Since its initial introduction, in 1985, the Mixshield TBM has taken on an essential role within the world of mechanical tunnel excavation. With its ability to operate as a classic slurry-shield, or change to Open or Earth Pressure Balance (EPB) mode mid-drive, the Mixshield offers a unique concept for dealing with mixed ground excavation conditions.
In order to appreciate how these machines have evolved, this article looks at the history and development of Mixshield technology over recent decades and the impact this has had on the underground construction industry.
Development of technology
The first European attempts to develop a bentonite shield began in England in the early 1960s, leading to the excavation of a short experimental tunnel in London, in 1971. German contractor Wayss & Freytag took these initial ideas and built upon them, incorporating an air bubble principle to control and regulate face support pressure.
These early machines were mainly used in gravel and sand under lower water pressure, such as the Wilhelmsburg CSO tunnel, in Germany, which was completed in 1974. The shields were equipped with a centre shaft drive and light spoke-type cutterheads with square drag tools, reflecting their limited range of application at that stage.
With a view to finding an alternative to conventional compressed air excavations, which were common at the time, Wayss & Freytag concentrated on the design of face support systems and the development of a tail shield seal that would stand up to the rough conditions of mechanical excavation.
In parallel with these developments, the evolution of water-resistant single-shell concrete segmental linings occurred.
Early Mixshield concepts focused both on the development of a shield that would provide a slurry supported tunnel face and also an open shield that could enable a change of operational mode[1]. That is why, even today, the term “Mixshield” is still used for “slurry only” machines as well as for convertible shields. In all cases, however, the use of a submerged wall/pressure bulkhead combination, to create an air bubble for face pressure control, has remained an essential design feature for this type of machine.
The Mixshield is ultimately distinguished by the following features:
• A modular design that allows adaption to different modes of operation (slurry, open, and EPB) for different projects, or a fully-equipped machine for changes mid-drive
• Centre-free cutterhead drive with three-axis roller bearing for a centrally arranged muck transportation system (screw or belt)
• Centre-free cutterhead drive to accommodate various media (water, bentonite, hydraulics, electrics, etc) into the cutterhead and excavation chamber via a rotary joint
• Integration of a cutterhead support system with axial and radial articulation by means of a spherical bearing
The first application of a true Mixshield machine was at the HERA tunnel, in Hamburg, in 1985. This 6.2km-long project accomplished advance rates of up to 20m/day. The developments introduced proved successful, following a few adaptations during tunnelling, and confirmed the suitability of the new design.
Like other early Mixshields, the HERA machine had a cutterhead that tilted forward at 3°. The idea was that a slightly inclined face would help with face stability. However, with growing experience and confidence in the principle of a bentonite-supported face, this mechanically cumbersome configuration eventually disappeared in favour of vertically mounted cutterheads.
Another obvious design feature on early machines was the wide open, light cutting wheel design and a large submerged wall opening. It was thought that these were essential requirements to ensure the best possible bentonite circulation and therefore stability of the face. As with the inclined face, however, it became obvious these were not mandatory requirements and that cutterhead designs more appropriate to mixed face conditions could be employed without negative effects on face stability or settlement. The difference in layout between slurry shields and EPB cutterheads therefore started to disappear[2]. Today’s cutterhead designs are driven by a much wider range of factors – including wear protection, muck flow, tool arrangement and tool access.
In the late 1980s, the potential of Mixshield machines became obvious to the tunnelling community. For the next 15 years, projects and orders were heavily influenced by the market enthusiastically embracing the concept. The development of other soft ground technology, such as compressed air shields and the membrane shield, were all but abandoned in favour of the Mixshield; which had already enabled the completion of projects previously considered impossible.
From a technical point of view, the step-by-step development of the Mixshield (Table 1) reads like a technical requirement catalogue for today’s slurry shields. The only difference being that the initial introduction of any new feature on a given project required a huge amount of dedication and commitment from all parties throughout the learning curve.
Early milestones included the application of a Mixshield, used as a slurry machine, in Duisburg, which adopted a jaw crusher for boulders up to 500mm; the use of a hard rock cutterhead in Mülheim; convertible operation modes (closed mode with slurry circuit – open mode with centre belt conveyor discharge system) for the Grauholz tunnel; and Mixshield operation in Strasbourg with slurry support and closed cutterhead in coarse sand and gravel.
Additional progress was made in the mid-1990s with projects such as the fourth Elbe tunnel and the Westerschelde crossing. Since then, advances to larger diameters, higher water pressures and shallower cover have presented a whole new dimension of challenges. Many engineering questions, such as lowering the bentonite level for face access, had to be revisited and new innovative solutions such as accessible cutterheads for cutter tool changes under atmospheric conditions were created.
As well as direct improvements to Mixshield technology, several more general TBM developments have also contributed to the advancement of Mixshields, including:
• Articulated cutterheads to allow overcutting and full control of cutterhead and main bearing loads
• Floating thrust cylinder systems to address increased segment length and difficult tunnel alignments
• Mixed and variable face cutterhead and backloading tool designs
• High-pressure mainbearing, articulation and tailseal concepts
• Vacuum systems for segment erection
• Advanced systems for data recording and processing or process automation
Many of these more general developments have to be viewed in combination with related advancements, e.g. segment design and manufacturing, soil conditioning, sealant materials, alignment control and survey systems, IT systems and data transfer.
In addition to the coventional use of Mixshields for face support in soft ground, the growing use of closed mode operation in rock tunnels has also been seen in recent years. In water bearing rock with the potential for high water inflows and/or pressure, the control of water inflow through the cutterhead is essential for operational reasons (segment backfill grouting, muck discharge) and, in many cases, even more so for environmental reasons.
The traditional approach to such ground conditions is pre-excavation grouting from within the shield ahead of the tunnel face. Depending on rock conditions, pre-grouting activities can be time-consuming and do not always guarantee success, especially against flowing water. To overcome these problems, a closeable single shield rock machine was developed that would allow pre-excavation grouting using preventer systems against static water pressure. The next logical step was to install an additional slurry circuit muck transport system to deal with worst-case scenarios in closed mode.
Operational modes for such hard rock Mixshields include: a) Open mode with dry primary muck discharge system (e.g. conveyor); b) Open mode with (cyclic) pre-excavation grouting; c) Open mode with (cyclic) pre-excavation grouting in closed static conditions; d) Closed mode with hydraulic muck discharge system under reduced face pressure; e) Closed mode under full-face pressure with potential for positive face support.
Closed mode, high-pressure operation in hard rock provides the most adverse conditions of operation for all components of the machine, but having options d) and e) available is a significant advantage in terms of mitigating potential risk.
The machines for the SMART project in Kuala Lumpur, in 2004, produced the second series of large Mixshield machines, this time partially in medium soft rock conditions.
The spotlight regarding large Mixshield machines is now currently focused on the world’s largest Mixshield machine, for the Chongming project, in Shanghai.
Face support
As a classic Hydro Shield, the Mixshield applies the necessary face support pressure via a pressure-controlled air bubble in the pressure chamber. A characteristic design feature of the Mixshield is the submerged wall, separating the pressurised front section of the shield into two areas. The area between the submerged wall and the pressure wall is called the “pressure chamber” (figure 2). The area in front of the submerged wall is defined as the “excavation chamber”. The required pressure exchange between excavation chamber and pressure chamber occurs via an opening in the bottom of the submerged wall.
Additional compensating pipes that connect the invert area of the pressure chamber with the excavation chamber ensure pressure exchange, even if there is a blockage of the submerged wall opening.
The air bubble is maintained by a self-regulating air pressure control system (with a back-up available if necessary). When under water, where groundwater pressure is influenced by tidal activity, water levels and pore water pressure readings are taken into consideration as additional parameters in the pressure control. For large diameters, the slurry density and level in the pressure chamber may also be used as input parameters. The required face support pressure settings must be determined by calculations for each individual section of the tunnel alignment before the excavation.
Cutterhead access & tool changes
Integral to the general Mixshield concept is the ability to lower the slurry level in the excavation chamber for cutterhead or face access, either remotely from the atmospheric area behind the pressure bulkhead or from inside the pressure chamber (figure 4). In locations with shallow cover and high water pressure, the level of slurry can only be lowered by a third of the diameter for access to the excavation chamber, in order to maintain sufficient safety against the risk of a blow out. In such cases, the required balance between excavation chamber and pressure chamber has to be controlled from the atmospheric area behind the pressure wall.
The use of a submerged wall gate enables the isolation of the pressure chamber from the excavation chamber. By closing the gate, maintenance work in the pressure chamber can be carried out under reduced pressure or even free air conditions. In this scenario, pressure regulation of the excavation chamber is carried out via a remote pressurised bentonite tank air bubble, usually mounted on the back-up gantries.
A new development for areas of sticky soil with a high clogging risk is the separation of the suction area from the rest of the pressure chamber. This was successfully implemented in the Mixshield used for the Weser tunnel, in northern Germany. Here, the invert area was isolated from the rest of the pressure chamber. Separate connection lines (or “compensation pipes”) provided the necessary pressure exchange to the excavation chamber for face support.
With this system, a large percentage of the total flow volume can be circulated through the excavation chamber reducing the slurry density in specific areas and making it more constant. This results in less muck accumulation/clogging, as well as less secondary wear and more even operational conditions for the slurry circuit and treatment plant. The isolated suction area created still accommodates the rock crusher and submerged wall gate.
Maintenance and service operations within working pressures of over 3.6 bar – and therefore outside of the normal framework of compressed air regulations – were successfully performed under special permits on the fourth Elbe tunnel and Weser tunnel at air pressures of up to 4.5 bar.
Professional divers can be employed for underwater operations or pressure levels beyond 3.6 bar for access into the suspension filled excavation chamber and/or for work to be performed in the invert area of the pressure chamber. Requirements for diving operations need to be established at an early stage of the project and have to be addressed in the design and installation of the TBM. Different applications also have to be defined. For example:
Short-term, submerged dives to explore the tunnel face or to inspect cutter tools: Divers enter the bentonite suspension from the air bubble in the pressure chamber. Submerged wall doors are used to access the excavation chamber. These are located below the suspension level in the pressure chamber. This procedure was successfully implemented at the fourth Elbe River tunnel.
Long-term, for major maintenance and repair work and/or pressures beyond 4.2 bar: Special dive techniques, such as those used in the offshore industry are used (involving mixed gas or saturation diving). In the case of saturation diving, dive crews remain under pressurised conditions for extended periods. The divers are transferred from an above ground “living chamber” to the TBM via a mobile shuttle lock. Depending on their tasks and the pressure levels at which they are operating, the divers can work for several hours before returning to their “living chamber”.
Whenever possible, maintenance work in the pressurised area is done in a dry compressed air environment using masks for the breathing gas mixture. The entire process, including breathing gas mixtures, atmosphere, durations, or individual pressure levels, has to be precisely planned and supervised by experienced specialists. Saturation diving for tunnelling operations was used for the first time on a large scale at the Westershelde project, in the Netherlands.
Crushing rocks and boulders
In principle, there are two ways to handle rocks and boulders. Firstly, if the matrix of the tunnel face is strong enough, disc cutters excavate rocks and boulders. In most cases, this excavation mechanism can be used successfully down to boulder sizes of 400mm to 600mm. The remaining rock entering the excavation chamber is then crushed into smaller particles by a rock crusher located in the invert of the machine. The maximum allowable grain size after crushing is dictated by the design of the slurry circuit, especially the size of discharge pipe, pump type and slurry flow speed.
As a rule of thumb the practical maximum grain size can be considered to be about 30-40% of the discharge pipe diameter. The typical arrangement in the suction area for conditions with boulders and cobbles is the installation of a grill for grain size limitation in front of the suction pipe and a hydraulic jaw crusher in front of the grill. Different size jaw crusher capacities are used in different machine diameters:
• 4m-6.5m: max boulder size 500mm
• 6m-10m: max boulder size 800mm
• 9m: max boulder size 1200mm
Early attempts to use “in-line” crushers or boulder traps in the discharge pipe were unsuccessful and have therefore nearly disappeared from modern designs.
The amount, size and consistency of the anticipated rock influences the choice of cutterhead configuration and cutter tools. Disc cutters are the most effective tools for excavating hard rock. However, the cutting tools for handling rocks and boulders with a Mixshield require different features in order to operate under pressurised slurry conditions. In particular, the cutter seal and seal gap design differs, to effectively prevent the penetration of muck and slurry (mud packing), but also provide the least possible friction to ensure the cutters are rolling properly across the tunnel face. For face pressures above 4 bar, compensating disc cutter systems have been developed that can handle high outside pressures as well as significant pressure variations, which on a 12m slurry machine is in the range of 1.5 bar from crown to invert.
If rolling is restricted due to inner friction, or the cutter is jammed, it will no longer be available for regular excavation and will only grind on one side. Two-ring cutters provide a better performance at lower single-ring thrust capacity, as they enable several cutting or face contact patterns for the same number of bearing seals and therefore a better relationship between cutting ring- and inner friction. The use of two-ring cutters also requires fewer housing positions on the cutterhead and, for this reason, provides more options for cutterhead openings to optimise muck flow.
In many cases, the use of two-ring cutters in the inner face and centre area and single-ring cutters for outer face and periphery area is a good compromise. Grain size limiters in the muck openings are installed to keep loose rock or large boulders at the tunnel face, so they can be broken down by the cutters. The design and layout of the size limiters has to be decided on the basis of anticipated ground conditions and installed crusher capacity. Special care also has to be given to the working levels of the different tool types on a mixed face cutterhead. Disk cutters should be positioned 30mm to 50mm ahead of the soft ground tools to ensure that hard rock or boulders are first attacked by the appropriate tool type.
Positive results have been achieved on several mixed face slurry machines using specially designed Monoblock cutters, with a reduced risk of secondary wear to non-cutting related elements of the cutters, such as split rings or hubs.
The combination of mixed face cutterhead tool arrangements and jaw crusher-suction grill arrangements has proven effective when dealing with variable face conditions or cobbles and boulders. The need to manually intervene in order to remove or split boulders has been reduced dramatically and can be considered an exception these days.
Clogging risks
The problem of clogging can be addressed in several ways: choice of tools, quantity of fresh suspension supply, flushing and/or agitation systems in the excavation chamber, flow in the chamber and the geometrical design and shape of cutterhead, excavation and pressure chamber.
The preferred method in adhesive ground conditions is the use of wide cutting tools, in order to achieve bigger cuttings or clay chips. This also reduces the number of tools required to cover the full face. The use of fewer cutting tools increases the free areas between the individual tool sockets and therefore reduces the risk of “bridge building” and adhesion at the cutterhead.
A high circulation or flushing quantity in the excavation chamber, in combination with a suitable cutterhead design, encourages free flow of excavated muck and reduces the time cuttings remain in the chamber to a minimum. Optimisation of the flow and a reduction in the time taken for muck to pass through the excavation chamber also have positive effects on wear reduction. This was demonstrated on the CTRL’s Thames Tunnel drives, in London, where two Mixshields were used to mine through chalk layers containing a large amount of abrasive flint.
Flushing nozzles at the centre of the cutterhead supply fresh suspension close to the tunnel face where the soil excavation takes place. These feed lines in the rotating cutterhead are supplied via single or multiple channel rotary joints in the cutterhead centre. Feed line outlet arrangements in front of the submerged wall ensure a sufficient quantity of supply to the rear face of the cutterhead in the excavation chamber. For Mixshields operating in adhesive ground, there is a general tendency to feed fresh suspension in front of the submerged wall.
Each individual supply line into the excavation chamber or the cutterhead can be controlled from the TBM’s cabin, with information about the flow and pressure of each individual line being fed back to the operator. Depending on the ground and the direction of cutterhead rotation, adaptation and optimisation of the feed-line flushing pattern is also possible. The installation of mixing arms behind the cutterhead is also a common solution to assist flushing.
There are two ways to avoid adhesion or muck settlement in the pressure chamber area. Mechanical agitator wheels in the invert area can assist muck flow. Alternatively, rotary sizers (see p39) can be used to cut clay chips to size, while not obscuring continuous conveying into the suction pipe. Additional flushing nozzles in the pressure chamber can also assist flow.
Part 2 of this article will focus on potential future developments of Mixshield technology.
Fig 1 – The HERA Mixshield Fig 1 – The HERA Mixshield Fig 2 – Mixshield concept Fig 2 – Mixshield concept Fig 3 – Mixshield face support principle Fig 3 – Mixshield face support principle Fig 4 – Reduced front level for face access Fig 4 – Reduced front level for face access Fig 5 – Example of suspension supply to excavation and pressure chamber Fig 5 – Example of suspension supply to excavation and pressure chamber HERA Mixshield HERA Mixshield Jaw crusher Jaw crusher Rotary sizer with agitators Rotary sizer with agitators 17″ monobloc cutter, on the ESCSO Project in Portland, USA 17″ monobloc cutter, on the ESCSO Project in Portland, USA Table 1
