GB2172632A

GB2172632A - Under-reaming of small diameter boreholes

Info

Publication number: GB2172632A
Application number: GB08507507A
Authority: GB
Inventors: Michael Thomas Durkan
Original assignee: Fondedile Foundations Ltd
Current assignee: Fondedile Foundations Ltd
Priority date: 1985-03-22
Filing date: 1985-03-22
Publication date: 1986-09-24
Also published as: GB2172632B; GB8507507D0

Abstract

An under-reaming machine for forming one or more enlargements below the surface of a borehole preformed in the ground comprises one or more pairs 3, 4, 5 of cutting blades 1, 2. The cutting blades 1, 2 are hinged together at one end, and respectively to an inner shaft 7 and an outer shaft 8 at their other ends. Relative axial movement of the shafts 7, 8 thus causes controlled opening and closing of the cutting blades 1, 2. In use the cutting blades are lowered into a borehole and further screw threaded sections 7A, 7B etc; 8A, 8B etc are added to the shafts so that they remain extending above the surface. The shafts are locked together by a dog clutch so that each shaft is driven to rotate directly from a driving head 6 in which they are mounted. The cutting blades are progressively opened by relative axial movement of the shafts, effected by a hydraulic ram at the surface. As the blades open and rotate, one or more enlargements are formed below the surface in the borehole, and spoil is carried away continuously by a flushing medium supplied through a passage in the inner shaft 7. The extent to which the cutting blades have opened and hence the size of the enlargements is indicated at the surface by the movement of the shaft 7 relative to the driving head 6, and this is measured by means of a scale. <IMAGE>

Description

SPECIFICATION Under-reaming of small diameter boreholes The invention concerns the under-reaming of boreholes formed in the ground, especially boreholes of relatively small diameter.

Piles and ground anchors may be formed in situ by first forming a bore in the ground, and then filling the bore with grout together with any desired reinforcement. It is well known that the load bearing capacity is increased by forming enlargements or bells in the bore at locations below the surface, prior to casting the grout. The pile or ground anchor is thus formed with integral bulbous expansions which considerably increase frictional engagement with the surrounding soil. By this means, a much shorter pile/anchor can be used to the same effect as a longer straight-shafted pile/anchor, and there is thus considerable economic advantage.

However, the formation of these bells below the ground is not a simple matter. In some methods, such as the load cell or tube à manchette techniques, a tube is lowered into a preformed borehole. Holes formed in the end of the tube are blocked, and pressurised fluid flushing medium is supplied to the interior of the tube. The pressure is raised until the fluid bursts through the blockage, thus producing an intense local compaction of the soil, forming a bell of irregular and unknown shape. The shape and properties of the resulting pile are thus unpredictable, resulting in a need for over-design and uncertainty. Furthermore, it is normally practicable only to form one bell in any pile by such a method. Another method having advantages over the foregoing is described in our prior patent No GB-A-1381575.An under-reaming tool having one or more pairs of spring biased hinged blades is attached to a drive shaft and lowered into a preformed borehole.

A thrust bearing is provided at the lower end of the tool, and the arrangement is such that when axial pressure is applied to the drive shaft, the thrust bearing is pushed against the bottom of the borehole, and the blades are thereby urged to open against their spring bias. The drive shaft is then rotated while maintaining the axial pressure, and each pair of blades thus progressively cuts away an enlargement or bell as it continues to open against the spring pressure. The spoil is flushed away continuously. When axial pressure is released, the blades collapse inwards under their spring pressure, and the tool can be withdrawn by means of the drive shaft.

This method has the advantage that several bells can be formed simultaneously, and to a certain extent the size and shape of the bells can be predetermined by the geometry of the blades and the degree to which they are opened, in that a gauge is provided on the tool which can indicate the extent to which the blades have opened during an under-reaming operation.

However, especially in the case of soft cohesive soils such as clay, the thrust bearing may continue to work its way deeper into the base of the bore. In that this permits axial movement of the tool during the under-reaming operation, the shape of the bells may be affected adversely. Furthermore, axial movement of the thrust bearing will result in a corresponding axial movement of the drive shaft into the borehole. Axial movement of the drive shaft thus cannot provide a reliable indication of the expansion of the cutting blades. For this reason, the size of the bells actually formed cannot be determined until the tool is completely withdrawn from the borehole to permit inspection of the gauge.

If it is then found that the bells have been insufficiently enlarged the tool must be re-inserted into the borehole, and the under-reaming process repeated. This is a long and therefore expensive process; for example one drilling rig is likely to produce only about two under-reamed bores in a complete day. Furthermore, in non-cohesive soils such as sand and gravel, removal and/or re-insertion of the tool is likely to damage the fragile structure of the under-reams already formed. For this reason under-reams have not previously been considered possible in non-cohesive soils.

There thus exists a requirement for an underreaming tool which is capable of producing bells in boreholes formed in various kinds of soils, the bells having a size and shape which can be accurately determined during the course of the underreaming operation.

Accordingly the present invention provides an under-reaming machine for forming one or more enlargements below the surface in a borehole preformed in the ground, the machine comprising at least one pair of elongate cutting blades, a driving head, a pair of co-axial shafts comprising an inner shaft and an outer shaft, a first pivotal connection by which one blade of each pair is pivotally connected at one end to the inner shaft, a second pivotal connection by which the other blade of each pair is pivotally connected at one end to the outer shaft, a third pivotal connection by which the pair of blades are pivotally connected together at their remaining ends, mechanical connections by which each shaft is mechanically connected to the driving head, means included within the driving head for effecting rotation of the two shafts, and means included within the driving head for effecting controlled relative axial movement of the two shafts.

A set comprising two or more pairs of blades may be provided at the same axial location on the shafts, the blades of each pair being similar to those of each other pair and symmetrically disposed with respect to the shaft axis.

Two or more sets of blades may be provided at separate axial locations.

The inner shaft may be provided with an axial passage there-through, by which fluid flushing medium may be passed down the inner shaft from the driving head to flush away spoil removed by the cutters.

Alternatively, there may be provided an axial passage between the inner and outer shafts, by which fluid flushing medium may be passed down between the shafts from the driving head to flush away spoil removed by the cutters.

The inner and outer shafts should preferably be formed in axial sections which can be locked together and separated to facilitate extension of the shafts into the borehole and their subsequent withdrawal.

Conveniently the shaft sections are provided with corresponding screw threaded sections at their ends by which the individual sections of each respective shaft can be locked together.

The means for effecting rotation of the two shafts should preferably be such as to provide a direct rotary drive of each shaft at the driving head, eg by a clutch which can lock the inner and outer shafts together.

The inner and outer shafts should preferably be freely rotatable one within the other, on disengagement of the clutch in order to facilitate assembly and separation of the sections.

The invention will now be described by way of example only with reference to the accompanying drawings, of which Figure 1 is a side view of an under-reaming machine in accordance with the invention, Figure 2 is a side view of a part of Figure 1 to an enlarged scale showing the driving head of the under-reaming machine in greater detail, Figure 3 is a side view of a part of Figure 1 to an enlarged scale, showing further detail of one set of cutting blades.

As shown in the Figures, an under-reaming machine comprises a plurality of pairs of elongate cutting blades 1, 2 disposed in sets in cutter sections 3, 4, 5. The cutting blades are rotationally driven from a driving head indicated generally at 6, by means of an inner shaft 7 and an outer shaft 8 which are co-axially disposed one within the other, and formed in sections 7A, 7B, 7C ....; 8A, 8B, 8C Although three axially separated, identical cutter sections 3, 4, 5 are shown, in practice in greater or lesser number may be provided according to the number of bells or enlargements which it is desired to produce.Similarly, although as illustrated each section 3, 4, 5 comprises two pairs of the blades 1, 2 diametrally opposed, other numbers of the pairs 1, 2 may be provided, (especially three), preferably being symmetrically disposed with respect to the axis of the shafts 7, 8.

Within the driving head 6 there is mounted a motor (not shown) which may be, for example, an electric motor or a diesel engine. The motor drives the uppermost section 9 of the outer shaft 8 through the medium of a gearbox 10. Mounted on top of the gear-box 10 is one half 11 of a dog clutch, and splined to the upper-most section 12 of the inner shaft is the corresponding half 13. As illustrated in solid lines, the dog clutch thus locks the inner and outer shafts 7, 8 so that they rotate together. When the dog clutch is disengaged (as shown in dotted lines) the inner and outer shafts are freed from the restraint of the dog clutch.

The upper end of the uppermost section 9 of the outer shaft 8 is received in a thrust bearing 14 which is mounted in the driving head 6. The bearing 14 permits free rotation of the shaft 8, but restrains it from axial movement relative to the driving head 6.

A hydraulic ram device 15 is also provided in the driving head 6, and can act on the end of the inner shaft 7 through a thrust race 16 to raise and lower the shaft 7 relative to the driving head. Controlled relative axial movement of the two shafts 7 and 8 can thus-be effected by actuation of the hydraulic ram device 15.

A water box 17 is atso provided in the driving head through which water or other fluid flushing medium can be supplied to an axial passageway 18 formed within the inner shaft 7. Rotation of the shaft 7 can be accomodated by the water box 17.

As shown in more detail in Figure 2, each set of blades, 1, 2 comprises an upper blade 1 pivoted at 19 to the inner shaft 7, and a lower blade 2 pivoted at 20 to the outer shaft 8. The two blades 1, 2 are pivoted together at 21, and the axes of the three pivots 19, 20 and 21 are mutually parallel and at right angles to the axis of the shafts 7, 8 The pivot 19 is contained in a block 22 fixed by means of screws 23 to a flat ground on the inner shaft 7. The outer shaft 8 is provided with an elongate slot 24 into which the blades 1, 2 can be received as shown in chain outline at 25 in Figure 2, and the pivot 20 is directly received in the side walls of the slot 24.

As already noted, the shaft 7 is slideable axially within the shaft 8, and a spacer 26 is provided to aid in keeping the shafts 7, 8 co-axially aligned. A cam 27 is welded to the outer surface of the shaft 7, and the cam 27 acts on a corresponding surface of the arm 1 and pushes the pivot 21 outwardly on initial axial downward movement of the shaft 7.

The pivot 21 is thus initially moved outside the line joining pivots 19 and 20.

The sections of the shafts 7 and 8, including those which carry the blades 1, 2 are each provided with a female screw threaded portion 28 at their upper ends, and a corresponding male screw 29 at their lower ends, by which the sections can be locked together and separated.

In use of the under-reaming machine, the desired number of cutter sections 3, 4, 5 are progressively assembled and fed down a preformed borehole (not shown). Further plain sections of shaft 7A, 7B ... 8A, 8B ... etc are then progressively assembled to the cutter sections until the latter reach the required depth in the borehole. When therequired depth is reached, with the dog clutch 11, 13 disengaged (dotted lines, Figure 1) to avoid damage to the blades 1, 2 and associated pivots, etc., the uppermost section of the outer shaft 8 is screwed tightly home into its connection in the gearbox 10, and uppermost section 12 of the inner shaft 7 (which is captive in the driving head) is screwed tightly to the topmost of the plain sections 7A, 7B, etc.

The dog clutch 11, 13 is then engaged by downward movement of the section 13 on its splines.

The supply of flushing fluid to the water box 17 is then turned on, and the motor (not shown) is operated to rotate the shafts 7, 8 via the gearbox 10.

The hydraulic ram device 15 is actuated progressively to lower the inner shaft 7 relative to the outer shaft 8. The initial movement causes the cam 27 to push the blade 24 outwards, thus pushing the pivot 21 over centre with respect to the pivots 19, 20. Further downward movement of the ram device causes the blades 1, 2 to move progressively outwards, thus enlarging the bells formed thereby in the borehole. The spoil produced thereby is washed away by the flushing medium passing down through the passage 18, outwardly at the lower end of the shaft 7, and upwardly in the space between the wall of the borehole and the shaft 8. Any suitable flushing medium will serve, for example water - although betonite mud or polymer flushing agents may be more appropriate in some instances.

The extent of opening of the bells formed can be simply and very accurately determined from the surface during the course of the under-reaming operation by observing the movement of the inner shaft 7 relative to the driving head 6, and from a knowledge of the geometry of the cutters. If desired a scale 30 may be provided on the driving head, by which is provided an accurate indication of the relative movement of a pointer carried by the shaft 7.

The invention thus provides an under-reaming machine suitable for use in many types of soil by which the progress and completion of the enlargements or bells can be confidently monitored without the inconvenience and expense of first removing the cutting tool to the surface. Bells can be accurately formed, one or more at a time, in cohesive soils with increased confidence. Furthermore, bells can also now be formed even in noncohesive soils such as sands or gravels where such a result has previously not been possible.

Claims

1. An under-reaming machine for forming one or more enlargements below the surface in a borehole preformed in the ground, the machine comprising at least one pair of elongate cutting blades, a driving head, a pair of co-axial shafts comprising an inner shaft and an outer shaft, a first pivotal connection by which one blade of each pair is pivotally connected at one end to the inner shaft, a second pivotal connection by which the other blade of each pair is pivotally connected at one end to the outer shaft, a third pivotal connection by which the pair of blades are pivotally connected together at their remaining ends, mechanical connections by which each shaft is mechanically connected to the driving head, means included within the driving head for effecting rotation of the two shafts, and means included within the driving head for effecting controlled relative axial movement of the two shafts.

2. An under-reaming machine according to claim 1 wherein a set comprising two or more pairs of blades are provided at the same axial location on the shafts, the blades of each pair being similar to those of each other pair and symmetrically disposed with respect to the shaft axis.

3. An under-reaming machine according to claim 2 wherein two or more sets of blades are provided at separate axial locations.

4. An under-reaming machine according to any one preceding claim wherein the inner shaft is provided with an axial passage there-through, by which fluid flushing medium may be passed down the inner shaft from the driving head to flush away spoil removed by the cutters.

5. An under-reaming machine according to any one of claims 1 to 3 wherein there is provided an axial passage between the inner and outer shafts, by which fluid flushing medium may be passed down between the shafts from the driving head to flush away spoil removed by the cutters.

6. An under-reaming machine according to any one preceding claim wherein the inner and outer shafts are formed in axial sections which can be locked together and separated to facilitate extension of the shafts into the borehole and their subsequent withdrawal.

7. An under-reaming machine according to claim 6 wherein the shaft sections are provided with corresponding screw threaded sections at their ends by which the individual sections of each respective shaft can be locked together.

8. An under-reaming machine according to any one preceding claim wherein the means for effecting rotation of the two shafts is such as to provide a direct rotary drive of each shaft at the driving head.

9. An under-reaming machine according to claim 8 wherein the means for effecting rotation of the two shafts comprises a clutch which can lock the inner and outer shafts together.

10. An under-reaming machine according to claim 9 wherein the inner and outer shafts are freely rotatable one within the other, on disengagement of the clutch in order to facilitate assembly and separation of the sections.

11. An under-reaming machine substantially as before herein described with reference to the accompanying drawings.

11. An under-reaming machine according to any one preceding claim wherein there is provided a scale on the driving head by which is provided an accurate indication of the axial movement of the shaft relative to the driving head.

12. An under-reaming machine substantially as herein before described with reference to the accompanying drawings.

Amendments to the claims have been filed, and have the following effect: (a) Claims 1, 8 to 12 above have been deleted or textually amended. (b) New or textually amended claims have been filed as follows: 1. An under-reaming machine for forming one or more enlargements below the surface in a borehole pre-formed in the ground, the machine comprising at least one pair of elongate cutting blades, a driving head, a pair of co-axial shafts comprising an inner shaft and an outer shaft, a first pivotal connection by which one blade of each pair is pivotally connected at one end to the inner shaft, a second pivotal connection by which the other blade of each pair is pivotally connected at one end to the outer shaft, a third pivotal connection by which the pair of blades are pivotally connected together at their remaining ends, means for applying a rotary drive to each shaft at the driving head, and means included within the driving head for effecting controlled relative axial movement of the two shafts.

8. An under-reaming machine according to any one preceding claim wherein the means for applying a rotary drive to each shaft comprises a clutch which can lock the inner and outer shafts together.

9. An under-reaming machine according to claim 8 as dependent on claim 7 wherein the inner and outer shafts are freely rotatable one within the other, on disengagement of the clutch in order to facilitate assembly and separation of the sections.

10. An under-reaming machine according to any one preceding claim wherein there is provided a scale on the driving head by which is provided an accurate indication of the axial movement of the shaft relative to the driving head.