Stability analysis of a cave excavated in granular cohesionless material

De Carvalho Faria Lima Lopes, Bruna and Tarantino, Alessandro (2020) Stability analysis of a cave excavated in granular cohesionless material. Géotechnique. ISSN 0016-8505 (In Press)

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    The paper presents the 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 such 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 owed to any cementation bonding the soil particles. Microstructural analyses, together with geomechanical testing produced enough evidence to suggest that the material is 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 3-D analysis based on the upper bound theorem of plasticity was successful in confirming the stability of the cave for the state in which the structure was at the time of field investigation. In addition, it was shown that 24 and 48hrs rainfalls of 100 years return period are not sufficient to relieve suction enough 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 on due to the potential adverse effect of rainwater infiltration. This case study demonstrates that suction can indeed naturally remain ‘active’ for long time contributing to the long-term stability of geo-structures.