GB2212196A

GB2212196A - Soil stabilisation

Info

Publication number: GB2212196A
Application number: GB8826473A
Authority: GB
Inventors: Ikram Haq
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1987-11-12
Filing date: 1988-11-11
Publication date: 1989-07-19
Anticipated expiration: 2008-11-11
Also published as: GB8726483D0; GB2212196B; GB8826473D0

Abstract

In a method of stabilising a slope or a side wall 21 of an excavation 20, which method does not require the use of heavy hammer action equipment and does not incorporate a grouting operation, a plurality of mutually spaced rigid ground anchors 22 are screwed into the ground at angles to the face of the slope or the side wall of the excavation. Each ground anchor 22 comprises a rod 1 having over at least a part of its length nearer its leading end 4 at least one helically extending fin 11 which cuts its way through the soil as the rod is rotated about its axis. The method is especially applicable to the soil nailing technique but it can also be employed in the reticulated micro-piling and dowling techniques. It is suitable for both permanent and temporary stabilisation of slopes and side walls of excavations and where temporary stabilisation is required each ground anchor 22 can be readily unscrewed from the ground and reused elsewhere. <IMAGE>

Description

SOIL STABILISATION This invention relates to the in situ reinforcement of soil and is particularly concerned with the stabilisation of slopes and of the side walls of excavations.

There are three main techniques currently employed to stabilise slopes and the side walls of excavations.

These techniques are generally known as soil nailing, reticulated micro-piling and dowelling. In soil nailing, a plurality of mutually spaced substantially rigid rods of metal or metal alloy are driven or otherwise inserted horizontally and/or at acute angles to the horizontal into the face of a slope or of a side wall of an excavation so that the rods improve the shearing resistance of the soil by acting in tension; in some cases, the rods are grouted into predrilled holes in the slope or side wall.In reticulated micro-piling, a plurality of substantially rigid rods of metal or metal alloy are driven downwardly into the ground behind the face of a slope or of a side wall of an excavation vertically and/or at various acute angles to the vertical, the rods serving to hold the soil together and to resist bending and shearing forces; again, in some cases, the rods are grouted into predrilled holes in the ground. In dowelling, which is usually employed with slopes and side walls of excavations that are less steep than those with which soil nailing and reticulated micro-piling are used, a plurality of mutually spaced substantially rigid piles of metal or metal alloy of substantially greater diameter and hence greater surface area than the rods employed in soil nailing and reticulated micro-piling, are driven downwardly behind the face of a slope or of a side wall of an excavation.These large piles improve mechanically the shearing resistance of a weakened shear surface.

All three of these techniques have the serious disadvantages that either heavy hammer action equipment is required to drive the rods or piles into the ground at the required angles causing undesirable noise and ground movement or, in addition to requiring equipment to predrill holes in the ground at the required positions for accommodating the rods or piles, after the rods or piles have been positioned in the predrilled holes, a grouting operation is required. Furthermore, an in situ reinforcement made by any one of these three techniques cannot be readily dismantled and, as a consequence, none of these techniques is really suitable for making temporary in situ reinforcement of a slope or of a side wall of an excavation.

It is an object of the present invention to provide an improved method of stabilising a slope or a side wall of an excavation, which method does not require the use of heavy hammer action equipment and does not incorporate a grouting operation.

According to the invention, the improved method comprises screwing into the ground at required angles to the face of a slope or a side wall of an excavation a plurality of-mutually spaced substantially rigid rods of metal or metal alloy, each of which rods has over at least a part of its length nearer its leading end at least one helically extending fin which effectively cuts its way through the soil as the rod is rotated about its axis.

Preferably, at least some of the rods are screwed into the face of the slope or of the side wall of the excavation horizontally and/or at acute angles to the horizontal. Alternatively, or additionally, at least some of the rods are screwed downwardly into the ground behind the face of the slope or of the side wall of the excavation vertically and/or at various angles to the vertical.

The or each helically extending fin on each rod may extend over substantially the entire length of the rod but, preferably, the or each helically extending fin extends over only a minor part of the length of the rod at its leading end. For example, where the rod has a length of x metres and a diameter of d mm, the minor part of the length of the rod over which the or each helical fin extends preferably lies in the range 1.25d to 0.125x.

Preferably, also, for a rod having a diameter of d mm, the or each helically extending fin has a diameter lying in the range 1.25d to 10d mm and has a pitch lying in the range 2d to 10d mm.

Each of at least some of the rods may consist of two or more substantially rigid rod units detachably connected together end to end, the leading rod unit having over at least a part of its length at its leading end at least one helically extending fin.

The or each helically extending fin preferably is made of metal plate which may be welded or otherwise permanently secured to the rod or to the leading rod unit but, preferably, the or each metal fin is welded or otherwise permanently secured to a sleeve or metal or metal alloy which is detachably secured to the leading end of the rod, or to the leading end of the leading rod unit1 in such a way as to prevent relative rotation between the rod and the sleeve when the rod is rotated about its axis to screw it into the ground.

The or each helically extending metal fin may be encapsulated in moulded rigidplastics material.

In a preferred embodiment, each of at least some of the rods, or each rod unit of said rod, has an external screw thread over a part of its length adjacent each of its ends and the screw threaded leading end of each rod, or the screw threaded leading end of the leading rod unit, has a longitudinally extending keyway into which engages a radially inwardly extending protrusion in the bore of the sleeve, the sleeve being clamped against a shoulder on the rod or rod unit by an internally screw threaded collar which screws on to the screw threaded leading end of the rod or rod unit. The shoulder itself may be constituted by an end face of an internally screw threaded collar which has been screwed on to the screw threaded leading end of the rod or leading rod unit prior to application of the sleeve.Where a rod consists of two or more rod units detachably secured together end to end, adjacent rod units may be detachably secured together by means of an internally screw threaded sleeve into opposite ends of which are screwed the neighbouring screw threaded ends of adjacent rod units.

In an alternative embodiment, the or each helically extending fin is an integral part of a sleeve of rigid plastics material which is moulded to the rod, or to the leading rod unit, over at least a minor part of length of the rod or rod unit at its leading end.

The trailing end of each of at least some of the rods, or the trailing end of the trailing rod unit of said rod, may have a transversely extending hole through which a bar can be passed for applying a screwing motion to the rod and/or it may have a cap of a shape suitable for use when applying a screwing motion to the rod. The rod, or the trailing rod unit, may have at or near it trailing end a substantially flat disc or plate which may or may not be secured to the rod or rod unit and which, when the rod is screwed into the face of a slope or of a side wall of an excavation or into any other surface of the ground, will bear against the face or surface when the rod is screwed fully home.

The improved method of the present invention is especially, but not exclusively, applicable to the soil nailing technique - and might be referred to as a soil "screwing" technique - but it is to be understood that the improved method of the application can also be employed in the reticulated micro-piling and dowelling techniques.

Where the improved method of the present invention is to be used in the soil nailing technique, prior to screwing the rods into the face of a slope or of a side wall of an excavation, a preformed sheet of mesh of metal or metal alloy having holes large enough for passage of the rods with their helically extending fins may be positioned on the face of the slope or side wall and, when the rods are screwed into the ground through holes of the mesh, is pressed against the face by the flat discs or plates The improved method of the present invention is suitable for both permanent and temporary stabilisation of slopes and side walls of excavations but it is especially suitable where temporary stabilisation is required because no grout or concrete is employed and each rod can be readily unscrewed from the ground and re-used elsewhere.

In addition to having all the known advantages of the three techniques of soil nailing, reticulated micro-piling and dowelling, the improved method of the present invention has the additional important advantages that the number of rods with helically extending fins required for stabilising a slope or a side wall of an excavation of a given area is substantially less than the number of rods which would be required in any one of the three known techniques employed in soil stabilisation and that, as a consequence, the cost of, and the time incurred in, stabilising any such slope or side wall of an excavation is substantially less than the cost of, and the time incurred by, the known techniques.

The invention also includes, for use in the improved method of soil stabilisation hereinbefore described, an improved ground anchor comprising a substantially rigid rod of metal or metal alloy having a sleeve of metal or metal alloy, to which at least one helically extending metal fin is welded or otherwise permanently secured, detachably secured to the leading end of the rod in such a way as to prevent relative rotation between the rod and the sleeve and having, at or near the extremity of the other end of the rod, means suitable for use when imparting a rotational motion to the rod about its longitudinal axis.

In addition to being suitable for use in the improved method of soil stabilisation hereinbefore described, the improved ground anchor may also be used as an individual anchoring device.

The invention will be further illustrated by a description, by way of example, of the preferred ground anchor for use in the improved method of stabilising a slope or a side wall of an excavation, and the preferred methods of stabilising a side wall of an excavation using the soil nailing and reticulated micro-piling techniques, with reference to the accompanying drawings, in which:: Figure 1 is a side view of the preferred ground anchor drawn on a greatly reduced scale; Figure 2 is a side view of the leading end of the rod of the ground anchor shown in Figure 1; Figure 3 is a side view of the trailing end of the rod of the ground anchor shown in Figure 1; Figure 4 is a plan view of the sleeve of the ground anchor shown in Figure 1; Figures 5 to 7 are diagrammatic side views of preferred methods of stabilising a side wall of an excavation using the soil nailing technique, and Figure 8 is a diagrammatic side view of the preferred method of stabilising a side wall of an excavation using the reticulated micro-piling technique.

Referring to Figures 1 to 4, the preferred ground anchor comprises a rigid steel rod 1 of circular transverse cross-section which, over a part of its length constituting the trailing end 2, has an external screw thread 3 and, over a part of its length constituting the leading end 4, has an external screw thread 5. At the extremity of its leading end 4, the rod 1 has an end face 6 which is inclined at an acute angle to the longitudinal axis of the rod to facilitate initial insertion of the rod into the ground.

Detachably secured to the leading end 4 of the rod 1 is a sleeve 10 to which is welded a helically extending fin 11 of metal plate. The sleeve 10 has a radially inwardly extending protrusion 12 which engages in a longitudinally extending keyway 7 in the leading end 4 of the rod 1 and the sleeve is clamped to the leading end of the rod between two internally screw threaded collars 14 each of which is screwed on to the external screw thread 5.

At the trailing end 2 of the rod 1 is a transversely extending hole 8 through which a bar can be inserted for effecting rotational motion to the ground anchor about its longitudinal axis, engagement of the protrusion 12 of the sleeve 10 in the keyway 7 preventing relative rotational motion between the rod and the sleeve.

In one preferred method of stabilising a side wall 21 of an excavation 20 shown in Figure 5, a plurality of mutually spaced ground anchors 22 as illustrated in Figures 1 to 4 are screwed into the face of the side wall at acute angles to the horizontal.

In another preferred method of stabilising the side wall 31 of an excavation 30 shown in Figure 6, a plurality of ground anchors 32 as illustrated in Figures 1 to 4 are screwed into the face of the side wall substantially horizontally.

In a third preferred method of stabilising a side wall 41 of an excavation 40 as shown in Figure 7, a preformed sheet 45 of mesh of metal or metal alloy having holes large enough for passage of the helically extending fins of modified forms 42 of the ground anchor shown in Figures 1 to 4 is positioned against the face of the side wall. Each of a plurality of modified ground anchors 42 is screwed into the side wall 41 until flat discs or plates 44 at the trailing ends of the rods of the ground anchors engage the outer surface of the sheet 45 of mesh and press it against the face of the side wall.

In the preferred method of stabilising a side wall 51 of an excavation 50 using the reticulated micro-piling technique as shown in Figure 8, a plurality of ground anchors 52 as shown in Figures 1 to 4 are screwed into the ground behind the face of the side wall vertically and at various acute angles to the vertical.

Claims

CLAIMS:

1. A method of stabilising a slope or a side wall of an excavation which comprises screwing into the ground at required angles to the face of the slope or the side wall of the excavation a plurality of mutually spaced substantially rigid rods of metal or metal alloy, each of which rods has over at least a part of its length nearer its leading end at least one helically extending fin which effectively cuts its way through the soil as the rod is rotated about its axis.

2. A method as claimed in Claim 1, wherein at least some of the rods are screwed into the face of the slope or of the side wall of the excavation horizontally and/or at acute angles to the horizontal.

3. A method as claimed in Claim 1 or 2, wherein at least some of the rods are screwed downwardly into the ground behind the face of the slope or of the side wall of the excavation vertically and/or at various angles to the vertical.

4. A method as claimed in any one of the preceding Claims, wherein the or each helically extending fin on each rod extends over only a minor part of the length of the rod at its leading end.

5. A method as claimed in any one of the preceding Claims, wherein each of at least some of the rods consists of two or more substantially rigid rod units detachably connected together end to end, the leading rod unit having over at least a part of its length at its leading end at least one helically extending fin.

6. A method as claimed in Claim 5, wherein adjacent rod units of the rod are detachably secured together by means of an internally screw threaded sleeve into opposite ends of which are screwed the neighbouring screw threaded ends of adjacent rod units.

7. A method as claimed in any one of the preceding Claims, wherein the or each helically extending fin of each of at least some of the rods is made of metal plate which is welded or otherwise permanently secured to a sleeve of metal or metal alloy, the sleeve being detachably secured to the leading end of the rod, or to the leading end of the leading rod unit, in such a way as to prevent relative rotation between the rod and the sleeve when the rod is rotated about its axis to screw the rod into the ground.

8. A method as claimed in Claim 7, wherein the rod, or each rod unit of the rod, has an external screw thread over a part of its length adjacent each of its ends and the screw threaded leading end of the rod, or the screw threaded leading end of the leading rod unit, has a longitudinally extending keyway into which engages a radially inwardly extending protrusion in the bore of the sleeve, the sleeve being clamped against a shoulder on the rod or rod unit by an internally screw threaded collar which screws on to the screw threaded leading end of the rod or rod unit.

9. A method as claimed in Claim 8, wherein the shoulder itself is constituted by an end face of an internally screw threaded collar which has been screwed on to the screw threaded leading end of the rod or leading rod unit prior to application of the sleeve.

10. A method as claimed in any one of Claims 1 to 6, wherein the or each helically extending fin of each of at least some of the rods is an integral part of a sleeve of rigid plastics material which is moulded to the rod, or to the leading rod unit, over at least a minor part of the length of the rod or rod unit at its leading end.

11. A method as claimed in any one of the preceding Claims, wherein the trailing end of each of at least some of the rods, or the trailing end of the trailing rod unit of said rod, has a transversely extending hole through which a bar can be passed for applying a screwing motion to the rod and/or has a cap of a shape suitable for use when applying a screwing motion to the rod.

12. A method as claimed in any one of the preceeding Claims, wherein the rod, or the trailing rod nit, has at or near its trailing end a substantially flat disc or plate which, when the rod is screwed into the face of the slope or the side wall of the excavation or into any other surface of the ground, will bear against the face or surface when the rod is screwed fully home.

13. A method as claimed in Claim 12 in which the rods are screwed into the face of the slope or of the side wall of the excavation, wherein a preformed sheet of mesh of metal or metal alloy having holes large enough for passage of the rods with their helically extending fins is positioned on the face of the slope or side wall and, when the rods are screwed into the ground through holes of the mesh, is pressed against the face by the flat discs or plates.

14. For use in the method of stabilising a slope or a side wall of an excavation as claimed in any one of Claims 1 to 4, a ground anchor comprising a substantially rigid rod of metal or metal alloy having a sleeve of metal or metal alloy, to which at least one helically extending metal fin is welded or otherwise permanently secured, detachably secured to the leading end of the rod in such a way as to prevent relative rotation between the rod and the sleeve and having, at or near the other end of the rod, means suitable for use when imparting a rotational motion to the rod about its longitudinal axis.

15. A ground anchor as claimed in Claim 14, wherein the rigid rod consists of two or more substantially rigid rod units detachably connected together end to end, the metal sleeve carrying the helically extending fin or fins being detachably secured to the leading rod unit.

16. A ground anchor as claimed in Claim 14 or 15, wherein the rod, or each rod unit of the rod, has an external screw thread over a part of its length adjacent each of its lengths and the screw threaded leading end of the rod, or the screw threaded leading end of the leading rod unit, has a longitudinally extending keyway into which engages a radially inwardly extending protrusion in the bore of the sleeve, the sleeve being clamped against a shoulder on the rod or rod unit by an internally screw threaded collar which screws on to the screw threaded leading end of the rod or rod unit.

17. A ground anchor as claimed in Claim 16, wherein the shoulder itself is constituted by an end face of an internally screw threaded collar which has been screwed on to the screw threaded leading end of the rod or leading rod unit prior to application of the sleeve.

18. For use in the method of stabilising a slope or a side wall of an excavation as claimed in any one of Claims 1 to 4, a ground anchor substantially as hereinbefore described with reference to and as shown in Figures 1 to 4 of the accompanying drawings.

19. A method of stabilising a slope or a side wall of an excavation substantially as hereinbefore described with reference to and as shown in Figures 1 to 5, Figures 1 to 4 and 6, Figures 1 to 4 and 7 or Figures 1 to 4 and 8 of the accompanying drawings.