Stability analysis of a cave excavated in granular cohesionless material

The paper presents an analysis of the long-term stability of a large unsupported cave excavated in granular cohesionless material in Ficulle, Italy. The stability of the cave arises from a ‘cohesive’ term in the shear strength criterion and this paper investigates the source of this cohesive term. Contrary to expectations, the material appeared to be granular (cohesionless) at the touch. The investigation started by determining whether the stability of the cave was attributable to any cementation bonding the soil particles. Microstructural analyses, together with geomechanical testing, produced enough evidence to suggest that the material was not naturally cemented. On the other hand, water-undrained direct shear tests on unsaturated intact specimens indicated the presence of significant apparent cohesion, which was then linked to the existence of suction in the material. In this way, the stability of the cave was assessed accounting for the beneficial effects of suction and partial saturation on shear strength. A three-dimensional analysis based on the upper bound theorem of plasticity was successful in confirming the stability of the cave in terms of the state of the structure at the time of field investigation. In addition, it was shown that 24 and 48 h rainfalls of 100 years return period are not sufficient to relieve suction sufficiently to bring the cave to collapse, thus justifying the observed long-term stability of the cave. Suction is rarely included in geotechnical design under the assumption that it cannot be relied upon owing to the potentially adverse effect of rainwater infiltration. This case study demonstrates that suction can indeed remain ‘active’ naturally for a long time, contributing to the long-term stability of geo-structures.