Tunnels can be designed for full over burden (i.e. the full weight of the soil acting above the tunnel) or a reduced over-burden case. In the case of deep tunnels in rock a reduced overburden approach is often adopted using Terzaghi equations. In the case of a shallower tunnel in soft ground an empircal approach such as the Curtis Muir Wood equations can provide an estimate of the hoop thrust and bending moment acting on the tunnel lining. These equations are used for designing circular segmental tunnel linings constructed as a ring. For more complex shapes and construction sequences and for detailed design, finite element software is used to calculate the forces applied to the tunnel lining. After the forces acting on the tunnel lining are calculated the tunnel segments are normally designed as short concrete columns.
How does this calculator work?
This calculator uses machine learning on historic tunnelling project data to calculate a reasonable prediction for tunnel lining design properties of a lining. Within this calculation your results can be filtered on ground condition type to find projects more similar to your project and refine your results.
How are tunnels built?
Tunnels can be constructed in a number of different ways with the most common being:
Segmentally lined tunnels constructed with the shield of a tunnel boring machine, here tunnel segments are transported to the front of the tunnel boring machine where they are assembled into circular rings using a hydraulically powered segment erector.
Mined tunnels using a mechanical excavator or explosives with a robot spraying a lining of concrete onto the exposed ground to create a support.
What is a tunnel boring machine?
A tunnel boring machine (or TBM) is a heavy-duty below ground drilling machine which excavates earth via a rotating cutterhead. This excavated earth is processed and either pumped or carried via a conveyor belt back to the ground surface level. Within the shield of a tunnel boring machine concrete tunnel segments are assembled into circular rings using a hydraulically powered segment erector. The TBM then pushes via hydraulic rams against the constructed tunnel lining to thrust forwards and continue cutting the earth. A face pressure is usually applied at the front of the machine to resist water pressure and prevent tunnel flooding. This face pressure can either be generated with the excavated earth itself extracted with an archimedes screw, know as earth pressure balance. Or the face pressure can be generated by pumping slurry to the face of the machine in a process known as slurry pressure.