Francesco Silvestri

Centrifuge Modeling of Seismic Loading on Tunnels in Sand

The purpose of the work is to provide an experimental benchmark on the seismic behavior of tunnels, with the final aim of calibrating numerical and analytical design methods. A series of plane-strain centrifuge tests with dynamic loading on a model tunnel was, therefore, carried out at the Schofield Centre of the Cambridge University Engineering Department (CUED). Four samples of dry uniform fine sand were prepared at two different densities, in which an aluminum-alloy tube was installed at two different depths. The tube was instrumented with strain gauges to measure hoop forces and bending moments at significant locations. To monitor the amplification of ground motion from the base to the surface, vertical arrays of accelerometers were placed in the soil model and along the box. The instrumentation also included linear variable differential transformers (LVDTs) that measured the soil surface settlement during all test phases. The test procedure and the results are described in this paper, showing the evolution of both accelerations and internal forces along the tunnel lining during the model earthquakes.

Geotechnical Aspects of the L’Aquila Earthquake

On April 6, 2009 an earthquake (ML = 5.8 and MW = 6.3) stroke the city of L’Aquila with MCS Intensity I = IX and the surrounding villages with I as high as XI. The earthquake was generated by a normal fault with a maximum vertical dislocation of 25 cm and hypocentral depth of about 8.8 km. The deaths were about 300, the injured were about 1,500 and the damage was estimated as high as about 25 billion €. Both maximum horizontal and vertical components of the accelerations recorded in the epicentral area were close to 0.65 g. The paper summarises the activities in the field of earthquake geotechnical engineering aimed to the emergency and reconstruction issues. The ground motion recorded in the epicentral area is analysed; the geotechnical properties measured by in-situ and laboratory tests before and after the earthquake are summarised; site effects are preliminarily evaluated at accelerometric stations locations and damaged villages; the outstanding cases of ground failure are finally shown.

Experimental and numerical study on circular tunnels under seismic loading

This paper compares the experimental results of a set of centrifuge models of tunnels in sand under seismic loadings with the predictions of finite element dynamic analyses and of simplified methods. In order to characterise the soil behaviour, mobilised shear stiffness and damping ratio of the sand model have been back-calculated from the experimental results according to two different procedures. Starting from the accelerometer measurements, one was based on the transfer functions from surface to base and the other one on the average shear stress–strain cycles along the sand layer. A series of viscoelastic 2D dynamic analyses were performed to simulate the model tests by a linear equivalent approach. The equivalent shear stiffness and damping ratio determined from stress–strain cycles were used as input values for the analyses. The shear stress transfer at the ground-lining interface was back-analysed to calibrate the interface elements used in the numerical code, in order to improve the assessment of the transient changes of hoop force. Finally, the numerical results have been compared to analytical solutions, widely adopted in the design, and to the experimental data in terms of transient increments of internal forces in the lining. Such a comparison indicates that the analytical formulations give a good estimation of the seismic increment of bending moment in the lining and a reasonable lower bound for the transient changes of hoop forces, provided that cyclic shear strains are correctly evaluated.

Analysis of Site Response and Building Damage Distribution Induced by the 31 October 2002 Earthquake at San Giuliano di Puglia (Italy)

This paper concerns the analysis of the site amplification that significantly influenced the non-uniform damage distribution observed at San Giuliano di Puglia (Italy) after the 2002 Molise earthquake (MW = 5.7). In fact, the historical core of the town, settled on outcropping rock, received less damage than the more recent buildings, founded on a clayey subsoil. Comprehensive geotechnical and geophysical investigations allowed a detailed definition of the subsoil model. The seismic response of the subsoil was analyzed through 2-D finite-element and 3-D spectral-element methods. The accuracy of such models was verified by comparing the numerical predictions to the aftershocks recorded by a temporary seismic network. After calibration, the seismic response to a synthetic input motion reproducing the main shock was simulated. The influence of site amplification on the damage distribution observed was finally interpreted by combining the predicted variation of ground motion parameters with the structural vulnerability of the buildings.

Prediction of Seismic Slope Displacements by Dynamic Stick-Slip Analyses

A good‐working balance between simplicity and reliability in assessing seismic slope stability is represented by displacement‐based methods, in which the effects of deformability and ductility can be either decoupled or coupled in the dynamic analyses. In this paper, a 1D lumped mass “stick‐slip” model is developed, accounting for soil heterogeneity and non‐linear behaviour, with a base sliding mechanism at a potential rupture surface. The results of the preliminary calibration show a good agreement with frequency‐domain site response analysis in no‐slip conditions. The comparison with rigid sliding block analyses and with the decoupled approach proves that the stick‐slip procedure can result increasingly unconservative for soft soils and deep sliding depths.

Lessons learned from two case histories of seismic microzonation in Italy

The prediction of the variability of the seismic ground motion in a given built-up area is considered an effective tool to plan appropriate urban development, to undertake actions on seismic risk mitigation and to understand the damage pattern caused by a strong-motion event. The procedures for studying the seismic response and the seismic microzonation of an urban area are well established; nevertheless, some controversial points still exists and are discussed here. In this paper, the selection of a reference input motion, the construction of a subsoil model and the seismic response analysis procedures are discussed in detail, based on the authors’ experience in two Italian case histories: the seismic microzonation of the city of Benevento, which was a predictive study, and the simulation of seismic response and damage distribution in the village of San Giuliano di Puglia, which was a retrospective analysis.

An uncoupled procedure for performance assessment of slopes in seismic conditions

To assess the seismic performance of slopes, the simplified displacement-based methods represent a good-working balance between simplicity and reliability. The so-called uncoupled methods permit to account for the effects of deformability and ductility by computing separately the dynamic site response and the sliding block displacements. In this paper the procedure proposed by Bray and Rathje (1998) was revised and adapted to Italian seismicity on a set of subsoil models, representative of the different soil classes specified by the Italian and European Codes. The relationship expressing the decrease of the equivalent acceleration with earthquake/soil frequency ratio was then obtained by means of dynamic 1D seismic response analyses. Statistical correlations between calculated Newmark displacements, significant ground motion parameters and the critical acceleration ratio were also derived. To estimate the reference ground motion parameters necessary for the full implementation of the proposed procedure, literature predictive equations, calibrated on strong motion records of international databases, were revised for the Italian seismicity. These ground motion prediction equations, together with simplified displacements relationships, allowed for developing an original quick procedure to evaluate the seismic slope performance by specifying the probability of exceedance of a threshold displacement, based only on few seismic input motion parameters.

Earthquake Triggered Landslides: The Case Study of a Roadway Network in Molise Region (Italy)

In this paper, a level II approach aimed at evaluating the impact of the triggering potential of seismic landslides on a regional roadway network is described. The case study is located within the Southern Apennines in Molise Region (Italy). A seismic vulnerability assessment through fragility curves was performed. Its capability to provide useful data in view of territory planning and/or emergency management was explored by means of the implementation in GIS environment.

Erratum to: Implications of non-synchronous excitation induced by nonlinear site amplification and of soil-structure interaction on the seismic response of multi-span bridges founded on piles

This work investigates the effects of soil-structure interaction and spatial variability of seismic motion due to nonlinear site amplification on the seismic behaviour of long multi-span bridges founded on piles. An analysis framework able to include the spatial variation of ground motion induced by specific geological and geomorphological scenarios in the seismic soil-structure interaction analysis of long bridges is adopted, exploiting advantages of the substructure approach. The methodology is applied to a case study constituted by a pile-supported multi-span bridge founded in a soft clay deposit overlaying a stiff bedrock with three different configurations: horizontal, inclined and wedge-shaped. The reference input motion at the outcropping bedrock is represented by a set of real accelerograms and different seismic response models are used to compute site amplification effects, discussing the contribution to the free-field ground motion of both the two-dimensional configuration of the deposit and the nonlinear soil behaviour. The ground motions obtained from the different models are then used for computing the foundation input motion accounting for the pile–soil kinematic interaction; thereafter, inertial interaction analyses are performed on structural models with either fixed or compliant base, considering the non-synchronous seismic actions at the piers foundation. The results, compared in terms of piers head displacements, ductility demand and deck transverse bending moments, finally show the relative importance of bedrock morphology, soil nonlinearity and soil-structure interaction on the structural response.

Development of a simplified model for pore water pressure build-up induced by cyclic loading

In this paper the formulation of a simplified model for predicting pore water pressure build-up under seismic loading is updated and applied to different soils. The model is directly based on the results of cyclic laboratory tests and it is based on the damage parameter concept, avoiding any arbitrary equivalence criterion necessary to compare the seismic demand to the cyclic strength of liquefiable soils. The model is suitable to be implemented into non-linear coupled seismic response analyses since it operates in the time domain. The analytical formulation is fully described and the calibration and the physical meaning of the model parameters are analysed in detail. Simple applications show the practical usefulness of the model with respect to other literature approaches.