Variable Density TBM – Combining Two Soft Ground TBM Technologies


The interest in today’s underground tunnel construction projects, in particular for clients and project financiers, is to complete the tunneling structures safely, on time and within the given cost frame as cost and time overruns weaken the confidence of clients and authorities and finally the confidence and acceptance of large-scale projects among the public. That’s how flexible, future-orientated solutions such as mechanized tunneling technology are becoming increasingly important to clients in achieving the targeted quality and functionality of a project on time and within budget.

As today’s projects are more and more designed and implemented in regions and geological conditions that would have been inconceivable a decade ago, specially adapted machine concepts are required to safely excavate the infrastructure below surface where needed regardless of subsoil conditions. Thus, tunnel structures are increasingly planned in heterogeneous geologies with sections that can comprise solid rock conditions, soft and water-bearing soils and/or mixed-face conditions composed of rocks and soils. Such conditions demand a specially adapted machine design to safely and reliably excavate and line the tunnel without the need for long conversion times.

This paper highlights the new generation of soft ground machines that combine the two basic soft ground technologies in one and the same machine. With the design of the new generation of multi-mode soft ground machine it is possible to smoothly switch between the different modes (EPB and Slurry) in the tunnel by maintaining permanent and full control of face pressure, and without the need for excavation chamber interventions. This new machine generation is called Variable Density TBM and offers maximum safety and flexibility in the choice of tunnel face support and discharge of muck.

Figure 1

Figure 1 – Multi-mode TBM for Socatop, slurry and earth pressure mode.

Development of Multi-mode Soft Ground TBMs

The first multi-mode TBM was designed back in the 1980s for a tunneling project that was built in variable ground conditions comprising stable to soft rock, mixed face and water-bearing soils. Since then the technology has been continually refined and updated to the high degree of maturity of today’s available machine design, the Variable Density TBM (K. Bäppler, W. Burger, 2016).

In 2000, a multi-mode TBM was successfully applied for the road tunnel project Socatop in Paris. The project comprised the construction of the A86 West Tunnel that forms the final link of the 80-km A86 ring road around Greater Paris. The tunnel has been built to relieve traffic congestion and improve traffic links between the suburbs of Paris. The multi-mode TBM that was used for the construction of this tunnel had a diameter of 11.56 m and was at that time the first innovative TBM that could be operated in EPB and slurry mode.

The tunnel was built by three of the biggest construction and road industry companies in France – Vinci, Eiffage Construction and Colas. To justify the significant effort of a machine design that is capable to completely change from EPB to slurry mode in the tunnel there must be special project conditions. The Socatop project featured such conditions. The tunnel has a length of 10 km of which approximately 60% of the subsurface conditions are soils suitable for the use of an EPB shield. The remaining 40% of the alignment comprised an optimum condition for a TBM operation with a slurry supported tunnel face. The TBM and tunneling concept for the project considered in particular that the respective geological formations occurred in long associated sections. Both slurry shields and EPB shields are operated with a filled excavation chamber and a controlled support pressure for the tunnel face. The major differences between the two operation modes are the properties of the chamber filling such as viscosity, shear strength, density and the type of chamber and face pressure control. With slurry shields, the face pressure is controlled by a remote pressurized air bubble that is in most cases provided by separating the excavation chamber in two compartments by means of a submerged wall. With an EPB shield the face pressure is controlled by the advance speed and muck extraction volume via the screw conveyor speed.

The design of both cutting wheel and excavation chamber does not require any compromises between the operation modes. The major mechanical differences are muck transportation and muck handling systems in the excavation chamber and in the tunnel. Slurry shields use a closed, pressurized slurry circuit with a slurry treatment plant at surface; EPB shields use a screw conveyor for controlled muck extraction out of the excavation chamber and an open tunnel transport system with muck cars or conveyors. The generous amount of space available with the large diameter soft ground TBM for the Socatop project allowed the parallel arrangements of both muck removal systems in the invert area of the excavation chamber with some minor functional compromises. If slurry operation was required in prevailing geological conditions with possible blocks, boulders or larger stones, a jaw crusher could be moved in from a parking position and activated in front of the suction grid. This required a manual intervention and additional mechanical effort to change the operation mode. With TBM diameters smaller than 8 m this becomes even more difficult.

Although the Socatop project was to stay a one-of-a-kind solution for a rather long time, it showed conclusively that a complex combination of different technologies can make sense if the project circumstances are right.

In May 2013, Bouygues Civil Works Florida successfully completed the excavation of the Port of Miami road tunnel in Florida. The project comprises 1.2-km long twin-tube tunnels that cross the shipping channel and cruise ship terminal with their ends curving off to tie-in the existing road alignments on Watson and Dodge islands. The tunnels accommodate two traffic lanes, curbs, walkways, ventilation fans and additional safety features.

Tunnelling for the Miami Port Tunnel in the Biscayne Bay was realized in porous and variable subsoil conditions of mainly sand and limestone and chloride groundwater. Due to high ground permeability in the porous formation and environmental concerns related to possible loss of slurry into the Biscayne Bay aquatic preserve, an EPBM (Ø12.87m) from Herrenknecht was selected to excavate and line the twin tube tunnels. The TBM could be adapted to deal with the variable ground conditions of locally high permeability where it was required to control the water and extract the rock at the tunnel face. The machine could be operated both in EPB mode with material discharge through the screw conveyor onto a continuous conveyor muck handling system and in a water controlled process (WCP) mode with hydraulic mucking as the machine crossed beneath the channel and entered into the highly permeable rocks.

The WCP mode bypasses the discharge gate and the muck is processed directly from the screw conveyor through a crusher and into a slurry pipeline to get pumped to a separation plant on surface. The system does not incorporate a crusher in the excavation chamber as done in a standard slurry machine. The cutterhead tool configuration is designed to limit the particles that can enter the excavation chamber to a size suitable for the installed screw conveyor. The WCP mode as designed for the EPBM that excavated the Port of Miami tunnels is a simplified system or preliminary step in the development of the Variable Density TBM where a rotary crusher-slurryfier box was designed in combination with the screw conveyor outlet. The rotary crusher-slurryfier box has to be moved into a parking position before the belt conveyor can be put in operation.

Figure 2

Figure 2 – Functional Principles used for The “WCP – Mode” on the Port of Miami TBM.

Variable Density TBM, a new generation of multi-mode soft ground TBM

The demand of today’s specific project conditions where frequent changes of soils, rocks and mixed face conditions of soils and rocks at the tunnel face are more and more common and this beneath the groundwater table had led to the development – also for small to medium sized TBMs – to change between operation modes, e.g. from a slurry supported face to an earth pressure supported face and this with full control of the face pressure also during the transition between modes.

This new generation of multi-mode TBMs, the Variable Density TBM, combines the individual advantages of each system in one machine. The development of the Variable Density TBM was focused on the goal to change between the closed operation modes (slurry and earth pressure face support) in the tunnel without any need of mechanical modification in the excavation chamber or behind the gantry in the tunnel area. The Variable Density TBM can be operated as classic Slurry TBM with an air bubble system to control the face pressure and in a full EPB mode. The change between the modes can be done gradually under permanent and full control of the tunnel face pressure and without any need of chamber interventions. This machine can also be operated using a high density in the excavation chamber that would be too dense for classic slurry operation but that would be too fluid for a classic EPB operation.

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If the Variable Density TBM is fully equipped it would require two muck transportation systems in the tunnel. In a classic Mixshield mode (air bubble for active face pressure control) a closed slurry circuit is required and if operated in a full EPB mode (dry system) muck cars or continuous conveyors are required. The Variable Density TBM can also be operated with classic earth pressure support in the excavation chamber and closed conveyance in the tunnel, e.g. in contaminated soils. In dependence on the specific project conditions one of the two systems may be selected to be the high performance primary system and the other, the secondary system of reduced performance. For all operation modes of the Variable Density TBM the muck is extracted from the excavation chamber via a screw conveyor. The further processing of the muck depends on the operation mode in place and the choice of logistics for material conveying. This can be changed from hydraulic transportation through pipes to belt-conveyor transport or muck car haulage.

Figure 3

Figure 3 – Variable Density TBM, different tunneling modes for optimum adaptability in difficult soft ground.

First Use of Variable Density TBMs (Ø6.62m) for Klang Valley MRT Line 1 in Kuala Lumpur

The first Variable Density TBM was used for the 9.5-km long underground section of the first line of the Klang Valley MRT Project in Greater Kuala Lumpur. This Metro Line 1 will be operated when commissioned below the surface as twin tubes running parallel and as stacked tunnel tubes due to space restrictions in the city area. The tunnel was built in Kenny Hill and Kuala Lumpur Limestone formation. The soil structure of the Kuala Lumpur Limestone is demanding due to its characterization of highly erratic karst features with eroded limestone rock beneath a layer of top soil. For a total of 8.6 km, six Variable Density TBMs (Ø 6.62m) were supplied to MMC-Gamuda KVMRT. The remaining section of about 1 km was excavated using EPB shields. The Variable Density machines were specially adapted to the specific subsurface conditions with a potential risk of suddenly encountering cavities. Using then the liquid supported tunnel face principle with an automatically controlled support pressure through air cushion that enables a precise control of the face pressure it could happen that the normal bentonite suspension could drain off continually into the ground or up to surface. So the idea was raised to use a thicker and heavier suspension to balance the earth and water pressure at the tunnel face. Using thicker and higher density suspensions requires design adaptations such as a High Density Slurry Material (HDSM) mixing plant on surface where the high density material is prepared.

Based on the positive application of the Variable Density TBMs for the challenging underground sections in the Kuala Lumpur Limestone formation of Metro Line 1, four additional Variable Density TBMs were to be delivered to Kuala Lumpur mid and end of 2017 to excavate and line approximately 7 km of the new Metro Line 2.

Figure 4

Figure 4 – Variable density technology for Kuala Lumpur.

Variable Density TBM (Ø10.21m) for Metro Lima Line 2/4

The consortium Consorcio “Constructor M2 Lima” comprising ACS, FCC from Madrid, Salini from Milan and COSAPI from Lima purchased two Herrenknecht TBMs, an EPBM and a Variable Density TBM, for the construction of 35 km of new urban rail for the Lima Metro Line 2 in Peru. The two TBMs will excavate and line a major east-west axis (Ate-Lima-Callao) of the Lima-Callao Metropolitan Region.

One section of the new urban rail line of 11.6 km in highly variable geological conditions that comprise mainly coarse gravel with sand, silty sands and clay and silt will be excavated by means of a Variable Density TBM. The machine has a shield diameter of 10.21 m. The Variable Density TBM can be operated in EPB and slurry mode and can change between operation modes with a continuous setting of face pressure support according to the prevailing geology and this without any need of chamber interventions.

The TBM is designed with a double screw conveyor of DN1150. The twin screw arrangement has a flat gate between the first and second screw and a muck discharge gate at the end of the first screw for the discharge of muck onto a belt conveyor in pressurized or open EPB mode. In slurry mode the discharge gate of the first screw is closed and the flat gate between the first and second screw is open thus that the muck can be discharged into a slurryfier box (capacity of 36 cubic m) that is installed at the end of the second screw. The slurryfier box contains a stone crusher (jaw crusher) that reduces larger particles to a size suitable for liquid transport through the attached slurry circuit to the Slurry Treatment Plant at surface. In slurry and high density mode the muck transfer along the screw conveyor is a combination of a mechanical and hydraulic transportation.

In case of required simplified crusher maintenance the screw conveyor flat gate has to be closed and the slurryfier box can be accessed in free air.

Figure 5

Figure 5 – Variable Density TBM for Metro Lima Line 2/4

Variable Density TBM (Ø7.41m) for Hong Kong’s Shatin to Central Link Contract 1128

The Shatin to Central Link (SCL) in Hong Kong is a strategic rail line that stretches from Tai Wai to Admiralty. It connects several existing rail lines and passes through multiple districts in Hong Kong. When finished it will serve areas in East Kowloon that currently do not have any MTR service and will also strengthen the linkage between the New Territories and Hong Kong Island.

The Dragages-Bouygues JV has won the contract to construct 2×2 tunnels that will form part of a 6-km long extension of the Shatin to Central Link. Tunneling works comprise two eastern tunnels (up-track and down-track) of each approximately 590 m in length that will run from the south ventilation building and the new Exhibition station on the Shatin to Central Link and two western tunnels (up-track and down-track) of each approximately 510 m that will be excavated between Fenwick Pier emergency egress point and the existing Admiralty station. The up-track tunnels are deeper tunnels; the down-track tunnels are shallow tunnels. All tunnels are to be constructed in complex geological conditions that comprise very variable geology that is mainly composed of completely decomposed granite (CDG) with the presence of boulders, corestones and transition zones in mixed face conditions and sections of alluvium and marine deposits. These heterogeneous conditions along the specific sections demand the use of two different types of TBMs, a Mixshield with liquid supported tunnel face and the city’s first Variable Density TBM.

Three sections, the two western tunnels and the up-track tunnel of the eastern section are planned to be excavated using a Mixshield. The first Mixshield drive started operation in March 2016. The Variable Density TBM (Ø7.41m), was launched in August 2016 to excavate the down-track shallow tunnel section of the eastern tunnels. This section is characterized by large portions within fill material that is frequently composed of rock fill blocks in the previous temporary seawall zone. The Variable Density TBM can continuously adapt to the face density to deal with the demand of the predicted heterogeneous and shallow ground conditions. The TBM can be operated both in a slurry and high density mode. The latter mode is used to cope with the very variable geology associated with shallow cover of locally less than 1 diameter where possible risks such as blow outs and settlements might have been an issue in slurry mode. As the TBM can be operated also with HDSM the Slurry Treatment Plant on surface is accordingly designed to supply the TBM with HDSM.

Two Variable Density TBMs (Ø7.05m) for Forrestfield Airport Link in Perth, Australia

Salini Impregilo S.p.A. and NRW Pty Ltd. JV was contracted to construct the twin tube rail tunnels for the Forrestfield Airport Link project in Perth, Australia. The Forrestfield Airport Link is a new rail line including two twin tube tunnels that have a bored tunnel length of about 7.14 km. This new rail line will connect Bayswater Junction with Forrestfield. A major section of the alignment crosses beneath the Perth airport precinct. This includes sensitive area beneath the airport runways, taxiways and buildings.

The expected subsurface conditions along the bored tunnel alignment are characterized to be variable, sand, clayey sand and cemented layers with varying degrees of strength and cementation. The majority of the tunneling ground is expected to be composed of mixed face conditions beneath the groundwater table.

With focus on the predicted varying geological units and their varying strength the application of two Variable Density TBMs is considered. The TBMs with a diameter of 7.05 m will be operated along the entire tunnel alignment in closed pressurized mode to guarantee ground stability during excavation. The machines will be launched at Forrestfield dive portal, extending westward and ending at the Bayswater dive portal.

The Variable Density TBMs for the Forrestfield Airport project are configured to operate in closed EPB and in slurry mode with a filled excavation chamber and a controlled support of the tunnel face pressure. Both in EPB and slurry mode the muck is extracted out of the pressurized excavation chamber via the screw conveyor and further processed with liquid mucking respectively hydraulic material transport via a closed, pressurized slurry circuit according to the slurry mode or HDSM operation with a slurry treatment plant at surface. To support the hydraulic muck transport, the muck is transferred at the end of the screw conveyor into a slurryfier box to liquefy the excavated material. A roller crusher is installed in the slurryfier box that processes the material to a size suitable for hydraulic mucking via slurry circuit.

The two Variable Density TBMs for Perth started tunneling in July and September 2017.

In the past a large number of tunneling projects were successfully completed in sensitive areas. The machines applied to date show the highest technical and quality standards of mechanized tunneling technology mastering project challenges and individual tasks in the interest of customers, clients and the environment.


With the first introduction of multi-mode TBM technology in 1980s and the further development in multi-mode technology with the possibility to change between various operation modes in the tunnel, e.g. from open to closed EPB and slurry mode and also to change between the two soft ground modes, the foundation was laid for the new generation of multi-mode soft ground TBMs, the Variable Density TBMs. These machines can smoothly and with continuous setting of the face pressure change between the operation modes from EPB to slurry, even in small and mid-range TBMs.

A major technological advantage of these machine types is that the set face pressure can be kept during the transition of operation mode and the need for working chamber interventions is avoided. The number of applications of this new generation of multi-mode soft ground TBMs shows the advantage of having a safe solution at hand even for potential high risk areas to support a settlement controlled operation with less impact to man and environment. Most of today’s tunneling projects feature complex heterogeneous geology and environment with low overburden or the demand of crossing beneath important and sensitive structures. With the state-of-the-art Variable Density TBM technology these projects can be safely and reliably implemented.

Karin Bäppler is Head of Geotechnics and Consulting-Traffic Tunnelling, Herrenknecht AG; Frédéric Battistoni is Division Manager-Traffic Tunnelling, Herrenknecht AG; and Werner Burger is Manager of the Design Division-Traffic Tunnelling, Herrenknecht AG.


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