EP0262146A1 - Improvements to ground anchors - Google Patents

Abstract

Un procédé servant à former un tirant d'ancrage (10) en milieu terrestre, par exemple dans de la roche ou dans un sol, consiste à faire avancer par son extrémité un élément allongé (11) dans le sol et à former un trou (15) dans le sol autour dudit élément (11), au fur et à mesure que celui-ci progresse, puis à ancrer l'élément (11) dans le trou (15). L'ancrage peut être obtenu en faisant passer une substance durcissable (16) à travers l'élément (11) jusque dans le trou (15). La présente invention peut également être utilisée pour stabiliser un milieu terrestre, destiné par exemple à recevoir par la suite un tirant d'ancrage.A method for forming a ground anchor (10), for example in rock or in soil, consists in advancing at its end an elongated element (11) in the ground and in forming a hole ( 15) in the ground around said element (11), as it progresses, then anchoring the element (11) in the hole (15). Anchoring can be achieved by passing a curable substance (16) through the element (11) into the hole (15). The present invention can also be used to stabilize a terrestrial environment, intended for example to subsequently receive an anchor.

Description

"IMPROVEMENTS TO GROUND ANCHORS" TECHNICAL FIELD OF THE INVENTION THIS INVENTION relates to improvements to ground anchors and in a first aspect to an improved method and means for providing ground anchors in a ground medium such as rock or soil. In a further aspect, the present invention provides a method and means for stabilising a medium say for the subsequent acceptance of ground anchors. BACKGROUND ART

In the common form of ground anchor an anchored tendon is established using a drilled hole with a tendon threaded therein and fixed in place, typically with a grout of a suitable form. Anchored tendons are used to hold back retaining walls; to consolidate rock faces in underground work; to resist bouyancy pressures on inground structures; and in the increasing of stability of dam walls. Ground anchors are provided into both soil and rock using adaptations taking into account the material in which the anchor is to be fixed. A basic sequence of steps in forming a typical ground anchor involves drilling a hole, removing the drill rods and flushing the hole, manufacturing an anchor, fitting a tendon carrying the anchor in place in the hole, grouting the hole around the anchor structure on the inner end of the tendon; allowing grout to cure; and tensioning the tendon. Drilling the hole for the anchor can be a time consuming process with much handling of drill strings and final clearing of the hole to allow receipt of the tendon. The tendon itself can involve a degree of labour in assembling which typically involves forming a sheathed bundle of cables which are alternately clamped and spaced at their exposed anchor end for their keying into a grout body filled into the drill hole to fix the tendon end in place .

OUTLINE OF THE INVENTION

The present invention aims to overcome or alleviate at least some of the above disadvantages by providing in a first aspect an improved method and means for providing ground anchors in a ground medium in a manner which results in a saving in time and labour both in manufacture of the anchor and securing the anchor in the medium. The present invention also aims to provide, in a further aspect, a reliable and efficient means for stabilising or consolidating in unstable medium such as sand and cohesionless soils. Other objects and advantages of the invention will become apparent hereunder. With the above and other objects in view, the present invention resides broadly in a method of forming a ground anchor in a ground medium, said method including the steps of providing at least one elongated member, advancing said member endwise into said ground medium and forming a hole in said medium about said member as said member is advanced thereinto, and anchoring said member in said hole.

The term "ground medium" used throughout the specification and claims includes soil, sands, muds, rock or any other earth medium including submerged mediums.

Suitably, the elongated member is a hollow elongated member and a plurality of elongated hollow members are arranged in end to end relationship and coupled together by coupling means preferably threaded coupling means- to enable the desired hole depth to be achieved. Preferably the member or members are secured in the hole by means of a settable substance such as cementitious or chemical grout or a resin such as an epoxy resin. Preferably also this substance is introduced into the hole through the interior of the hollow members. The hole in the ground medium may be formed by means of a drilling bit, suitably a consumable drilling bit on the leading end of the hollow member or members, the member or members being rotated as by a drilling rig as they are advanced into the medium. Alternatively the hole may be formed by means of a jetting fluid forced through the interior of the members. To further anchor the members and assist in hole formation, the coupling means may be provided with auger flights. If requried, the above method may be used with a casing which is disposed about the hollow members and is advanced into the ground medium as the hollow members are advanced.

The present invention also provides ground anchors formed in accordance with the above method.

In a further aspect, the present invention provides a method of stabilising a ground medium, said method including the steps of providing an alongated hollow member, advancing said member endwise into said medium, and passing through said member for passage into said ground medium, ground medium stablising matter. In accordance with a first embodiment of the present invention, a drilling rod is formed from a plurality of hollow members, preferably hollow steel bars which are joined in an end to end relationship by means of respective threaded couplings, the bars and couplings suitably carrying complementary threads.

In providing anchors in rock, the hollow steel drill rods can be used on standard or modified rotary percussion drill rigs and a consumable drilling bit screwed or fitted on the forward or leading end of the drill rods, or a cutting edge formed integrally thereat. The centre hole of the rod allows for flushing by either air or water or any other medium that may be required. Drilling in rock is commenced and in practice there are no limitations to the depth that can be achieved other than that set by the equipment in use as the respective drill rods may be joined together to form a multiple drill string by the use of respective threaded couplers between each rod section. When the required hole depth is achieved, the drilling rig is uncoupled from the hollow drill rods at the top of the hole so that the tendon which is defined by the uncoupled drill rods is now manufactured and homed in place. Typically the drill bit or cutting edge bores a hole larger in diameter than the drill string to enable downward movement as externally applied couplings to the hollow bars need be accommodated and the gap provides one means of applying grout to anchor the drill string or tendon in place.

The hole if required can be thoroughly washed and flushed with a suitable fluid through the hollow rods. After completion of this operation the anchorage is formed by grouting the rods in situ by using either cementitious or chemical type grouts or epoxy resins or other settable substances. This anchorage medium is preferably introduced by injection through the hollow rods and passage out of the leading end of the rods or apertures formed in the walls thereof or in the walls of the couplings.

After curing of this anchorage medium, the hollow rods are stressed by stressing equipment most commonly hydraulic as is employed in prior systems. The load is typically transferred to the ground from the stressed rods by a plate and nut system or a plate and wedge system. Alternatively self-centering washers may also be employed. In providing a ground anchor in soils, the same principles are applied. Additionally or alternatively however, the hollow steel rods may be jetted into the soil by a high pressure system. Rotation forces and feed pressure to home these rods are suitably provided by a rotation head on a drill mast assembly. The jetting medium is commonly air, water or grouts or any combination of these or other fluids. The jetting medium is typically pumped under pressures up to 15,000 PSI through the centre of the hollow rods to a special directional bit attached to the end of the hollow rods. The jets at this directional head are directed radially out into the soils while being rotated and fed into the ground simultaneously so as to form the hole to accept the rods.

A continuous or intermittent bulb arrangement of treated soil up to three (3) metres in diameter can also be formed around the rods. This in situ treated soil bonds to the rods and couplers and can provide the pull out resistance for the anchor system to function. Again in this system the rods are homed in one operation, the grouting medium is introduced through the hollow rods which as well as being the drill string for the drilling operation, subsequently become the stressed tendon. After curing the anchor or tendon is stressed by stressing equipment most commonly hydraulic with the load being typically transferred to the ground by a plate and nut system, a plate and wedge system or a self-centering washer may also be used.

.In the above system, non-slump mortars of Epoxies can be forced at high pressures through the hollow rods in cohesive or non-cohesive soils to form densely compacted bulbs to both stabilise and solidify the area of the anchorage zone. This procedure can occur as an "up stage" or "down stage" method with the rods being bonded into the soil as placement proceeds. The compacted bulbs give the anchor pullout resistance and may be used as an alternative or in conjunction with jetting. Alternatively, the mortar or epoxy may be placed and allowed to set and thereafter re-drilled to install a tendon in any suitable fashion.

An alternative to jetting is to place the couplers at closed centres of say one (1) metre. To each of these couplers is fitted a single flight auger suitably formed from a high tensile or other type of steel or alloy. The drill rods and couplers carrying the auger flights can then be assembled and fed into the ground with rotation being provided by a drilling head to provide a mechanical anchorage. Additional flights and lengths of drilling rod can be added readily to achieve the desired pull out resistance. The rotation head is detached after the required depth has been reached to permit stressing of the rods to take place. If required, bulbs of stabilising material may be formed at or adjacent the couplings by high pressure injection of grouts. The load from these anchors can also be transferred to the ground in the same manner as described above.

When soil overlays rock the hollow drilling rods can be used to drill through the soil down to the rock level by a rotary percussion drill rig with either water or air flushing through the centre of the hollow rods. No casing is required as the hollow rods will form the anchorage. The anchorage is formed by drilling a socket into the rock to the length required. The drill rig may then be detached and the hole flushed with high pressure water. Grout (cementitious or chemical) or epoxy resin can then be introduced through the centre of the hollow bars to push out the water leaving the drill rods and rock clean for grout bond. After curing of the grout medium the formed anchor can then be tensioned as previously described.

The ground anchors and their applications described above can be adapted for either temporary or permanent use and in many cases may be retrievable and re-useable. Free lengths can be formed by greasing, grease wrapping or sheathing the upper bar lengths as in normal anchor configurations. For permanent applications corrosion resistance is of prime concern. Multiple lengths of the hollow drill rods can be encapsulated and similarly installed as above but with provision for the desired degree of corrosion protection.

BRIEF DESCRIPTION OF DRAWING In order that the invention may be more readily understood and put into practical effect, reference will now be made to the accompanying drawing which illustrates the general features of a ground anchor formed in accordance with the present invention. METHOD OF PERFORMANCE

As shown, the ground anchor 10 inlcudes a tendon 11 formed of a plurality of hollow drill rods 12 arranged in an end to end relationship and coupled together at their opposite adjacent end portions by means of respective annular couplings 13 provided with internal threads for engagement with external threads on the opposite ends of the rods 12. The leading end of the tendon 11 is provided with a consumable drill bit 14 so that when the rods 12 are coupled to a drilling rig and advanced endwise, the drill bit 14 will form a bore or hole 15 at the position in the ground where the anchor is to be formed.

As shown the drill bit 14 is of such a size as to form a hole of greater diameter than the couplings 13 so as to accommodate the couplings 13 in use.

After the hole 15 is drilled to the required depth, the drilling rig is detached from the drill rods

12, and the lower end of the tendon 11 is grouted in place as at 16, the grout being introduced through the rods 12. A structure 17 to be held by the anchor 10 is attached to the upper end of the tendon 11 by suitable connection means 18 in the form of a plate and nut which transfer the load to the ground. To prevent corrosion, the area 19 of the hole 15 above the anchored portion of the tendon 11 may be sealed or filled to exclude water. This seal or fill may comprise grout. Grout for example may be introduced at a variety of points along the tendon 11 and this may be achieved by providing outlet means at specific points along the drill rods 12 or couplings 13. In an alternative drill rod configuration, the rods 12 may be provided with complementary male and female threaded portions at the opposite ends to enable their coupling and thereby eliminating the separate couplings 13. It will be seen that the above system allows the drilling of the hole, the manufacture of the tendon and the homing of the tendon to be carried out in the same time as drilling of the hole in the normal circumstances. Thus time reduction and saving in labour is clearly apparent.

Labour is significantly reduced as the total installation, manufacture, grouting, and drilling can be carried out by as few as two (2) people (or one (1) if required) at a rate not achievable by conventional methods. Drilling consumables are essentially removed from the operation to reduce the drilling costs giving another significant advantage. In most cases, casing materials can also be done away with.

More expensive materials such as chemicals grounts and epoxy resins can be employed as hole sizes can be reduced using hollow bar anchors as the size of the hole required for their accommodation is reduced by a lack of the grout tubes and diverging spacers that are required for conventional ground anchors. These materials can be used to achieve twenty-four (24) hour cycles for stressing operations thus providing an anchorage programme not realised before due to high costs. Conventional grouts require a period of days to cure as opposed to the faster times achieved with epoxies and other chemical grouts.

"Draw in" losses of prior strand anchorage system do not apply to the above system which allows a reduction in anchor length in most cases, another area of cost reduction. The hollow bar or rods to be used in the present system can be chosen to meet or exceed the specifications set out in any Standards which may be applied to ground anchors. The threading of the rods is preferably achieved by cold roll form threading. In some circumstances where the performance of the anchors are temporary and of less consequence, cut threads could be used.

Whilst the use of casings is not required as stated above, the method and means of the present invention may be applied to the use of casings in certain situations such as in soft, unstable mud, sand or cohesionless soil. In this application, the drill string formed from coupled drilling rods having a drilling bit at the leading end thereof is placed within a suitable casing preferably concentrically with the drill bit protuding from the end of the casing suitably a distance of approximately 50 mm. The drill string may then be ortated and moved into the soil and at the same time flushing liquids or air may be pumped through the rods so that the casing can be fed into the soil. The flushing fluid causes the mud soil or overburden to be reduced to a "soup" or finely dispensed particles which are flushed to the surface as the casing follows the drill string down into the soil. When the casing reaches rock so as to be unable to further advance, drilling is continued with percussion is necessary to a required depth in the rock. The casing is thus placed at the same time as the anchor .so as to result in considerable time saving. The casing may suitably be set up in joinable lengths substantially equal to the drill rod lengths so as to provide a continuous system.

In a modification, the flushing fluid may comprise a binding fluid so as to stabilise the soil or sand about the casing. Furthermore, if required, the anchor can be removed leaving a "clean" hole within the casing. In some cases, the casing can be installed with percussion, and by rotating the casing if desired to break up the soil cohesion from inside and outside. In some circumstances, the ground anchors may be required to be in the form of strand tendons rather than a hollow rod system. Placement of strand tendons may also be made using the method and means of the present invention. The soil into which the strand tendon is to be placed may be drilled using a drill string as formed above. During drilling and thereafter, chemical grouts may be pumped through the drilling rods to stabilise the soil in a local area so as to be self-supporting when set and after the drill string is removed. The hole can then be redrilled without the need for casings. This arrangement, which is suitable primarily on sand and cohesionless soils, consolidates a large mass to provide anchor pull out resistance so that a normal anchor system can be used. While the above has been given by way of illustrative example, many modifications and variations as would be apparent to persons skilled in the art may be made thereto without departing from the broad scope and ambit of the invention as herein defined in the appended claims.

Claims

1. A method of forming a ground anchor in a ground medium, said method including the steps of providing at least one elongated member, advancing said member endwise into said ground medium and forming a hole in said medium about said member as said member is advanced thereinto, and anchoring said member in said hole.

2. A method according to Claim 1, wherein said elongated member is a hollow elongated member and wherein a settable substance is applied about said member to anchor said member in said hole.

3. A method according to Claim 2, wherein said settable substance is passed through said hollow member into said hole.

4. A method according to Claim 2 and further including the steps of coupling a plurality of said members in end to end relationship through respective couplings means to permit the required depth of hole to be reached.

5. A method according to Claim 1, wherein the leading end of said elongated member is provided with cutting means and wherein said member is rotated whilst being advanced into said ground medium so that said cutting means drills said hole.

6. A method according to Claim 2 and including the step of applying a jetting fluid through said member as said member is advanced into said ground medium whereby to form said hole therein.

7. A method according to Claim 1 and including the step of locating a casing about said elongated member and advancing said casing into said medium as said hole is formed therein.

8. A method according to Claim 1 and including the step of tensioning said member subsequent to its anchoring in said hole and maintaining said tension therein.

9. A ground anchor formed in accordance with the method of any one of the preceding claims.

10. A method of stabilising a ground medium, said method including the steps of providing an elongated hollow member, advancing said member endwise into said medium, and passing ground medium stabilising matter through said member for passage into said ground medium.