Fracture behaviour of clayey soils overlying cavities

The frequent appearance of sinkholes has become a hazard due to their instantaneous development and the damage they can cause to infrastructure. The term sinkhole indicates a location in which the ground is sinking into cavities. The process is caused by water penetration into the ground. Water causes dissolution of soluble rocks in the bedrock resulting in the creation of a void and consequent collapse of the overlying soil. Water also affects the stability of voids from former old mine works. Despite the increasing interest in this phenomenon, rarely the geotechnical aspects of the event are studied. In particular, the characteristics of the soil standing over the cavity are usually neglected. This research aimed to study clay behaviour in tensile conditions, investigating the effects of moisture content and plasticity index. The results will aid future research on the determination of a predictive model for sinkhole appearance. The project was divided into two parts: the first was based on the collection of data of previous events, the second was related to laboratory tests performed on small clay models. The creation of the historical database showed the importance of studying cohesive soils behaviour and common features of past sinkholes. The laboratory tests allowed the effect of moisture content and plasticity index on the fracture failure modes of clay to be determined. Two types of experiments were performed to study the instantaneous failure caused by cracks propagation: direct tensile and bending tests. The results found showed that fracture in clay is affected by both moisture content and plasticity index. High moisture contents determined gradual collapses in which clays underwent large deformations before reaching failure. This explained why sinkholes usually form after some time from the rain event. Instead, dry clays represent a more dangerous situation for their unexpected collapse without any deformation.