Daniela Boldini

International Benchmark on Numerical Simulations for 1D, Nonlinear Site Response (PRENOLIN): Verification Phase Based on Canonical Cases

PREdiction of NOn‐LINear soil behavior (PRENOLIN) is an international benchmark aiming to test multiple numerical simulation codes that are capable of predicting nonlinear seismic site response with various constitutive models. One of the objectives of this project is the assessment of the uncertainties associated with nonlinear simulation of 1D site effects. A first verification phase (i.e., comparison between numerical codes on simple idealistic cases) will be followed by a validation phase, comparing the predictions of such numerical estimations with actual strong‐motion recordings obtained at well‐known sites. The benchmark presently involves 21 teams and 23 different computational codes. We present here the main results of the verification phase dealing with simple cases. Three different idealized soil profiles were tested over a wide range of shear strains with different input motions and different boundary conditions at the sediment/bedrock interface. A first iteration focusing on the elastic and viscoelastic cases was proved to be useful to ensure a common understanding and to identify numerical issues before pursuing the nonlinear modeling. Besides minor mistakes in the implementation of input parameters and output units, the initial discrepancies between the numerical results can be attributed to (1) different understanding of the expression “input motion” in different communities, and (2) different implementations of material damping and possible numerical energy dissipation. The second round of computations thus allowed a convergence of all teams to the Haskell–Thomson analytical solution in elastic and viscoelastic cases. For nonlinear computations, we investigate the epistemic uncertainties related only to wave propagation modeling using different nonlinear constitutive models. Such epistemic uncertainties are shown to increase with the strain level and to reach values around 0.2 (log_(10) scale) for a peak ground acceleration of 5  m/s^2 at the base of the soil column, which may be reduced by almost 50% when the various constitutive models used the same shear strength and damping implementation.

Geomechanical Characterization of the Volcaniclastic Material Involved in the 2002 Landslides at Stromboli

During the 2002 eruption, the NW flank of Stromboli volcano (Southern Italy) experienced large landslides that produced tsunami waves with a maximum runup of 10 m in height. This paper focuses on the geomechanical behavior of the loose volcaniclastic layers, the weakest component of the deposit forming the volcano flank. Tests on the coarse-grained volcaniclastic soil and on the rock material forming the grains were conducted in dry conditions, a feature of the subaerial slope. The rock material, in spite of its high porosity, exhibits relatively high stiffness and strength due to the remarkable continuity of the solid skeleton. Accordingly, the volcaniclastic soil is characterized by high shear strength. During shear tests, however, significant grain crushing occurs, which partly explains the nonlinear shear strength envelope and the strain softening associated with contractive behavior. Grain crushing was also found to affect compressibility and its time dependency, investigated under oedometric conditions.

Influence of hydro-mechanical coupling on tunnel response in clays

The design of deep tunnels underneath the water-table in clayey soils should carefully take into account the influence of pore pressures and seepage forces on the stability of the excavation and on the loading conditions of the support systems. This paper focuses on the application of analytical solutions and numerical models to the analysis of stress and deformation around deep tunnels, in the short and long term, considering the idealized situation of axisymmetric conditions. General remarks on the influence of hydro-mechanical coupling, artificial boundary conditions and lining permeability are presented. Theoretical predictions are compared with measurements made in tunnels excavated in the Boom clay formation at Mol (Belgium).

Mechanism of Landslide Causing the December 2002 Tsunami at Stromboli Volcano (Italy)

Between 29 and 30 December 2002 the NW flank of Stromboli Volcano (Sciara del Fuoco) was involved in a series of large-scale instability phenomena which culminated in submarine and subaerial destructive landslides provoking two tsunami waves with a maximum run-up of 10 m. In this paper, part of the results of a joint research between the National Research Council (Italy) and the Disaster Prevention Research Institute of Kyoto University (Japan) are presented. The activity has focused on the mechanical characterization of the volcanoclastic material forming the Sciara del Fuoco depression and the interpretation of landslide mechanisms on the basis of large-scale ring shear tests. Attention is given here to the initiation and propagation of the submarine landslide which caused the first tsunami. In order to investigate the material response to different displacement rates in terms of shear resistance, pore pressure generation and grain crushing, ring shear tests were conducted in both undrained and drained conditions. Experimental results indicate that a fully or partial liquefaction mechanism can be invoked to explain the failure of the submarine flank of the Sciara del Fuoco and the long run-out which followed, as it was suggested by comparing pre- and post-failure in situ observations.

Three-Dimensional Numerical Modelling of Historical Masonry Structures Affected by Tunnelling-Induced Settlements

This paper focuses on the interaction between tunnelling and historical masonry structures. These latter often characterise the centre of many cities and should be preserved from possible tunnelling-induced damage. In recent years the Authors of this contribution have adopted an advanced numerical approach to investigate this issue in the two-dimensional domain, schematising the block masonry structure as a homogenised anisotropic medium [1, 2]. This study extends the approach to three-dimensional conditions. The behaviour of masonry is described by a modified version of the Jointed Rock model, named hereafter as Jointed Masonry model, an anisotropic elastic perfectly plastic constitutive model implemented in the code Plaxis 3D. This model takes into account the directional properties of the medium, identifying the orientation of three planes along which the Mohr-Coulomb yield criterion applies. The paper first briefly describes how the original Jointed Rock model was modified to more realistically account for some specific features of the nonlinear mechanics of masonry. This is followed by the 3D analysis of a tunnelling-structure interaction problem, aimed at highlighting the key features of the proposed masonry model.

Analysis of convergence data and 3D numerical modelling of tunnels excavated in fine-grained soils

This paper focuses on the convergence measurements of twin motorway tunnels under excavation in a Pliocene formation in the South of Italy. The tunnels are driven full face, with a cross-section of 120 m, by means of a conventional excavation method, up to a maximum depth of 120 m. The primary lining is an open arch made up of shotcrete and steel sets; the definitive lining consists of concrete cast in three different stages. Convergence measurements were analyzed on the basis of the curve-fitting technique proposed by Sulem et al. (1987). Then, the influence on tunnel deformation of overburden and lining construction sequence is investigated. Finally, the role of the primary and final linings in reducing tunnel convergence is investigated by means of a three-dimensional stress-strain analysis, performed with FLAC3D code. Ags Asa Santa Maria Tunnel North q (m) a.s.l. 1000

Tunnel behaviour under seismic loads: Analysis by means of uncoupled and coupled approaches

In this paper different approaches to investigate the behaviour of tunnel under seismic loads are presented. They include one-dimensional (1D) numerical analyses performed modelling the soil as a single phase non-linear visco-elastic medium, the results of which are then used to evaluate the input data for selected analytical solutions proposed in the literature (uncoupled approach), and 2D fully coupled Finite Element (FEM) simulations adopting a visco-elastic effective stress model for the soil (coupled approach).