Multi-mode TBM

Multi-mode TBMs in operation

With their specific methods, in particularly variable geologies along the tunnel alignment, conventional shield tunnelling machines can reach their technical or economic limits. Within a tunnel alignment to be excavated, the ground can change over longer sections, for example from stable rock to soft, water-bearing unconsolidated rock and vice versa. Such alignments are among the most complex challenges there are in tunnelling. To ensure maximum safety and flexibility throughout the advance, Herrenknecht has developed so-called Multi-Mode tunnel boring machines for such cases. They are designed to be flexible in terms of their support and excavation methods. The tunnelling mode can be adapted to changing ground conditions, with relatively short conversion times and minimal effort. With Herrenknecht's innovative Multi-Mode TBMs, tunnels can thus be built safely and economically even under varying geological and hydrogeological conditions.

One basic idea, two concepts

In mechanized tunnelling, a fundamental distinction is made between three different shield types: single shields, earth pressure balance (EPB) shields, and slurry-supported shields. Each of these proven methods has advantages in its particular area of application. In recent years, technological advances have further expanded the respective range of geological applications. The aim is to use the optimal machine technology for the prevailing geology and thus optimize the economic efficiency of each project.

Many tunnel alignments pass through highly variable ground conditions where conventional methods reach their technical or economic limits. For these special cases, Herrenknecht engineers have developed the so-called Multi-Mode TBM. The basic concept allows switching between slurry support, earth pressure balance support and open mode by means of various conversions within a tunnel section. Fundamentally, two design implementations are possible.

• Multi-Mode TBM with a modular basic structure: a change in the tunnelling method inside the tunnel is achieved through extensive modifications of individual assemblies.
• Multi-mode TBM with components for multiple tunnelling methods integrated in parallel: changing the tunnelling mode is possible with relatively little effort and time. These machines are much more complex technically.

1. Combination of open mode (Single Shield) and slurry-supported TBM

Multi-Mode TBMs with open and slurry-supported mode have already been used successfully in many projects. The biggest challenge when changing the tunnelling mode lies in the different ways of conveying the muck. While a slurry circuit is installed for the slurry-supported shield, in open mode the muck is removed using a central belt conveyor with a retractable muck ring. To implement this combination, both conveyor systems are therefore installed on the TBM, the back-up and in the tunnel.

2. Combination of EPB and open TBM

EPB Shields with a screw conveyor in the invert area can also be operated in open mode in addition to the conventional closed EPB mode. The excavation chamber and screw are only partially filled. In this mode, no active support pressure can be built up against the tunnel face; the screw conveyor is only used to transport the muck. Alternatively, a belt conveyor with a retractable muck ring can be installed in the center. This requires additional modifications to the cutting wheel, including the installation of material chutes, and the screw is retracted in open tunnelling mode. The combination of EPB Shield and open TBM can be designed with either a modular basic structure or with a screw and central belt conveyor installed in parallel.

The special case of the TBM Variable Density®

A Multi-mode TBM with EPB and slurry-supported mode is the most complex form of a convertible machine so its use is only cost effective in very special cases. For this reason, Herrenknecht has developed the Variable Density TBM which applies a totally unique tunnelling technology. It combines the advantages of both methods in one machine. Without major mechanical modifications, the machine can switch between four different tunnelling modes directly in the tunnel. This means that geological and hydrogeological changes along the alignment can be managed with extreme flexibility. 

Gearbox for the optimal combination of EPB and open TBM 

High advance speeds for open drives require high cutting wheel speeds in order to repeat the penetration of the disc cutters as often as possible.  Earth pressure balance operation with the EPB Shield also requires high torque to overcome the friction of the pasty chamber filling. An economical solution for this is the use of gearboxes in the cutting wheel drive.