ZTM Warsaw Public Transport Authority reports the TBM Anna already bored almost a kilometer of metro tunnel and lined it with concrete rings from Karolina - towards the Chrzanów station. In the coming days, its twin colleague - the Krystyna TBM will set off on a similar journey.

The TBM Anna shield is a real colossus. The cutting head alone weighs 55 tons, and the entire machine about 600 tons. The length of the Warsaw machines is 97 meters, of which 12 meters is the head itself. The cutting shield has a diameter of 6.26 meters.

This large, underground "drill" will reach the C2 Chrzanów station at the turn of September and October. Just before breaking through the diaphragm wall of the future station, the shield will stop in a specially prepared block (so-called plug-in) made of soil compacted with a cement mixture. In the meantime, the personnel responsible for the drilling will start work with the second shield, Krystyna. The actual breakthrough to the Chrzanów station will be performed by the Anna TBM during the Krystyna shield's technical stop. During such a stop, the conveyor belts carrying the mined material are extended, among other things. After breakthrough to the Chrzanów station, the Anna TBM will undergo an inspection, be moved along the entire length of the station and wait there for its colleague - until Krystyna reaches the Chrzanów station. Then the machines will move towards the Lazurowa station.

Two revolutions per minute

The M2 line in Warsaw was bored by four TBM shields from the German company Herrenknecht AG - Maria and Elisabetta and - working on the Bemowo section of the M2 line - Anna and Krystyna. The choice of the TBM (Tunnel Boring Machine) technology, which uses a machine that simultaneously digs and lays subsequent tunnel rings, is dictated by, among other things, the very low risk of ground subsidence.

The shield maintains the stability of the so-called face and prevents soil and groundwater from getting into the tunnel under construction. In addition, the TBM can also excavate underground tunnels under industrial and residential infrastructure that is sensitive to ground movement.

TBM shields operate using the Earth Pressure Balance technique. It consists of two phases: tunnel excavation and installation of its lining. The TBM excavates the tunnel using a front cutting disc rotating twice a minute, which is moved forward by hydraulic cylinders. The disc pressure on the ground is 3.5 bar.

The blades, knives and cutting discs located on the excavation disc are made of very high-strength steel, thanks to which they can crush any material they encounter. Using nozzles located at the front of the disc, the machine can also inject water, hardeners, foam or concrete into the ground, which, depending on the characteristics of the ground, soften it or compact it.

First it excavates, then it installs

And what happens to the material from the borehole? The material cut by the shield is transported by a screw conveyor to the belt conveyors, from where it reaches the tank in the station body. Then the boring shield and the screw conveyor stop, and the module laying the tunnel ring is started.

The ring consists of 5 elements and a so-called wedge, which is attached last and provides the structure with tightness and stiffens the tunnel.

After laying the ring, the machine pushes away from it and starts drilling the next section. Thanks to the covers, the interior of the shield is completely tight and safe.

Anna (and soon also Krystyna) is controlled on the command bridge, where all data and parameters of the shield's operation are collected and analyzed. The speed of tunneling by the TBM is about 15-20 m per day.

Power consumption like in a town

The TBM shield consumes 2.5 MW/h, which is as much energy as a city of 15 thousand people needs in the same time. The main drive of this huge "drill" is a hydraulic drive in the form of 16 pairs of actuators. The installed power is 1200 kW.

The machine is also equipped with sensors: six sensors at the front of the shield, two sensors at the top of the shield, two sensors in the middle, two sensors at the bottom of the shield. They record parameters such as: the torque of the cutting head, the force of the actuators, the speed of the TBM movement, the pressure in the working chamber, the volume of foam in the working chamber and the cement mortar "injected" behind the housing.

People and machines

Once started, the shield does not stop - it works 24 hours a day. The only reason for stopping work may be a problem with the encountered ground, a machine failure or the replacement of cutting blades.

The shield is fully mechanized, but its work must be supervised by personnel - four teams of 12 employees each. In total, this is 48 people (four shifts a day).

The progress of the TBM shield excavation is updated every Wednesday on the Warsaw Metro website. As of this morning, the Anna TBM has excavated 922 meters of tunnel behind the C1 Karolin station and laid 610 lining rings.

source: ZTM Warsaw Public Transport Authority.