Real time determination of the utilisation of a sprayed concrete lining during tunnelling

The displacements of the sprayed concrete lining must be measured during tunnel excavation and checked against predictions in order to assess stability. Generally, if the measured displacements are equal or less than the predictions, stable and safe conditions are assumed.

However, just looking at single displacements is insufficient to assess stability. Displacements are not necessarily creating stress in the lining, such as in the case of whole body movement of the lining. Time-displacement graphs are important to analyse in order to determine whether a new equilibrium has been established and stable conditions can be assumed. However, they do not tell anything about the actual stress state in the lining.