Using pea gravel instead of the usual cementitious grout to fill the annulus gap between the ring structure and natural ground, the Ibbenbüren Mine Dewatering Tunnel project presented significant challenges. These included controlling the clean bedding of the ring, which was indirectly assessed through minimal or non-existent convergences. While innovative, the use of pea gravel could have potentially caused issues in maintaining the stability of the ring structure, but these were successfully overcome through careful monitoring and adjustment of the RCMS.
The Ibbenbüren coal mine, one of the last remaining anthracite coal mines in Germany, ceased operations around 2018. Following the closure, the owner became responsible for the long-term upkeep of the mine’s shafts and tunnels to prevent collapse. As part of this ongoing responsibility, the mine requires a new dewatering tunnel to transport mine water pumped from the mine to a treatment plant at Gravenhorst.
Tunnelling through varied ground conditions
The 7.1 km long, 4.7 m outside diameter tunnel uses a Herrenknecht TBM, type Variable Density Shield, with a drilling diameter of approximately 4,800 mm with segmental lining. An additional 230 m of tunnel requires using open-cut techniques. Underground tunnelling commenced in December 2022. The overall underground construction comprises two tunnelling operations: one in a westerly direction of 3,230 m and one in an easterly direction of 3,870 m.
The tunnelling process encounters varied ground conditions, including sections with pea gravel, where mine water drainage is required, and sections using 2K-grout as segment lining backfill, where mine water drainage is unnecessary.
Putting our ring convergence measurement system to the test
One of the key requirements from the client for the tunnelling operation was the accurate monitoring of ring convergence. Our Ring Convergence Measurement System (RCMS) was not just a solution, but a crucial component in ensuring the project’s success. The sensors, particularly valuable in the small and very compact TBM where there is rarely a line of sight between the individual points, have been instrumental in maintaining the project’s progress.
This project represents one of the largest applications of our RCMS to date, showcasing its performance under challenging tunnelling conditions. The RCMS, featuring 10 measuring cross-sections spaced 10 m apart and spanning at least 100 m, provided an opportunity to optimise the sensor transmission rate. This not only enhanced the accuracy of our measurements but also significantly improved the efficiency of our operations, contributing to the overall success of the project.
Our automated system, a significant technological advancement, has played a pivotal role in the project’s success. By eliminating the need for separate evaluation and documentation of the measurements, we have not only streamlined the process but also contributed significantly to its overall success. We are proud to have been part of this journey.