When Cables Get Wet Feet
The German Telekom AG maintains a man-sized supply tunnel in the Stuttgart city for the distribution of data lines between various locations which is approx. 1.7 km long.
Extensive repair measures have been implemented on the joints of the precast concrete elements in order to protect the cables from penetrating humidity. It was decided to implement an injection procedure to seal the defective joints for the tunnel's repair. Drill holes were made into the joints via lateral drill channels without damaging the existent sealing profiles. These profiles had an insulating effect during the injection because they prevented the injection material from being washed out.
Only large leaks were patched with epoxy putty. Depending on the respective requirements, the rigid WEBAC® 4505 and the flexible WEBAC® 4520 were used. Injection packers were screwed into the drill holes and sealing material injected into the joints. Two different methods were used: The PU injection resin WEBAC® 1403 was injected into cracks and narrow pressed joints. Upon contact with water, the material forms an impermeable pore structure which is capable of absorbing slight movements of the construction component.
The large number of partly highly sensible cable looms in the supply tunnel was a particular challenge for the company in charge of the repair (BST Erich Kühfuß GmbH, 71732 Tamm/Germany).
The use of electric pumps, solvents and cleaners was prohibited due to the risk of explosion. Therefore compressed air driven pumps were used, and the compressor for the air supply was installed outside the shaft. The pumps used for the injection of the two-component PU injection resin were cleaned with the A-component of the respective resin. This way no solvents at all were used in the shaft. The polyacrylic gel WEBAC® 240 was used to fill and seal wide joints and areas with large cavities. This three-component material is characterized by its high elasticity and flexibility. Its short reaction time prevents the material from flowing into the adjacent soil in an uncontrolled manner. A sealing gel curtain formed behind the respective joint. The repair procedure took 9 years altogether and comprised two construction sections. A total of over 3,000 defective joints was repaired.
The second construction section was inspected and accepted by the client so that the cable shaft can now be accessed without getting wet feet.