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J. Macht, R. Lackner, Ch. Hellmich, H.A. Mang:
"Quantification of Stress States in Shotcrete Shells";
in: "Numerical simulation in tunnelling", G. Beer (ed.); Springer, Wien, 2003, ISBN: 3-211-00515-3, 225 - 248.

As reported in Chap. 3, monitoring of deformations which occur during the excavation of tunnels is essential for the success of the NATM. Nowadays it is performed at every NATM construction site. The combination of these monitored displacements with the material model for shotcrete outlined in Chap. 5 has led to the development of a hybrid method for the analysis of closed shotcrete tunnel shells by means of nonlinear Finite Element (FE) analyses. It allows determination of the stress state in the shotcrete shell. Knowing this stress states a level of loading can be computed, amounting to 0 % for the unloaded shell and to 100 % for shotcrete (locally) loaded up to
its compressive strength. The analysis of segmented tunnel shells required further development of this hybrid method. The desire for real-time monitoring of the level of loading led to the development of a shell-theory-based hybrid method avoiding time-consuming FE analyses.