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Stump-Franki applies innovative construction technique: “ground freezing” in action for the extension to the U5 U-bahn line in Frankfurt am Main

“Build safely on every foundation” is Stump-Franki Spezialtiefbau’s service promise. Meeting this promise sometimes requires considerable creativity from the construction experts. This was certainly the case for the extension of the U5 U-bahn line in Frankfurt am Main, a complex project in a highly sensitive city-centre zone. Their solution involved a technology which has been tried and tested in the past, but remains an innovative approach: ground freezing.

Bauvorhaben U5 Tunnelblick

Construction project on Frankfurt’s U5 U-Bahn line


What is ground freezing? In this technique, groundwater is frozen by technical means, solidifying the subsoil and making it fully impermeable. The ice content in the subsoil makes it very stable and protects it against water ingress until these functions can be taken over by the structure being built above it. Once construction is complete, the subsoil can be left to thaw naturally. Other than the injection rods, ground freezing does not leave any artificial additions to the subsoil.

Ground freezing at the connection to the existing U5 U-Bahn tunnel

As part of the work to upgrade line B in Frankfurt’s U-Bahn network to double track, the ground freezing technique was used where the construction site connects to the existing U-Bahn tunnel below Platz der Republik. This method involves drilling special rods into the soil and running refrigerants through them to extract heat from the soil, freezing the groundwater and creating “ice cylinders” which gradually widen around the freezing rods. Rows of these expanding ice cylinders eventually merge to form closed watertight systems with strong structural qualities. A calcium chloride solution (CaCl) was used as the refrigerant for the Frankfurt U5 project. Using a solution is one of the most economic refrigerant options if the ground is to be kept frozen for a longer period of time.