Excavation works in stiff jointed clay material: examples from the Trubi formation, southern Italy

The construction work of the DG22 mega lot, an 11-km segment of the new Jonica national road (southern Italy), mainly involves the Pliocene deposits belonging to the Trubi stiff jointed formation. This paper focuses on the geotechnical studies carried out to explain the kinematics of the instability processes which were observed during excavation work. Two different cases are presented. The first one refers to the movements caused by a 20-m-high slope excavated for a cut and cover tunnel. The second one describes the kinematics of the failure of a shallow tunnel which caused the subsidence of a widely extended area. In both cases, the response of the soil mass to the excavation was governed by the system of discontinuities that characterizes the Trubi clay formation. The influence of the structural features, beyond the knowledge of the mechanical behavior of the soil at the scale of the laboratory sample, was an essential ingredient to find a convincing explanation for both phenomena observed.