In soft rock or ground with minimal cohesion, conventional tunnelling with an excavator and shotcrete support is generally regarded as the economically superior method as relatively small investments are needed. However, this advantage is often overshadowed by low advance rates and unfavourable working conditions.

German manufacturer, Tachus has recently developed an automated tunnelling system, the ‘Tachusmatic’ as an alternative. A doubled advance rate to NATM is estimated, as a result of synchronised processes. Within certain restrictions, the system can also be adapted to suit various types of tunnel cross sections and the cost can therefore be written off over several projects, meaning investment is moderate.

The individual modules of the Tachusmatic are sufficiently well known from other techniques. The innovation stems from the new construction process optimisation and a high degree of automation.

The cutter removes the tunnel contour and thereby produces a preliminary profile, approximately 1m long, in advance of the actual tunnelling operation. The machinery is enclosed longitudinally by six to eight segments, depending on the diameter, known as slipformers (figure 1). As soon as the preliminary profile is of sufficient size, a slipformer is driven forwards into it. A chamber is thereby created, bounded by the slipformer, the rock and the concrete from the previous concreting cycle (figure 3). This chamber is immediately filled with steel fibre concrete, so the area surrounding the slipformer needs only stand unsupported for a short period of time. As soon as the concrete support is installed around the whole circumference, the cutter removes the remaining tunnel face (figure 4).

Mounted on the slipformers are different shields, which vary according to their function. The first shield serves as boxing for the new concrete, and thereby encloses part of the concreting chamber (concreting shield); the shields situated further back are braced into the concrete tube (spreader shields). At the same time they apply an even resistance of approximately 100KN/m2. The second shield (steering shield) is also braced against the concrete tube, but it is this shield that applies the steering forces for driving curves.

At first glance curve-driving may seem impossible, due to the tunnelling system’s longitudinal direction and permanent tension in the concrete tube. But, the solution is as follows: The cutter is attached to the machine frame and drives the tunnel parallel to the axis of the frame. Arranged between the frame and the slipformers are hydraulic aggregates which enable the frame to shift against the slipformers in such a way that the frame points in the new direction. As each slipformer is then pushed forward into the preliminary profile it is adjusted back parallel to the machine frame. Afterwards, the shields on the slipformers are braced against the concrete tube. The system compensates for the over-cut, which is created by all systems during curve driving, by radially fitting its shields (into the curve) with the help of hydraulic cylinders.

Once the support in the tunnel side walls and the ridge is installed around the whole circumference and the tunnel has advanced by one cycle, the machine frame and the floor-former are pulled along. In the process, the friction between the slipformers and the concrete tube serves as an abutment. The floor-former can be lifted, as all the forces (as well as the net weight of the machine) are carried by the hydraulic cylinders in the spreader or steering shields in the lower side-wall-formers.

The tip of the floor-former is shaped like the shovel of a wheel-loader, so that in a similar fashion to a hard rock machine it can pick up drilling debris found in the invert and transfer it to conventional scraper chain conveyors or conveyor belts.

Support

The concrete support requirements relate to primary support NATM. Secondary support is installed in a separate procedure. Once the tunnel face core has been removed (figure 4), the concrete is installed from the bottom up; in the tunnel side walls, followed by the crown and finally the invert. The concreting chambers in the lower tunnel side-wall-shields are closed off by ‘concrete flaps’, allowing the concrete level to rise freely. Rock debris is prevented from falling into each chamber by a ‘dirt flap’.

Since, at the assumed maximum rate of advance, it takes approximately one hour to complete one full advancing and concreting cycle, the slipformer needs to be lowered, moved forward into the newly cut profile and re-braced after about an hour and a half. Therefore the main requirement of the concrete, is for it to have reached a hardness of 1.5MN/m2 after this period, so that it can carry its own weight without being supported. After approximately 12 hours, when the supporting effect of the Tachusmatic is lost, the concrete must have reached a hardness of approximately 20MN/m2. A further requirement of the concrete lies in its workability before the curing process begins. The concrete needs an open time of approximately 30 minutes to be sure that the curing does not start while the concrete is still in the feed line. Generally there are two ways to obtain this; employing a fast cement or using a liquid accelerator. The advantages and disadvantages of each method must be weighed up based on the job specifications.

When employing a fast cement, a specially prepared cement is used which has the required setting behaviour without additional additives. When using this method the concrete must be mixed on a dolly, as its rapid setting behaviour means it has to be used immediately. When using liquid accelerators, the concrete can either be pumped ready-mixed (without accelerant) directly to the machine, or it can be mixed on a dolly, depending on circumstances. The liquid accelerator is added to the concrete before it reaches the concreting chamber in the ‘Wet-in-Wet Mixer’.

The Tachusmatic utilities dolly supports either a transfer station for the muck material, or silos with the required concrete components and equipment (batch mixer, etc.) for production of the concrete. Should the concrete be pumped ready-mixed from the portal directly to the machine, then the dolly supports amongst other things a silo of ready-mixed concrete as a buffer, as well as containers of accelerant.

The required geological resistance can be established alongside construction by monitoring the face and taking readings of force and deformation of the hydraulic cylinders. Depending on the required shell gauge, the cutting head can excavate varying chamber sizes, allowing concrete thickness to vary.

Should the telemetry suggest that the cast concrete tube alone is not capable of the required resistance, ground anchors can be installed at the end of the tunnelling unit. The tunnel face is naturally supported in that it is removed at an angle of 60° to the horizontal. Following this, crown support is installed in advance of the invert support. For a typical subway tunnel this means an estimated advance of around 4m. Additional advance support measures, such as bars or forepoling irons, are unnecessary.

Should the geological conditions turn adverse an additional shotcreting unit is at hand to secure the exposed rock face with shotcrete before the slipformer is driven forwards. This helps to simulate the load-bearing performance of a standard tunnelling set-up, but advances the tunnel sidewalls first without having to alter the building processes on site. According to static calculations it can be shown that tunnelling can be carried out even in geological conditions with an angle of repose of 20° and cohesion of 10KN/m2 without the need for shotcrete. Another advantageous feature is the early floor closure that takes place about 1m from the tunnel face. A drilling rig is also mounted in the machine, used for probing. Should, in exceptional cases, additional measures be necessary these holes can be grout-injected.

The automation system

A wide-ranging automation system, which permits reduction of personnel to about four, is Integrated into the machine. However, it is possible to switch to manual mode and drive the cutter using a joystick. In this mode the control system prevents the operator from driving the cutter into other components and damaging or destroying the machine. By allowing the operator to manually intervene into the standard cycle, more flexibility is gained, especially in difficult ground. Through the concept of an integrated automation system, all building process data can be collected in a database. Analysing this data enables the user to counteract logistical bottlenecks at an early stage or to optimise the production process.

The Tachusmatic has been conceived to form the central element of a building block system. Technological adaptations to the required tunnelling approach are built up around central features. Delivery times are short in comparison to standard shield machines or TBMs. Various configuration designs have been collected in a database, that can be used during process planning and construction. To a large extent ‘Tachusmatic’ assemblies can be used repeatedly, cross-sectional dependent parts which exist as 3D CAD drawings filed in a EDM system can be scaled to size according to the geometrical conditions. Due to the time-dependent costs of construction sites, availability of spare parts at short notice are crucial. The Tachusmatic is built up as a kit, with a high level of standardisation making it easy to obtain parts immediately.

Conclusion

From a tunnel length of 750m upwards and bearing in mind that the Tachusmatic can be adapted to various cross-sections by adding or removing slipformers, or even to short tunnels featuring a constant cross-sectional geometry, Tachus believes that estimated savings will compensate for the total investment or rental cost of the machine when compared to using NATM techniques.

Related Files
Figure 3
Figure 5
Figure 4
Figure 2