Comparison of Cut-and-Cover Tunneling Method vs. New Austrian Tunneling Method (NATM) for Urban Tunnels with Shallow Overburden
TU Wien
ABSTRACT
In urban tunneling both the New Austrian Tunneling Method (NATM) and the Cut-and-Cover Tunneling Method are possible options, if shallow overburden and favorable geology for either method exist. Typical applications are subway or metro running and station tunnels. While running tunnels are often built using a TBM (Tunnel Boring Machine), NATM and Cut-and-Cover techniques are common worldwide today for station construction.
The Cut-and-Cover Method entails many disadvantages in urban areas, resulting especially from surface interruptions. Both methods differ in terms of surface settlement, restrictions with the alignment, direct and indirect construction cost, construction risk and other. A risk comparison example in this thesis shows, that Cut-and-Cover Tunneling is not necessarily less risky - as might be expected - but may involve much higher risks than NATM.
Historically, NATM was always been considered to be costlier than Cut-and-Cover Tunneling, if the tunnel is situated at a depth of not more than one and a half times the tunnel height. Studies have shown, that NATM can be competitive even above this depth. By counting all the costs like utility relocation and reinstatement of the surface, NATM is usually the cheaper solution.
As an example, a subway station was designed in Open-Cut, Top-Down and using NATM. The quantities of the main materials were compared, construction schedules were developed and cost estimates prepared. The results show, that the Cut-and-Cover options have a higher demand of reinforcement, excavation and backfilling material. The construction duration is shorter with the NATM, not only because of shift-work, but also because less time is needed for relocation of utilities and backfilling works. Concerning the construction cost, NATM is the more favorable method in this example.
This thesis does not only explain the different advantages and disadvantages of the two construction methods. By citing international studies and real world examples and by developing a simple calculation example it also illustrates that the NATM represents an economic and advantageous alternative even for tunnels in urban areas and with shallow overburden.
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