Soil nailing

Abstract: A method of soil nailing is shown in Figure 13 in which a soil nail (50) is placed in the ground by being fired from a barrel (3) of a launcher (1), the method including the steps of loading the nail into the barrel with the nail being a loose fit in the barrel, providing the nail with a sabot (58) which is a sliding fit in the barrel, supporting the barrel such that a forward end of the barrel is spaced from the surface (13) of the ground (14) into which the nail is to be fired, admitting pressurised gas to the barrel to apply an accelerating force to the sabot so as to drive the sabot and with it the nail towards the forward end of the barrel, allowing the nail to travel towards the ground surface such that the sabot exits from the barrel thereby discontinuing the accelerating force and allowing the nail to thereafter continue travelling to penetrate and become embedded in the ground under its own momentum.

Description: This invention relates to soil nailing and in particular to a method and apparatus for placing a soil nail in the ground. It is known to provide ground strengthening by driving elongate reinforcing members known as soil nails into the ground in an array in order to improve the bulk properties of the ground. GB-A-1580142 for example discloses the use of solid rods or tubes of 3 to 15 metres length arranged in a grid distributed over soil to be stabilized. EP-A-0239258 discloses the use of pickets fired from a launcher with sufficient momentum to penetrate and become embedded in the ground to provide a ground strengthening array. Disadvantages of such methods are that where deep penetration is required the length of soil nail may lead to handling difficulties particularly when the nails are to be fired from a launcher. The launcher must be designed to fire long and slender soil nails without bending or buckling during firing and the launcher should ideally be able to fire nails of different lengths. A further problem when firing from a launcher is to control the depth of penetration. EP-A-0239258 discloses the use of pickets having enlarged heads which are fired through surface cladding so that the picket is arrested by impact with the head. There are many applications however where surface cladding is not required and in such cases the depth of penetration is difficult to control. According to the present invention there is disclosed a method of soil nailing in which a soil nail is placed in the ground by being fired from a barrel of a launcher, the method including the steps of loading the nail into the barrel with the nail being a loose fit in the barrel, providing the nail with a sabot which is a sliding fit in the barrel, supporting the barrel such that a forward end of the barrel is spaced from the surface of the ground into which the nail is to be fired, admitting pressurised gas to the barrel to apply an accelerating force to the sabot so as to drive the sabot and with it the nail towards the forward end of the barrel, allowing the nail to travel towards the ground surface such that the sabot exits from the barrel thereby discontinuing the accelerating force and allowing the nail to thereafter continue travelling to penetrate and become embedded in the ground under its own momentum. An advantage of such a method is that the removal of the accelerating force prior to impact with the ground avoids placing the nail under axial compressive loading which would otherwise occur if for example the nail continued to be pushed into the ground by a force applied to the rear of the nail. Axial compressive loading would tend to bend and possibly buckle the nail thereby reducing the accuracy with which the nail could be placed in the ground. Preferably the launcher includes a breech defining an aperture through which a rearward portion of the nail projects prior to being fired and preferably the sabot is connected to the nail adjacent a forward end of the nail during firing. An advantage of such a method is that during firing the accelerating force is applied to the forward end of the nail thereby placing the nail in axial tension. This tension removes any in-built deformations in the nail which might otherwise cause it to buckle during firing. The straightness of the nail on impact with the ground is of great importance because any slight bending will tend to be exacerbated by the resulting compression on impact so that straightness of the nail is essential if bending and buckling are to be avoided. According to a further aspect of the invention there is disclosed apparatus for use in soil nailing comprising a barrel receiving in use a soil nail, support means supporting the barrel such that a forward end of the barrel is spaced from the surface of the ground, the soil nail being a loose fit in the barrel and being provided with a sabot which is slidable in the barrel, the barrel having a rearward end provided with a breech defining a firing chamber closed by the sabot, and firing means operable to admit pressurised gas to the chamber so as to apply an accelerating force to the sabot whereby in use the soil nail is accelerated towards the ground surface by the accelerating force until the sabot exits the barrel, the soil nail thereafter travelling under its own momentum to penetrate and become embedded in the ground. Conveniently the apparatus includes guide means located rearwardly of the barrel and maintaining a rearward portion of the nail in coaxial alignment with the barrel. The support means may comprise an articulated arm mounted on a vehicle. The apparatus may include a generally cylindrical baffle of greater internal diameter than the barrel and extending in use between the forward end of the barrel and the ground surface. The baffle may include a bellows portion. The soil nails are preferably embedded in the ground so as to cross critical failure surfaces in the ground. Embodiments of the present invention will now be disclosed by way of example only and with reference to the accompanying drawings, of which:- Figure 1 is a partly sectioned elevation of a launcher loaded with a first tubular member ready for firing into the ground; Figure 2 shows the launcher of Figure 1 after firing in which the first member is embedded in the ground and a second member is loaded in the launcher; Figure 3 a is an end view of the tip of the first member of Figure 1; Figure 3 b is an elevation of the tip of Figure 3 a ; Figure 4 is an elevation showing the first and second members of Figure 2 after firing and jointing to form a soil nail; Figure 5 is a detailed partly sectioned view of the joint between the first and second members of Figure 4; Figure 6 is an enlarged view of the firing chamber of the launcher of Figure 1 showing the first tubular member connected to its sabot; Figure 7 a is an end view of the sabot mounted on the second member of Figure 2; Figure 7 b is an elevation of the sabot of Figure 7 a , Figure 8 is an elevation of a soil nail comprising first, second and third members; Figure 9 is a partly sectioned elevation of a solid member in its loaded position in a launcher; Figure 10 is a front end view of the sabot of Figure 9; Figure 11 is a rear end view of the sabot of Figures 9 and 10; Figure 12 is a rear end view of the member of Figure 9; Figure 13 is a partly sectioned elevation of the launcher and member of Figure 9 shortly after firing; Figure 14 is a further view of the apparatus of Figure 13 showing detail of the initial penetration of the ground by the member; Figure 15 is a similar view of the apparatus of Figures 9 to 14 showing detail of the breech seal being engaged by the rearward end of the member; Figure 16 is a similar view of the apparatus of Figures 9 to 15 showing detail of the sabot and seal after ground penetration; Figure 17 is a perspective view of a launcher mounted on an articulated arm of a vehicle; Figure 18 is a schematic elevation showing a launcher having a baffle with a bellows portion; Figure 19 is a perspective view of an alternative sabot; Figure 20 is a schematic sectional elevation of apparatus including a rail means spanning an excavation comprising a trench; Figure 21 is a sectional elevation schematically showing apparatus having rail means spanning an excavation comprising a concrete foundation; and Figure 22 is a sectional elevation showing a soil nail having perforations for drainage purposes engaging a surface cladding. In Figure 1 a launcher 1 is shown in its loaded condition in which a first elongate tubular member 2 is located in the launcher ready for firing. The launcher 1 comprises a barrel 3 communicating with a chamber 4 defined by a breech 5. A forward end 6 of the member 2 is received within an annular sabot 7 of a plastics material which is slidably received in the barrel 3 adjacent to the chamber 4. The first member 2 extends through the chamber 4 and projects rearwardly of the launcher 1 through an aperture 9 formed in the breech 5 in axial alignment with the barrel 3. An annular breech seal 10 of a plastics material provides sealing between the member 2 and the breech 5 at the aperture 9. The first member 2 is of 6 metres length of which 5 metres projects rearwardly of the launcher. A gas inlet tube 11 is connected to the chamber 4 for the admission of compressed gas. A baffle 12 of larger diameter than the barrel 3 forms an axial projection of the barrel extending into contact with the surface 13 of a body of ground 14. As shown in Figure 1 the direction of travel of the first member 2 is vertically downwards and the travel between the lower end 15 of the barrel 3 and the ground surface 13 is 2 metres. The sequence of events on firing the launcher will be described with reference to later Figures but results in the first member 2 being embedded in the ground 14 to an extent such that the sabot 7 is located at the surface 13 as shown in Figure 2. The baffle 12 includes a locating ring 16 which is a snug fit around the sabot 7 such that the launcher 1 is then aligned with the previously fired first member 2. In this position a bore 17 defined by the first member 2 is coaxially in alignment with the barrel 3 and hence in alignment with a second member 18 loaded in the launcher as shown in Figure 2. The forward end 6 of the first member 2 has a tapered tip 19 as shown in Figures 3 a and 3 b in the form of a truncated cone which is segmented by means of axially extending slots 20. The slots 20 do not extend fully to the forward extremity of the tip 19 such that a thin retaining ring (not shown) maintains the shape of the cone during firing and gives sufficient strength to act as a stop which arrests the second member 18 as described below when fired through the bore 17. A detachable conical cap 21 overlays the tip 19 so as to close the forward end against the ingress of soil during ground penetration. The second member 18 as shown in Figure 2 in its loaded position has a forward end 22 which is carried in an annular sabot 23 of a plastics material which is received as a sliding fit within the barrel 3 and located adjacent to the chamber 4. The sabot 23 is different from the sabot 7 associated with the first member 2 in that the sabot 23 is segmented as shown in Figures 7 a and 7 b by radially extending cuts 24. For handling purposes prior to firing the segments are held together by means of adhesive tape (not shown). The second member 18 is similarly provided with a breech seal 25 forming a seal in the aperture 9 of the breech 5 although the dimensions of the seal 25 are different to those of seal 10 since the diameter of the second member 18 is less than that of the first member 2 in order to facilitate penetration of the second member into the bore 17. The second member 18 has a collar 26 adjacent to but spaced from its rear end 27 which constitutes a radial enlargement dimensioned such that it will pass through the breech aperture 9 and will pass through the bore 17 but will not pass through the opening defined by the truncated tip 19 of the first member 2. The tip 19 thereby acts as a stop which arrests the second member 18 by engagement with the collar 26. After firing the second member 18 into the bore 17 it comes to rest in the position shown in Figure 4 in which it forms an extension to the first member 2 thereby projecting deeper into the ground 14 by almost the full length of the second member. As shown in detail in Figure 5 the collar 26 is arrested by contact with the tapered tip 19 of the first member 2 since it is too large to pass through the opening 28 defined by the truncated tip. The rear end 27 of the second member 18 includes a threaded portion 29 engageable with a reinforcing member 30 shown in chain dot in Figure 5 which optionally may be inserted within the bore 17 to provide additional strength. The bore 17 is filled with grout or resin 31 adjacent to the tip 19 to form a joint 32 between the first and second members which is resistant to shear forces and to both tension and compression forces applied along the soil nail 33 constituted by the combined first and second members. For this purpose the collar 26 is deliberately spaced from the rear end 27 of the second member 18 in order to provide adequate bonding surfaces. The cap 21 is removed from the tip 19 on impact by the second member 18 during travel through the bore 17 and is therefore not shown in Figure 4. In Figure 6 the sabot 7 associated with the first member 2 is shown to be connected to the forward end 6 by means of cooperating circumferential ribs 34 and grooves 35 formed on the first member 2 and the sabot 7 respectively. The ribs and grooves 34 and 35 are ramped to permit movement of the sabot 7 relative to the first member 2 only in a direction towards the rear end 8 of the member. The sabot 7 and the breech seal 10 are connected by four circumferentially spaced thin straps 36 which are formed of a resilient plastics material and are bowed to exert a separating force between the breech seal 10 and the sabot 7. The breech seal and sabot are thereby held in position prior to firing. The straps also serve to maintain the breech seal 10 in position during handling of the first member 2. The straps 36 are formed so as to have a weak point adjacent to the junction between each strap with a breech seal 10. The sequence of events in placing the soil nail of Figure 4 in the ground is to first position the launcher 1 such that the baffle 12 is in contact with the ground surface 13 at the required location. A first member 2 is loaded in the launcher in the loaded position as shown in Figure 1 and the launcher fired by admitting compressed gas to the chamber 4 via the gas inlet tube 11. Gas pressure acting on the sabot 7 causes the member 2 to accelerate along the barrel 3 whilst the breech seal 10 is forced upwardly by gas pressure so as to remain held within the aperture 9 so that the straps 36 are broken. Continued downward motion of the tubular first member 2 is accompanied by further acceleration until the sabot emerges from the barrel into the baffle 12 at a speed in the range 110 to 400 km/h (70 m.p.h. to 250 m.p.h.). The member 2 continues its linear trajectory by virtue of its acquired momentum and travels into penetration with the ground surface 13. Any bowing of the member 2 or tendency to deflect the member by collision with hard objects in the ground 14 is limited by contact between the member 2 and the walls of the barrel 3. At the moment of initial ground penetration the mid-point of the member 2 is still contained within the barrel so that bowing is effectively controlled. After initial penetration the member 2 continues along the trajectory established by initial penetration. When the rear end 8 of the first member 2 encounters the breech seal 10 an enlargement 37 formed on the rear end of the member forces the seal 10 out of the aperture 9 such that the seal is carried with the member 2 on its trajectory. The sabot 7 is dissociated from its initial position at the forward end 6 of the first member 2 at the time of initial ground penetration because the diameter of the sabot 7 is much greater than that of the member 2 and hence has greater resistance to penetration of the ground. Consequently the sabot 7 comes to rest in a position as shown in Figure 2 in which it is embedded partially into the ground 14. The seal 10 carried on the member 2 in contact with the enlargement 37 finally collides with the sabot 7 and this results in the member 2 being arrested. The member 2 thereby comes to rest at the position shown in Figure 2 with the rear end 8 projecting slightly above the ground so that the bore 17 is accessible. The launcher is then reloaded with the second member 18 together with its sabot 23 and breech seal 25 as shown in Figure 2. The launcher is aligned by locating the locating ring 16 of the baffle 12 over the sabot 7. The launcher is then fired by admitting compressed gas to the chamber 4 resulting in gas pressure being applied to the sabot 23 and downward acceleration of the second member 18. Downward motion of the second member 18 continues such that the sabot 23 enters the baffle 12 and fragments so as to fall away from the member. The second member 18 continues under its own momentum towards the first member 2 and enters the bore 17. The breech seal 25 is picked up by collision with the collar 26 and carried at the rear end 27 of the second member 18 into collision with the enlargement 37 at the mouth of the bore 17. The breech seal 25 shatters on impact and falls away from the second member 18 which continues to travel downwardly in the bore 17 until the collar 26 is arrested by contact with the tapered tip 19. Grout or resin 31 is then inserted into the bore 17 to a depth sufficient to form a joint 32 as shown in Figures 4 and 5. Optionally the steel reinforcing member 30 may then be inserted into the bore 17 and engaged with the threaded portion 29. An extended soil nail 33 as shown in Figure 4 is then provided. An array of such nails embedded in the soil will enhance the bulk properties of the ground. Typically the nails will be inserted in positions such that the nails intersect a critical slip surface at which the ground is expected to fall. Although the ground surface has been shown as horizontal in the above Figures the method may similarly be used in strengthening ground having inclined surfaces or vertical surface where for example a wall of earth is to be reinforced. Where an inclined bank or wall is provided already with surface cladding then the nails may be fired through apertures formed in the surface cladding or may be fired directly through the cladding. In an alternative method the second member 18 may be replaced by a further hollow member (not shown) of structure generally similar to that of the first member 2 referred to above but having a reduced outer diameter sufficient to be accommodated within the bore 17 of the first member 2. Such a modified second member 38 as shown in Figure 8 will form an extension to the bore 17 by providing an additional bore section 39 through which a further member 40 may be fired using the launcher 2. The further member 40 may be a solid rod or may again be modified to be tubular to thereby extend the bore and accommodate a further member fired from the launcher (not shown). The resulting soil nail may thereby comprise any number of members which are telescopically jointed in accordance with the above method. A further aspect of the present invention is illustrated with reference to Figures 9 to 16 where corresponding reference numerals to those used in the preceding Figures are used where appropriate for corresponding elements. In Figure 9 an elongate member 50 is shown in its loaded position in a launcher 1. The member 50 may be of any required length but in this example is 6 metres in length and made of steel. The member 50 has a conical tip 51 formed integrally with the member which is recessed immediately behind the tip to define the shoulder 52. A frusto-conical portion 53 of the member 50 extends between the shoulder 52 and an elongate cylindrical portion 54 which extends through the chamber 4 and projects rearwardly of the breech 5. A rear end 55 of the member is crimped to form a plurality of circumferentially spaced axially extending ribs 56 which progressively project radially to form a tapered enlargement 57 at the rear end 55. The launcher 1 has a breech 5 defining a chamber 4 communicating with a barrel 3 and the breech is provided with an aperture 9 in axial alignment with the barrel 3 and through which the member 50 extends rearwardly of the breech. A gas inlet tube 11 communicates with the chamber 4 for the admission of pressurised gas to the chamber. The member 50 is a loose fit within the barrel 3 and carries a sabot 58 in the form of a collet fitting snugly on the forward end of the cylindrical portion 54 and dimensioned such that the sabot is a sliding fit within the barrel 3. The sabot 58 has a rear end 59 projecting into the chamber 4 and four radially extending lugs 60 project from the sabot into the chamber which is formed with a greater diameter than that of the barrel 3. The sabot 58 is thereby restrained prior to firing from movement along the barrel by the lugs 60. An annular breech seal 61 is mounted coaxially on the cylindrical portion 54 of the member 50 and is of stepped diameter such that it projects partially into the aperture 9 formed in the breech 5. The breech seal 61 is dimensioned to be a sliding fit on the cylindrical portion 54 of the member 50. Four circumferentially spaced straps 36 connect the seal 61 with the sabot 58 and are dimensioned such that in the loaded position of the member 50 the straps are bowed and possess sufficient stiffness to exert a separating force between the seal and the sabot. The seal and sabot are thereby retained in their desired positions before firing. The straps 36 are made sufficiently thin such that they are frangible on firing to permit separation of the sabot 58 and the seal 61 and are provided with a weak point adjacent to the point of connection with the seal so that on firing the straps remain attached to the sabot as shown in Figure 13. The sabot 58 is generally cylindrical having a forward face 62 having a central recess 63. An annular segmented truncated cone portion 64 projects forwardly within the recess 63 into contact with the shoulder 52 to thereby resist movement of the sabot relative to the member 50 in a direction towards its forward end. The cone portion 64 is segmented by slots 65 formed radially and extending longitudinally of the member. The relative dimensions of the recess 63 and the cone portion 64 are such that the cylindrical portion 54 can be accommodated within the cone portion by radially outward deflection of the segmented portion such that the segments 65 are accommodated within the recess 63. The member 50 can thereby be driven through the sabot by downward movement of the member if the sabot 58 is held stationary but movement in the upward direction is prevented by abutment between the shoulder 52 and the cone portion 64. By this arrangement the cone portion 64 and cooperating features of the member 50 constitute stop means permitting relative movement in one direction only. The launcher 1 includes a baffle 12 which forms an extension to the barrel 3 of enlarged diameter as shown in Figure 13. To fire the launcher 1 the baffle 12 is placed in contact with the surface 13 of a body of ground 14 at the required location and the launcher is fired by admitting compressed gas to the chamber 4 through the gas inlet tube 11. The gas pressure contained within the chamber 4 exerts on the sabot 58 a downward force resulting in acceleration of the sabot and with it the member 50 which is constrained to move with the sabot by the stop means 66. Figure 13 shows the position of the sabot 58 shortly after it begins to move downwardly within the barrel. Gas pressure within the chamber 4 presses the breech seal 61 into sealing engagement with the breech 5 and the relative movement between the sabot 58 and the seal results in breakage of the straps 36 which continue to be attached to the sabot. Downward motion of the member 50 in line with the barrel 3 continues until the sabot 58 passes out of the barrel 3 into the baffle 12 during which travel the member continues to be accelerated. Thereafter the member 50 continues its trajectory under its acquired momentum and penetrates the ground as shown in Figure 14. Gas escaping from the barrel 3 into the baffle 12 is then able to expand freely so that the barrel serves as a silencer. Because the sabot 58 is of larger diameter than the member 50 it has a greater resistance to ground penetration and will penetrate only slightly the ground surface 13 as shown in Figure 14. Relative movement of the sabot towards the rear end 58 of the member 50 is accommodated by the stop means 66 since the conical portion 53 of the member forces apart the segments 65 of the cone portion 64 and permits the cylindrical portion 54 to be passed through the sabot. Continued travel of the member 50 results in the enlargement 57 at the rear end 55 of the member engaging the breech seal 61 which is carried with it as shown in Figure 15 through the chamber 4 and into the barrel 3. Continued travel of the member 50 into the ground results in impact between the seal 61 and the sabot 58. The seal 61 is formed of a material sufficiently strong to withstand the resultant radial pressure exerted by the enlargement 57 so that the member 50 is arrested. Some further penetration into the ground of the sabot 58 at this instant will occur but generally this travel will be less than the thickness of the sabot. The seal 61 and the enlargement 57 together constitute a second stop means preventing movement of the sabot relative to the member 50 in a direction towards the rear end 55 beyond the position shown in Figure 16 where the sabot is adjacent to but spaced from the rear end. The penetration of the soil nail placed by firing from the launcher may thereby be accurately controlled. The member 50 in the example shown in Figures 9 to 16 is a solid steel rod but may alternatively be tubular with the conical tip 51 being formed as a separate element connected by welding or otherwise to the cylindrical portion of the member. The first stop means may alternatively comprise cooperating ribs and grooves 34 and 35 as shown in Figure 6 which are ramped to permit movement of the sabot relative to the member only in a direction towards the rear end. The use of a sabot permits members of different diameter to be accommodated within a launcher of a given barrel size simply by changing the dimensions of the sabot and breech seal. The arrangement of a launcher in which the bulk of the member projects rearwardly of the breech allows the launcher of a given barrel length to be used with members of greatly varying length. A further advantage of the above apparatus is that the gas pressure admitted to the chamber need not be closely controlled in order to match a required penetration depth since it is sufficient to provide a level of gas pressure which will ensure penetration for all applications and rely on the sabot to arrest the member at the required depth of penetration. Figure 17 shows a vehicle 70 suitable for deploying a launcher 1 in soil nailing applications where the ground surface is horizontal, vertical or inclined to the horizontal. The vehicle 70 includes an articulated arm 71 allowing the launcher 1 to be oriented as required. The launcher 1 in Figure 17 is seen to include a baffle 12 which is shown in contact with a horizontal ground surface 13. The launcher barrel (not shown) is shrouded in a jacket 72 with a further shroud 73 surrounding the breech (not shown) in order to lessen the effects of noise on firing. A guide 74 projects rearwardly of the launcher 1 and consists of a guide arm 75 and guide ring 76 supported by the guide arm in a position such that a soil nail 77 loaded in the launcher 1 projects rearwardly of the breech and is supported within the guide ring 76. The guide 74 ensures that the soil nail 77 is in coaxial alignment with the barrel prior to and during the initial stages of the launch thereby reducing the likelihood of bending and possible buckling of the soil nail during firing. The vehicle 70 includes a rack 78 containing a supply of further soil nails 79 of different lengths for future use. Figure 18 shows schematically how the launcher 1 might be deployed to fire soil nails 77 into a vertical ground surface 13 in order to stabilise a body of ground 14 against failure at a critical failure surface 80. The arm 71 is articulated to a position in which the launcher 1 aims the soil nail 77 at an angle such that it lies approximately at right angles to the critical failure surface. The embedded position 77 a of the soil nail 77 after firing is shown in broken lines in Figure 18. The embedded soil nail 77 a extends on either side of the critical failure surface 80 so as to prevent relative ground movement on opposite sides of the critical failure surface. The launcher 1 is used to fire an array of similar nails 77 into the ground 14 such that collectively the array of soil nails provides ground strengthening and improved resistance to shear failure. In Figure 18 the nail 77 is received in a sabot 81 of a type which fragments on leaving the barrel 82 and fragments of the sabot are retained within a baffle 83 which encloses a generally cylindrical space 84 between the barrel and the ground surface 13. The baffle 83 also contains any flying debris created on impact of the nail 77 with the ground surface 13 in addition to reducing noise created during firing. The baffle 83 includes a cylindrical portion 85 which is connected to the barrel 82 and a bellows portion 86 which extends between the cylindrical portion and a disc 87 which is maintained in contact with the ground surface 13. The bellows portion 86 is sufficiently flexible to accommodate the launcher being deployed at angles other than 90 o relative to the ground surface 13. The bellows portion 86 is however sufficiently stiff to ensure that flying debris created within the space 84 remains trapped during firing. Figure 19 shows schematically the way in which a sabot 81 can be preformed with cuts 88 and 89 which ensure that the sabot fragments on leaving the launcher barrel. The sabot 81 has a rear surface 90 which during firing is exposed to pressurised gas. The sabot 81 has cuts 88 and 89 which meet in a diametric line 91 and divide the sabot into three portions 92, 93 and 94. The cuts 88, 89 are orthogonal to one another and arranged at 45 o to the cylindrical axis of the sabot such that a central portion 93 of the sabot has a triangular cross-section and acts as a wedge to force apart side portions 92 and 94 of the sabot when accelerating gas pressure is applied to the rear surface 90. On leaving the barrel the side portions 92 and 94 tend to fragment radially away from the nail 77. The central portion 93 is formed with further cuts (not shown) allowing the central portion to fragment once the side portions 92 and 94 have separated. In Figure 20 an alternative apparatus 100 includes a number of features in common with apparatus of preceding Figures so that corresponding reference numerals are used where appropriate for corresponding elements. The apparatus 100 includes a launcher 1 which is shown schematically in three alternative positions 1 a , 1 b and 1 c . The launcher 1 is connected to a horizontal rail 101 which is deployed so as to span a trench 102. The rail 101 is mounted at each end on wheeled conveyors represented schematically at 103 and 104 respectively. The trench 102 has vertical left and right-hand side walls 105 and 106 respectively each of which is covered with a corrugated cladding 107. In the absence of ground strengthening the ground 108 and 109 adjoining the excavation defining the trench 102 will tend to fail in shear at critical failure surfaces 110. In accordance with known soil nailing technique the ground can be strengthened against shear failure by emplacing an array of soil nails which intersects the critical failure surface 110 at approximately right angles to the surface, the array being such as to form a grid pattern over the critical failure surface. The apparatus 100 enables such an array of soil nails to be emplaced by first deploying the rail at a first location as shown in Figure 20. The launcher is deployed at position 1 a in which it is offset to one side of the trench adjacent to the left-hand side wall 105 and so that it directs the soil nail towards the right-hand side wall. The launcher is pivotal about a pivot 111 which is slidably movable along the rail 101 and can be clamped at any desired position. The launcher 1 a is tilted so as to direct a soil nail 77 at a downwardly directed angle such that on firing it penetrates the corrugated cladding 107 on the right-hand side wall 106 and penetrates the ground 108. Penetration of the soil nail 77 is arrested by means of a radial enlargement at the rear end of the soil nail (not shown) the length of nail being selected such that the soil nail crosses the critical failure surface 110 and extends on either side of the surface. The launcher 1 a is then tilted to a steeper angle and a second soil nail 77 a is fired into the ground 108. The launcher is again reloaded and tilted to an even steeper angle and a further soil nail 77 b fired into the ground. The critical failure surface 110 extends upwardly from the toe 112 of the side wall 106 at a progressively increasing angle to the horizontal. As the successive soil nails 77 are fired at progressively steeper angles to the horizontal the tendency for the critical failure surface to change in gradient is compensated at least partially by the change in gradient of successive soil nails such that, to an approximation, the angle of intersection between the soil nail and critical failure surface is maintained at a right angle. The launcher is then redeployed to position 1 b at which it is central to the trench and is pointed directly downwards to fire a soil nail 77 c into the ground underlying the trench. The launcher 1 is then re-deployed to position 1 c in which it is offset to one side of the trench adjacent to the right-hand side wall 106 and is downwardly directed towards the left-hand side wall 105. A corresponding number of soil nails 77 to those fired from location 1 a are again fired at different angles of inclination to the horizontal so as to cross the critical failure surface on the left-hand side of the trench. The rail 101 is then moved in a direction at right angles to the rail to a new location and the above process repeated. This procedure is repeated until a suitable array of soil nails 77 extends the full length of the trench 102. This procedure can be carried out not only to strengthen the ground adjoining trenches but can be used to support the sides of an excavation for a foundation and to increase the foundation strength as shown schematically in Figure 21. The apparatus and method described above with reference to Figures 20 and 21 typically employs relatively short soil nails perhaps of 1.5 metres length, the precise length required being determined by an analysis of the location of the critical failure surfaces. When firing relatively long soil nails such as 6 metres in length or greater it is desirable to use a sabot of reinforced strength since a greater firing gas pressure will be required. Suitable construction of such a reinforced sabot would be to make use of high strength tensile fibres reinforcing a plastics material or to form the sabot of cast aluminium or other metals. When firing through cladding such as shown in Figure 20 it is also important to ensure that the sabot fragments before impacting the cladding and a sabot of the type shown schematically in Figure 19 is suitable for such use. If the sabot fails to fragment it may impede full penetration of the soil nail particularly if the angle of incidence between the soil nail and the ground surface is other than a right angle since the sabot or a fragment of the sabot may become wedged between the nail and the ground surface and provide increased friction against penetration. Penetration of a soil nail may in the above embodiments be arrested by means of a radial enlargement of the soil nail encountering a ground surface engaging member which can be a disc or surface cladding applied to the ground surface. Figure 22 shows a tubular soil nail 120 having a radially enlarged head 121 which on firing is arrested by impact with a surface cladding 122. The nail 120 is fired through a preformed bore 123 in the cladding 122 The nail 120 includes a plurality of perforations 124 distributed along its length, these perforations being provided for the purpose of drainage. Such a disc or surface cladding will preferably have a preformed hole into which the soil nail is fired and may have elastic putty or the like surrounding the hole such that the radial enlargement of the soil nail compresses the putty on impact. The sabot may be connected to the soil nail at positions intermediate the front end and rear end. The sabot may for example be located adjacent to but spaced from the front end of the nail where the nail is to be fired through corrugated sheeting or any other type of surface cladding material. When a collet of the fragmenting type is positioned in this manner it has been found that there is a reduced likelihood that the sabot (or fragments of the sabot) will cling to the nail at the point of penetration due to plastic deformation of the cladding. When a non-fragmenting sabot is used as an arresting mechanism it may also be desirable to locate the radial enlargement of the soil nail at a position adjacent to but spaced from the rear end of the nail such that the sabot comes to rest some distance from the rear end. When embedded in the ground this provides for a rear portion of the nail to remain projecting from the ground surface where such projection is desirable. The term sabot used throughout the description and claims should be understood to have its normal meaning of being an attachment to guide a projectile through a bore (in this case the barrel of a launcher). It should be understood however that in certain contexts the element referred to as being a sabot might equally be referred to as being a collet (i.e. a ring or a collar) particularly where the element provides a separate function after having left the barrel so that it is in fact no longer acting as a sabot.

Claims: A method of soil nailing in which a soil nail (2,18,50,77) is placed in the ground by being fired from a barrel (3) of a launcher (1), the method including the steps of loading the nail into the barrel with the nail being a loose fit in the barrel, providing the nail with a sabot (7,23,58,81) which is a sliding fit in the barrel, supporting the barrel by a support means (71) such that a forward end (15) of the barrel is spaced from the surface (13) of the ground into which the nail is to be fired, admitting pressurised gas to the barrel to apply an accelerating force to the sabot so as to drive the sabot and with it the nail towards the forward end of the barrel, allowing the nail to travel towards the ground surface such that the sabot exits from the barrel thereby discontinuing the accelerating force and allowing the nail to thereafter continue travelling to penetrate and become embedded in the ground under its own momentum. A method as claimed in claim 1 including the step of arresting the nail when it has penetrated the ground to a predetermined extent, the nail being arrested by engagement between a radial enlargement (37,57,121) of the soil nail with a ground surface engaging member (7,122) which has a substantially greater cross-sectional area than the nail. A method as claimed in claim 2 wherein the ground surface engaging member comprises a surface cladding or disc (121) having an aperture (123) through which the soil nail is fired. A method as claimed in claim 2 wherein the ground surface engaging member is constituted by the sabot (7). A method as claimed in any preceding claim wherein the launcher includes a breech (5) defining a chamber (4) and having an aperture (9) through which a rearward portion of the nail projects prior to being fired, the method including the step of slidably mounting on the nail an annular seal (10,25,61) forming a breech seal between the nail and the breech so as to close a rearward end of the chamber. A method as claimed in any of claims 1 to 5 in which an array of soil nails are fired into the ground so as to cross a critical failure surface (80,110) in the ground. A method as claimed in any of claims 1 to 6 including the step of connecting the sabot to the nail adjacent a forward end (6) of the nail such that an accelerating force is applied to the forward end of the nail during firing thereby placing the nail in axial tension. A method as claimed in claim 1 including the step of providing the sabot with fragmenting means (88,89) whereby the sabot fragments on leaving the barrel. A method as claimed in any preceding claim including the step of positioning the barrel relative to the ground surface such that the travel of the nail between the open end of the barrel and the ground surface is no more than one half the length of the nail to thereby resist buckling and/or deflection of the nail. Apparatus for carrying out the method of soil nailing as defined in the claims 1 to 7 comprising a barrel receiving in use a soil nail, support means (71) supporting the barrel such that a forward end (15) of the barrel is spaced from the surface of the ground, the soil nail being a loose fit in the barrel and being provided with a sabot (7,23,58,81) which is slidable in the barrel, the barrel having a rearward end provided with a breech (5) defining a firing chamber (4) closed by the sabot, and firing means operable to admit pressurised gas to the chamber so as to apply an accelerating force to the sabot whereby in use the soil nail is accelerated towards the ground surface by the accelerating force until the sabot exits the barrel, the soil nail thereafter travelling under its own momentum to penetrate and become embedded in the ground. Apparatus as claimed in claim 10 wherein the breech defines an aperture (9) through which the soil nail projects rearwardly of the chamber when the nail is in a loaded position, an annular seal (10,25,61) being slidably mounted on the nail and forming a breech seal between the nail and the breech so as to close a rearward end of the chamber. Apparatus as claimed in claim 11 wherein the nail includes stop means comprises a radially projecting formation (37,57) at the rear end of the nail over which the seal cannot pass such that on being fired the seal engages the formation. Apparatus as claimed in any of claims 11 and 12 including guide means (74) located rearwardly of the barrel and maintaining a rearward portion of the nail in coaxial alignment with the barrel. Apparatus as claimed in any of claims 10 to 13 wherein the support means comprises an articulated arm (71) mounted on a vehicle (70). Apparatus as claimed in any of claims 10 to 14 comprising a generally cylindrical baffle (12,83) of greater internal diameter than the barrel and extending in use between the forward end of the barrel and the ground surface. Apparatus as claimed in claim 15 wherein the baffle (83) includes a bellows portion (84).

Inventors: BARR BENJAMIN ISAAC GEORGE, BRIDLE RONALD JARMAN, CAMPBELL COLIN IAN, MYLES BERNARD, BARR, BENJAMIN ISAAC GEORGE, BRIDLE, RONALD JARMAN, CAMPBELL, COLIN IAN, MYLES, BERNARD
Applicants: UNIV CARDIFF, UNIVERSITY COLLEGE CARDIFF CONSULTANTS LTD.
Application year: 1990
Main classification: E02D5
IPC codes: E02D17, E02D7, E02D5, E02D3