Are long-term loads shared between primary and secondary lining?

Various design approaches have been historically applied to the long-term load sharing between primary and secondary lining. The primary lining is in almost all cases considered permeable so that the secondary lining is designed to carry the full water pressures (if any). Regarding the long-term ground pressures, the primary lining may be designed to carry from zero (sacrificial) to full pressures and the secondary lining the remainder. The friction of the interface between the two layers which in most tunnels comprises a waterproofing layer also affects the load-sharing considerations.

Advanced design tools can support load-sharing assumptions through simulation-testing-calibration cycles, resulting in more efficient designs and reduced material quantities.