PhD student Rodolfo Javier Williams Moises from the Department of Statics and Dynamics explains why it is advantageous to deep-freeze a tunnel construction site for a short period of time in the current issue of the science magazine Rubin.

While the benefits of tunnels are huge, as they make travelling easier, tunnel construction is still subject to particular challenges, such as insufficiently load-bearing subsoil or groundwater flows. Ground freezing is often used for temporary stabilisation and sealing of the ground. This involves placing 30 to 100 metre long freezing lances in the ground. "A cold fluid flows through them constantly, causing the temperature around them to drop so much after a while that the pore water in the soil freezes into ice. Either brine or liquid nitrogen can be used as a refrigerant for ground freezing. In brine freezing, a salt solution circulates in a closed circuit, which usually creates a freezing pipe temperature between minus 40 degrees Celsius and minus 25 degrees Celsius," says Rodolfo Javier Williams Moises. It can take up to four weeks for the soil to freeze sufficiently.

The freezing lances are often positioned in a semi-circle around the planned tunnel, which is slightly larger than the tunnel cross-section. Prof Dr Günther Meschke, Head of the Chair of Statics and Dynamics, explains: "The tunnel is then driven inside the frozen zone, only touching its inner edge. This allows you to work in a kind of protective ice shell." With this environmentally friendly method, groundwater flows inside the ice shell can no longer disrupt the tunnelling process and any subsidence and thus damage to buildings due to the frozen ground can no longer be caused.

The research work is currently focusing on the simulation of potential risks associated with icing on tunnel construction sites, particularly in mechanised tunnelling. "Because this process takes place automatically without being able to see the conditions on site - unlike in mining construction - we want to use simulations to find out what possible conditions exist that could influence icing," says Rodolfo Javier Williams Moises. The Chair of Statics and Dynamics is the first in the world to realise such simulations on a complete model. All possible influencing factors are taken into account for the simulation.