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

Abstract

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

Physically based approaches incorporating evaporation for early warning predictions of rainfall-induced landslides

“…-the one-dimensionality of water fluxes; this hypothesis, formulated in order to save analysis time in early warning 5 applications, should also lead to realistic estimations of the hydrological state of the cover in line with indications for similar soils provided by interpreting field measurements (Pirone et al, 2015) and theoretical studies ; -the rigidity of the domain; neglecting the effects of deformational processes induced by suction changes is suggested by the stiff volumetric response observed under swelling-reloading paths, which should speed up re-equilibrium processes so that hydraulic response is unaffected by the consolidation delay; 10 -precipitation history applied with hourly resolution at the top boundary; an incoming flux corresponding to rainfall intensity is applied at the top boundary in the meanwhile that at the same boundary the analysis returns pore water pressure values less than zero (atmospheric pressure); the top boundary condition is switched to null pore water to prevent pore water pressure from exceeding atmospheric pressure; this condition may be maintained until the analysis returns incoming fluxes less than rainfall intensity, otherwise it is again switched so that the incoming flux is equal to the rainfall intensity; 15 -surface seepage is applied at the bottom boundary; this condition corresponds to the hypothesis that the bedrock the volcanic layer rests on is intensely fractured and that the fractures are filled only by air (Reder et al, 2017).…”

A new technique for deep in situ measurements of soil water retention behaviour

“…Since their developments, these instruments have also found utilisation in some geotechnical problems, most commonly in the field of monitoring landslides (e.g. Bordoni et al, 2015;Cascini et al, 2014;Pirone et al, 2015;Springman et al, 2013) or changes in the soil volume due to seasonal variations of water content for the purpose of foundation design (Harris et al, 2013;Nguyen et al, 2010) and river embankments (Casagli et al, 1999).…”

Estimation of hydraulic conductivity function in unsaturated pyroclastic soils

“…The tensiometers were installed at different depths along all the vertical sections, TDR and Casagrande piezometers along the verticals only at the central section, B-B'. However, the test site itself has been extensively described elsewhere; readers can refer to [7] and [4] for further detailed information about the instrumented area. The simplified mean soil profile obtained from experimental investigation through some trenches and boreholes, mean physical properties (specific gravity G s , porosity Φ, dry soil unit weight γ d ) and instrumentation installed along the central longitudinal section, B-B', of the test site are reported in Figure 1.…”

“…Here a method to detect a hydraulic conductivity function by processing in situ data is provided, in particular, the identification of the HCFs operating on site by applying the Soil Water Balance (SWB) to a pyroclastic soil cover is carried out. These features have been investigated by processing data from the test site at Monteforte Irpino [4] where meteorological data, matric suction and volumetric soil water content measurements were collected for about four years. Moreover the saturated hydraulic conductivity and the hydraulic conductivity function obtained in the laboratory [5] are compared with the unsaturated hydraulic conductivity values derived from in situ measurements to show that laboratory testing results are not always representative of the effective hydraulic conductivity operating at the site scale.…”