to claim 10, wherein the tip of said auger protrudes from the longitudinal casing.
12. A method according to claim 10 or 11, wherein the means for providing said crowd force comprises at least one crowd ram which exerts a downward force on the top of said auger, wherein the force exerted by said crowd ram reacts against the side of said casing by means of a thrust plate which may be pressurised or operated by a separate ram.
13. A method according to claim 12, wherein said pressurised thrust plate(s) is or are hydraulically operated.
14. A method according to any one of claims 10 to 13, wherein said tool further comprises a hydraulic motor arranged inside the casing of said tool to supply the power necessary to rotate the auger and/or to provide the downward crowd force.

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15. A method according to any one of claims 10 to 14, wherein said tool is provided with a pair of said thrust plates which are positioned on opposite sides of said casing.
16. A method according to any one of claims 10 to 15, wherein said casing has a square or rectangular profile.
17. A method according to any one of claims 10 to 15, wherein said casing,has a substantially circular profile..
18. A method according to claim 17, wherein the outer surface of said casing is further provided with at least one stabiliser fin which serves, in use, to suppress the rotational motion of said casing.
19. A hole forming tool substantially as herein described with reference to the accompanying drawings.