Tunnel Boring Machines (TBMs) are often buried and abandoned at the end of a tunnel project due to practical and economic considerations. While it may seem wasteful, this approach can be more cost-effective and technically feasible than dismantling or extracting the entire machine.
Key Reasons for Abandoning TBMs:
Cost
TBMs are massive, specialized custom made machines custom designed for a specific tunnel's sizes and geology. Recovering and storing the machine is often more expensive and time-consuming than manufacturing new parts for future projects.Complexity of Retrieval
Retrieving a TBM, especially after completing a long and deep tunnel, would require significant effort, including constructing new shafts and complex logistics. This is because the tunnel behind the machine is smaller than the cutting head, as the tunnel walls are built behind it, making it impossible for the TBM to reverse. Additionally, when TBMs work in pairs from both ends of the tunnel, neither machine can advance once the tunnel is completed.Time Efficiency
Abandoning the TBM can allow the project to progress faster, particularly for tunnels that need to be operational quickly. Retrieving the machine would delay finishing touches such as lining, ventilation, and paving.Reusing Salvageable Components
In most cases, only the front section of the TBM, including the cutterhead and machine shields, is abandoned. The rest of the complex machine, including motors, hydraulics, and control systems which trail behind it, is dismantled and reused for future projects.Planned Disposal
TBMs are deliberately abandoned as part of the project’s design and only designed for a single project. For example, in the Channel Tunnel project, a TBM was driven into a side passage and encased as part of the tunnel lining. Similarly, during Brisbane's Airportlink project, the TBM's was buried and encased in cement at the tunnel's end as part of the planned construction process. In some cases, such as the HS2 railway tunnel the TBM is being stored instead at the end of the tunnel so it can be reused should the tunnel ever be expanded.Illustration: HS2 TBM
