U5 Europaviertel, Frankfurt am Main

Soil freezing works were required for the double-track extension of Frankfurt’s underground rail network in the connection area of the existing tunnel at Platz der Republik. Soil freezing is a state-of-the-art construction method that significantly reduces disturbance to the site soil and surrounding areas. This procedure is used in a variety of circumstances, including where dense construction or geographic constraints make drilling from the surface impossible. This was exactly the scenario with the extension project on underground rail line 5 in Frankfurt am Main’s Europa district: in the highly sensitive, densely built-up inner-city location, tunnel construction works could not be permitted to endanger existing buildings or restrict surface traffic.

Soil freezing involves the use of freezing lances drilled into the soil. These lances inject cold and extract heat from the earth. The groundwater then freezes until individual ice cylinders are created, growing over time to match the diameter of the freezing lances used. These ice cylinders can be lined up to create capped, waterproof systems with extensive static effects. Calcium chloride brine (CaCl / to approx. -37°C) is used to transfer the cold. For long term solutions, brine is a more economical choice in soil freezing. Nitrogen, a relatively expensive cooling agent, is primarily used for short-term special situations.

Precision freezing boreholes: accuracy leads to success

Technical and economic considerations dictated the use of a

brine-based freezer unit with a refrigeration capacity of 140kW for this project. Freezing boreholes were drilled to enable the cold to be transmitted into the soil. This was the most difficult part of the process, as very high precision was required in calculating borehole variations and ensuring the impermeability of the inserted drill strings. Preventers and previously mounted standpipes were used to enable drilling against the pressure of the groundwater. Extreme precision was essential, with multiple work processes and extremely limited room to work adding to the difficulty of the process.

An innovative special solution: active cooling reduces freezing time

On the U5 construction scheme in Frankfurt, the tunnel boring machine was virtually drilling into a funnel composed of the previously frozen body of earth. This effect was achieved via a specially shaped flange on the freezing tube. Active cooling, which recycles the already “used” and warmed brine from the freezing pipe return flow, was applied to the surface of the striking wall to assist the freezing process. It took around 50 days before the required temperature was achieved throughout the affected soil and a frozen body of sufficient diameter was created. Once the soil was successfully frozen, our experts were able to seal and compact the soil so that the tunnel boring process could be safely executed. An uninterrupted freezing process was ensured at all times with two redundant cooling cycles and an emergency backup power supply on hand throughout the process.