Tunnel excavation with the shield tunnelling technique (using a TBM full face tunnel boring machinery) in rock or rock masses with high convergence (i.e. where the rock has a strong tendency to expand after excavation, thereby reducing the cross-section excavated) require special care at the design stage and during construction, to ensure that there is no excessive thrust from the rock on the tunnel lining segments.
Laterlite Expanded Clay is the ideal material for gap filling in this type of application because it is sufficiently rigid to ensure that the tunnel lining ring remains stable, but it is also able to collapse when a predetermined level of stress is reached, deforming plastically to reduce the thrust on the lining.
Annular gap filling with Laterlite Expanded Clay in tunnel liner segments construction was developed for the Gran San Bernardo Tunnel under the Alps, between Italy and Switzerland, and won the Best Technological Solution of the Year award at the 2012 World Tunnel Congress.
Expanded clay is widely used to fill various types of cavity between the rock and the lining of new and existing tunnels, whether or not the rock is convergent.