A glance at the development of Mixshields over the past two decades (figure 8) shows an impressive increase in tunnel diameters. What’s more, from a TBM technology and manufacturing point of view, there is no obvious technical limit on further increases.

Diameter increases are closely connected to the planned purpose of a tunnel. Two and three lane road tunnels have now been constructed with diameters of 11.2m (A86 Road Tunnel, Paris) and 14.2m (Lefortovo Tunnel, Moscow), and a three-lane road tunnel is currently being built in China with a diameter of more than 15m (Chongming, Shanghai).

With these diameter increases, multi-purpose or combined-use tunnels such as road/water storage (SMART, Kuala Lumpur) or road/subway (Silberwald, Moscow) are also becoming more widespread. The ability to excavate very large diameters also creates additional potential for new usage concepts, like subway station platform tunnels.

Twin-track rail tunnels with diameters of 11.4m-12.6m already exist, and the increasing speed of trains and higher demands of operational safety (emergency rescue/escape concepts) will create further need for larger tunnel and machine diameters.

Increasing performance demands, combined with experience from past projects, has also contributed to a continued increase in Mixshield operating pressures (see figure 9). Compared with EPB machines, the Mixshield’s use of a closed slurry circuit as the mucking system enables higher face pressures to be effectively dealt with. Controlling a large pressure drop in a continuous mucking system is also easier with a slurry circuit than with a screw conveyor, especially in heterogeneous or highly permeable ground conditions.

A significant increase in face pressure affects all components of the shield that are exposed to the surrounding soil or groundwater. In particular, it affects:

• Shield structure

• Tail seal systems

• Main bearing seal systems

• Articulation seals

• Shield thrust system

• Slurry circuit

• Equipment (and procedures) for face access

While it is possible to accomplish the required shield thrust by changing the number or diameter of the thrust cylinders, far more sophisticated technical solutions are required for seal systems. This is especially true of the main bearing seal system, which is one of the most sensitive design elements in high-pressure applications. For support pressures beyond 4 bar, pre-stressed cascade systems are used with the individual cascade chamber pressures automatically following the face pressure.

These systems can handle pressures far beyond 10 bar for an extended period of time in dynamic mode without the risk of overloading the individual lip seals. Long-term field experience with large diameter drive systems (bearing diameter range of 6m) with face pressures of 7 bar to 10 bar already exist and full scale workshop and commissioning test programmes with pressures of 15 bar have been performed successfully. In emergencies or extended stoppages (long-term static mode), additional inflatable seals are included.

While it is now possible to address high-pressure operations by using appropriately designed equipment, the key questions now relate more to the potential, and the limitations, for chamber access under hyperbaric conditions.

Technical solutions to reduce the need for man access to the excavation chamber are available and currently include:

• Accessible cutterheads for atmospheric cutter tool change (larger machines only)

• Remotely activated standby cutter tools

• Load detection and wear sensor systems

However, these technical features will not totally eliminate the need for a “Plan B” for manual intervention to cover unforeseen conditions or worst-case scenarios.

Based on the system of excavation and face support, a Mixshield requires lower cutterhead torque compared with an EPB shield (figure 10), as the cutterhead is only excavating the ground at the tunnel face into the suspension-filled excavation chamber. The excavated soil sinks towards the submerged wall opening in the invert due to gravity, assisted by the flow direction of the circulated slurry, and is carried to the suction pipe after clearing the rock crusher and suction grille.

An EPB shield requires a comparatively high torque at the cutterhead because, in addition to the soil excavation, the cutterhead itself acts as a mixing tool inside the excavation chamber, which is completely filled with muck.

Therefore by adopting high torque EPB drive systems that have been developed for large diameter machines, such as that used on the M30 project, in Madrid (with 125,000kNm), there is huge potential for the development of larger diameter Mixshield machines.

Examples of projects

The following presentation of the Chongming and A86 tunnel projects demonstrates the efficiency of current Mixshields and the value of development.

Mixshield used as a shield with slurry supported face – Chongming, China: A twin tube road tunnel is currently being built beneath the Yangtze River in the city of Shanghai, comprising two 7160m-long bores with three lanes each. The tunnel, along with a new bridge, will link the islands of Changxing and Chongming to the freeway system and city. The geology of the tunnel is defined by its position in the river delta, consisting of soft clay deposits and thin sand layers. The tunnel has an outside diameter of 15m. The pre-cast concrete ring consists of 9+1 segments with a length of 2m. The segments are 640mm thick and weigh up to 16.7 tons. The basic concept of the two Mixshield machines for the project is based on experiences from the Mixshield used at the fourth Elbe Tunnel and advancements in large diameter shield developments in high water pressure conditions. With a shield diameter of 15.43m, the two machines are currently the world’s largest diameter shields.

The Mixshield machines have following technical features:

• The shields are designed for an anticipated operational pressure of 6 bar at springline level. Due to the underwater application, and nearly straight alignment, (Rmin = 4.000m), a shield articulation joint was not included

• The invert area of the Mixshield is equipped with two agitator wheels (Ø1.900mm), which assist the material flow to the grille and a 500mm diameter suction pipe. Submerged wall gate, bentonite nozzles, cutting wheel and extensive excavation chamber flushing arrangements complete the Mixshield configuration to address the soft soil conditions and potential clogging risks

• The double shell tailskin with integrated grout lines has a three-row wire brush seal and an inflatable emergency seal system. Furthermore, freezing lines are integrated into the tail shield, which, in case of emergency, can be used for ground freezing around the machine to minimise the risk of water inrush during brush seal changes or repair works

• The cutterhead is designed with six main spokes accessible under atmospheric pressure. To reduce the need for pressurised face access, one complete set of cutting tools (covering the entire face area) is exchangeable under atmospheric conditions from within the cutterhead spokes. To suit to the anticipated geology, the cutterhead was equipped with massive scrapers. Two hydraulically operated overcutters can create an overcut of 40mm in radius. The cutterhead front and outer areas, as well as the rear, are designed to be durable and wear resistant to cope with the single drives of more than 7000m (see p31).

As an additional safety feature, the Mixshields are equipped with all components – such as air locks and installations – necessary for pressurised face access including saturation diving activities.

The installed cutterhead drive power is 3750kW and the bearing diameter is 7.6m. The torque of the variable frequency electrical drive is 34800kNm, the shield thrust capacity is 203000kN and the TBM system is designed for a nominal mining speed of 45mm/min.

The three-section backup system has an overall length of 118m and is divided into primary backup, bridge section and secondary backup.

The primary backup, or first three-deck trailer, contains all the hydraulic power packs and electrical systems for the supply and operation of the shield, along with slurry pumps and backfill grout system. For an even distribution of the wheel loads the trailer contains an integrated support system of auxiliary rail elements (steel invert slabs) and multi-wheel sets. The prefabricated 35 ton invert elements are installed in the area under the 67m bridge section. The supply crane system is installed inside the bridge cross-section to transfer segments, grout and other consumables to the TBM. All installations and workplaces for extension of services are located in the third section, along with ancillary equipment.

The machine is supplied with segments and grout by rubber-tired transport vehicles, which travel in convoy and carry either segments only or segments and grout tanks. The segment transfer on the backup is done by segment crane and a segment feeder. The grout is supplied in transfer tanks to the first backup.

The shield structures and assemblies of the 132m-long and 2,300 ton TBMs were manufactured in Shanghai. Cutterheads and other main components such as drive assemblies and thrust cylinders were manufactured in Germany and shipped to China. After shop acceptance, the TBM was disassembled and transported to the start shaft about 6km from the workshop.

Tunnelling started for the first tube in September 2006, and in January 2007 for the second. In March 2008, the first 7160m tunnel was about 90% complete and the second about 70%. Constant weekly performances of 90-120m are now being achieved by each TBM. Both drives are scheduled to finish in 2008 (the first TBM in May, and second in September), almost a year ahead of the project schedule.

Mixshield used in differing operational modes – A86 tunnel: To close the gap in the A86 orbital motorway, a 10.1km-long, two-deck road tunnel for cars has been built to the West of Paris. A second tunnel for trucks is planned for construction at a later stage. Two levels, with three lanes each, require an outer diameter of 11.565m. The tunnel crosses the entire spectrum of geological formations under Paris: Marl, clay, limestone, chalk and sand as well as three different groundwater levels. For optimum adaptation to the geological conditions, the machine had to operate in different modes:

• As slurry shield with slurry supported face (slurry operation – see figure 13a)

• As earth pressure balance (EPB)?shield with face support provided by conditioned muck (see figure 13b)

• In Semi-EPB, or compressed air, mode

• In open mode (muck discharge via screw conveyor, non pressurised excavation chamber)

The change between different operational modes is carried out within the tunnel and, depending upon the level of preparations, can take between 1.5 to 3 days. Shield and backup are equipped with the full range of equipment for each mode. For slurry mode, this included a full slurry circuit with submerged wall/pressure wall installation and also a rock crusher. For EPB mode, components such as screw conveyor and TBM conveyor were installed.

The cutterhead is designed for use in all modes of operation without the need for modification. The cutterhead concept is a closed wheel type with a full set of mixed tool equipment including 17” backloading disc cutters and ripper tools for two directions of rotation.

In slurry mode, the excavation chamber and the lower part of the pressure chamber are filled with bentonite slurry; the upper part of the pressure chamber contains the air bubble, and the entire area is pressurised. In EPB mode only, the excavation chamber is pressurised so the submerged wall becomes a pressure bulkhead. The pressure chamber is then at atmospheric pressure and can be used as a working chamber, only pressurised during face access. To change from EPB to slurry mode, the entire screw casing is moved back, thus clearing the submerged wall opening in the invert and the suction grille below. After this, a specially designed jaw crusher moves from parked position to operational mode.

Some of the slurry mode installations, such as the air bubble pressure regulation system or the bentonite circulation systems, can also be used in EPB mode when required. Having the two systems permanently available provides potential synergy.

Apart from the ability to change modes of operation, the TBM also has the following technical key features:

• To cater for EPB mode, installed cutterhead power is 4000kW and the available cutterhead torque is 35000kN/m. Shield thrust is 150000kN, and the designed advance speed is 80mm/min

• The slurry circuit with 1900m³/h flow volume is designed for a mining speed of 50mm/min in slurry mode. The tunnel is runs uphill and the largest difference in height between portal and TBM is 160m. This configuration needed to be addressed in the design of the slurry circuit as, under some conditions, the friction losses in the discharge line are less than the geometrical height between TBM and treatment plant

• A specially designed camera system for the excavation chamber was installed for the first time and successfully tested in semi EPB/compressed air or open mode

• Due to the steep tunnel gradient of 4.5% rubber tired vehicles were used for segment and grout transport. At the tunnel portal a semi-automatic loading station for the vehicles was installed loading one complete multi stack truckload at the same time, which together with a quick unloading system in the gantry reduced the turnaround cycles

The pre-cast invert slab elements for the final lower road deck were installed 200m behind the trailing gear, concurrent to the advance of the TBM. In November 2000, the machine started excavating the VL1 tunnel (figure 14) in open mode EPB configuration. The first 150m in an incomplete starting configuration through chalk containing a high amount of flint was excavated in two-shift operation, quickly reaching mining speeds of 80mm/min. After having installed the TBM and portal systems in their final configuration, the operation was changed to three shifts.

Following a fire in the rear section of the tunnel in 2002, mining activities were halted for three months. By October 2002, the TBM had operated in open mode, closed mode EPB with face pressures of 1-2 bar, and semi EPB mode. The semi EPB mode proved to be the most appropriate method for excavating the stable but water bearing material, using the compressed air to control the water and achieving dry excavated material.

With the ground conditions changing into Fontainebleau sand, the machine was changed in-tunnel to slurry mode and operated in that mode for one year achieving mining speeds of 50mm/min. The breakthrough of the first tunnel was in October 2003. The TBM was disassembled, transported and reassembled at the Pont Colbert starting portal for the VL2 tunnel.

For the VL2 tunnel the TBM began excavation in slurry mode. Immediately, around 10m after the portal, a major six-lane motorway had to be passed beneath with shallow cover. Launch and passing under the freeway was completed after just nine days with no problems. After 1.2km in slurry mode, the TBM was changed back to EPB mode, and after passing an escape and ventilation shaft at the deepest point of the VL2 tunnel the TBM mode was changed back to slurry again. The machine arrived at the portal in August 2007.

Conclusion

Initiated by the requirements of numerous large scale projects around the world, the development of Mixshield technology has taken major steps forward, as illustrated in this and the previous article (T&TI, May p35). Numerous additional features are also currently on the drawing board or being used for the first time. These include:

• Advanced wear detection systems for cutting tools and structure

• Positive ground support of the tunnel wall along the shield skin

• Advanced ground improvement scenarios for closed mode from within the machine

• Total integration of the whole package of above ground and underground measurement, process and alignment control data for a controlled boring process (CBP)

• Approaching diameters of 18m to 20m

• Fully variable, multi-mode concepts (EPB/HD slurry/LD slurry)

The ability to handle high water pressures, the potential for crusher installation, low power requirements, high accuracy of face pressure and settlement control, and favourable face configurations, are just some of the current advantages of Mixshield technology. The combination of these advantages along with the ability to change modes of operation, brings the concept close to combining the best of both worlds. Nevertheless, there is also still huge potential for future development of the technology, that will see even greater tunnelling challenges conquered.


The 14.2m diameter Lefortovo Mixshield, before it was shipped to Moscow, Russia 14.2m diameter Lefortovo Mixshield Westerschelde’s Mixshield incorporated an accessible cutterhead for tool changes Westerschelde’s Mixshield Fig 12a – Accessible cutterhead design Fig 12a – Accessible cutterhead design Fig 12b – Accessible cutterhead design – front view Fig 12b – Accessible cutterhead design – front view Fig 12c – Accessible cutterhead design – view from inside Fig 12c – Accessible cutterhead design – view from inside Fig 8 – Diameter development of Herrenknecht Mixshields Fig 8 – Diameter development of Herrenknecht Mixshields Fig 9 – Operating pressure of Herrenknecht Mixshields Fig 9 – Operating pressure of Herrenknecht Mixshields Fig 10 – Torque comparison of cutterhead drives (Mixshield vs EPB-shield) Fig 10 – Torque comparison of cutterhead drives (Mixshield vs EPB-shield) Fig 11 – Chongming alignment Fig 11 – Chongming alignment Fig 13 – Machine concept of the A86 Mixshield, used in Paris, France: Left – Operation mode slurry, right – operation mode EPB Fig 13 – Machine concept of the A86 Mixshield, used in Paris, France: Left – Operation mode slurry, right – operation mode EPB Fig 14 – A86 Tunnel alignment Fig 14 – A86 Tunnel alignment The A86 machine breaks through The A86 machine breaks through