chapter  9
Lining
Pages 12

Lining is provided inside the tunnel section with masonry or concrete and/or by shotcreting to provide support to the cavity against thrust from surrounding ground. The internal dimensions and shapes of tunnel after lining should be in conformity with the minimum section required for the tunnel. One rule of thumb for the thickness of lining in rock was 1 mm / 1.2 mm of diameter of the circle of the tunnel and twice as much in soft rocks. Primary lining is provided to help the exposed face to resist and reduce convergence of the surface and conserve the integrity of the structure. The correct time for providing lining is governed by the nature and condition of the soil bored through. The mix of concrete for lining depends on the location and design requirements. Curing of concrete should be done by spraying water at short intervals to maintain a wet surface.