Ground freezing is a state-of-the-art construction method that significantly reduces the impact on the building site and the surrounding area. The process is used, among other things, when dense development or geographical constraints make drilling from the surface impossible. This was precisely the case in the project to extend subway line 5 to the Europaviertel district in Frankfurt am Main: in the highly sensitive, densely built-up city center location, the tunnel construction work had to be carried out without endangering the existing buildings or obstructing above-ground traffic.

In ground freezing, heat is extracted from the ground by supplying cold using frozen ground probes. The groundwater freezes and individual “ice cylinders” are formed, which increase in diameter around the frozen ground probe over time. By stringing these ice cylinders together, closed, watertight systems with an extensive static effect can be created. Calcium chloride brine (CaCl /up to approx. -37 °C) was used as the “conveyor” of the cold. The reason: for longer applications, ground freezing with brine is the more economical option. The relatively expensive coolant nitrogen is primarily used for short-term special cases. Once the construction work is complete, the ground thaws naturally. Apart from the injection lances, ground freezing leaves no artificial additives in the building ground.

Precision work Freezing boreholes: Drilling accuracy leads to success

Based on technical and economic considerations, a brine freezing unit with a cooling capacity of 140 kW was invested in for this freezing measure. Freezing boreholes were drilled to “bring the cold to the ground.” This was the most difficult part of the task, as very high demands were placed on the accuracy of the boreholes and the tightness of the drilled boreholes. In addition, the drilling had to be carried out against pressurized groundwater, which had to be done using preventers and standpipes that had to be installed beforehand. This required precision work – many work steps and very confined working spaces made the work difficult.

Innovative special solution: “Active cooling” reduces the freezing time

In the U5 construction project in Frankfurt, the tunnel boring machine virtually drove into a funnel created from previously frozen soil bodies, which had a special collar-shaped arrangement of the freezing pipes. Active cooling, which additionally utilized the residual cold from the brine that had already been “used” and thus warmed up from the freezing pipe return, was installed on the surface of the stop wall and supported the freezing process. The freezing time until the required temperatures or ice body diameters were reached was around 50 days. With the successful ground freezing, the ground was sealed and solidified so that the mining tunnel could be safely excavated using compressed air. Two redundant cooling circuits and an emergency power supply ensured that the freezing process was secure at all times.