WO2003035988A1

WO2003035988A1 - Soil reinforcing device and method

Info

Publication number: WO2003035988A1
Application number: PCT/GB2002/004836
Authority: WO
Inventors: Gerald O'brien; Anthony Stephen Miner
Original assignee: Phi Group Limited
Priority date: 2001-10-26
Filing date: 2002-10-25
Publication date: 2003-05-01
Also published as: GB0125790D0; GB2398589A; GB2398589B; GB0408797D0

Abstract

A soil reinforcing device comprises an elongate, substantially rigid, soil reinforcing element (23), and a drive tube (27) surrounding the reinforcing element (23) and adapted to move into soil with the reinforcing element. Subsequently the drive tube (27) is withdrawn from the soil while leaving the reinforcing element (23) in the hole (30) left after withdrawal of the drive tube (27). A helix (24) is secured to the end of the reinforcing element (23) and is adapted to move into the soil with the reinforcing element (23) when the helix is rotated. The drive tube (27) is removably secured to the helix (24) at the distal end of the drive tube (27) and is arranged to drive the helix (24) into the soil when the drive tube (27) is rotated. The helix (24) has dimensions and configuration such as to draw the reinforcing element (23) into the soil by screwing into the soil without auguring soil out of the hole (30) in the soil.

Description

SOIL REINFORCING DEVICE AND METHOD

The present invention relates to a soil reinforcing device and to a method of reinforcing soil, in particular but not exclusively for slope stabilisation.

The invention is concerned in particular, but not exclusively, with soil stabilisation where a steep slope or wall is formed for example in cuttings for a motorway or rail transport. It is known to reinforce such soil slopes in a number of ways by inserting various metal soil reinforcing elements, known by various terms such as soil nails or ground anchors. Typically a soil nail is a solid steel rod having a length of three or more metres. These are placed at a spacing of approximately two metres apart and held in place at their outer ends by various surface retaining components, e.g. welded wire metal mesh. A number of forms of soil nail are used. In one form, known as a drill and grouted soil nail, a hole is first formed in the soil by a full length helical auger which augers out soil from the hole. A reinforcing steel rod is then threaded into the hole, with spacers to hold the rod from the edges of the hole, and grouting material such as grouting cement is then pumped into the hole to surround the steel rod. The spacers are known as centralisers, and are used to maintain an even thickness of grout around the bar. For permanent applications, the steel rod may be galvanised and/or provided with a protective sheath for corrosion protection.

In the use of another form of soil nails, known as screw anchor soil nails, the soil reinforcing element, which may typically be a five centimetre square solid steel shaft, carries at its distal end a short helix, and may carry spaced along the shaft a series of further short helices. Each helix typically consists of a single turn of helix and the spacing of the helices is typically about 3.6 times the helix diameter. Such screw anchor soil nails screw into the soil without auguring out the soil, and are retained in the soil by the helix bearing on the soil.

A number of disadvantages arise with each form of soil nail. The main disadvantage of a grouted soil nail is that it is necessary to wait until the grout in the soil nail has set, before any substantial reinforcing strength is provided for the soil. Typically this takes at least twenty-four hours during which time the soil is unreinforced and face of the slope is exposed and unsupported. The main disadvantage of screw anchor soil nails is the lack of corrosion protection to the reinforcing element compared with grouted soil nails where the reinforcing element is encapsulated in the cement grout.

There have also been described in the past modifications of these known soil nails, and other devices for anchoring screw devices in soil. In WO 01/77444A (Cap Number One Trust) there is described a screw form anchor device intended for screwed penetration into a host material which may include earth. The anchor device includes a screw thread formed by a plurality of tapering angular helix thread forms. In one embodiment the anchor device has a flexible member secured to it, such as a guy rope, and is inserted into the host material (i.e. the ground) by use of a hollow tool through which the flexible member is threaded. The tool engages with the anchor device so that it can be rotated by the tool. In another embodiment, the anchor device may be positioned on the end of an earth nail for stabilising unstable earth.

In US-A-3,363,422 (Turzillo) a tie-down bar for anchoring structures in earth extends through a hollow auger shaft and has a driving bit at the distal end thereof. When the hole is fully drilled, a quantity of pressurised grout is pumped through the auger shaft to pass into the bottom of the drilled hole and around the drill bit. The anchor bar is then released from the hollow auger shaft which is then withdrawn from the hole. Simultaneously, progressive withdrawal of the auger to remove the earth from the hole, and injection of grout into the hole, is continued until the hole is filled with grout. Upon hardening of the grout, the tie- down bar is permanently embedded and anchored in axially centred condition in the hole.

It is an object of the present invention to provide a soil reinforcing device and a method of slope stabilisation, which combines advantages of different types of previously known soil nails, whilst avoiding or reducing disadvantages of known forms of soil nails.

In this patent specification the term soil includes a wide variety of forms of soil including clays, sandy soils, gravel based soil, or soft rocks such as chalk, and includes any other soils requiring stabilisation during earth working operations.

According to the present invention there is provided a soil reinforcing device comprising an elongate, substantially rigid, soil reinforcing element; a drive tube surrounding the reinforcing element and adapted to move into soil with the reinforcing element in operation and subsequently to be withdrawn from the soil while leaving the reinforcing element in the hole left after withdrawal of the drive tube; and a helix secured to the end of the reinforcing element and adapted to move into the soil with the reinforcing element when the helix is rotated, the drive tube being removably secured to the helix at the distal end of the drive tube and being arranged to drive the helix into the soil when the drive tube is rotated in operation; the helix having dimensions and configuration such as to draw the reinforcing element into the soil by screwing into the soil without auguring soil out of the hole in the soil.

It is an advantage of such an arrangement that the helix enters the soil by compacting the earth, rather than auguring soil out of the hole in the soil. This allows an arrangement in which after the tube has been withdrawn from the soil the helix is left secured in the soil so as to give an immediately useful secure soil reinforcing element in the soil which can be used without awaiting for grouting material to set.

In preferred forms according to the invention, the helix extends angularly around its axis to an extent less than 720°. Preferably the helix extends angularly around its axis to an extent greater than 180°. Most preferably the helix extends angularly around its axis for approximately one turn of 360°. Preferably the outer perimeter of the helix has a helix angle of less than 45°. Conveniently the outer perimeter of the helix has a helix angle in the range 40° to 20°.

A line forming a helix can be defined as a three-dimensional curve that lies on a cylinder or cone, so that its angle to a plane perpendicular to the axis is constant. This angle is the helix angle. In the present invention, the helix angle of the helix is the helix angle of the outer perimeter of the flight of the helix. Thus the helix angle can be defined as the angle between a tangent to the perimeter of the helical flight and a plane perpendicular to the axis of the helix.

It is particularly preferred that the soil reinforcing device is arranged to have an outer surface such as to allow the drive tube to be withdrawn from the hole substantially without removal of earth from the hole. In one preferred form the drive tube has a substantially smooth cylindrical outer surface.

It is much preferred that the apparatus includes arrangements for pumping grouting material into the hole in the soil. In one preferred form the helix is mounted on support shaft including a passage leading from the drive tube to an opening in the support shaft to allow grouting material to be pumped through the drive tube into the soil in the region of the helix during driving of the drive tube into the soil.

It is particularly preferred feature that the drive tube may be open ended and have at the proximal end thereof a rotatably coupled inlet device, such as a grout swivel attachment, to allow grouting material to be pumped through the drive tube into the hole during driving and/or withdrawal of the drive tube into and from the soil. In other arrangements it may be provided that grouting material is pumped into the hole after the drive tube has been removed, but it is much preferred that the grouting material is pumped through the drive tube into the hole during the withdrawal of the drive tube. Most preferably the rate of withdrawal of the drive tube is arranged to match the rate of pumping of the grouting material so that the hole is uniformly filled by the grouting material without any voids being left, and without the need for the grouting material to travel along the hole after the drive tube has been withdrawn.

Conveniently the drive tube will include a drive coupling at the proximal end of the tube adapted to allow a torque to be applied to the drive tube to rotate the drive tube. The drive coupling may consist of or include a steel cross member secured to the tube and extending beyond the diameter of the tube to assist in applying torque to the tube.

Conveniently the reinforcing device includes a rotary coupling between the helix and the distal end of the drive tube, such that rotation of the drive tube in a first direction drives the helix into the soil, and rotation of the drive tube in the opposite direction releases the drive tube from the helix. Conveniently the rotary coupling is a driving engaging device, for example a profiled connection on the proximal end of a shaft of the helix, which engages with an opposing matching profile on the distal end of the drive tube.

A reinforcing device embodying the invention may be included in a soil reinforcing apparatus which also includes a drive head for operating the reinforcing device, the drive head being adapted to rotate the tube to drive the helix into the soil, and to release the tube from the helix and withdraw the tube from the hole formed by the tube in the soil. Such apparatus may include a pump connected to the drive tube via an inlet device such as a grout swivel to pump grouting material through the tube into the hole during driving and/or withdrawal of the tube from the hole.

Although in some circumstances the soil reinforcing element may comprise a hollow element, it is preferred that the elongate soil reinforcing element comprises a solid rod. By way of example this may consist of a round section steel rod. Dimensions which may be used conveniently may be as follows. The soil reinforcing element may conveniently have a maximum diameter in the range 12mm to 32mm, preferably in the range 20mm to 28mm, for example 25mm. The drive tube may have an outside dimension in the range 76mm to 127mm, preferably 89mm to 114mm, for example 89mm. The drive tube may have a thickness in the range 6mm to 13mm, preferably 7mm to 11mm, for example 8mm. Conveniently the spacing between the maximum diameter of the elongate soil reinforcing rod and the inner surface of the drive tube may be in the range 25mm to 40mm, preferably 25mm to 30mm, for example 25mm. Conveniently the soil reinforcing element and helix may be constructed either in steel or non-ferrous, non-corrosion high strength materials and the drive tube typically in steel. The soil reinforcing element is constructed to be substantially rigid. Although some small amount of bending may be possible in the reinforcing element in some circumstances, the element must be sufficiently rigid to act as a substantial soil reinforcing component in operation to restrain soil movement in normal conditions of a soil slope.

In accordance with a preferred feature of the soil reinforcing device, the device may include a corrosion protection tube positioned inside the drive tube, and surrounding the elongate steel reinforcing rod the protection tube being arranged to remain in the hole after the removal of the drive tube. Conveniently annular spacers are provided with openings arranged to allow grouting material to be pumped between the corrosion protected tube and the elongate reinforcing rod, and the protection tube and the boundary of the hole.

It is to be appreciated that where features of the invention are set out herein with regard to apparatus according to the invention, such features may also be provided with regard to a method according to the invention, and vice a versa.

In particular there is provided in accordance with the invention in another aspect a method of soil reinforcement comprising inserting into soil an elongate soil reinforcing element surrounded by a drive tube; and withdrawing the drive tube from the soil while leaving the reinforcing element in the hole left after withdrawal of the tube; in which the method includes drawing the reinforcing element into the soil by rotating a helix secured to a distal end of the reinforcing element, the helix being rotated by the drive tube which is removably secured to the helix at the distal end of the drive tube, and the helix being moved into the soil by screwing the helix into the soil without auguring soil out of the hole; releasing the drive tube from the helix and withdrawing the drive tube from the soil; and leaving the helix secured in the soil by the effect of the helix being screwed into the soil.

Preferably the method includes withdrawing the tube from the soil substantially without removal of earth from the hole.

Preferably the method includes the step of pumping grouting material through the drive tube into the hole during driving of the drive tube into the soil. In addition, or alternatively, the method preferably includes the step of pumping grouting material into the hole during the withdrawal of the drive tube.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:-

Figure 1 is a diagrammatic representation of a known soil nail, shown in cross section when grouted in a hole in a soil embankment;

Figure 2 is a diagrammatic representation in side view of a known screw anchor form of soil nail;

Figure 3 is a diagrammatic representation of a soil nail embodying the present invention shown partly in side view and partly in section, at a stage of operation when the soil nail is partly inserted into soil having an excavated face forming a steep slope; Figure 3a is an enlarged view of a helix shown in Figure 3, and illustrates a passage for grouting during operation;

Figure 4 is a diagrammatic representation of the soil nail of Figure 3 embodying the invention, when inserted in soil in an embankment, showing a finished arrangement of such a soil nail;

Figures 4a and 4b show details of a drive head which forms with the soil nail shown in Figure 3, a soil reinforcing apparatus;

Figures 5 and 6 are diagrammatic representation of a modification of the soil nail of Figure 4 embodying the invention, modified by the addition of a corrosion protection tube, shown during insertion and in the final position, respectively.

The requirements to work within existing site boundaries, and to utilise fully existing space, has led to the adoption of steep slopes to cuttings and excavations. An example of this is a motorway widening scheme, i.e. adding extra lanes but keeping within the existing boundaries. In addition, the adoption of "top down" excavation techniques means that steep slopes are formed and stabilised at each level of the excavation of a site before proceeding with the next level of the excavation.

Currently the usual technique adopted to stabilise each level of excavation is to use grouted soil nails which, simply described, are steel bars inserted into drilled holes and cement grouted into the ground. Such an arrangement is shown in Figure 1 which is a diagrammatic view partly in section and partly in side view of a grouted soil nail when in its final position. An elongate soil reinforcing element 11 , for example a steel bar, is positioned in a pre-drilled hole 12 and is located centrally by spacers 13. The bar 11 is surrounded by grouting material 14, for example grouting cement. The method of construction is that the bar 11 is inserted into the pre-drilled hole 12 and located by the spacers 13, and then the cement grouting is pumped into the hole 12. In a typical use, the excavated face 15 of the slope to be stabilised is covered with a temporary supporting cover 16 which may be for example a wire mesh and polythene sheet, and this is held in place by a plate 17 secured on the bar 11 and held in place by a nut 18 threaded on the outer or proximal end of the bar 11.

The soil nails stabilise the whole block of soil, forming the original ground, which would otherwise slip into the excavation. The actual face of the excavation is restrained and kept in place by fixing the polythene sheets and steel mesh to the steel bars 11 using the steel plates 17 and nuts 18. In loose soils plywood sheets can be used also, held in place by the soil nails. This temporary facing is only required until the whole of the excavation is complete, after which event the permanent face is built up from the new ground level, and is usually attached to the soil nails to form a completely integrated retaining wall system.

The drawback with the system is that before the temporary facing can be attached to support the exposed face of the excavation, the grout in the soil nail has to set and gain some initial strength. This takes at least 24 hours, during which time the cut face is exposed and unsupported. This can lead to significant surface spalling and slumping especially if there is rain during that time. In order to try to avoid this problem, particularly in weaker soils, it is necessary to introduce additional temporary mechanical anchors, such as those known as duckbill anchors, to which the temporary face support can be attached immediately, and before carrying out the soil nailing.

In Figure 2 there is shown another form of soil nail, known as a screw anchor. This consists of an elongate reinforcing element 19 which is typically a solid or hollow, steel shaft, of greater diameter than the bar 11 in Figure 1. At the distal end of the shaft 19, is a drive helix 20 which typically extends for one complete turn of 360 degrees of the helix. Conveniently the end of the shaft 19 has a sloping face 21 , giving a sharpened tip. The operation of the screw anchor soil nail 10 is that the shaft 19 is rotated by a drive head (not shown) and the helix 20 is driven into the soil, without auguring out any of the earth. Thus the shaft 19 is drawn into the soil by the rotation of the helix 20, typically with additional axial force applied continuously or periodically, along the shaft 19 in the inward direction. Various forms of drive connections to the drive head may be provided to rotate the shaft 19, the simplest including a cross pin inserted in an aperture 22 in the proximal end of the shaft 19. The advantage of such a screw anchor is that the soil nail has an immediate effect after installation in stabilising the soil, and the excavated face 15 can be restrained by a mesh and plastics sheet which can be attached immediately to the outer or proximal end of the shaft 19. Immediate usability is provided because the helix 20 secures the shaft 19 in the soil. However the disadvantage is that no corrosion protection is provided to the reinforcing element compared with that provided by a conventional grouted soil nail such as shown in Figure 1 , and the shaft 19 is generally more expensive than a solid reinforcing rod.

In some cases it is necessary to provide additional helices spaced apart along the length of the shaft 19, to give additional strength. Each such additional helix would again typically extend for approximately a single turn of 360 degrees around the shaft. If such measures are taken, a further disadvantage arises in that the soil nail becomes more costly than a single simple bar as used in a grouted soil nail.

Figures 3 and 4 show an embodiment of the present invention, which combines the advantages of the known forms of grouted soil nail and screw anchor soil nail, and avoids or reduces the disadvantages of each. Figure 3 shows a soil nail 10 embodying the present invention, during the insertion of the soil nail into the soil. The soil nail comprises an elongate soil reinforcing element 23 and a helix 24 secured to the distal end of the reinforcing element 23. The soil reinforcing element 33 is sufficiently rigid to form a substantial reinforcing component when inserted in a soil slope. Conveniently the reinforcing, element 23 is a deformed circular-section steel bar of diameter substantially the same as a conventional grouted soil nail, for example in the range 12mm to 32mm, for example 25mm. Conveniently the helix 24 extends for approximately one turn of 360 degrees around a shaft 25 and has the general construction and dimensions of a typical helix of a screw anchor as shown in Figure 2, for example having an outer diameter of the helix in the range 200 to 400mm, for example 300mm, and a helix pitch in the range 80 to 140mm, for example 100mm. Conveniently the helix 24 is formed around the short hollow or solid shaft 25 which may be of the form and dimensions of a typical shaft of a screw anchor as shown in Figure 2, conveniently having a diameter in the range 76 to 127mm, for example 89mm. The end of the shaft 25 has an oblique face 26, giving a sharpened end similar to a conventional soil nail screw anchor. The distal end of the bar 23 is permanently attached to the helix 24 for example by a screw thread and/or by welding.

The soil nail also comprises a drive tube 27 which conveniently is a hollow steel tube of diameter similar to a conventional diameter of a screw anchor soil nail such as shown in Figure 2, and may be of slightly larger diameter than the shaft 25. The drive tube 27 surrounds the bar 23 and is removably secured to the helix 24 at the distal end of the drive tube. The securement is such that the helix 24 can be driven into the soil by rotation of the drive tube 27, and when the soil nail is in the required position, the drive tube 27 can be released from the helix 24 to be withdrawn from the soil. Conveniently this is achieved by a rotary coupling 28 which conveniently may be an engaging device on the end surface of the drive tube 27, connecting with a corresponding opposing matching profile on the outer end of the shaft 25.

Figure 3a shows in detail the helix 24 shown in Figure 3. The helix 24 is mounted on a support shaft 25 which includes a passage 25A which leads from the interior of the drive tube 27 to one or more openings 25B in the side of the helix 24. This arrangement allows grouting material to be pumped down the drive shaft 27 and through the opening 25B during driving of the helix 24 into the soil. This allows grouting material to be fed under pressure into the soil into which the helix 24 is being screwed.

The soil nail forms part of a soil reinforcing apparatus which includes a drive head (shown in Figures 4a and 4b) adapted to rotate the drive tube to drive the helix into the soil, and to release the tube from the helix and withdraw the tube 27 from the hole 30 formed by the tube 27 in the soil. The apparatus also includes a pump (not shown) connected to the drive tube 27 via an inlet device such as a swivel head to pump grouting material into an opening 29 and through the tube 27 into the hole 30 during driving and withdrawal of the tube from the hole. The drive head may conveniently have the general features of a drive head used for driving a typical soil nail screw anchor such as shown in Figure 2.

Referring to Figures 4a and 4b, the drive head 40 comprises a torque head 41 for driving the shaft 27 in rotation in known manner. A hitch 42 allows the torque head 41 to be coupled to the tube 27 via a grout swivel 43, also of known form. Referring to Figure 4b, the grout swivel 43 is rotatably mounted on a cylinder 44 coupled by top and bottom flanges 45 and 46 to the hitch 42 and tube 27 respectively. The grout swivel 43 is located axially by circlips 47 and 48 and is rotatably sealed to the cylinder 44 by upper and lower seals 49 and 50. The interior of the grout swivel 43 is hollow, and communicates with the interior of the cylinder 44 through two apertures 51 and 52 in the cylinder 44. Grouting material is pumped under pressure into the interior of the grout swivel 43 through a grout inlet tube 53 from a pump (not shown).

The method of use of the soil nail of Figure 3 consists of the following main steps. First the helix 24 is driven into the soil by rotating the drive tube 27 and grouting at the same time until the soil nail is in the required position. The direction of rotation of the tube 27 is then reversed, so as to release the drive tube 27 from the shaft 25. The drive tube 27 is then withdrawn from the soil while leaving the bar 23 in the hole 30 left after the withdrawal of the tube 27. During the withdrawal of the drive tube 27, grouting cement is pumped through the drive tube 27 into the hole left by the withdrawal of the drive tube, so as to leave the bar 23 in the hole 30 surrounded by grouting material 31. The soil nail in the position shown in Figure 4 can then be used immediately in the same manner as described in with regard to Figure 2.

The helix 24 may either be formed of material sufficiently durable merely to draw the steel bar 23 into the soil, and may then be allowed to deteriorate with time, the main strength of the soil nail being provided by the bar 23 and the cement grouting. In such a case the purpose of the helix is to install the soil nail, and to give initial strength until the grouting has set. Alternatively, the design and material of the helix 24 can be chosen to be such that the helix has a durability comparable to that of the bar 23 when protected by grouting, so that the helix 24 remains a structural component of the soil nail throughout the life of the soil nail. In such circumstances, in addition to giving immediate ability to apply a load (because of the presence of the helix), the overall load characteristics of the soil nail can be increased significantly by utilising the positive effects of the helix permanently.

The method of soil reinforcement which has been described embodying the invention allows a number of advantages over previously known systems. As mentioned, grout is pumped while the driving tube is driving and when withdrawn to fill the void formed by the tube. On completion of the grouting the temporary face support can be fitted, plates fixed and nuts tightened immediately. This is because the steel bar is locked in place by the helix which is buried in the soil at the end of the bar. The ability to do this at this stage reduces considerably the risk of localised face failure after soil nailing.

The screw-in system requires a tube of sufficient torque strength to develop the torque necessary to drive the spiral helix at the end into the ground. The steel bar 23 is centrally attached to the helix 24 and taken down with it, remaining central in the hole 30 in the drilling process. Other systems which involve putting a bar down a pre-drilled hole require spacers to ensure the bar may remain in the centre. Displacement of the spacers during construction may cause the bar to be in contact with the soil, and therefore not surrounded by grout as required.

In contrast, in the embodiment described, as the drive tube 27 is withdrawn grout is pumped down under pressure and fills the hole left by the tube itself. This technique ensures the soil is supported whilst the grout flows in and fills the void around the bar as the tube is withdrawn, preventing soil collapsing into the hole. Such a collapse, in known systems, is a real concern where an unlined pre-drilled hole is used and the bar inserted afterwards. If it occurs, either water or air is used to flush the hole clean. This can have detrimental effects to the soil strength. An air flush, in particular, can "blow" the face of the excavation local to the hole causing the very problem which the soil nail is intended to avoid.

A further advantage of the embodiments described compared with a conventional grouted soil nail is that there is generally less vibrations when drilling into the soil the helix 24 and tube 27 than when auguring out material to form a pre-drilled hole for a soil nail.

Known systems for installing grouted soil nails all rely on auguring and removal of soil to form the hole for the soil nail. It is an advantage of embodiments of the present invention that the method described uses displacement of soil within the hole (i.e. compression of the soil) and the soil is pushed aside as the tube is drawn down into the ground, rather than the soil being augured out of the hole. This has the advantage of improving the strength of the soil around the soil nail, increasing the frictional strength of the soil, and also can be advantageous in some special circumstances, for example on a contaminated site where disposal of excavated contaminated material could be expensive.

Figure 5 and 6 show a modification of the embodiment of Figures 3 and 4. Figure 6 shows the modified soil nail 10 in its final position. The modification is that there is provided inside the drive tube 27 a corrosion protection corrugated plastic tube 32. Figure 5 shows the modified soil nail during insertion into the soil. The diameter of the corrugated plastic tube 32 is approximately mid way between the diameter of the drive tube 27 and the steel bar 23. After the drive tube 27 and helix 24 have been driven into the soil to the required depth, the drive tube 27 is released from the helix 24, and withdrawn from the remaining hole 30. During this withdrawal grouting cement 31 is pumped into the spaces between the steel bar 23 and the corrosion protection tube 32, and the space between the corrosion protection tube 32 and the inner surface of the hole 30 in the soil. As shown in Figure 6, the end result is that the steel bar 23 is surrounded by both grouting 31 and the corrosion protection tube 32.

In a further modified embodiment of the invention (not shown) it is arranged that on very long soil nails, driving tubes with a helix on may be used to assist in driving the tube into the soil. This helix will enhance the draw-down power on the tube to overcome the extra amount of skin friction which will develop on the longer tubes.

As has been mentioned, it is a particularly preferred feature also to pump in grout whilst driving. The grout is pumped into the driving tube and then into the ground via holes in shaft 25. This not only assists the driving by acting as a lubricant whilst it is fluid, but also by being stirred and effectively penetrating any void created by the helix as it progresses, the grout increases the strength of the soils immediately around the soil nail when it sets.

Claims

1. A soil reinforcing device comprising:

an elongate, substantially rigid, soil reinforcing element (23);

a drive tube (27) surrounding the reinforcing element (23) and adapted to move into soil with the reinforcing element in operation and subsequently to be withdrawn from the soil while leaving the reinforcing element (23) in the hole (30) left after withdrawal of the drive tube (27); and

a helix (24) secured to the end of the reinforcing element (23) and adapted to move into the soil with the reinforcing element (23) when the helix is rotated, the drive tube (27) being removably secured to the helix (24) at the distal end of the drive tube (27) and being arranged to drive the helix (24) into the soil when the drive tube (27) is rotated in operation;

the helix (24) having dimensions and configuration such as to draw the reinforcing element (23) into the soil by screwing into the soil without auguring soil out of the hole (30) in the soil.

2. A reinforcing device according to Claim 1 in which the helix (24) extends angularly around its axis to an extent less than 720°.

3. A reinforcing device according to Claim 1 or 2 in which the helix (24) extends angularly around its axis to an extent greater than 180°.

4. A reinforcing device according to Claim 1 in which the helix (24) extends angularly around its axis for approximately one turn of 360°.

5. A reinforcing device according to any preceding claim in which the outer perimeter of the helix (24) has a helix angle of less than 45°.

6. A reinforcing device according to Claim 5 in which the outer perimeter of the helix (24) has a helix angle in the range 40° to 20°.

7. A reinforcing device according to any preceding claim in which the drive tube (27) has an outer surface such as to allow the drive tube to be withdrawn from the hole (30) substantially without removal of earth from the hole.

8. A reinforcing device according to Claim 7 in which the drive tube (27) has a substantially smooth cylindrical outer surface.

9. A reinforcing device according to any preceding claim in which the helix (24) is mounted on support shaft (25) including a passage (25A) leading from the drive tube to an opening (25B) in the support to allow grouting material to be pumped through the drive tube into the soil in the region of the helix during driving of the drive tube into the soil.

10. A reinforcing device according to any preceding claim in which the drive tube (27) has at the proximal end thereof an inlet device (43) arranged to allow grouting material to be pumped through the drive tube into the hole in the soil.

11. A reinforcing device according to Claim 10 in which the inlet device (43) is rotatably coupled to the drive tube to allow grouting material to be pumped through the drive tube into the hole during driving of the drive tube into the soil and/or during withdrawal of the drive tube from the soil.

12. A reinforcing device according to any preceding claim in which the drive tube includes a drive coupling (42) at the proximal end of the tube adapted to allow a torque to be applied to the drive tube to rotate the drive tube.

13. A reinforcing device according to any preceding claim including a rotary coupling between the helix and the distal end of the drive tube (27), such that rotation of the drive tube in a first direction drives the helix into the soil, and rotation of the drive tube in the opposite direction releases the drive tube from the helix.

14. A soil reinforcing apparatus comprising a reinforcing_device according to any preceding claim, and a drive head (40) for operating the reinforcing device, the drive head being adapted to rotate the tube to drive the helix into the soil, and to release the tube from the helix and withdraw the tube from the hole formed by the tube in the soil.

15. A reinforcing_apparatus according to Claim 14 in which the apparatus includes a pump connected to the drive tube to pump grouting material through the tube (27) into the hole during driving and/or withdrawal of the tube into and from the soil.

16. A method of soil reinforcement comprising:

inserting into soil an elongate soil reinforcing element (23) surrounded by a drive tube (27); and

withdrawing the drive tube (27) from the soil while leaving the reinforcing element (23) in the hole (30) left after withdrawal of the tube;

in which the method includes:

drawing the reinforcing element (23) into the soil by rotating a helix (24) secured to a distal end of the reinforcing element (23), the helix (24) being rotated by the drive tube (27) which is removably secured to the helix (24) at the distal end of the drive tube (27), and the helix (24) being moved into the soil by screwing the helix into the soil without auguring soil out of the hole; releasing the drive tube (27) from the helix (24) and withdrawing the drive tube (27) from the soil; and

leaving the helix (23) secured in the soil by the effect of the helix being screwed into the soil.

17. A method according to Claim 16, including withdrawing the tube (27) from the soil substantially without removal of earth from the hole (30).

18. A method according to Claim 16 or 17, including pumping grouting material through the drive tube (27) into the hole during driving of the drive tube into the soil.

19. A method according to Claim 16, 17 or 18 including pumping grouting material through the drive tube (27) into the hole during the withdrawal of the drive tube (27) from the soil.

20. A method according to Claim 16, 17, 18 or 19 in which the drive tube (27) is released from the helix by rotating the drive tube in the opposite sense to the rotation of the drive tube which drives the helix into the soil.