GB2177741A

GB2177741A - Excavation shield

Info

Publication number: GB2177741A
Application number: GB08617489A
Authority: GB
Inventors: Pierre Blanc
Original assignee: GTM ETS SA
Current assignee: GTM ETS SA
Priority date: 1985-07-19
Filing date: 1986-07-17
Publication date: 1987-01-28
Also published as: FR2585067B1; GB8617489D0; FR2585067A1; GB2177741B

Abstract

An excavation shield for a cylindrical subterranean gallery comprises a protection component (2) in the form of a metal cylinder, a device (4) for removing material and a device (5) for positioning prefabricated supports inside the protection component. Watertightness between the rotating extraction head and the fixed protection component is ensured by a rotating joint (6). Behind the joint (6), a fixed watertight bulkhead having no rotating joint, forms the boundary of a watertight caisson (70) in which a counterpressure is established such that the differential pressure exerted on the rotating joint is maintained at a value which the joint can stand. <IMAGE>

Description

SPECIFICATION Excavation shield The invention relates to an excavation shield suitable for use in subterranean galleries at great depth, and in particular, at depths of the order of 200 m.

In order to construct cylindrical galleries in ground containing water, that is to say, in general, ground situated beneath subterranean water, use is made of devices known as shields. Such devices generally include a protection component or casing in the form of a metal cylinder, of a diameter corresponding to that of the desired excavation, provided for the removal of the excavated earth, in the form of a pump or a screw, and then a device for placing prefabricated voussoirs or supports to form a lining inside the protection component. A seal between the protection component and the lining prevents the ingress of water into the tunnel thus constructed. The shield assembly is moved with the aid of thrust jacks which bear against the support lining positioned inside the protection component.

In order to maintain pressure against the material which is being excavated, compressed air has been used and now for some time mud, such as bentonite, in order to stabilise the surface of the excavated earth (bentonite shield). Instead, the quantities of material which have been excavated and the quantities of material which have been removed are carefully balanced (confined earth shield). The excavation component or extraction head which is situated in front of the protection component includes cutting tools having various dimensions and shapes and is rotated by one or more motors, by means of a toothed wheel and a bearing wheel. In order to prevent mud or earth from entering the interior of the protection component, it is necessary to provide a rotating joint or seal between the fixed and rotating parts.The hydrostatic pressure under which this type of apparatus can operate is limited by the rotat ing joint which consists of a delicate compo nent, especially for large diameter shields. At the present time rotating joints or seals can withstand, in use, pressures of the order of 5 kg/cm2 (500 kPa). It is not possible then to excavate submarine galleries at depths greater than 50 m by using such shields. In point of fact, the presence of faults in the ground would give rise to pressure greater than those which the rotating joint could withstand. For example, at a depth of 100 m, there is a pressure greater than 10 kg/cm2 (1000 kPa).

The development of large diameter rotating joints, or seals which can withstand pressures greater than those indicated above, has proved to be an extremely delicate and complex task and has not been completed at the present time.

In order to mitigate this inconvenience and to use a shield of this type to excavate submarine galleries at great depths, the invention provides a shield which comprises, behind the rotating joint, a fixed bulkhead, having no rotating joint and a forming of a watertight compartment or caisson in which a counter-pressure is established such that the differential pressure exerted on the rotating joint is kept at a predetermined value within the limits which the joint can withstand.

The bulkhead may be situated at the rear of the motor or motors used for rotating the extraction head, and may comprise a watertight door and/or one or more manholes permitting inspection of the watertight zone thus created.

The component used for the removal of material, which must pass through the bulkhead, but which is fixed, is connected to the latter by means of a fixed joint or seal which can be designed to withstand pressures much greater than those which will occur.

The watertight compartment or chamber is put under pressure by means of water or bentonite mud or similar products.

Pressure sensing heads at the front of the extraction apparatus determine the pressure to which the rotating joint will be subjected and an automatic regulation system for pumps which put the watertight chamber under pressure, enables the necessary counter-pressure to be established.

The invention will now be described in more detail, and by way of example, with reference to the accompanying drawing in which the single Figure is a schematic representation of an excavation shield embodying the present invention.

The excavation shields of subterranean galleries are well known and their structure and operation will be described only in the general way in order that the significant features of the present invention may be appreciated.

The illustrated shield 1 consists of a protection component 2 consisting of a metal cylinder, the diameter of which corresponds to that of the excavation to be dug. An extraction head 3 for the subterranean material is situated at the front of the protection component and is rotated by one or more motors 31 (only one motor being shown). The head 3 is provided with cutting tools 30 and is connected by a drive shaft 36 to a fixed compo nent 37. The motor 31 rotates a toothed wheel 32 which engages the rotating part 33 of the extraction head as the assembly is ro tated. Ball races 34 are provided between the rotating part 33 and the fixed mounting inside the protection component 2. The exterior con tour of the rotating part 33 and the corre sponding contour of a mounting 14 inside the protection component 2 may vary and need not have the shape represented in the draw ing.In order to prevent the extracted ma terials, or the mud used to stabilise the ground, passing between the fixed and rotating components, one or two rotating seals or joints 6 are provided. During extraction of materials at the front, a device 5 positions supports 11 over the whole periphery of the excavated tunnel, inside the protection component 2. The watertight joints 12 between the protection component 2 and the last support placed in position, prevent the ingress of water inside the tunnel. Advance heading jacks 13 bear on the positioned supports, thus enabling the assembly to be pushed forward.

A device 4 for removing the extracted materials extends through a seal in the fixed component 37 of the extraction head 3 and operates to take away the materials, by means of a screw 40, for example.

If the shield 1 is a bentonite shield, betonite is introduced into the space 35 within the head 3 in order to maintain the pressure of the earth constant. If a confined earth shield is involved, the equilibrium between the volume of earth extracted and the volume of earth taken away is maintained, thanks to an appropriate installation. The presence of the rotating joint 6 limits the present machines to use at a pressure of the order of 500 kPa, which corresponds to about 50 m water gauge. If a submarine gallery is excavated to a depth greater than 50 m, penetration of the extraction head into a fault in the ground brings about a sharp increase in the pressure to which the joint is subjected.The present invention keeps the effective pressure experienced by the joint substantially constant by creating such a pressure within the entire protection components, this counter-pressure is created within a watertight caisson 70 situated behind the joint.

The watertight caisson 70 is bounded at the front by a fixed part 37 of the extraction head and by the rotating part 33 of this head, by a mounting 14 of the component 2 which houses the rotating part 33, by a mounting 15 of component 2 which is situated behind the mounting 14, and by a bulkhead 7 which rests in a watertight manner against a step 16 in the mounting 15. The extractor device 4 for removing the earth, extends through the bulkhead 7, watertightness being ensured by means of a fixed seal 41.

The watertight caisson 70 thus formed, may be put under pressure by the introduction of pressurises water or mud. Pressure detectors determine the pressure existing in the cavity 35 and relay this value to a calculating unit (for example a computer controlling the operation of the shield 1) which then reguiates the operation of the pumps feeding the caisson 70 in order to maintain the rotating joint 6 at an appropriate differential pressure. Thus, if the pressure in the cavity 35 is 1200kPa (12 kg/cm2), corresponding to a water gauge pressure of about 120 m, it will be sufficient to create in the watertight caisson a pressure of 800 kPa (8 kg/cm2) for the joint to work at an effective differential pressure of 400 kPa (4 kg/cm2).

The bulkhead 7 includes, if necessary, a watertight door and/or watertight manholes allowing access to the interior of the caisson 70 when the device is not working. This allows for maintenance, inspection or repairs to the motors 31. For practical reasons, it is simpler to place the motors 31 totally inside the watertight caisson 70 in order to avoid transmission problems, since it is known how to construct motors which can function in water or mud.

It is evident that the invention is not limited to the sole example illustrated and described and that many variations are possible. For example, the extraction head may be oblique in relation to the protection component. Several material extraction devices can be used and the location of the rotating joint or seal may be modified, as well as the location of the rotating motor or motors.

Claims

1. An excavation shield for a cylindrical subterranean gallery, the shield comprising a protection component in the form of a metal cylinder, an extraction head for materials rotatably mounted in front of the metal cylinder, a first device for removing material, a second device for positioning prefabricated supports inside the protection component, a watertight seal between the rotating extraction head and the protection component a fixed watertight bulkhead behind the seal and means for reating a counter-pressure in the watertight compartment between the bulkhead and the extraction head, such that the differential pressure exerted on the seal is maintained at a value which the seal can withstand.

2. An excavation shield as claimed in claim 1 wherein the device for removal of material extends through the watertight bulkhead by way of a fixed seal.

3. An excavation shield as claimed in claim 1 or 2 having at least one watertight door and/or at least one watertight manhole in the watertight bulkhead.

4. An excavation shield as claimed in 1, 2 or 3 having pressure of materials in front of the extraction head and transmitting the value to a calculating unit which thereby regulates the operation of the means which pressurises the watertight compartment.

5. An excavation shield as claimed in claim 1, 2, 3 or 4 wherein the waterproof chamber is pressurised with water.

6. An excavation shield as claimed in claim 1, 2, 3, 4 or 5 wherein the waterproof chamber is pressurised with bentonite or a mud.

7. An excavation shield comprising a casing rotatably mounting a cutting head, a seal between the casing and the cutting head, and means pressurising the seal from within the casing.

8. An excavation shield substantially as herein described with reference to the accompanying drawing.