2 Technology of closed-face tunnelling machines

Transcription

1 2 Technology of closed-face tunnelling machines 2.1 Machine types There are two basic types of pressurised closed-face tunnelling systems slurry tunnelling machines (STMs) and earth pressure balance machines (EPBMs). Slurry tunnelling machines were developed specifically for use in cohesionless soils that contain little or no silt or clay. These machines operate with piped hydraulic spoil removal and the slurry is regenerated in a separation plant that removes the excavated spoil from the slurry. Earth pressure balance machines were developed for use in weak cohesive soils that are capable of filling completely the excavation chamber ahead of the pressure containing bulkhead or plenum in order to provide positive pressure support to the tunnel face. The ground or soil is required to be plasticised during the excavation process so that it is capable of creating a plug or seal in the system s spoil extrusion screw conveyor in order to maintain pressure support of the face, resist ground water pressure, and discharge spoil at atmospheric pressure for loading into muck haulage skips or onto continuous conveyor haulage systems. Pure cohesionless soils and pure weak cohesive soils, however, are rare. Consequently it is necessary to extend the application of STMs into cohesive soils and of EPBMs into cohesionless soils. The result is the necessity to provide a more extensive separation plant for STMs in cohesive soils and for EPBMs the need to inject conditioning agents such as chemical foams and polymers into the plenum and the screw conveyor housing to produce a more fluid/plasticised spoil. Slurry tunnelling machines and EPBMs were developed initially in Japan and Europe. In Japan, STMs were developed in the 1960s with EPBMs introduced in the mid to late 1970s. In Europe, STMs were in use in the 1970s with a similar gap of about 10 years before the first EPBMs were employed. Slurry tunnelling machines and EPBMs have seen numerous developments and improvements since their inception and the modern machines, as typified by those used to excavate the of the Channel Tunnel Rail Link (CTRL) under east London, have continued the evolution of closed-face TBMs. This section describes recent developments of closed-face TBM technology designed to improve the ability of the systems to control ground stability and operate safely Slurry machines With an STM, excavation is carried out by a rotating cutterhead fitted with picks or disc cutters or a combination of both. The excavated material is mixed with the frictionless support fluid, usually a bentonite suspension that is maintained at a predetermined positive pressure in the plenum. This pressure reacts to any imbalances between the volume of suspension supplied to the plenum and the amount of suspension combined with excavated material removed from it. These volume variations are unavoidable and pressure is maintained either by controlling the volume differences with interlinked pumps and valves or by the provision of either a compressed air reservoir or air bubble. The bentonite tunnelling machine developed in the early 2 Closed-face tunnelling machines and ground stability. Thomas Telford, London, 2005

2 Figure 2.1 Illustration of an STM (slurry tunnelling machine) system 1970s in the UK (Bartlett et al., 1974) and the Hydroshield developed in Germany somewhat later (West, 1988) are examples of slurry machine types. There have been many developments and improvements to STMs from their beginnings and modern STMs, such as the system used on the CTRL to excavate the under the River Thames and the larger-diameter machines in use in Europe, can excavate a considerably greater range of soil types than before Earth pressure balance machines Excavation using an EPBM is also by a rotating cutterhead fitted with picks or disc cutters or a combination of both. The soil excavated from the face enters the plenum directly behind the cutterhead in a fluid/ plasticised state having been mixed with varying proportions of conditioning agents. The plasticised spoil is removed from the plenum via an Archimedean screw. While in the screw flights, the plasticised spoil forms an effective plug to ensure there is no loss of pressure in the plenum. Pressure is maintained on the face to balance the soil pressure by a combination of propulsion thrust and removal of material at the correct rate to match the rate of advance. At the end of the Archimedean screw there is a guillotine discharge gate that can be used in exceptionally fluid soils to assist in maintaining the correct earth pressure. In addition, some machines have been fitted with positive Figure 2.2 Illustration of an EPBM (earth pressure balance machine) system Closed-face tunnelling machines and ground stability. Thomas Telford, London,

3 displacement devices such as, for example, piston pumps, at the end of the screw to further control the discharge. 2.2 Cutterhead and cutters Through experience, manufacturers have evolved cutterheads that have the optimum open area in terms of the ability to give added mechanical support to the ground at the tunnel face and to allow excavated material to flow through the cutterhead. Modern cutterheads are highly abrasion-resistant leading to a reduced need for entry into the pressurised plenum for necessary maintenance. The ability of cutterheads to break up boulders and pass them through the head has been greatly improved. This largely avoids the need for entry to the pressurised plenum to break up or remove boulders. Cutter technology has also improved considerably. All cutters are now back-loading which removes the need for personnel to go ahead of the cutterhead to undertake cutter changes. At least one manufacturer is in the process of testing intelligent disc cutters that give an early indication of wear. Wear indicators on picks are now in common use. Copycutters, or cutters that can be moved in and out from the body of the cutterhead to provide an overcut, can be useful in assisting the steering of a TBM particularly where ground conditions are giving rise to problems. Modern copycutters are designed to overcut all or part of the tunnel face. Copycutters are normally used as a last resort but they can also act as gauge cutters in the event that the fitted gauge cutters become badly worn. A disadvantage of using a copycutter is the risk that the cutter becomes jammed in the extended position. This risk, however, is small as the copycutter is usually a very simple and robust item of equipment. Figures 2.3, 2.4, 2.5 and 2.6 show cutterheads of the four types of closed-face tunnelling machines used recently to excavate the Channel Tunnel Rail Link Section Spoil conditioning It is a fundamental requirement of EPBM operation to optimise the conditioning of the excavated material in the plenum. Modern practice is to use surfactant foam and/or polymers. This greatly enhances the EPBM s ability to maintain the correct pressure in the plenum and transmit this pressure to the tunnel face. Its further effect is to improve the ability of the spoil to flow from the plenum into and through the Figure 2.3 Kawasaki EPBM used on Contract 220 of the CTRL Section 2 4 Closed-face tunnelling machines and ground stability. Thomas Telford, London, 2005

4 Figure 2.4 Wirth EPBM used on Contract 240 of the CTRL Section 2 Figure 2.5 Lovat EPBM used on Contract 250 of the CTRL Section 2 Figure 2.6 Herrenknecht STM used under the River Thames on Contract 320 of the CTRL Section 2 Closed-face tunnelling machines and ground stability. Thomas Telford, London,

5 screw conveyor. Together, these lead to greatly improved face stability and settlement control. Spoil conditioning also reduces cutterhead torque, wear of machine components, and maintenance requirements. With STMs the traditional use of a bentonite slurry has been improved by the addition of long chain polymers for more efficient application in certain soil types. 2.4 Dealing with boulders 2.5 Screw conveyor design Boulders dispersed randomly in the ground are difficult to identify with traditional site investigation methods. Meeting boulders, of varying sizes, in the path of a closed-face TBM tunnel heading is therefore often a surprise. As a result, it is important that TBMs are equipped with the means to deal with unexpected as well as expected boulders. Earth pressure balance machines include the ability of the cutterhead to break down boulders to sizes suitable for passing through the screw conveyor. On STMs the practice is to install powerful crushers at the intake of the out-bound slurry circulation pipe to reduce boulders to fragments that can be pumped. In addition, the cutterhead should be capable of having disc cutters fitted without the need for structural modification. There should be a facility to grade boulder fragments and screen their passage through the cutterhead openings. This can be achieved by practical design of the cutterhead openings and the addition of grizzly bars across the openings. In the case of an EPBM this presents the screw conveyor with small enough pieces for transportation and on STM systems with pieces that will fit into the crusher. Nowadays screw conveyors in EPBMs are usually of the Archimedean type. Conditioning agents can be injected to reduce screw torque, however care has to be taken that, in conditioning the spoil in the screw, the vital pressure-maintaining plug is not destroyed. Screw conveyors normally enter the plenum at the invert of the pressure bulkhead. This aids pressure control and helps to clear the plenum of excavated material for required maintenance and repairs. Protection against screw wear has been improved but a screw used without a good spoil conditioning agent will wear rapidly in abrasive soils. 2.6 TBM articulation Various methods of articulation have evolved to improve alignment control. These include articulated cutterheads and active or passive articulation of the shield body. 2.7 Seal systems The design, technology and manufacture of seals that protect the main bearings on which the cutterheads of TBMs are assembled and driven has improved significantly in recent years and particularly to keep pace with increasing machine and cutterhead diameters and higher operating pressures. 2.8 Tail seals The introduction of grease-fed wire brush tail seals, in sets of either three or four, has reduced the risk of an in-rush of water and material at the trailing end of STM and EPB TBMs. This is one of the more significant improvements in terms of ground control. For TBMs subjected to high hydrostatic heads an emergency inflatable seal can be fitted if considered necessary. 2.9 Control systems Computers are at the heart of the control system on modern closed-face TBMs. They provide the logic that is used to manage the different functions of the machines and the interlocking facilities that prevent 6 Closed-face tunnelling machines and ground stability. Thomas Telford, London, 2005

6 TBM functions activating if the parameters are not correct. Through an interface they also receive many of the control instructions and display the information used to operate the machines. They are likely to be linked to computers on the surface that duplicate the display in the TBM operator s cabin. They can also be linked to remote locations, such as the offices of the TBM manufacturer or the head offices of the contractor or the client. The underlying data streams can be analysed in a variety of locations to monitor the performance of the machine, predict maintenance requirements, provide data for early warnings of impending problems, and allow later back-analysis of any problems. A very large number of critical machine functions are likely to be monitored in real time and recorded in a tabular format against time to give the raw information that can be examined in detail or presented graphically as an aid to management or investigation. All critical data should be automatically backed up at the TBM and elsewhere. The list of functions that can and should be recorded may run into hundreds. Every hydraulic pressure, every electrical circuit and every moving part can generate data if fitted with a sensor. Often the raw data is processed to provide management information. As such it is important that all data is produced and stored in its most basic form, as well as in processed forms to provide useful management reports. Machine functions that can be reported graphically include cutterhead thrust, torque and rotation, total power demand, annular grout volumes and pressures, rates of advance etc. For an STM, monitored data will also include flows and pressures in the slurry circulation system and the operation of the separation plant. On EPBMs additional monitored data would include spoil pressure, screw conveyor pressure, discharge gate opening, spoil quantity removal, operation of the belt conveyors that transfer spoil to the haulage loading station, injection of water and conditioning agent etc Improved general reliability 2.11 Future development It is estimated that more than 3000 closed-face TBMs have been manufactured and used in the world up to 2005 and that growth over the years up to 2005 has been exponential as the applicability of the systems has expanded and urban soft-ground tunnelling has increased. This has resulted in considerable improvement in the quality and reliability of each new machine and an increase in the ability of each new system to maintain ground stability and improve operational safety. It has also expanded the pool of experienced personnel although this must be maintained to continue to service this particular sector of civil engineering as the market grows. When excavating in unstable, water-bearing, granular and soft cohesive materials, the best method for controlling ground stability is to use a closed-face TBM. When excavating in fissured rocks closedface TBMs can be used as an effective method of controlling groundwater inflows. There has been considerable development and improvement of both STMs and EPBMs since their inception in the 1960s and 1970s. Most recent developments include a closed-face tunnelling machine that uses compressed air pressure as well as earth pressure to support a tunnel face. Such a machine system was used for construction of the 1.3 km-long twin-bore 8.1 m-internal diameter tunnel through London Clay at Heathrow Airport in the UK (Sam et al., 2003). At the same time in Paris, France a dual-mode machine that can operate Closed-face tunnelling machines and ground stability. Thomas Telford, London,

7 either as an EPBM or as an STM was used on the 10 km-long, 10.4 m- internal diameter double-decked A86 highway tunnel through the chalk, clay and sands of the Seine River Basin (British Tunnelling Society, 2004). Development of closed-face systems to further refine the advantages of these techniques can be expected with the ordering, manufacture and application of each new machine. As a result of this continuing technical development, it is important to provide suitable training of both staff and operatives. This training and ongoing development is necessary to ensure that there is a pool of experienced staff and labour available who understand the complexity of modern tunnelling machines and understand the requirements for operating them to reduce the risks of instability and collapse. As will be seen throughout this document the management and operation of a closed-face tunnelling machine is a task that requires a highly skilled and educated staff and workforce one that is motivated and understands the requirements of the work. 8 Closed-face tunnelling machines and ground stability. Thomas Telford, London, 2005