Pre-failure suction-induced deformation to inform early warning of shallow landslides: proof of concept at slope model scale

The majority of the Landslide Early Warning Systems (LEWS) currently in operation are based on the monitoring of rainfall data alone and this limits their performance due to false alarms generated by rainfall thresholds that are inevitably set conservative. The accuracy of LEWS may be significantly enhanced by monitoring soil-based variables associated with the stress-strain response of the ground. This paper investigates whether slope pre-failure deformation can be used as additional precursor of landslide initiation. This would lead to a substantial improvement of LEWS accuracy especially if pre-failure deformation is combined with suction monitoring. Tests were carried out using a small-scale physical model of a slope built with unsaturated volcanic silt subjected to artificial rainfall. A new device named tensio-inclinometer was purposely developed to monitor simultaneously suction and suction-induced deformation. It combines a conventional tensiometer and an accelerometer installed at the top of the tensiometer shaft. It is shown that pre-failure deformation detected by the tilting of the tensiometer shaft is an adequate landslide precursor and that, combined with suction, can provide soil-based thresholds for early warning systems.