GB2373268A

GB2373268A - Drilling and installing a ground anchor below groundwater level

Info

Publication number: GB2373268A
Application number: GB0106616A
Authority: GB
Inventors: Charles J Mcanally
Original assignee: Keller Ltd
Current assignee: Keller Ltd
Priority date: 2001-03-16
Filing date: 2001-03-16
Publication date: 2002-09-18
Anticipated expiration: 2021-03-16
Also published as: GB0106616D0; GB2373268B

Abstract

A geotechnical installation such as a ground anchor, tube or a manchette is installed in a soil retained by a wall (3) by advancing into the soil, through a sealing assembly (16) and a hole in the wall (3), a drill string having a series of drilling elements, each comprising a rod (24) and a casing (26). The drill rods (24) are then withdrawn, leaving the casings (26) in place. The installation is then installed through the casings (26). The casings (26) are withdrawn and the borehole is filled with grout while a seal (19) is provided around the installation. The procedure is said to allow the installation of a ground anchor below ground water level whilst preventing excessive water ingress.

Description

Drilling below ground water level This invention relates to the provision of geotechnical installations in possibly waterbearing soils, particularly below ground water level.

It is frequently necessary to carry out geotechnical processes below ground water level, for example the installation of ground anchors or soil nails, jet grouting or using a tube à manchette, soil mixing, or soil washing. It would be desirable to provide a method which enables such processes to be carried out without undue loss of water or soil.

The present invention provides, in particular, a method of providing a geotechnical installation in soil retained by a wall, the method preferably comprising: mounting a sealing assembly on the wall; advancing into the soil, through the sealing assembly and a hole in the wall, a drill string having a series of drilling elements, each comprising a drill rod and a drill casing, while providing a seal around the drill string within the sealing assembly ; withdrawing the drill rods from the drill string while leaving the series of drill casings in place; installing the geotechnical installation through the series of drill casings ; withdrawing all but one of the drill casings through the sealing assembly while leaving the geotechnical installation in place ; performing at least one operation on the soil by way of the geotechnical installation while providing a seal around part of the geotechnical installation within the sealing assembly; and removing the sealing assembly and closing the hole in the wall.

The preferred technique is based on duplex drill casings (typically 75-300 mm diameter) and may drill to depths of up to 100 m (typically 20-50 m). The drill casings can be drilled horizontally or at any angle to suit, but typically 2 : 300 from horizontal can be used. The drill casings can be drilled into water-bearing soils using water, bentonite, or cement-grout flushing medium. Groundwater and soil is prevented from discharging from the borehole by the use of seals around the casing and between the casing and inner drill rod.

On reaching the required installation drill depth, the inner drill rod is withdrawn from the borehole using a double sealing arrangement in a'stuffing-box'which is bolted, welded, or otherwise fixed to the face of the wall. The geotechnical installation (e. g. anchor or tube à manchette) is then positioned to the required depth whilst using the sealing arrangement. Once the geotechnical installation has been positioned to depth, the outer drill casing is withdrawn from the borehole (apart from the last 1 m of casing) and the borehole is grouted or the installation can be used for jet grouting, soil mixing or soil washing applications. The remaining 1 m length of drill casing is removed after the geotechnical process has been completed and the borehole sealed at the wall face.

The invention will be described further, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a section through a shaft extending below ground water level and containing a drilling rig; and Figures 2 to 5 are schematic view of the bottom of the shaft, showing successive stages in a geotechnical process.

As shown in Figure 1, a shaft 1 (for example 4 to 6 m in diameter) in granular soil 2 has a concrete soil-retaining wall 3 (for example 15 m deep) extending below the ground water level 4. A pedestal 6 fixed to a concrete base 7 mounts a drilling rig 8 which includes a boom 9 pivotable through + 30'from the horizontal about an axis 11 by means of an hydraulic ram 12. The boom 9 mounts a drive assembly 13, which traverses along the boom, and a movable guide member 14.

As shown in Figure 2, an assembly 16 is mounted on the wall 3 and comprises a wall plate 17, a shut-off valve 18, a pinch valve 19 (constricting seal), and a casing seal 21, all arranged on an axis 22 coincident with that of the drive assembly 13. Subsequently, a core drill (not shown) is advanced through the assembly 16 by means of the drilling rig 8, cuts a hole through the reinforced concrete of the wall 3, and removes the resulting wall plug. The shut-off valve 18 is then closed.

Referring to Figure 3, next, an initial section of a drill string is mounted on the drive assembly 13 and supported by the guide member 14. This initial section comprises a bit 23, a starter rod 24, a cutting shoe 25, and an initial casing 26 with a length of 1 m. The initial section is advanced into the casing seal 21 before the shut-off valve 18 is opened.

Drilling through the wall into the water-bearing soil then commences, and it continues with the addition to the drill string of a series of drilling elements, each comprising a rod 24 and a casing 26, with seals (not shown) between the rods on the one hand and the casings on the other hand.

Referring to Figure 4, after the required depth (typically 20 to 50 m but possibly up to 100 m) has been reached, the inner drill rods 24 are pulled out. Then a geotechnical installation 27, such as a ground anchor or a tube à manchette, is installed through the casing 26.

Referring to Figure 5, the casings 26 are then withdrawn from the borehole, apart from the initial short casing 26. This short casing 26 is pushed back until it is clear of the shut-off valve 18. The pinch valve 19 is inflated so as to seal around the installation 27 projecting through it, before grout is introduced into the borehole through the installation 27.

The assembly 16 is finally removed from the wall 3, the projecting casing 26 is also removed, and the wall 3 is made good. Alternatively, the short casing with the cutting shoe may be removed before final grouting at the wall.

The skilled reader will appreciate that the geotechnical installations which can be used are not limited to anchors and tubes à manchettes and that the equipment and techniques described above are applicable to many geotechnical processes, particularly when working below ground water level or in water-bearing soils.

Claims

Claims: 1. A method of providing a geotechnical installation in soil retained by a wall, the method comprising: (a) mounting a sealing assembly on the wall; (b) advancing into the soil, through the sealing assembly and a hole in the wall, a drill string having a series of drilling elements, each comprising a drill rod and a drill casing, while providing a seal around the drill string within the sealing assembly; (c) withdrawing the drill rods from the drill string while leaving the series of drill casings in place; and (d) installing the geotechnical installation through the series of drill casings.
2. A method as claimed in claim 1, further comprising: performing at least one operation on the soil by way of the geotechnical installation while providing a seal around part of the geotechnical installation within the sealing assembly.
3. A method as claimed in claim 1, further comprising : withdrawing all but one of the drill casings through the sealing assembly while leaving the geotechnical installation in place; and performing at least one operation on the soil by way of the geotechnical installation while providing a seal around part of the geotechnical installation within the sealing assembly.
4. A method as claimed in claim 2 or 3, further comprising: removing the sealing assembly and closing the hole in the wall.
5. A method as claimed in any preceding claim, in which the hole is cut in the wall after the sealing assembly is mounted on the wall.
6. A method as claimed in claim 5, in which the hole is cut by a core drill.
7. A method as claimed in any preceding claim, in which the sealing assembly comprises a constricting seal.
8. A method as claimed in any preceding claim, in which the sealing assembly includes a casing seal which seals around the drill casings that pass through it.
9. A method as claimed in any preceding claim, in which the sealing assembly includes a shut-off valve.
10. A method as claimed in any preceding claim, in which the drill string is advanced into the soil to a depth of at least 20 m.
11. A method as claimed in any preceding claim, in which the geotechnical installation is a ground anchor.
12. A method as claimed in any of claims 1 to 10, in which the geotechnical installation is a tube à manchette.
13. A method as claimed in any preceding claim, in which the drill string is advanced by means of a drilling rig comprising a boom, a drive assembly which traverses along the boom, and a guide member mounted on the boom, the guide member supporting the drill string between the drive assembly and the sealing assembly.
14. A method as claimed in claim 13, in which the boom is pivotable about a horizontal axis.
15. A method of providing a geotechnical installation in soil retained by a wall, substantially as described with reference to the accompanying drawings.

16, A geotechnical installation provided by a method according to any preceding claim.