Roberto Vassallo

Surface displacements of two landslides evaluated by GPS and inclinometer systems: a case study in Southern Apennines, Italy

This paper reports experimental data on surface and deep displacements evaluated by means of GPS stations and inclinometers in two rototranslational deep landslides in a clayey slope of the Italian Southern Apennines. The displacements of the landslides cause continuous damage to buildings and infrastructures. To study these phenomena and control their effects, the local public administration provided financial support for a geotechnical investigation that started in 2004. Laboratory tests, in situ pore pressure and inclinometer measurements were carried out. In July 2006, systems of fixed-in-place inclinometer probes with continuous data acquisition were installed in two of the eleven guide casings, in correspondence to the slip surfaces detected by previous periodical measurements. In the meanwhile, a GPS network was installed, consisting in six permanent stations and ten non-permanent ones. Among the latter, five were installed on the top of five inclinometer casings. The experimental results show that, in the case under study, the surface displacements evaluated by means of the GPS stations are consistent with the surface displacements evaluated by means of the inclinometer measurements. This implies mutual data validation, availability of considerable amount of continuous data, as well as monitoring continuity when, for some reason, one of the instruments goes out of use.

Pore water pressures and slope stability: a joint geophysical and geotechnical analysis

Slope stability is influenced by many factors, among which are subsoil structure and pore water pressure distribution. This paper presents a multi-disciplinary approach for the determination of these two factors and for the construction of a reliable model of the subsoil for the slope stability analysis. The case of a clay slope located in the Southern Apennines (Italy) is presented and discussed. Geophysical imaging (2D electrical resistivity tomography—ERT), in situ geotechnical monitoring (measurements of pore pressures and horizontal displacements) and laboratory geotechnical tests (for the determination of index, hydraulic and mechanical properties of soils) have been carried out. The comparison and the integration between ERT images and direct observations of the material extracted from boreholes have allowed us to reconstruct the subsoil stratigraphy with continuity. Thus, a reliable 2D model of the subsoil has been obtained, with well-defined boundaries on which it has been possible to apply appropriate hydraulic conditions. This geotechnical model has been used for studying the pore water pressure distribution and for analysing how the hydraulic boundary conditions—among which rain events—influence the slope stability. Our findings demonstrate the powerful skill of the ERT, if integrated with borehole data, to generate an accurate subsoil model. It is also evident that geophysical imaging can be a source of ambiguity and misjudgement if interpreted without a comparison with geotechnical data.

Surface and Deep Displacements Evaluated by GPS and Inclinometers in a Clayey Slope

The continuous deformations of two landslides of a clayey slope of the Italian Southern Apennines cause damage to buildings and infrastructures. In 2004, 11 inclinometer casings were installed to monitor them. In 2006, fixed-in-place probes with continuous data acquisition were installed in two of those casings, in correspondence to the detected slip surfaces. In 2010, such probes went out of use, thus were removed and periodical measurements restarted. The validity of the interpretation of previous localized measurements could thus be ascertained.

Interpretation of the Behaviour of Compacted Soils Using Cam-Clay Extended to Unsaturated Conditions

This paper is focused on the interpretation of experimental results obtained at the Department of Geotechnical Engineering of the University of Naples Federico II (Italy) to investigate the effects of partial saturation on the volumetric behaviour and the initial shear stiffness of a compacted silt. Tests were performed by using suction-controlled triaxial and resonant column cells. Herein, the compatibility of the results with a Single Stress Model (SSM) is discussed. The SSM allows to highlight that suction can have two effects on the mechanical behaviour of an unsaturated soil: it increases the average volumetric stress acting on the soil skeleton and it has a sort of cementing effect on the soil packing (hardening and cementation).

Relationships between rain and displacements of an active earthflow: a data-driven approach by EPRMOGA

Inclinometer and piezometer measurements have been carried out since 2005 in a slow active earthflow in a clay shale formation of the Italian Southern Apennines. Previous studies outlined the main geometrical and kinematic features of the landslide and the pore pressure response to rainfall. Displacement rates seem to depend on the hydrological conditions as suggested by their seasonal variations. The availability of long time series of data, in some periods recorded in continuum, allows the use of a data mining approach to evaluate the relations among displacement rates in different points of the landslide, and between displacement rates and rainfall. To define such relations, the evolutionary modelling technique EPRMOGA, based on a genetic algorithm, has been used in this paper. The results give a deeper insight into the landslide behaviour on the one hand and, on the other hand, show the reliability of the technique, also in building up management scenarios. In particular, the results show that the landslide displacement rates in different points of the slip surface, although characterized by different values, are linearly dependent and thus have the same time trend, supporting the hypothesis of a constant soil discharge mechanism of movement. Piezometric data in single points cannot be used, in the considered case, to forecast displacements. The obtained relations allow to quantify the displacement rate variations due to contemporary rainfall. The influence of past rainfall is shown to decrease exponentially with temporal distance. Furthermore, the EPRMOGA simulations seem to confirm that there are no other dominant causes, besides rainfall, responsible of displacement rate variations in time.

Fiber optic based inclinometer for remote monitoring of landslides: On site comparison with traditional inclinometers

Distributed optical fiber sensors, and in particular those based on stimulated Brillouin scattering, have in recent years been the object of a growing attention for structural and environmental monitoring of large areas because they allow to measure strain and temperature profiles up to tens of kilometers with a strain accuracy of ±10ue and a temperature accuracy of ±1°C. In this paper, we present the application of the above sensing principle to the realization and field testing of a novel inclinometer for the measurement of 3D deformation of soil.

A Possible Mechanism of Movement of an Ancient Clayey Landslide

The displacement rate of the considered ancient clayey earth slide varies noticeably along its longitudinal axis, however it is practically constant from the slip surface to the ground along four inclinometer profiles. In this paper, the displacement uniformity and the absence of viscous deformation are discussed and justified. The comparison among the theoretical shear stress distribution—within the landslide body and on the slip surface—the experimental peak shear strength and the residual shear strength shows that shear stresses are lower than or equal to residual strength. This latter, in turn, is much lower than the peak strength. Furthermore, long term shear tests carried out under controlled shear forces suggest that only primary creep occurs at shear stresses lower than residual strength. So, the landslide displacements can be hypothesized uniform in each entire transversal section of the channel.

Influence of Pore Fluid Composition on the Residual Strength of a Clayey Landslide

This paper reports the results of some laboratory tests aimed to evaluate the influence of pore solution composition on the residual shear strength of a clayey landslide (Costa della Gaveta—Southern Italy). The natural pore fluid of several samples extracted from the subsoil was analysed. Direct shear tests were carried out on specimens reconstituted with—and/or exposed to—salt solutions at various concentrations. A solution reproducing the undisturbed in situ one at about 26 m depth was also used. The results of shear tests, carried out at constant displacement rate, show that the residual shear strength greatly increases with the pore solution concentration. On the other hand, depending on the composition of both solid skeleton and pore fluid, exposure to distilled water of solution saturated specimens can cause a noticeable decrease of the residual shear strength. Exposure to distilled water of the solution saturated material during a shear test at constant shear stresses lower than the residual strength evaluated for the solution-clay system can cause a creep-like process. Such behaviour is reasonably due to strength decrease caused by ion diffusion outward from the pores. An outward ion diffusion is probably among the causes of the current ion concentration profiles in the landslide, with the values decreasing dramatically from the deeper soil to the ground surface.

Distributed Fiber Optic Sensors for the Monitoring of a Tunnel Crossing a Landslide

This work reports on the application of a distributed fiber-optic strain sensor for long-term monitoring of a railway tunnel affected by an active earthflow. The sensor has been applied to detect the strain distribution along an optical fiber attached along the two walls of the tunnel. The experimental results, relative to a two-year monitoring campaign, demonstrate that the sensor is able to detect localized strains, identify their location along the tunnel walls, and follow their temporal evolution.

Analysis of Transient Pore Pressure Distribution and Safety Factor of a Slow Clayey Deep-Seated Landslide by 2D and 3D Models

This paper is focused on pore pressure response to rainfall in a clayey, deep seated landslide of the Southern Italian Apennines. The displacements of the studied landslide, measured by several techniques, including fixed-in-place inclinometer probes with continuous acquisition, are very slow and show seasonal rate variations of about an order of magnitude. The relationship between such variations, pore pressure changes and rainfall history was investigated. A finite difference code was used in 2D and 3D conditions, carrying out transient analyses with the historical rainfall of the site as boundary condition. We analyzed how the pore pressure time trend and distribution along the slip surface, whose depth varies significantly in the three dimensions, can influence the safety factor. The comparison between calculated and measured pore pressure time trends reveals that the best agreement and a clearer picture of the global response of the landslide are achieved by using a daily discretization.