Marianna Pirone

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.

Hydro-mechanical Analysis of an Unsaturated Pyroclastic Slope Based on Monitoring Data

In recent decades in Campania (Italy) there have been rapid landslides involving unsaturated soil covers resting on steep bedrock. The paper summarises the results of both monitoring at a pilot site in the municipality of Monteforte Irpino (AV) in southern Italy and hydro-mechanical properties of soils from the same site. The first part of the paper concerns the description of the pilot site and the hydro-mechanical characterization of the soils based on wide-ranging laboratory investigation. In the second part of the paper the seasonal features of the pore water regime are described. Measurements are elaborated to estimate the groundwater head spatial distribution and, in turn, head gradients. Finally, a simple stability analysis is presented, schematizing the slope as infinite.

Review on the Methods for Evaluation of Root Reinforcement in Shallow Landslides

Open image in new window Recently geotechnical engineers aim to adopt more environmental-friendly solutions (not harmful to the environment), therefore the interest on the use of vegetation as a measure to improve slope stability is increasing. The mechanical reinforcement due to roots against shallow landslides occurs when the fibres intersect the shear surface, usually at depths lower than 2 m. In the literature, the presence of roots is often taken into account by modelling the soil as an equivalent composite material: ‘the root-permeated soil’, by including an additional cohesion term in the Mohr-Coulomb equation. The models used to estimate the root additional cohesion are presented in the first part of the paper. In some cases, root cohesion is calculated based on the resistant properties of the fibres and assuming an order for the progressive roots failure, either breaking, slipping out or buckling. On the other hand, some authors used structural models of the roots investigating not only the stresses in the roots, but also in the surrounding soil to obtain a better estimation of the root cohesion. In the second part of the paper, the calculation of the root reinforcement is used to assess the safety factor (SF) of the slope. Both Limit Equilibrium analyses (LE) and Finite Element Methods (FEM) are discussed, stressing the limitations of both the approaches.

Review on Types of Root Failures in Shallow Landslides

Nowadays the interest of geotechnical engineers for green solutions is being developed and the use of vegetation as a reinforcement to improve slope stability is growing. The sliding surface of shallow landslides tends to not exceed 1.5–2 m depth, and as a consequence it can be crossed by roots that, in this case, work as a stabilizing measure. Therefore, the study of the soil-roots interaction is necessary to quantify the contribution of vegetation to the stability of shallow landslides. The goal of this paper is to provide an overview of the root failure mechanisms that can occur along the sliding surface and of the forces applied by roots, in order to evaluate the safety factor of a reinforced slope. Several prevailing stress states occur along a shallow landslide failure surface: tension stress at the slide crest, shear stresses along the base of the unstable soil layer and passive earth pressures at the slope toe. Some considerations are also made regarding acceptable simplifications, in terms of root geometry and soil-root friction strength, that are currently assumed in the literature.

Subsurface Drainage for Slope Stabilization

In saturated soils, drainage systems are one of the most effective remedial measures against slope instability due to their capacity to reduce pore-water pressure in the subsoil, increasing the shear strength of the soil. Due to their relative cheapness, subsurface drainage systems are widely used, also in combination with other stabilization works. Moreover, they provide a suitable solution to stabilization in a large number of cases, even when the landslide is very deep and structural measures are not effective (Popescu (2002) Landslide causal factors and landslide remedial options. Keynote lecture. In: Proceedings of the third international conference on landslides, slope stability and safety of infra-structures, Singapore, pp 61–81). In this paper the main subsurface drainage systems are described in terms of their conventional and innovative technologies, their limits and advantages. The design approaches in the relevant literature are also reviewed.

Seasonal Effects in the Groundwater Regime of Unsaturated Slopes

Landslides in partially saturated pyroclastic slopes are strongly linked to rainwater infiltration, which is the main factor responsible for pore pressure and shear strength variations in shallow subsoil. Monitoring of the groundwater regime allows the most critical period for slope stability to be identified and the relationship between critical pore pressure and rainfall to be explored. This paper describes the monitoring of an unsaturated pyroclastic slope instrumented at Monteforte Irpino in southern Italy (Pirone et al., 2010), where suction and volumetric water content measured at different depths were collected from 2006 to 2011. Using this database, typical aspects of the groundwater regime are described. The maximum water content is observed to be significantly lower than the saturated value measured in the laboratory (Nicotera et al., 2010). This condition affects permeability attained in situ during the wet period of the year and hence infiltration.

Suction Fluctuations in Unsaturated Slopes: Evidences from Two Test Sites in Southern Italy

Slope failures in partially saturated pyroclastic slopes are often induced by rainwater infiltration. Monitoring pore pressure in the subsoil makes it possible to identify the most critical period for slope stability and to determine whether current site conditions are critical for slope failure (according to a Civil Protection approach). This paper deals with the behaviour observed from 2002 to 2011 in two test sites in southern Italy. Using the data obtained, typical aspects of groundwater regime are described. The effect of single rainfall events is recognized only in the shallowest layers, with a relatively small perturbation in the suction trend compared to the amplitude of seasonal fluctuations (that reach 80 kPa). Minimum suction values occur during winter in all layers. Therefore, we can conclude that the critical period for landslide triggering is between January and April.