The global tunnelling market is typically driven by unrelenting urbanisation, population growth and globalisation. When combined, these factors necessitate the creation of new infrastructure, particularly new transport systems to connect the newly-created urban and regional networks.
Key to this expansion is the innovative technology associated with mechanised tunnelling and exemplified by the now well-proven Variable Density tunnel boring machine (VDTBM). Continuing to facilitate the construction of sustainable tunnel infrastructure, the VDTBM – thanks to its low-settlement-inducing nature – is making tunnelling safer even in extremely challenging and variable ground conditions.
This article will look at the design and development of the VDTBM, as well as its application areas, focusing on the Lyon Metro, which was one of the first projects to exploit the technology.
CHALLENGES AND SOLUTIONS
Seen as some of the greatest challenges in tunnelling, highly variable ground conditions can present an array of problematic scenarios: changes in soil and rock types, thicknesses and extent; changing layers, not to mention variations from sticky, cohesive, coarse-grained soils to transitions from loose to solid rock; all these can work against the expected progress of a tunnel project.
To provide a safe and cost-effective solution to such variable ground challenges, Herrenknecht developed the Variable Density TBM. Able to switch easily as required between slurry-supported mode and earth pressure balance (EPB) mode, the VDTBM offers significantly greater flexibility with regard to the consistency of the chamber fill than the original versions of these two closed tunnelling methods.
SELECTING THE MOST SUITABLE TECHNOLOGY
Choosing the most appropriate tunnelling method is a critical consideration for any project. But it is not the only one. In mechanised tunnelling, other important criteria must be considered, such as cost effectiveness of the technique, sustainability, muck recovery, landfill that does not harm the environment, not to mention the logistics that will be involved in the project1.
For example, the VDTBM’s soil recovery system with slurry circuit and slurry treatment plant allows solid-liquid mixtures to be separated and reused. Large solid fractions are separated from the support medium which is then recirculated. Extracted solids typically have very low moisture contents and can thus be disposed of as landfill at reduced cost.
Developed around 10 years ago, the VDTBM has been applied mainly to projects with highly heterogeneous ground2. This paper focuses on the extension of Metro Line B in Lyon, France which involved a mechanised drive by a Herrenknecht VDTBM.
Because of the highly variable geology of the Line B tunnel alignment and the way it was successfully overcome, the project constitutes an excellent case study for this new machine technology. The VDTBM bored safely through highly permeable soil above the water table, as well as through a granite horizon with minimal settlement.
Variable density technology is also being used on other projects, including sections of the UK’s HS2 high-speed rail link; some sections of the Grand Paris Express megaproject; and the large diameter Hampton Roads Bridge Tunnel project in the US.
AN ADVANCED TECHNOLOGY
As an advanced form of hybrid machine, the VDTBM combines EPB shield and slurry-shield technologies, offering maximum flexibility when selecting the support and excavation method. This enables tunnel sections with variable geological conditions to be excavated safely and cost effectively. The designers’ objective in developing the new technology was to enable tunnelling modes to be changed inside the tunnel; this means there is no need for mechanical modifications or interventions, whether inside the excavation chamber, to the machine itself, or in the backup area.
DESIGN FEATURES AND APPLICATIONS
The ability to allow switching between tunnelling modes is a major benefit of the VDTBM (Figure 1). This extends the capabilities of mechanised tunnelling and allows underground infrastructure to be built even in very heterogeneous ground – from solid rock to cohesive, coarse grained and highly permeable soil – above and below the water table3.
The VDTBM can be operated both in slurry-supported mode using an air bubble to control the support pressure, or in EPB-supported mode by means of earth pressure gauges and control of the discharge volume from the screw conveyor. The seamless switch between operating modes allows full control of support pressure during the transition without any requirement for chamber interventions.
While in operation, muck at the tunnel face is removed from the VDTBM’s excavation chamber by a screw conveyor in both operating modes, i.e. slurry and EPB shield mode with active face support. In the basic machine configuration, the support and removal system includes a closed and pressurised slurry circuit located behind the screw conveyor and a slurry treatment plant at the surface.
Depending on the geology encountered, in slurry-supported mode, the TBM can be driven either with a normal bentonite suspension or with a high-density suspension for excavating highly permeable ground or karst formations along the route. In EPB-supported operation, i.e. conventional, closed EPB mode, the hydraulic slurry circuit can be used with or without the addition of bentonite in the excavation chamber. If foams or polymers are used for muck conditioning in EPB mode, the hydraulic slurry circuit and slurry treatment plant at the surface can be dispensed with, and the muck transported out of the tunnel by belt conveyor.
A crusher can also be incorporated in the VDTBM design: mounted immediately behind the screw conveyor, it breaks (or ‘treats’) the aggregate into a manageable size for the downstream slurry circuit. With this configuration, maintenance work on the crusher can be carried out under atmospheric conditions when the screw conveyor gate valve is closed. In the same way, pressurised chamber interventions for crusher maintenance can also be dispensed with in slurry-supported mode.
A closed ‘slurryfier box’ – capable of processing suspensions of varying density in the hydraulic slurry circuit – is connected to the crusher. If the density of the conveying medium is too high for the slurry circuit, it can be diluted to an appropriate density for hydraulic transportation inside the slurryfier box.
A further development of the variable density concept is the inclusion of a compact screw conveyor in the shield: this allows operation of the TBM with increased chamber density and with hydraulic muck removal via the screw conveyor without a reduction in pressure.
However, a compact screw conveyor is not suitable for use in EPB mode when using foam or polymer conditioners. The advantages of a compact screw conveyor are that it reduces wear inside the conveyor in abrasive ground conditions and thus eliminates the need for a support and interface in the backup area. In this case, the crusher is connected directly to the compact conveying system in the shield and can be accessed under atmospheric conditions, if required. The VDTBM for the Hampton Roads Bridge Tunnel project in the US was configured in this way.
LESSONS FROM LYON
Ground conditions and tunnel design
The 2.4km extension connects the Lyons Metro Line B to two new stations at Oullins Centre and Saint-Genis- Laval/Hopitaux Sud. Segmentally lined, the tunnel has an internal diameter of 8.55m. Predicted conditions along the tunnel alignment include a variation of mostly heterogeneous gravel layers and a section of solid granite. Running mainly above the water table, the tunnel encountered overburdens of between 9m–24m along a route that can be divided into three sections, according to the conditions at the surface (Figure 2):
- Km 1+800 to km 2+411 Saint-Genis-Laval: sparsely built-up area, almost rural.
- Km 1+100 to km 1+800 Grand Revoyet: built-up area comprising one- and two-story buildings.
- Km 0+012 to km 1+100 Oullins: densely built-up area with particularly sensitive old buildings.
Very heterogeneous gravel layers display distinctly different mechanical properties, including one section with crystalline bedrock comprising granite with a uniaxial compressive strength (UCS) of up to an encountered 189MPa (Figure 3). Soft ground in the Oullins section is mainly highly permeable (up to 10m/s-2m/s) with a fines content of less than 10%, apart from some sections which contain silt lenses (Figure 4). The geological report also indicated the presence of erratic blocks, mainly between Saint Genis and Grand Revoyet.
Predicted geological conditions and water pressure along the route were influential in the design of the 8.55m internal diameter tunnel lining comprising six precast concrete segments plus key. The segments are 400mm thick and the backfill material is injected behind the tail skin through six grout lines (DN65mm). Tunnelling commenced at the Pahls start shaft in the Saint-Genis-Laval district towards the Ortsteil target shaft in Oullins.
TBM CONFIGURATION AND OTHER REQUIREMENTS
The contract for the Lyon Metro Line B extension was awarded to a consortium comprising Implenia and Demathieu Bard. Due to the expected geological conditions (highly permeable ground above the water table) and the exceptionally diverse geology, the consortium opted for a Variable Density TBM instead of a Mixshield for the 2.4km-long tunnel section. With a diameter of 9.68m, the machine was designed to operate at 4bar.
The Lyon TBM was configured to remove muck solely via a pressurised slurry circuit (feed and discharge line), which means that it must operate in closed (slurry) mode. This means the excavation chamber is continuously supplied and flushed with bentonite; and support pressure is controlled by an air bubble with an upstream air pressure control system (active face support). This guarantees the stability of the working face during excavation. In addition, a high-density suspension (high-density support medium – HDSM) can be supplied directly to the excavation chamber via a separate supply line.
Once the screw conveyor has removed muck from the excavation chamber, it is then liquefied in the slurryfier box which has a jaw crusher to reduce the material to a suitable size for hydraulic conveyance through the slurry circuit. The position of the jaw crusher enables regular maintenance work to be carried out under atmospheric pressure, providing corresponding cost savings. Muck is pumped through the conveyor pipeline to the slurry treatment system at the surface. There, the suspension is cleaned and then pumped back to the TBM.
KEY TECHNICAL DATA OF THE S1204 VDTBM
- Shield diameter 9.68m
- Active cutting-wheel displacement
- Operating pressure: 4bar on TBM axis
- 20m-long screw conveyor supported by the first backup unit
- Slurryfier box with integral jaw crusher behind screw conveyor
- Conveying/feed circuit/DN400/1,800m3/hr
- Backup system comprising four units
- Overall length: 125m approx.
- Multiservice vehicle (MSV9) for tunnel logistics.
SITE EXPERIENCE
The JV launched the TBM on 29 November 2019, advancing toward the Hopitaux Sud station and on to the target shaft in Orsel via Gare d’Oullins. Final breakthrough occurred on 15 May 2021. Launched in a short 37 × 20m starter shaft, the machine had backup unit 1 (assembled in the shaft) and backup units 2 and 3 assembled at the surface using hose and cable extensions. The machine was subsequently disassembled at the reception shaft in Orsel, although the shield skin was left in the ground.
Faced with the alignment’s difficult ground and coarse soil structures, the contractor had to develop (before tunnelling could begin) a suitable suspension to cope with the anticipated conditions. Lyon’s mainly highly permeable coarse-to-very-coarse soil structures required a suspension that could mitigate the risk of losses into the ground and improve face-support behaviour. Having tested a variety of suspensions, the consortium created a support medium which was tailored specifically for the project. Such a bespoke suspension can be combined with a mixer at the surface as required and piped to the TBM by pumping at a maximum rate of 120m3/hr.
In tunnelling stretches where the soil proved to be highly permeable, the TBM was operated in HDSM mode which involves a minimum 20% supply of bentonite to the excavation chamber. Thus, the main liquefaction process (80%) takes place in the slurryfier box. Precise control of support pressure can be achieved using the air bubble system with upstream air pressure control for active face support.
Also, during tunnelling breaks and at regular intervals – such as for ring building or maintenance – the specially developed suspension (HDSM) was fed directly into the excavation chamber. This approach allowed suspension losses to be minimised and so successfully prevented undue settlement at the surface. Indeed, the highest recorded settlement of 3mm was well below the 5mm maximum stipulated in the contract.
Low-density support medium (LDSM) mode was used to excavate the stretch of solid granite. In this mode, the supply of bentonite to the excavation chamber can be increased to the maximum slurry volume. LDSM mode sees the main liquefaction process take place in the excavation chamber (60%–80%). The remaining slurry is conveyed to the slurryfier box for residual liquefaction and for flushing out the crusher (20%–40%). This mode ensured optimum muck removal via the screw conveyor and by the downstream crusher and slurry circuit.
OUTLOOK
The range of applications for mechanised tunnelling encountered in extremely challenging, highly variable ground conditions are extended by Variable Density technology. Several successful reference projects since the first use in Kuala Lumpur, Malaysia have demonstrated the benefits of the VDTBM.
The primary focus in its first deployment was to control suspension losses when driving through karst limestone. However, subsequent deployments resulted in further refinements to the ability to customise the consistency and density of the chamber fill according to the project, and to flexibly adjust these parameters during the drive. In predominantly cohesive soils, the combination of mechanical and hydraulic removal of muck from the excavation chamber proved a major advance, compared with the pure Mixshield operation with its associated clogging and deposits in the invert of the excavation chamber.
Variable Density TBMs can be adapted to changing ground conditions without modifications in the excavation chamber or to the cutting. This significantly improves tunnelling safety for personnel, since the operating mode can be changed without the need for chamber interventions.
The fully equipped VDTBM has two material handling systems inside the tunnel (hydraulic slurry circuit and dry removal via conveyor belt or muck car) and spans the complete range of applications – from EPB support and slurry support to open drive. However, a ‘slurry only’ version has also been developed which features all the essential components of a VDTBM within the shield itself but uses a hydraulic slurry circuit only for conveyance inside the tunnel.
With this design, the screw conveyor (the primary conveying system) is not employed for support pressure control or pressure reduction in EPB mode. This facilitates the use of applications at support pressures above the current normal limits for EPB machines.
The main design criterion governing screw length is no longer support pressure reduction, and this has allowed the development of a machine configuration with a very compact, short screw conveyor and integrated roller or jaw crusher, if required.
NEW POSSIBILITIES
This new ‘compact screw’ version of the VDTBM allows the entire primary conveying system comprising short screw and slurrifier box (with crusher) to be installed in the front shield area. This has the effect of reducing the geometric constraints in the ring-build area typically associated with long screw conveyors. Fewer geometric limitations at the transition between the shield and the backup system facilitate the move towards highpressure applications which require saturation diving and transfer shuttles.
Furthermore, all the operational benefits offered by VDTBMs are also provided by the ‘slurry only’ concept, such as flexible adjustment of the chamber fill density, hydraulic -mechanical removal of muck from the chamber, and improved access to the crusher under atmospheric conditions.
For large -diameter machines, a further evolutionary step in variable density technology is offered by the ‘slurry only’ design combined with an accessible cutting wheel for atmospheric tool changes. Up until now, for conventional EPB operations, the use of large-volume accessible cutting wheels was considered problematic in view of the uncertainties surrounding mixing behaviour, material flow and clogging in the excavation chamber. But now, such a combination is possible, thanks to the greater flexibility with regard to the consistency of the chamber fill afforded by variable density machines. This has significantly improved personnel safety in applications involving higher support pressures, since few interventions are required under pressurised conditions.
As in many walks of life, so also in mechanised tunnelling, technological advances are achieved incrementally. Real- life projects (rather than test tunnels or similar contexts) typically provide the opportunity to test prototypes. This means that the maxim of ‘controlled and controllable risk’ should be applied to each subsequent development to ensure the project’s success. Ideally, this can be achieved by developing close, trust-based working relationships between operators, machine manufacturers, clients and designers. Such collaborations can foster real advances in safety and application possibilities, as well as in the cost effectiveness of mechanised tunnelling.
It was precisely under these conditions that Variable Density technology was developed by Herrenknecht.
