178 research outputs found
The influence of soil plasticity on the seismic performance of bridge piers on caisson foundations
No full textThis paper investigates the role of soil plasticity on the seismic performance of bridge piers founded on cylindrical caissons through a numerical study where the geometrical and mechanical properties of the caisson and the pier, as well as the weight of the deck, are varied. Numerical models of the soil-caisson-pier-deck systems are subjected to six real acceleration time histories differing in frequency content, strong-motion duration and intensity. Three-dimensional coupled dynamic analyses are carried out in the time domain in terms of effective stresses, assuming an undrained response for the foundation soils. The studied systems are designed to be characterised by the same safety factors under static vertical loads, to evaluate the seismic performance of different systems endowed with similar mobilised shear strength, and by a low pseudo-static factor of safety to promote the activation of plastic mechanisms during seismic shaking. To reproduce the initial state of stress around the caissons, the effects induced by the construction stages are also simulated in the analyses through a simplified procedure.
Seismic performance of the systems is evaluated comparing the maximum and the permanent deck drift ratios with corresponding threshold values related to ultimate limit states. It is shown that seismic performance is strongly related to the ratio between the fundamental periods of the flexible-base system and the soil, as well as to the strong motion duration. The range of systems and input properties for which soil plasticity is significant is identified, thus avoiding excessive overestimates of earthquake-induced displacements and inertial forces on the superstructure
Equivalent seismic coefficients for caisson foundations suppoting bridge piers
No full textSafety of a foundation under seismic loading is strongly dependent on the inertial forces transmitted by the superstructure, exchanged with the surrounding soil and acting into the foundation itself. The latter contribution, typically neglected for shallow and pile foundations, should be considered for caisson foundations, much more massive and rigid than the foundation soil. In this paper, the inertial forces acting in caisson foundations during seismic shaking are extracted from the results of a parametric study where different caissons supporting bridge piers are subjected to severe ground motions. The parametric study was carried out in the time domain via 3D Finite Element (FE) dynamic analyses performed in terms of effective stresses but assuming an undrained response of the foundation soil. Non-linear and inelastic soil behaviour was described in the analyses by an elastic-plastic constitutive model with isotropic hardening. In the framework of a pseudo-static approach, the caisson inertia is represented by equivalent horizontal and rotational seismic coefficients, kh eq and krot eq, relating the generalised inertial forces to the caisson weight. The coefficient kh eq turns out to be always remarkably smaller than the maximum value computed at ground surface in free-field conditions, kh max(g.s.). The equivalent seismic coefficients kh eq and krot eq are expressed via empirical relationships as a function of the dynamic properties of the whole system and the seismic input, through dimensionless parameters. Calculation examples are finally given, where safety assessment of bearing capacity is made for different systems using the pseudo-static approach, showing that use of the seismic coefficients computed by the proposed relationships yields results consistent with the ones obtained from the dynamic analyses
A simplified procedure for the evaluation of the seismic performance of bridge piers on caisson foundations
No full textIn this paper, a simplified procedure for the evaluation of the seismic performance of bridge piers founded on caissons subjected to strong ground motions is outlined. To this end, the up-per-bound semi-empirical relationships proposed in [1] are considered for the estimation of the seismic performance, expressed in terms of the maximum and permanent values of the deck drift ratio attained during and at the end of the seismic event. These drifts were related to the period ratio Teq/T0 between the fundamental periods of the deck-pier-caisson-soil sys-tem and of the soil column in free-field conditions. The deck drift and the period ratios were extracted from the results of an extensive parametric study, where 14 different systems were subjected to 6 real high-intensity seismic records. In the parametric study, 3D dynamic anal-yses were performed with the Finite Element Method in the time domain, in terms of effective stresses but assuming undrained conditions and adopting an elastic-plastic constitutive model to reproduce the irreversible soil behaviour under cyclic loading. As 3D dynamic numerical analyses are not expected to become an every-day design tool, the period ratios Teq/T0 are evaluated through empirical and analytical relationships available in the literature as well and then compared with the ratios obtained from the parametric study, to assess the possibil-ity of using simplified relationships while still getting a reliable estimate of the deck drift ra-tio. It is shown that these relationships can be profitably adopted provided that a fair estimate of the equivalent shear wave velocity, depending on the intensity of the seismic inputs, is used
Deformabilità non-drenata statica e dinamica di un'argilla fortemente sovraconsolidata.
No full textInfluence of input assumptions on evaluation of seismic performance of an earth dam
No full textSeismic performance of earth dams can nowadays be performed via dynamic analyses using constitutive models of sufficient accuracy in describing cyclic soil behaviour. However, a number of assumptions different from soil model affect the problem as a whole. In this paper, a 2D model of a simplified scheme of a homogeneous earth dam is developed, to assess the influence of bedrock compliance, vertical component of ground motion and initial pore water pressure distribution on its seismic performance, thus providing some guidance on the choices that should be adopted for a safe assessment of the seismic performance of existing earth dams
Riduzione degli spostamenti indotti da scavi profondi mediante pannelli sacrificali: simulazione numerica 3D
No full text3D finite element analysis of deep excavations with cross-walls
No full textGround movements induced by deep open excavations retained by diaphragm walls may be substantially reduced using sacrificial cross-walls installed as props between the retaining walls. In this paper, a 3D finite element study is presented in which the retaining walls and the cross-walls are modelled using brick elements and accounting for the interfaces between the panels of diaphragm walls and of cross-walls. Influence of cross-walls spacing and length in reducing the horizontal deflection of diaphragm walls and the ground settlements behind the excavation is evaluated. Companion plane strain analyses, in which an homogeneous equivalent medium is considered in between the retaining walls, are also presented to assess the capability of 2D analyses to predict the performance of deep excavations in which cross-walls are used as a mitigation measure to reduce wall deflections and ground movements
An experimental investigation into the mechanical behaviour of a structured stiff clay
No full textIn recent years, fundamental research has been carried out into the properties of some natural stiff clays and the corresponding reconstituted materials, highlighting the role of microstructural features in the observed differences. In this paper the results of an experimental investigation into the mechanical behaviour of an Italian stiff clay of marine origin are presented. Medium-pressure and high-pressure stress-controlled triaxial cells were used in which natural samples underwent isotropic and anisotropic compression and swelling before drained or undrained shearing. Comparison of soil behaviour observed after different compression histories up to different values of maximum effective stress allowed the following aspects to be discussed: the effects of the initially structured state on the medium to large strain response and shear strength characteristics of the soil; the relevance of volumetric and deviatoric plastic strain to the structure degradation; the role and implications of the imposed non-isotropic stress histories; the permanent differences between reconstituted samples and fully destructured natural samples; and the uniqueness of the critical state condition
An experimental investigation of the mechanical behaviour of a pyroclastic soft rock
No full textThis paper presents the results of an experimental investigation of the mechanical behaviour of a pyroclastic soft rock, locally known as Pozzolana Nera, from the volcanic complex of the Colli Albani (Roma). The laboratory testing programme consisted of standard drained and undrained triaxial compression tests, at increasing values of cell pressure. One-dimensional compression tests were also carried out in oedometers with reduced cross-section. The testing programme made it possible to investigate the full range of behaviour of the material from contractant to dilatant. Attention was focused on assessing the influence of natural microstructure on the shear strength parameters. The stress-dilatancy curves in triaxial compression were examined in an attempt to isolate the dilatancy component and the bonding component of peak strength
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