<p>This paper discusses the deformations measured, the extent of the yielded zone and the tunnel support performance around the right bore of a 12-m span tunnel in soft flysch under a 40-m overburden. The tunnel has been constructed under a construct-only contract, from 2001 to 2002 using the sequential excavation method. It comprises a temporary support and a final reinforced concrete lining. In both bores, the temporary support measures at the location examined, have been a sprayed concrete shell, steel sets, rock bolts and forepoles combined with fiberglass anchors as face reinforcement. Shotcrete inverts have been also installed at both the top heading and the bench. At the location of interest, the tunnel has been excavated in a folded siltstone including alternating shear zones of soft clayey material. The groundwater table has been found to be a few meters below the ground surface. Geomonitoring has been conducted by means of convergence measurements and extensometers. By adopting the strength and deformability parameters of the clay component of the ground—as measured from laboratory testing—a simplified numerical back analysis has been performed to calibrate the outcome of the geomonitoring results as regard the deformation pattern of the tunnel and the extent of the yielded zone around it. The timely installation of the invert at the top heading and the impact of the excavation of the left bore on the right one have also been examined and discussed. At the particular location considered, the presence of the invert has been found to better control the deformations of the top heading shell, rather than the stresses developed in it. Some observations have also been made for the extent of the yielded ground between the tunnel bores under fictitious higher overburdens.</p>