Luca Pagano

Soil suction monitoring for landslides and slopes

Abstract Rainfall is the most frequent triggering factor for landslides and the development of early warning systems has to take account of this. It is suggested that direct measurement of pore pressure gives the most reliable prediction of failure of a slope. The amount of rainfall can be only a crude indicator of failure as the processes that occur between rain falling on a slope and the resulting pore water pressure change are complex, highly non-linear and hysteretic. The paper describes high-capacity tensiometers developed within the EU-funded MUSE Research Training Network that have been used for measuring suctions in slopes. High-capacity tensiometers are capable of direct measurement of pore water pressure down to −2 MPa and are also able to record positive pore water pressures. Two methods of field installation are discussed; one developed by ENPC in France uses a single tensiometer per hole, and the second technique, developed by Durham University in the UK, allows multiple tensiometers to be used at different depths within a single borehole. Continuous monitoring of pore water pressure has been carried out over several months and shows the responses to climatic events.

The role of the lowermost boundary conditions in the hydrological response of shallow sloping covers

In many areas of the world, slopes covered by shallow unsaturated non-plastic soil layers experience rainfall-induced landslides causing heavy damage and casualties every year. Landslide occurrence depends on the amount of water infiltrated and stored. Among the contributing factors are the hydraulic conditions at the lowermost boundary, a feature that is often disregarded. The paper focuses on this topic, presenting the results of some laboratory and numerical experiments on ash-pumice interfaces. A strategy is then proposed for selecting the lowermost boundary condition, and some studies are carried out to compare the results obtained with the proposed solution and other more popular ones.

A Physical Model to Investigate the Influence of Atmospheric Variables on Soil Suction in Pyroclastic Soils

This paper investigates the interaction between soil and atmosphere in pyroclastic soils with a view to understanding the influence of meteorological factors on soil variables (essentially water content and suction). Such variables are known to have a great effect upon slope stability. As particular interest of the work lies in the influence of evaporation, a physical model has been set to quantify evaporation fluxes and the influence they have on fluctuations in soil water content and suction. The physical model consists of a 1 m3 tank filled with pyroclastic soil and exposed to natural weather elements. The system is extensively monitored to record atmospheric and soil variables during the tests.

Earthquake early warning for earth dams: concepts and objectives

In the geotechnical field, the risk related to slope instabilities or collapse of geotechnical structures are increasingly being faced by early warning systems, capable of: (1) predicting the incipient collapse based on the interpretation of a continuous monitoring of the structure and (2) spreading alarm promptly to reduce people exposure. Compared with structural approaches, early warning systems have two important advantages: a faster, simpler and less expensive implementation and environmental compatibility. Past experience indicates that vulnerability of earth dams is generally low under both static and seismic loading conditions. In spite of this, earth dams are characterized by a high-risk level, due to the high exposure factor. Nowadays, the application of early warning systems to dams is fully supported by the technological progress achieved in the telecommunication field, since it is possible to install and automate recordings and transmission of all physical variables significant to check dam safety: accelerations, displacements, pore-water pressures, total stresses, seepage flows. A considerable lack still arises in the predictive models for interpreting monitoring data and providing indicators on dam safety soon after a strong earthquake. The present work illustrates the basic concepts of an earthquake early warning (EEW) system for earth dams and the main features that should characterize a predictive model to such a scope. An application to a real case is finally provided, enhancing the role played by each monitored physical variable for the aims of EEW.

Basic features of the predictive tools of early warning systems for water-related natural hazards: examples for shallow landslides

To manage natural risks, an increasing effort is being put in the development of early warning systems (EWS), namely, approaches facing catastrophic phenomena by timely forecasting and alarm spreading throughout exposed population. Research efforts aimed at the development and implementation of effective EWS should especially concern the definition and calibration of the interpretative model. This paper analyses the main features characterizing predictive models working in EWS by discussing their aims and their features in terms of model accuracy, evolutionary stage of the phenomenon at which the prediction is carried out and model architecture. Original classification criteria based on these features are developed throughout the paper and shown in their practical implementation through examples of flowlike landslides and earth flows, both of which are characterized by rapid evolution and quite representative of many applications of EWS.

Pore Water Pressure Distribution for Use in Stability Analyses of Earth Dams

In the assessment of a dam safety with respect to global instability, nominal pore water pressure distributions are often adopted for each stage of the dam life, referring to a hypothetical “expected performance” of the different dam components. The seepage phenomena taking place within the dam may, however, modify substantially the pore water pressure distributions up to make them inconsistent with the nominal ones. With reference to this subject, the present paper represents and interprets the singular pore water pressure distributions measured within an Italian earth dam, pertaining to the typology of zoned earth dam with internal clay core. Pore water pressure interpretation is supported by the representation of the evolution of the measured seepage flows. The work highlights how pore water pressure may differ strongly from the nominal distribution during the first stages of the dam life, when, if nearly undrained conditions take place, pore water pressures are discontinuous as a result of their dependency on total stresses. The work also shows how pore water pressure may assume distributions differing from the expected ones also after some decades of the operational stages, this time as a result of suffusion phenomena induced by the seepage processes within the dam embankment. Measured pore water pressure distribution, suitably interpreted as piezometric heads contours, clearly show how part of the downstream shell contributes to the embankment watertightness.