Guido Gottardi

A control-volume finite-element model for two-dimensional overland flow

Abstract A control-volume finite-element (CVFE) formulation for the numerical integration of the two-dimensional diffusion hydrodynamic model is presented. Three different methods for computing the transmissivity terms are compared on the basis of the global boundary-outflow error for both the finite-element (FE) and the CVFE formulation of the model. As is well known, the two-dimensional diffusion-hydrodynamic model (DHM) is obtained from the more general overland-flow model, based on momentum and mass-balance equations, with inertia terms neglected. The applied CVFE technique uses linear triangular finiteelement shape functions to estimate the value of the variables at the boundaries of the subcontrol volume of each node. The use of linear-interpolation functions allows the elemental matrices of the model to be expressed in a simple polynomial form. Integration in time is obtained by a finite-difference (FD) Picard scheme. Some numerical examples are presented to compare, on the basis of globaloutflow errors and analytical solution, the performances of six schemes of the CVFE method with that of the equivalent Galerkin FE formulations.

Fracturing Pressure in Clay

AbstractHydraulic fracturing in clayey soils can be triggered by either tensile or shear failure. In this paper, the physical meanings of various equations to predict fracture initiation pressure proposed in the past are discussed using the cavity expansion theory. In particular, when fracturing pressure is plotted against initial confining pressure, published laboratory test results as well as analytical models show a linear relationship. When the slope is close to 2, fracture is initiated by tensile failure of the clay, whereas when the slope is close to 1, it is initiated by shear failure of the clay. In this study, the analytical models, validated only on laboratory test data to date, were applied to unique data from field grouting work in which extensive soil fracturing was carried out to improve the mechanical characteristics of the soft silty clay underlying a bell tower in Venice, Italy. By a careful assessment of initial confining pressure in the field, the variation in recorded injection pressur...

Probabilistic Calibration of a Dynamic Model for Predicting Rainfall-Controlled Landslides

AbstractItaly has a number of regions with mid to high vulnerable areas from a hydrogeological point of view. The causes are the result of both the fragility of territory and the anthropic influence on its continuous modifications. A quantitative landslide risk analysis is then necessary to avoid or reduce human life and property losses. In particular, the prediction of landslide occurrence should be estimated taking into account the uncertainties affecting the analysis process. In this paper, a specific type of landslide, triggered by rainfall and characterized by the viscous behavior of soil, is discussed and analyzed. The goal is to illustrate the applicability of a probabilistic approach, based on Bayesian theorem, which aims at developing an advanced analysis, and to predict slow-slope movements. The proposed methodology relies on the probabilistic calibration of a well-defined, viscoplastic-dynamic model that is able to predict the soil mass displacement evolution from groundwater level inputs and r...

Interpretation of Piezocone Tests in Venetian Silty Soils and the Issue of Partial Drainage

The paper reports results of an extensive piezocone testing campaign performed at the Treporti Test Site (Venice, Italy) within a long-lasting research project aimed at developing a reliable geotechnical model of the heterogeneous and highly stratified Venetian lagoon subsoil. The analysis and the interpretation of the large amount of piezocone data in terms of compressibility and consolidation properties of the Venetian silty sediments reveal limitations of standard approaches for the characterization of such intermediate soils. Potentially dilative behaviour as well as partial drainage effects during cone penetration in silts must be taken in careful consideration in order to derive reliable soil parameters from in-situ measurements. According to recently proposed procedures, a preliminary attempt to assess drainage degree during piezocone tests is finally proposed.

Monumentations of control points for the measurement of soil vertical movements and their interactions with ground water contents

Historically, one of the techniques used in the study of vertical movements of soil is that of high precision spirit levelling. Nowadays, this technique is often accomplished by satellite interferometric synthetic aperture radar (SAR) analysis that requires a calibration phase, i.e. through the connection to a spirit levelling network or by means of a long time series acquired by global navigation satellite systems permanent stations in order to transform relative velocities derived by SAR in absolute terms. An important aspect of this process consists of the materialization of the object used as a control point, while its selection may depend on the geological context. Typically, due to the inevitably high cost of installation, very accurate monumentation is reserved only for a limited density of points on the territory; therefore, a type of negative correlation exists, in terms of reliability in the monumentation of the control points, with respect to their density in a real distribution within a specific monitoring technique. For example, in levelling networks, a density of one benchmark every 700–1000 m of line is often desired in order to reach a compromise among costs, practical operative requirements and precision. Levelling benchmarks are usually fixed on existing structures (i.e. buildings or concrete structures). This aspect is even more evident in the case of SAR interferometric analysis, where the persistent scatterers (PS) are identified based on the coherence in the radar response. Therefore, the PS display movements are measured by a variety of different structures, characterized by foundations fixed at different depths. Starting from repeated levelling measurements, we verified the order of magnitude of movements of control points characterized by shallow foundations in cohesive soils. In practice, we observed their behaviour in relation to the depth of the foundation under simple and very common conditions, such as the presence of periods of drought or rainfall. The results indicate movements in the order of 3–7 mm in the first metre of depth that occurred in a week during the transition between the period of summer drought and the first rains. The magnitude of the total uplift observed between the end of the summer drought and the beginning of successive springtime (30/08/2012–18/04/2013) exceeded 10 cm of movement in very shallow layers of the soil surface for the same test field.

Introducing Meta-models for a More Efficient Hazard Mitigation Strategy with Rockfall Protection Barriers

AbstractThe paper presents a new approach to assess the effecctiveness of rockfall protection barriers, accounting for the wide variety of impact conditions observed on natural sites. This approach makes use of meta-models, considering a widely used rockfall barrier type and was developed from on FE simulation results. Six input parameters relevant to the block impact conditions have been considered. Two meta-models were developed concerning the barrier capability either of stopping the block or in reducing its kinetic energy. The outcome of the parameters range on the meta-model accuracy has been also investigated. The results of the study reveal that the meta-models are effective in reproducing with accuracy the response of the barrier to any impact conditions, providing a formidable tool to support the design of these structures. Furthermore, allowing to accommodate the effects of the impact conditions on the prediction of the block–barrier interaction, the approach can be successfully used in combination with rockfall trajectory simulation tools to improve rockfall quantitative hazard assessment and optimise rockfall mitigation strategies.

Simplification of the Stratigraphic Profile in Geotechnical Models of Landslides: An Analysis Through a Stochastic Approach

The uncertainties in the determination of the stratigraphic profile of natural soils is one of the main problems in geotechnics, in particular for landslide characterization and modelling. The study deals with a new approach in geotechnical modelling which relays on a stochastic generation of different soil layers distributions, following a boolean logic. In this way, it is possible to randomize the presence of a specific material interdigitated in a uniform matrix. In fact in the building of a geotechnical model it is generally common to discard some stratigraphic data in order to simplify the model itself, assuming that the significativity of the results of the modelling procedure would not be affected. With the proposed technique it is possible to quantify the error associated with this simplification. Moreover, it could be used to determine the most significant zones where possible further investigations and surveys would be more effective. The commercial software FLAC 6.0 was used for the 2D geotechnical model. The distribution of the materials was randomized through a specifically coded Matlab program that automatically generates text files, each of them representing a specific soil configuration. Besides a routine was designed to automate the computation of FLAC with the different data files in order to maximize the sample number. In this paper, the methodology is applied with reference to a simplified slope. However, it could be then extended to numerous cases, especially for hydrogeological analysis and landslide stability assessment, in different geological and geomorphological contexts.

Seismic Stability Analyses of the Po River Banks

The Po River is the major Italian watercourse. Over half its length is controlled with embankments as protection measures against heavy floodings. Recently, the Italian Government has funded a project for the evaluation of the seismic stability of about 90 km of embankments of the Po River. The project mainly aims at the seismic stability analyses of the river banks, with assessment of local site response and evaluation of the liquefaction potential. Hundreds of geotechnical investigations within the study area were performed and the water level variations in the embankment and subsoil were investigated using piezometers. This paper describes the methodology and the main results of the analyses. The safety of 43 significant sections in static and seismic conditions was investigated using limit equilibrium analyses. Dynamic effects in the seismic condition were considered using the pseudostatic method. Local seismic hazard and effects of site conditions on the ground motion are taken into account in the definition of the expected seismic action. Eventually, the analysis results are summarized in a static and seismic stability map of the investigated area, a useful tool for the local Authority in the prevention and mitigation.

An Equivalent Continuum Approach to Efficiently Model the Response of Steel Wire Meshes to Rockfall Impacts

Steel wire meshes are a key component of rockfall protection barriers. The efficiency in reproducing the structure response with numerical methods relies upon the specific modelling technique employed to capture the wire mesh behaviour. The fabric of some rockfall meshes, such as chain-links is quite complex, which leads to sophisticated and costly numerical models, if modelled accurately. This paper presents an efficient approach to model the response of steel wire meshes to rockfall impacts by using shell elements to develop an equivalent continuum model. An elastoplastic behaviour is prescribed to the shell elements to reproduce the results of a set of experimental data, carried out on mesh portions under various load paths and boundary conditions. The idea is that simple laboratory tests can be used to calibrate an effective numerical model of the steel wire mesh with a significantly lower computational cost if compared to other effective solutions. The model’s ability in yielding consistent results when implemented at the structure scale is then assessed, based on the data of full-scale impact tests on a three-span low-energy rockfall barrier. The method can be extended to other wire mesh types and can find convenient application on exploring the response of a rockfall barrier with a cost-effective tool.

Closure to "fracturing pressure in clay" by M. Marchi, G. Gottardi, and K. Soga

M. Marchi; G. Gottardi; and K. Soga Research Assistant, Dept. of Civil, Chemical, Environmental, and Materials Engineering (DICAM), Univ. of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy (corresponding author). E-mail: michela.marchi@unibo.it Professor, Dept. of Civil, Chemical, Environmental, and Materials Engineering (DICAM), Univ. of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy. E-mail: guido.gottardi2@unibo.it Professor, Dept. of Engineering, Univ. of Cambridge, Cambridge CB2 1PZ, U.K. E-mail: ks207@cam.ac.uk