Safety Aspects of Flexible New Austrian Tunneling Method Versus Stiff Shotcrete Linings

When driving large underground cavities such as metro stations in urban areas, the method of excavation and the type of lining must be adjusted to minimize the surface movements. Two approaches can be envisaged—a flexible lining or a stiff lining. These two approaches can be associated with the original new Austrian tunneling method (NATM) concept (flexible lining) and the sprayed concrete lining (or stiff lining). On the basis of the historical background and definition of these two concepts, typical site movements encountered at two underground stations located under the same soft rock conditions were studied. A theoretical analysis was undertaken to try to explain the reasons why the interaction rock lining support behaves in the way observed.