AlpTransit - Lötschberg Base Tunnel

The use of load-bearing yet at the same time yielding sprayed concrete linings in tunnelling is based on the following basic principle: In squeezing rock the rock pressure decreases with increasing rock deformation.

The practical application of this knowledge is found in the use of the system of a sprayed concrete lining with integrated deformable hiDCon elements, whereby a load-bearing but yielding lining is obtained, thereby reducing the rock pressure.

The geometry and number of elements in the cross section determine the allowable controlled radial displacement that the sprayed concrete lining can withstand with a given lining resistance without overstressing. Once the compression range of the elements has been exhausted, the lining exhibits normal shotcrete characteristics, and now provides the full resistance of the sprayed concrete lining.

Figure 1 shows the sprayed concrete lining with beam-shaped hiDCon elements in the Lötschberg Base Tunnel in Switzerland. In the detail (Figure 2) it is possible to see that the elements are installed between two steel arches. The connections of the steel arch segments consist of sliding locks.

1) Yieldable shotcrete lining		2) Elements after installation		3) Deformed element

Even with large convergences the sprayed concrete lining is protected from overstressing by the deformation capacity of the elements (see Figure 3).

In the Lötschberg Base Tunnel, in a zone of highly squeezing rock with a great height of overburden it was necessary to install hiDCon beam elements over a tunnel length of 180 metres.