The underground operation carries out mining of large-profile tunnels using full-face tunneling machines (TBM). Metrostav owns a complex array of machinery, enabling mechanical drilling of an excavation of a circular cross-section of the required size into the advance core and ensuring it against caving in.

This operation also carries out the conventional way of tunnel construction, the “New Austrian Tunneling Method” (NATM), which is characterized by its work organization or the Drill & Blast method.

New Austrian Tunneling Method – NATM

NATM is a tunneling method that deliberately and purposefully uses the load-bearing properties of the advance core to optimize the mining process and secure the excavation and to minimize the associated economic costs. During construction of tunnels using NATM, the stability of the excavation is usually ensured by the primary lining and the definitive construction of the tunnel tube (secondary lining) is built only after the stress-strain state stabilization around the excavation.

The main structural elements of the primary lining are sprayed concrete and the anchorage system. An integral part of the NATM is geotechnical monitoring based mainly on deformation measurements of the tunnel excavation. NATM belongs to a group of observation methods based on a geotechnics perspective, where the course of construction is continuously monitored, and the method of mining and excavation securing by the primary lining are adjusted according to the actual behaviour of the excavation and the advance core.

Tunnelling with Tunnel Boring Machines (TBM)

Tunnel boring machines are efficient mechanisms that can be used for continuous ground breaking in the entire tunnel face without the use of blasting and at the same time to almost continuously build a tunnel lining. In fact, even the tunneling machines break the ground per the rounds given by the maximum possible extension of the axial thrust presses, and after exhaustion of the extension, it is necessary to relocate the machine, or the gripper pistons. At this stage, ground breaking in the face is interrupted.

In solid rocks, open tunneling machines are used, equipped with grippers that stabilize the machine and provide support for machine thrust into the face by means of axial presses. If necessary, the excavation is provided by a bolt reinforcement combined with sprayed concrete. For non-solid, faulty and generally problematic rocks or in the soil, tunneling machines with the protective shield are used, to ensure the stability of the excavation during mining and to allow the safe execution of the final reinforcement of the tunnel. The excavation is secured by lining from segments folded into individual lining rings. At present, prefabricated reinforced concrete units are predominantly used, but steel fibre concrete units are also increasingly used. Soft rocks and soils do not allow the use of grippers, and therefore the machine is supported by axial presses on the last ring of the segment lining during tunnelling.

Drill & Blast Tunnelling

This method of mining is based on the principles of the Norwegian tunneling method, which has proved to be very efficient and successful over the last forty years. Mining is usually performed using blasting, in exceptional cases also with machine ground breaking. The convergence measurement is not systematic, it is carried out mainly in fault-zones.

Another basic aspect of the Drill & Blast method is the use - as temporary reinforcement – of bolts and sprayed concrete with scattered reinforcement (steel wires or polypropylene fibres). The temporary reinforcement is not separated from the final lining and it forms one unit. The final reinforcement usually consists of another layer of sprayed concrete, and addition of bolts into a defined raster. The reinforcement design of the work is carried out round by round on the basis of a Q-system. The Q Index is determined on the basis of an empirical system that uses the RQD Index supplemented by five other parameters, such as the number of crack systems, cracks misalignments, alteration of discontinuity surfaces or fillings, water pressure, and conditions of the pressure manifestations of the advance core.

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