Translational Landslides on Slopes under Transient Rainfall Conditions: Estimating Time to Failure as Guidance for Alerts and Alarms

Abstract

Intense, increasingly recurrent rainfall events, combined with population growth in recent decades, have resulted in tragedies caused by translational landslides. In this sense, the search for measures to prevent and mitigate damage has become a priority for public bodies. One action that aims to minimize the consequences of this geological process is using warning systems. However, a warning cannot be generalized temporally or spatially. This study, therefore, estimated the factor of safety (FS) for portions of slopes with different gradients in an area located in Campos do Jordão municipality to propose a warning system. The potential rupture times of hillsides were estimated for the different slope classes considering geological, geotechnical, and rainfall data. The geological-geotechnical units in the study area showed different behaviors, leading to the adoption of specific warning systems. There are units where the FS decreases and remains lower than 2 for a long time (up to 27 days) before reaching values lower than or equal to 1, indicating potential instability. In these cases, it is possible to issue warnings of a potential rupture, allowing decisions to be taken and damage to be mitigated in good time. In other units, cohesion decreases rapidly, and the FS reduces from 2 to 1 in a very short time (up to 60 minutes), so there is no significant time to issue warnings, only alarms. The study results show that with the proper use of large-scale geological models, quality geological and geotechnical data, and detailed rainfall monitoring, it is possible to draw up maps of unstable areas dynamically in transient and continuous conditions.