Lyon Metro Line B extension: A variable density TBM for an underground mission in a remarkably diverse geology
The metro of the eastern French city of Lyon the third largest city in France, after Paris and Lille has a share of around 50 percent of passenger movements, and it is by far the most important means of public transport in the city.
Part of the metro system in greater Lyon is the Metro Line B Extension that is currently under construction. The project is located west of the Rhone Valley and extends to Hôpitaux Sud. It comprises a bored tunnel section of 2.4 km (1.5 miles), a new metro station in Oullins Centre and the start shaft at Saint-Genis-Laval. This new metro section is not one of the basic and ordinary metro extension projects with favorable geological conditions and simple machine solutions. The subsurface conditions along the 2.4-km (1.5-mile) long bored tunnel section are anything but ordinary and demand a solution of applied tailored machine technology. The project sets a benchmark for the use of variable density tunnel boring machine (TBM) technology in view of the forecast geological conditions along the tunnel route. They comprise sections of average highly permeable and extremely highly abrasive alluvial deposits of mainly coarse-grained soils that will be excavated mainly above the water table, as well as areas of crystalline substratum (fresh granite) with UCS of up to 164 MPa. This article will concentrate on this special area of application with sophisticated mechanized tunneling technology. This applied technology is not only scoring with high levels of safety and quality, it also helps to master tunnel construction in built-up area without impacting the aboveground environment during the construction phase.
Subsurface conditions and tunnel design
TBM tunnel operations started in 2020 beneath the builtup area. The tunnel will connect the Metro Line B to two new stations at Oullins Centre and at Saint-Genis-Laval/ Hôpitaux Sud with a total length of 2.4 km (1.5 miles). The tunnel is segmentally lined and designed with an inner diameter of 8.55 m (27.8 ft). The predicted ground conditions along the tunnel corridor comprise a variation of mostly highly permeable alluvial deposits and a section of fresh granite. The tunnel runs mainly above the groundwater table with ground cover in the range of 9 to 24 m (30 to 79 ft).
The tunnel route can be divided into three sections with regard to the conditions on surface:
- Km 1+800 to km 2+411 “Arrière-gare”: No densely built-up area, almost rural.
- Km 1+100 to km 1+800 “Saint Genies”: Built-up area with one- and two-story buildings.
- Km 0+012 to km 1+100 “Oullins”: Densely built-up with particularly sensitive old buildings.
The overall geology at tunnel level is predicted to be of high heterogeneity alluvial deposits and with diverse mechanical properties including a section of crystalline substratum that is composed of granite with UCS of up to 164 MPa. The majority of the soft soils are highly permeable with fines content of less than 15 percent except the portions that are composed of silty lenses. The geological report also indicates the possibility of occurrence of erratic blocks mainly through the alignment between Saint Genies and Arrière-gare. Both soil and rock sections are classified as extremely abrasive. Therefore, one of the key construction considerations was that the design and operation of the TBM must accommodate variable excavation conditions for face stability, muck handling and discharge.
The design of the tunnel lining is based on the predicted geological conditions and water pressure along the alignment. It consists of six precast concrete segments plus a key segment having an inner diameter of 8.55 m (27.8 ft). The segments have a thickness of 400 mm (15.8 in.). The annular gap between the outside of the segmental lining and the excavated surface of the ground is backfilled as the TBM advances to provide the bedding of the lining and to prevent subsidence. The backfill material is injected through six grout lines (DN 65 mm or 2.5 in.) incorporated in the tailskin at the rear of the shield structure. Tunneling started at the start shaft Pahls in the district Saint-Genis-Laval toward the reception shaft Orsel in Oullins.
TBM layout and project-specific design features
The contract for the construction of the Lyon Metro Line B Extension was awarded to the joint venture (JV) of Implenia and Demathieu Bard. According to the predicted geological conditions and exceptionally diverse geology the JV decided to use a variable density TBM for the 2.4-km (1.5-mile) long bored tunnel section. The machine has a diameter of 9.68 m (31.8 ft) and is designed to operate at 4 bars. The variable density TBM in use for this project is the basic version of this machine type and consists only of one muck transportation system in the tunnel. It is designed with a slurry circuit and always functions in the corresponding closed operating mode. Thereby the excavation chamber is filled with material and the tunnel face pressure is controlled. This guarantees the ground stability during excavation. The special characteristic of the variable density technology is that the individual advantages of both systems, EPB and slurry, are combined in one machine. It is possible to change between operation modes from earth pressure supported tunnel face to a slurry supported face with full control of the face pressure. The transition between the operating modes can be achieved without the need for chamber interventions and without any need of mechanical modification in the excavation chamber or in the gantry area. The machine for Lyon can also be operated using a high-density material in the excavation chamber that would be too dense for classical slurry operation but that would be too fluid for a classical EPB operation. In both, EPB and slurry operating modes the muck is extracted from the pressurized excavation chamber by a screw conveyor. The excavated muck is then transported via a closed, pressurized slurry circuit in the slurry mode or HDSM operation to a slurry treatment plant on surface. At the discharge end of the screw conveyor the excavated material passes into a slurrifier box where the excavated muck is liquefied. A jaw crusher installed in the box processes the material to a size suitable for hydraulic mucking through the slurry circuit.
The predicted diverse highly abrasive geological conditions demanded an adapted cutterhead design. The cutterhead has a bore diameter of 9.75 m (32 ft) and is designed with a nominal torque of 15,000 kNm. Wear protection is provided by grillbars, protection wedges and hardox plates in the face and gauge area. The wear detection system has four hydraulic structural wear detectors for the face area. The cutterhead tooling consists of 482-mm (19-in.) disc cutters (4x double discs and 49x single discs) and soft ground tools (130x).
One of the specific design features of the TBM is the disc cutter rotation monitoring (DCRM) system that is fitted to 15 x of the disc cutters. The DCRM system is used to optimize the maintenance intervals of the disc cutters on the cutterhead. This is particularly of interest in highly abrasive geology. One main benefit is that an operator does not have to enter the pressurized chamber for regular disc cutter inspections but only when needed. In the past, the TBM advance had to be stopped for disc cutter inspections. The DRCM system monitors the rotation and temperatures of the disc cutters that are equipped with the DCRM units during tunneling. Thus, stoppages for disc cutter inspection can be reduced to a minimum. This independent monitoring system is used for immediate detection of blocked disc cutters with the following further benefits:
- Service and maintenance work can be minimized by failure detection and track identification in real time, e.g. with the occurrence of abruptly blocked disc cutters or gradual bearing damage.
- Avoidance of disc cutter overload through the possibility to adjust cutterhead rotation speed and thrust force in case of detection of unstable or blocky tunnel face conditions.
- Optimization of disc cutter lifetime and decrease of downtime periods and thus achieving an efficient tunneling process.
- Load impact of all instrumented disc cutters is visualized continuously and displayed in simple traffic light colours on a radar-like circular guided image.
Another specific design feature of the machine is the possibility of making the invisible visible with a sonic soft ground probing system. This seismic system is called SSP-E. The measurement hardware is mounted within the shield and can be maintained in free air conditions. It is composed of two sources installed within the shield at about 5 and 7 o’clock positions, three receiver cylinders that are pushed horizontally through the excavation chamber and the stationary cutterhead into the tunnel face and two receiver cylinders that are installed radially in the shield. During each ring building phase, a signal is sent from the transmitter positioned in the shield into the ground. This signal or swipe induces seismic waves. The travel paths of the seismic waves in direction of TBM advance are the ones used by the SSP-E. The signal energy is propagated at the relevant wave speed of the specific geology and is reflected by any contrast in seismic impedance such as boulders, cavities or abrupt changes of geological conditions. The reflected signals are picked up by the receivers and evaluated. On the basis of the detected density contrasts it is possible to obtain a three-dimensional visualization of the ground up to 40 m (131 ft) ahead of the tunnel face within the tunnel alignment and making obstacles visible. The advantage of this system is, that it is largely integrated in the boring process, enabling continuous, ring-by-ring preliminary exploration parallel to tunneling. The measured data are processed and evaluated nearly in real time.
Site experience to date
In July 2018 Sytral (Syndicat mixte des transports pour le Rhône et l’agglomération Lyonnaise) awarded the contract to build the Metro Line B extension in Lyon to the JV Implenia and Demathieu Bard. The 9.68-m (32-ft) diameter variable density TBM that is excavating the 2.4-km (1.5-mile) long tunnel section toward Oullins was designed and manufactured by Herrenknecht in Germany. The machine was accepted in the TBMs manufacturer’s headquarters beginning of August 2019 and was then directly transported to the jobsite in Lyon by trucks. The cutterhead arrived on site end of August 2019. Because of its size and weight of about 117 tons, a barge was used only to transfer the cutterhead center, loaded on a truck, to transport it from one side of the river Rhone to the other to the Edouard Herriot port in Lyon and then further on the road to the launch shaft in Saint-Genis-Laval.
The TBM started tunneling on November 29, 2019 toward the Station Hôpitaux Sud via Gare d’Oullins and will end its operation at the Orsel shaft. The 9.68 m (32 ft) diameter TBM started out of a short shaft (37 x 20 m) with the first back-up assembled in the shaft and the gantries two and three assembled on surface using umbilical lines. The machine will be disassembled in the reception shaft Orsel with the shield being left in the ground.
Mid of May 2020, at the time of writing this publication, the shutdown of the jobsite due to the Covid-19 virus was slowly deactivated. Till then, the machine operated only a few hundred meters so that TBM operation experiences cannot be shared within the framework of this publication.
Before TBM operations could begin the contractors needed to find an appropriate slurry or high-density suspension to cope with the predicted difficult ground conditions and coarse soil structures along the tunnel route.
There was already experience of this demand for an appropriate high-density suspension from previous projects where this Variable Density TBM technology was applied, in particular with the first use of this technology for a metro project in Malaysia where the subsurface conditions included karstic characteristics.
Coarse to very coarse soil structures of high permeability, such as prevalent in Lyon, demand a suspension that avoids the risk of suspension loss into the ground and can improve face support behavior. The contractor did several suspension tests and developed a support medium made of technical mud of confidential composition, with a density close to 1 t/m3 that can block grain sizes of less than 0.5 m. They developed several technical muds or support medium according to the needs such as for example for hyperbaric intervention, mining operation or emergency situations. These different mixtures are stored on surface and are sent to the TBM with a dedicated pump and pipe at max. 100 m3/h.
Conclusion
The project Metro Line B extension in Lyon is part of an intelligent underground solution that will enhance the mobility and living standards of people in Greater Lyon. It will support sustainable urban development. The project is with focus on its remarkably singular diverse subsurface conditions one of the few challenging projects to be realized in built-up area. It will set a benchmark when successfully finished related to TBM works in difficult grounds characterized by a significant variation in ground conditions of compact rock and highly permeable and abrasive soils.
References
Dr. Karin Bäppler, Michael Straesser. Certainty and reliability in mechanized tunnelling, RETC 2015, New Orleans, June 2015
Dr. Karin Bäppler, Frédéric Battistoni, Werner Burger, Variable Density TBMcombining two soft ground TBM technologies, AFTES, Congrès International, Paris, 13-15 November 2017
Dr. Karin Bäppler, Michael Straesser, Forrestfield Airport Link-Project Challenges and TBM solution, RETC 2019, Chicago, June 2019
