The idea is to reduce costs while improving safety. Cross-section of weak zone of weakness after boring and just before lining with cast concrete. The deformation is highly exaggerated in the figure. In this example the tunnel floor is not lined, and estimates show a significant degree of rise. Tunnel boring can be a risky business if insufficient attention is paid to safety measures.
Risks are particularly high when there are weak zones in the rock.
For this reason, it is important to pay special attention to safety in such areas. Weak zones are stabilized by lining them or spraying them with concrete. The extent of these safety measures are often based on swelling tests using soil samples taken from the weak zone. These tests measure the pressure developed by the clay when it has free access to water, but is prevented from swelling. However, these estimates are often very conservative, resulting in a conservative design for the stabilizing system.