In situ monitoring of the groundwater field in an unsaturated pyroclastic slope for slope stability evaluation

In the last 15 years, a series of catastrophic flowslides involving pyroclastic unsaturated soils has caused severe damage and a number of fatalities in the Campania region (southern Italy), where flowslides occur within the pyroclastic cover resting on the limestone massif. Rainwater infiltration is considered to be the triggering mechanism that leads to slope failure by reducing matric suction in unsaturated soils, hence reducing its shear strength. Therefore, knowledge of the groundwater regime is an important factor for slope stability evaluation as well as for predicting slope conditions prone to landslides triggered by water infiltration. The quality of prediction can be greatly improved if observations from in situ monitoring are available. This paper describes the results of an ongoing experimental research project on flowslides based on monitoring a test site in a typical geological environment in western Campania. The site was selected to carry out extensive laboratory and in situ testing. The test programme consisted of field monitoring of climatic conditions, matric suction and volumetric water content. The instrumentation and measuring techniques used in this research are systematically described to provide an example of good practice for use at other sites with similar features. With the use of the collected data, the seasonal fluctuations of the hydraulic regime in the subsoil may be observed and the critical periods for flowslide triggering identified. Measurements of matric suction from the test site are interpreted and directions of groundwater flow vectors in the subsoil are shown. Moreover, once the current state of the subsoil is known, slope stability conditions can be calculated using a simple infinite slope model that would provide continuously updated information on the current slope safety level.