Ground freezing: An efficient method to control the settlements of buildings

The construction of the new soccer stadium in Munich-Fröttmaning made it necessary to extend the platforms of the subway station U6 "Marienplatz" below the historical town hall of Munich, as up to 32 000 passengers per hour use this station. For this, two new tunnels for pedestrians were driven parallel to the existing subway tunnels. They were planned as shotcrete construction and driven under atmospheric conditions. In order to reduce the ground displacements, the tunnels were driven below caps of artificially frozen soil. As frozen soil has higher stiffness and bearing capacity compared to its unfrozen condition, the displacements could be reduced by use of this frozen soil cap. However, the well known - but difficult to quantify - phenomenon of frost heave could lead in unscheduled additional displacements, which might reduce or even nullify the positive effects of the frozen cap. Therefore the magnitude of the frost heave displacements had to be predicted prior to undertake the construction. The following steps were taken to analyse the problem at hand: - Laboratory tests to establish bearing-capacity, creep-behaviour, and frost heave response - Finite-Element calculations - Installation of a monitoring system (automatic levelling system) on the construction site - Strain gauge measurements in the shotcrete lining of the tunnel.