Modelling tunnel behaviour under seismic actions: An integrated approach

This paper intends to describe the integration of physical and numerical modelling, focusing on tunnels under seismic actions. It shows how numerical calculations can be used in association with centrifuge testing to model different aspects of tunnel behaviour during earthquakes. The scope of the paper has been limited to a few aspects, mainly concerning the change of internal forces in the tunnel lining during shaking and the effect of soil liquefaction. The interaction between a tunnel and a building in a soil layer undergoing liquefaction has also been taken into account

A numerical study on the solution of the Cauchy problem in elasticity

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Quasi-isospectral Sturm-Liouville operators and applications to system identification

Abstract Quasi-isospectral Sturm-Liouville operators play an important role in inverse spectral theory and are typically used for determining exact solutions to suitable classes of eigenvalue problems with variable coefficients. In this work we investigate on alternative applications of quasi-isospectral operators as key tool for structural identification purposes. We review some recent results concerned with the construction of rods with a given finite number of natural frequencies and we present some generalization to beams under bending vibration and to the identification of damages from natural frequency data

The use of quasi-isospectral operators for damage detection in rods

We consider the inverse problem of reconstructing the axial stiffness of a damaged rod from the knowledge of a finite number of resonant frequencies of the free axial vibration under supported end conditions. The damage is described as a reduction of the axial stiffness, and the undamaged and damaged configurations of the rod are assumed to be symmetric. The method is based on repeated determination of quasi-isospectral rod operators, that is rods which have the same spectrum of a given rod with the exception of a single resonant frequency which is free to move in a prescribed interval. The reconstruction procedure is explicit and it is numerically implemented and tested for the identification of single and multiple localized damages. The sensitivity of the technique to the number of frequencies used and to the shape, intensity and position of the damages, as well as to the presence of noise in the data, is evaluated and discussed. The effect of suitable filtering of the results based on a priori information on the physics of the problem is proposed. An experimental application to the identification of localized damage in a free-free steel rod is also presented

Monitoring an Excavation in an Urban Area

A new building recently constructed in the centre of the city of Salerno, Italy, comprises a two level underground park with a maximum depth of 8 m below the ground surface and around 5 m below the ground water table. The excavation was carried out within a cast in situ reinforced concrete diaphragm wall, with a thickness of 0.6 m and a length of 18 m. The subsoil profile consists essentially of sand and gravel, with horizontal layers of silt and silty sand. The main silty layer, located near the toe of the diaphragm, appears to be continuous over the excavation area. Other thinner layers are found between the main one and the bottom of the excavation, giving rise to some concern for a possible bottom heave. To eliminate such a risk, the water was pumped from wells with the filter above the main silty layer. The excavation is close to existing buildings and infrastructures; accordingly, a monitoring program has been carried out to control the effects of excavation and dewatering on the surroundings. The horizontal displacements of the diaphragm wall have been measured by means of inclinometers, while vertical displacements of points at the ground surface and on the nearby buildings have been observed by means of precision levelling. Finally, the ground water level has been monitored by standpipe piezometers inside and outside the excavation. The paper reports the results of the measurements and compares them to the previous available experimental evidence

a pore water pressure model calibration based on in situ test results

Abstract This paper proposes a procedure for the calibration of a simplified pore water pressure model implemented in 1D effective stress dynamic analyses. The calibration procedure is based on the cyclic strength of the soils as quantified using empirical correlations with in-situ tests, CPT and SPT. Specific relationships have been directly defined among the parameters of the pore water pressure model and the results of in-situ tests. All the steps for the definition of these relationships are described in detail. The proposed calibration procedure has been used to simulate the seismic response of two well-documented reclaimed sites where widespread liquefaction occurred: Port Island, in Kobe (Japan) and Treasure Island in California (US), struck by the 1995 Kobe and the 1989 Loma Prieta earthquakes, respectively. The results of the analyses have been compared to the actual site response as recorded by downhole acceleration arrays, showing that the proposed approach leads to a good estimate of the site response. Guidelines for calibration of the proposed model are finally provided, and the advantages and limitations of its use are discussed in detail

Prediction of stresses and strains around model tunnels with adjacent embedded walls in overconsolidated clay

This paper presents the results of finite element analyses carried out using different constitutive models for overconsolidated clay: the Modified Cam clay model and the Three-Surface Kinematic Hardening (3-SKH) model. These analyses are evaluated against data from an extensive series of physical model tests examining the influence of an embedded wall placed near a tunnel on ground movements and tunnel stability. It is shown that for heavily overconsolidated soils reasonable predictions of both deformations and failure can be obtained from kinematic hardening models such as the 3-SKH model, which allow plastic deformation inside a Modified Cam clay state boundary surface

Experimental Assessment of the Effect of Vertical Earthquake Motion on Underground Metro Station

This paper presents experimental assessment of the effect of the ratio of vertical to horizontal peak ground acceleration (RVH) on underground metro station. An atrium-style metro station embedded in artificial soil subjected to earthquake loading is examined through shaking table tests. The experimental results for three different RVH, including soil acceleration, soil-structure acceleration difference, dynamic soil normal stress (DSNS), and structural dynamic strain, are presented and the results are compared with the case of horizontal-only excitation. It is found that for an atrium-style metro station, the differences in horizontal acceleration amplitude between the structure and the adjacent soil rise with increasing RVH, which are different at different depths. The most significant differences occur at the depth of the ceiling slab. It is also observed that both the amplitude and distribution of peak DSNS have obvious differences between the left and right side walls at all levels. It is therefore concluded that the RVH has a significant influence on dynamic soil-structure interaction. It is believed that under extreme earthquake loading, such as near fault zones, RVH is a parameter of paramount importance and should be accounted for in the seismic analyses and seismic performance assessments of underground structures, especially for those with zero or near-zero buried depth, such as atrium-style metro stations

Seismic vulnerability of circular tunnels in sand

The paper is focused on the assessment of seismic fragility curves for circular tunnels under moderate to severe earthquakes with the aim of improving the reliability of the risk assessment of unde..