Glossary.

TBM without pressure bulkhead behind the excavation chamber. Open shields are normally only employed above groundwater level. Open TBMs include, among other machines, TBMs and partial face excavation machines.

Distance between the upper edge of the pipeline and the surface of the ground and/or the bottom of a body of water located above the pipeline. Often also called cover or overlay.

The remaining ring-shaped gap between the excavation tunnel and the exterior diameter of the shield.