Current Projects / By-pass Sissach - Chienberg road tunnel:
Provisions against heave

Rock containing clay minerals and anhydrite increase in volume (swell) due to water absorption. The swelling results in a build up of pressure in the bottom of the tunnel lining.

Under low overburden conditions (10-50 m) pressures at the bottom of the tunnel lining could cause the tunnel to heave, resulting in vertical displacements at the ground surface above the tunnel. Additionally, damages to the tunnel lining could occur because of varying degrees of heaving along the tunnel axis (primarily in the lower part of the cross section).

Significant heave in the 60 and 370 m sections of the Chienberg road tunnel near Basel (Switzerland) occurred before the tunnel was even finished. The observed heave rates and resulting heave damage on the tunnel lining were so serious abandonment of the tunnel project was discussed.

It was decided to use the "Modular Yielding Support" method developed by Prof. Dr. K. Kovári (Zürich) after reviewing numerous proposed solutions. Figure 1 shows the application of this method in a schematic manner. The existing bottom lining was removed in stages and the tunnel profile was extended to form a flat base. The newly concreted sidewalls of the tunnel were supported by hiDCon-elements (ø 90 cm, h=100cm) placed on a foundation.

The hiDCon® foundation-elements reduce the swelling pressure on the tunnel lining and simultaneously allow a limited amount of heave in the bottom of the tunnel. Pre-stressed 25 m long anchors with tensile strength of 1750 kN were built into the flat tunnel base. Cylindrical hiDCon - elements were placed between the natural rock and the anchor plates to permit the floor to heave in a controlled manner.

Finally the concrete carriage-way was attached to the existing tunnel lining as shown in Picture 2. The space beneath the carriage-way is accessible anytime providing the possibility of maintenance - even with construction machinery - without interference of the traffic. When the deformation range of the hiDCon® foundation and anchor-elements is reached the floor can be re-excavated and the elements replaced.