Observations from monitoring of tunnel excavations in clayey silt

The paper illustrates the application of practical methods for analyzing tunnel convergence and deformation of support systems. The case-history considered deals with the excavation of several tunnels in a clayey silt formation for a new highway under construction in South Italy. Convergence measurements were back analyzed by empirical models in order to calculate separately the deformation caused by the face advance and by the time-dependent soil behavior. Statistical analysis of a large set of monitoring sections shows that displacements are significantly correlated with the orientation of the stratification of the clayey silt formation, the single phases of excavation and support installation and the rate of face advance. The distribution of stress resultants (axial force and bending moment) within the lining was also investigated in some monitoring sections. This was done by two different techniques, respectively based on back-analysis of displacements and direct measurement by local strain-gauges. The axial force at the tunnel crown was found to be regularly higher than at the side walls. Furthermore, the shotcrete rings seem to be more strained than the steel sets.