GROUND REINFORCEMENT OR STABILISATION METHOD AND APPARATUS

Abstract: A method of ground reinforcement uses a screw-in anchor element (16) which has a shaft (17), helical flight (18) adjacent a leading end, a helical flight (19) adjacent a trailing end, and a drive coupling (20). The anchor element is screwed into the ground with the leading end placed in relatively stable soil and the trailing end placed in relatively unstable soil. The flight (18) may be of greater pitch than the flight (19). The shaft (17) may be hollow. When the shaft (17) is hollow, it may be used to drain liquids or gases from the soil, or to introduce active agents to treat polluted ground.

Description: Title of Invention: Ground Reinforcement or Stabilisation Method and Apparatus Technical Field: This invention relates to reinforcement of earth structures, both naturally occurring and man-made. Background Art: Ground reinforcement systems such as geogrids, metal strips and fabrics have been used to improve the properties and behaviour of placed earth fills. These systems are practicable only with man-made earth structures, as they are placed in position during construction. For reinforcement of natural ground (that is, not placed fill) the "soil nail" has been developed in recent years. The soil nails are in general a cylindrical shaft of constant dimension throughout. Initially soil nails were in the form of a metal reinforcing shaft grouted into a cylindrical hole in the ground. Ballistic soil nails have been developed more recently. A ballistic soil nail is a metal reinforcing shaft which has been percussively driven into the ground. Both forms of soil nails operate by way of both: adhesion of soil to the shaft of the nail; and friction between the soil and the shaft of the nail. These systems of ground reinforcement suffer several disadvantages. Reinforcing geogrids, metal strips and fabrics can only be utilised in the course of constructing man- made earthen structures. They cannot be used to reinforce naturally occurring structures. Soil nails in the form of a shaft grouted into the ground suffer the disadvantage that grouting can take several days to cure and develop maximum strength. Although ballistic soil nails, unlike grouted soil nails, are fully effective immediately on insertion, they are limited in their length to about six metres. This limitation in their length results in a limitation in their effectiveness, because they operate by way of soil adhesion and friction along the length of the shaft. In a different field, screw-in ground anchors have been known for some time. These anchors generally comprise a shaft carrying one or more helical flights. Rotation of the shaft of such an anchor causes the anchor to screw into the ground. Typical descriptions of such anchors include the following patent documents. United States Patents 3,011,597 and 3,011,598 to Samuel R. Galloway and William H. Galloway, published 5 December 1961. This specification relates to a supporting post. It discloses an open-ended tube carrying one screw thread. British Patent 1,098,555 to Trefileries Leon Bekaert P.V.B.A, published 10 January 1968. This specification relates to a post suitable for use as a fence- post. It discloses an open-ended tube carrying one or more screw threads. When multiple screw threads are provided, they may form a multi-start screw thread. Hereto, screw-in ground anchors have been used either as tension anchors or as foundation anchors, they have not been used in the field of ground reinforcement. For example, the A. B. Chance Company subsidiary of Emerson Electric Co (Centralia, Missouri, USA) has for some years manufactured screw-in tension anchors for anchoring guy-wires of electrical reticulation poles and radio transmission towers. The A. B. Chance Company has also manufactured screw-in foundation anchors for some years. Forms of these anchors specifically designed for street lighting generally comprise a hollow steel pipe, carrying at least one helical flight or auger adjacent one end. Rotation of the pipe screws the foundation into the ground. The leading end of the pipe may be open, or it may be closed off with a shaped blanking plate. The other end of the pipe generally carries a plate to which a street light may be bolted. Some other forms of screw-in foundation anchors produced by the A. B. Chance Company are illustrated in United States Patent 4,339,899 (issued 20 July 1982). In contrast, the present invention utilises screw-in ground anchors in the field of ground reinforcement, where the anchors are normally neither under tension or compression. The present invention also provides alternative types of screw-in ground anchors. The present invention seeks to provide soil reinforcement apparatus • which can be placed in existing earth structures; • some embodiments of which are effective immediately on insertion; and • which is readily placed in the soil. The present invention also seeks to provide: • a method of soil reinforcement; • a method of treating polluted ground; • a method of draining soil structures; • a method of investigating sub-soil conditions; and • a method of constructing wells. Summary of the Invention: This invention provides a ground anchor element comprising a shaft having at least one substantially helical flight adjacent each end. This invention also provides a ground reinforcement method which may utilise a ground anchor element according to the invention. According to this method, an anchor element is placed with one end and at least one adjacent helical flight in relatively stable ground, and with the other end in relatively unstable ground. If ground movement or sliding fracture occurs, the resistance of the anchor element is mobilised to improve the resistance against ground movement or sliding fracture. The anchor resistance is in the form of tension in the shaft of the anchor element and compression bearing onto the soil mass, thereby anchoring the potentially sliding soil down to the deeper stable structure. The compression bearing onto the soil mass is provided at least in part by the helical flight which is in the relatively stable ground bearing against that ground. This invention also provides a method of treating polluted soil, comprising placing a ground anchor element according to this invention into the ground and introducing an active agent or active agents into the ground through a hollow shaft of the anchor element. This invention also provides a method of draining soil, comprising placing a ground anchor element according to this invention into the ground and draining water out of the soil through a hollow shaft of the anchor element. This invention also provides a method of inspecting sub-soil conditions, comprising placing an anchor element which has a removable tip according to this invention into the ground, removing the tip, and sampling the undisturbed soil ahead of the anchor element. In a preferred form, apparatus according to this invention may have more than one flights adj acent either or both ends of the shaft. The shaft may be hollow. When the shaft is hollow, the leading end of the shaft may be provided with a tip which prevents soil from occluding the hollow shaft. The tip may be fixed or removable. When this invention is utilised in draining soil, it is especially preferred that the anchor member be or carry an electrode to create an electric field to provide an electro-osmosis effect enhancing movement of moisture through the soil. When the shaft is hollow, it may be either or both of: • slotted; and / or • provided with a porous tip to allow the ingress of moisture from a soil mass. When the shaft is slotted, it is preferred to fill the shaft with a porous material to prevent ingress of soil. The shaft of an anchor element according to the present invention may: • be of steel or other metal; • be of high tensile strength plastics material; or • may be of concrete reinforced with steel or other reinforcing. In the method of ground reinforcement according to this invention, it is preferred that the compression bearing onto the soil mass be provided by the helical flights of a ground anchor element according to this invention. However, in some applications of the method according to this invention, compression may be provided by a helical flight adjacent one end of a shaft and at least in part by some other means adjacent the other end of the shaft. In one form of this method, particularly preferred where an exposed face is accessible, the other means takes the form of a substantially flat plate. Specific embodiments of anchor elements and methods in accordance with this invention will now be described by way of non-limiting examples with reference to the accompanying drawings. Brief Description of the Drawings: Figure 1 is a vertical sectional view illustrating a ground reinforcement method and apparatus according to the prior art. Figure 2 is a side elevational view of a ground anchor element according to the present invention. Figures 3 and 4 are vertical sectional views illustrating a method of ground reinforcement according to the present invention. Figure 5 is a side elevational view of a ground anchor element according to the present invention. Figure 6 is a vertical sectional view illustrating both a ground anchor element and a method of ground reinforcement according to the present invention. Figure 7 is a vertical sectional view illustrating both a method of ground reinforcement and a method of ground water reduction according to the present invention. Figure 8 is an elevational view, partly in section, of an embodiment of a ground anchor according to the present invention which is particularly adapted for use in corrosive conditions. Figures 9, 10 and 11 are vertical sectional views illustrating the use of the method and apparatus of the present invention to stabilise existing structures. Figure 12 is a vertical sectional view of use of apparatus and a method according to the present invention used to rehabilitate an earth structure which has failed. Figure 13 is a vertical sectional view of apparatus according to the present invention which allows sampling of soil conditions. Figure 14 is a vertical sectional view of apparatus according to the present invention which is used in the construction of a well. Detailed Description of the Preferred Embodiments: Prior art Figure 1 shows the use of prior art soil nails as soil reinforcement. In this figure, an embankment 2 has an exposed, inclined earthen face 3 and has a roadway 4 on the top 7 of the embankment. Geomechanical failure of a structure such as embankment 2 tends to take place by slippage of soil mass 8 along surface 9 which may be circular as in the vertical section illustrated. Initial failure may occur by rotation of soil mass 8 as a coherent structure about point 11 , which is the centre point of the circular arc which defines the surface 9. Soil nails 12, 13 and 14 have been placed into the embankment from the direction of exposed face 3. When the soil mass 8 is in equilibrium, there is no resultant stress applied by the soil in the direction of the length of the nail. When the soil mass 8 begins to move, there is adhesion of soil to the surfaces of the nails, and friction between the soil and the surface of the nail. The consequent adhesive and frictional forces put the nails into tension along the direction of their length and restrains the soil mass 8 from moving. Apparatus and methods according to the present invention Figure 2 shows a screw-in earth anchor element 16 with shaft 17 and flights 18, 19 adjacent each end. The element 16 may be of hot dipped galvanised steel, black steel, or other suitable material. The flights 18 and 19 are substantially helical, and are welded to shaft 17. The shaft 17 is of substantially uniform diameter throughout its length between flights 18 and 19. The coupling 20 transfers torque in a known manner from a known driving element which is not shown in the figure. The application of rotation to the end of the shaft via coupling 20 advances the anchor element into the ground in the well-know manner of a screw, allowing the screw-in ground anchor element to be installed. Flights 18 and 19 may be of the same pitch, or of different pitch. When the flights are not of the same pitch, rotation of the anchor element tends to advance each flight a different distance into the soil, causing either compression or tension in shaft 17 between flights 18 and 19. The pitches of flights 18 and 19 may be chosen to be different such that advancement of the anchor element into the soil causes compression of the soil between the two flights. This compressive pre-loading changes the characteristics of the soil and increases the stabilising effect of the anchor element. It has been found that it is convenient for the dimensions of the anchor elements to be in the following ranges: shaft diameter, 60 mm to 220 mm; flight diameter 150 mm to 600 mm; flight spacing along the shaft at least 2.5 times the flight diameter; and flight pitch typically one-third the helix diameter. The configuration and placement of anchor elements according to the invention are site specific. That is, the dimensions, number and placement of the anchor elements depend on the site and are determined using standard engineering techniques. The present invention can be used to support steep excavations, to improve slope stability and to improve the carrying capacity of foundations. Figure 3 illustrates the use of ground anchor elements according to Figure 2 to support a steep excavation. In this figure, the original ground line 21 had been excavated to produce vertical face 22. Anchor elements 23 have been screwed into earth 24 in a substantially horizontal orientation to provide reinforcement. If the mass of earth 24 begins to slip in direction of face 22, then adhesion of soil to shaft 17 and friction between the soil and the surface of shaft 17 will, in a manner similar to the operation of the prior art soil nails, provide some degree of stabilisation of the soil. However, in addition to these forces of adhesion and friction, the movement of soil in direction 25 will tend to put the shaft 17 of reinforcing element 16 into tension, and the shaft 17 acting through flights 18 and 19 will tend to place the soil mass between flights 18 and 19 into compression. Flights 18 are embedded in relatively stable soil remote from moving face 22, tending to anchor the anchor element in place. The compressive forces acting on moving soil in the vicinity of flights 19 will in turn tend to hold that soil back from moving. Figure 4 illustrates how ground anchor elements according to the present invention may be used in a different manner to support a steep excavation. In this figure, the original ground line 26 has been excavated to produce vertical face 27. Anchor elements 28 have been screwed into earth 29 in a substantially vertical orientation to provide reinforcement. In any given structure, the orientation of the anchor elements can be chosen having regard to the degree and placement of reinforcement required, and availability of access for installation. Figure 5 illustrates an alternative embodiment of ground anchor elements according to the present invention. Shaft 31 carries three substantially helical flights 32, 33 and 34 adjacent the coupling 35. Two substantially helical flights 36 and 37 are carried adjacent the end 38 of shaft 31 remote from the coupling 35. When installed in the earth, the two flights adjacent end 38 are deeper in the earth structure than are the three flights adjacent coupling 35. In general, the soil deeper in the structure will be stronger than will be the soil closer to the surface. Fewer flights will be necessary to develop a given load bearing capacity in this stronger soil than will be necessary to develop the same load bearing capacity in the weaker soil nearer the surface. In the embodiment of the invention as illustrated in Figure 6, screw-in anchors 40 have additionally been provided with restraining plates 41, 42 and 43. Each restraining plate is attached at or adjacent an end of an anchor, and each restraining plate bears against the exposed face 44 of the excavation. In this embodiment of the invention, the installation of the anchor is adjusted so that the shaft of the anchor is under tension, the restraining plate and the anchor operating to place a positive compressive load to the earth. Each screw-in anchor element is installed as a single operation and supports load immediately without delays for grouts to set. In yet another embodiment of the invention similar to figure 6 but not illustrated, screw-in anchor elements are provided with a helical flight or flights similar to 18 adjacent their ground penetrating end only and restraining plates similar to 41-43 adjacent their other end. Figure 7 illustrates the use of the present invention to improve stability of a slope (either natural or man-made) by both mechanical reinforcement and pore water pressure reduction. In this embodiment, the shaft 46 of each screw-in anchor is hollow and protrudes from the face 47 of the slope 48. Water enters the hollow shaft through a porous tip 49 of each shaft 46. The porous tip may be made of any suitable material such as sintered bronze or porous ceramics. By installation of each screw-in anchor on a slight upwards slope and providing a porous tip the hollow shaft can be used to provide drainage deep within the slope, improving stability significantly. In an alternative embodiment of the invention (not illustrated) water enters the hollow shaft through slots or holes in the surface of the shaft. In this embodiment the hollow shaft is preferably filled with a porous filter material to prevent soil from washing into the shaft. Suitable porous filter materials include filter paper, geo-textile fabric, and sand glued together with epoxy glue. When this invention is utilised in draining soil, it is especially preferred that at least one anchor element be or carry an electrode to create an electric field to provide an electro- osmosis effect enhancing movement of moisture through the soil. Yet other embodiments of the invention are useful in ground conditions where corrosion of the shaft of the reinforcing element could be a problem. Figure 8 illustrates one such embodiment. This figure shows an anchor element 51 with a single flight 18. The flight 18 is not welded to a shaft, but instead it is welded to a pilot section 52 which in turn is detachably connected to a drive tool 53 in the form of a hollow shaft. A tie bar or reinforcement bar 54 runs down the hollow shaft of the drive tool 53 and is connected to the pilot section 52. Tie bar or reinforcement bar 54 may be made of any suitable material such as steel or high tensile strength pultruded plastics material. Anchor element 51 is screwed into the ground in the usual manner. Depending on how firm the soil is, the drive tool 53 then may or may not be removed. If the soil is firm enough to not collapse into the resulting void as the drive tool 53 is removed, then that drive tool is removed as grouting is pumped into the resulting void to fill it. If the soil is not firm enough to not collapse, then the drive tool is not removed and grouting is pumped down the hollow of the drive tool to fill the space between the bar 54 and the drive tool 53. When the grouting cures, the anchor element comprises flight 18 cemented to a reinforced concrete rod. A restraining plate as described with reference to figure 6 is attached to the end of the anchor element remote from the helical flight 18. Figure 9 illustrates the use of screw-in anchors 55 to strengthen the foundations 56 of new or existing buildings. Figure 10 similarly illustrates the use of the screw-in anchors 57 to improve foundations under an embankment or large tank 58. The anchors 57 can either be installed during construction or as a retrofit. Figure 11 illustrates how an embodiment of this invention can be used to stabilise the foundations of existing structures without the need for physically interfering with the existing structure. The figure illustrates a structure 61 which has developed a "lean" because of differential compression of the foundation soil. Anchor elements 62, 63 and 64 have been installed at one side of the structure where the soil is more compressible. In the embodiment of the invention as illustrated in figure 11, the loads on anchor elements 62, 63 and 64 are compression loads. Figure 12 is a vertical sectional view of use of apparatus and a method according to the present invention to rehabilitate an earth structure which has failed. Figure 12 shows an earth structure which has failed by slip failure along face 67 which is circular in the vertical section illustrated. Anchor elements 68 according to the present invention have been placed into structure 66 with one end protruding from face 67. These protruding ends have been attached to a reinforcing mesh 69 laid over the face 67. Fill 70 has then been laid over the reinforcing mesh 69 to form a face 71. Further anchor elements 73 according to the present invention have been inserted through fill 69 and into the structure 66, with one end protruding from face 71. These protruding ends have been attached to a reinforcing mesh 72 laid over face 71. The face 71 and the reinforcing mesh 72 have then been sprayed with a thin layer of concrete. Alternative embodiments of apparatus according to the present invention allow investigation of ground conditions. One such embodiment is illustrated in figure 13. Anchor element 75 of figure 13 has hollow shaft 76 with an open leading end 77. Shaft 76 carries helical flight 18 adjacent leading end 77. A removable rod 78 carrying closure plug 79 extends the length of shaft 76. To sample soil conditions, the anchor element 75 carrying rod 78 with closure plug 79 is advanced into the soil. At the desired depth, the rod 78 and plug 79 are removed, and the soil ahead of the anchor element 75 is sampled. The rod 78 and plug 79 may then be replaced, and the anchor element advanced further into the soil to take another soil sample. Yet further embodiments of apparatus and methods according to the present invention provide for the construction of wells. One such embodiment is illustrated in figure 14. This figure shows an anchor element 81 with a single flight 18. The flight 18 is not welded to a shaft, but instead it is welded to a pilot section 82 which in turn is detachably connected to a drive tool 83 in the form of a hollow shaft. A well-liner 84 extends the length of the anchor element 81. In use, anchor element 81 is advanced into the ground carrying well-liner 84 to the desired depth. Drive tool 83 is then detached from pilot section 82, and removed from the ground, leaving well-liner 84 in place.

Claims: Claims 1. A method of ground reinforcement which uses an anchor element which includes a shaft having a leading end and a trailing end, and which anchor element includes at least one substantially helical flight adjacent the leading end, the method including: placing the anchor element in the ground which is to be reinforced with the leading end of the shaft and at least one adjacent substantially helical flight in relatively stable ground and with the trailing end of the shaft in relatively unstable ground. 2. A method of ground reinforcement as claimed in claim 1 wherein the anchor element includes at least one substantially helical flight adjacent the trailing end of the shaft. 3. A method of ground reinforcement as claimed in claim 2 wherein at least one flight adjacent the leading end of the shaft and at least one flight adjacent the trailing end of the shaft are of different pitches to cause compression of the soil between the leading and trailing ends of the shaft as the anchor element is screwed into the ground. 4. A method of ground reinforcement as claimed in any preceding claim in which the shaft is hollow. 5. A method of ground reinforcement as claimed in claim 4 in which the leading end of the shaft is provided with a tip which prevents soil from occluding the hollow shaft. 6. A method of ground reinforcement as claimed in claim 5 in which the tip is removable. 7. A method of ground reinforcement as claimed in claim 5 or claim 6 in which the tip is porous to allow percolation of liquids into the hollow shaft. 8. A method of ground reinforcement as claimed in any one of claims 4 to 7 in which the shaft is perforated to allow the flow of liquids into the hollow shaft. 9. A method of ground reinforcement as claimed in any preceding claim wherein the anchor element is an electrode for an electro-osmosis field. 10. A method of ground reinforcement as claimed in any preceding claim wherein the anchor element carries an electrode for an electro-osmosis field. 11. A method of ground reinforcement as claimed in any preceding claim wherein the anchor element includes a plate attached to the shaft proximate its trailing end and adapted to apply a compressive load to an adjacent exposed earth face. 12. An element for a method of ground reinforcement, which element includes a shaft having a leading end and a trailing end and which includes at least one substantially helical flight adjacent the leading end of the shaft. 13. An element as claimed in claim 12 further including at least one substantially helical flight adjacent the trailing end of the shaft. 14. An element as claimed in claim 13 wherein at least one substantially helical flight adjacent the leading end of the shaft has a greater pitch than does at least one substantially helical flight adjacent the trailing end of the shaft. 15. An element as claimed in any one of claims 12 to 14 which element includes a plate fixed to the shaft substantially at its trailing end. 16. An element as claimed in any one of claims 13 to 15 in which the shaft is hollow. 17. An element as claimed in claim 16 in which the leading end of the shaft is provided with a tip which prevents soil from occluding the hollow shaft. 18. An element as claimed in claim 17 in which the tip is removable. 19. An element as claimed in claim 17 or claim 18 in which the tip is porous. 20. An element as claimed in any one of claims 16 to 19 in which the shaft is perforated. 21. An element as claimed in any one of claims 12 to 15 in which the shaft comprises high tensile strength pultruded plastics material. 22. An anchor element as claimed in any one of claims 12 to 15 in which the shaft comprises reinforced concrete. 23. A method of investigation of sub-soil conditions which uses an element as claimed in claim 18. 24. A method of constructing a well which uses an element as claimed in claim 19 or claim 20. 25. A method of treating polluted ground which introduces at least one active agent into the ground through the hollow shaft of an element as claimed in any one of claims 18 to 20. 26. A method of draining gases or liquids from ground which uses an element as claimed in any one of claims 18 to 20. 27. A method of ground reinforcement substantially as herein described with reference to any one of figures 3, 4, 6, 7, 9, 10, 11 or 12 of the accompanying drawings. 28. An anchor element substantially as herein described with reference to any one of figures 2, 5, 8, 13 or 14 of the accompanying drawings. 29. A method of ground water reduction substantially as herein described with reference to figure 7. 30. A method of investigation of sub-soil conditions substantially as herein described with reference to figure 13. 31. A method of construction of a well substantially as herein described with reference to figure 14.

Inventors: ANDERSON DALE JOHN, YTTRUP PETER JOHN, ANDERSON, DALE, JOHN, YTTRUP, PETER, JOHN
Applicants: ANDERSON DALE JOHN, SCREW IN TECH PTY LTD, YTTRUP PETER JOHN, ANDERSON, DALE, JOHN, SCREW IN TECHNOLOGIES PTY. LTD., YTTRUP, PETER, JOHN
Application year: 1998
Main classification: E02D5
IPC codes: E02D5