Tunnel Business Magazine

FEB 2018

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Page 15 of 43

TUNNELINGONLINE.COM TBM: TUNNEL BUSINESS MAGAZINE // FEBRUARY 2018 F E A T U R E S T O RY 1 6 T he interest in today's underground tunnel construction projects, in par- ticular for clients and project finan- ciers, 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 au- thorities and finally the confidence and ac- ceptance of large-scale projects among the public. That's how flexible, future-orientat- ed solutions such as mechanized tunneling technology are becoming increasingly im- portant 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 in- creasingly planned in heterogeneous geol- ogies with sections that can comprise solid rock conditions, soft and water-bearing soils and/or mixed-face conditions com- posed of rocks and soils. Such conditions demand a specially adapted machine de- sign to safely and reliably excavate and line the tunnel without the need for long con- version times. This paper highlights the new genera- tion 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 pres- sure, and without the need for excavation chamber interventions. This new machine generation is called Variable D and offers maximum safety and flexibility in the choice of tunnel face support and discharge of muck. Development of Multi-mode Soft Ground TBMs The first multi-mo d back in the 1980s for a tunneling project that was built in variable ground condi- tions 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 matu- rity of today's available machine design, the Variable D , W. Burger, 2016). In 2000, a multi-mo cess- fully 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-mo t was used for the construction of this tunnel had a diameter of 11.56 m and was at that time the first innovativ t could be operated in EPB and slurry mode. The tunnel was built by three of the big- gest construction and road industry com- panies in France – Vinci, Eiffage Construc- tion 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 f eration with a slurry supported tunnel face. The oncept for the project considered in particular that the respective geological formations occurred in long as- sociated sections. Both slurry shields and EPB shields are operated with a filled exca- vation chamber and a controlled support pressure for the tunnel face. The major dif- ferences 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 pres- sure 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 con- veyor 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 han- dling systems in the excavation cham- ber 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 gr or the Socatop project allowed the parallel ar- rangements of both muck removal systems in the invert area of the excavation cham- ber with some minor functional compro- mises. If slurry operation was required in prevailing geological conditions with pos- Variable Density TBM – Combining Two Sof t Gr ound TBM Technologies BY KARIN BÄPPLER, FRÉDÉRIC BATTISTONI AND WERNER BURGER Figure 1 - Multi-mode TBM for Socatop, slurry and earth pressure mode.

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