Seismic Performance of the San Pietro Dam

Abstract The paper focuses on the seismic performance of the San Pietro dam (located in Southern Italy) evaluated, with different levels of accuracy, through pseudo-static, simplified-displacement and dynamic 2D finite difference analyses. For critical mechanisms, detected through pseudo-static analyses, simplified displacement analyses were performed assessing the horizontal and vertical components of the expected permanent displacements. Dynamic analyses were carried out accounting for non-linear soil behavior under cyclic loading. The adopted input motions consist of several sets of accelerograms selected, from a worldwide database, assuming as a reference the seismicity of the area where the dam is located. The results of the analyses show a satisfactory behavior of the dam for the selected input motions

Seismic Response of Submerged Cohesionless Slopes

Seismic slope stability analysis is a topic of great interest in geotechnical and geoenvironmental engineering particularly in seismic area in fact the occurrence of earthquake induced landslides is documented in many recent post-earthquake damage report (Japan 1993- 199 Greece 1995, Turkey 1999). Generally saturated slopes of loose sand or silty-sand and earth dams and embankment resting on low cohesionless soil deposit are highly susceptible to liquefaction-induced damage and during strong earthquake several landslides caused soil liquefaction may occur. In this paper a numerical model to evaluate seismic response of submerged cohesionless slopes is described. Slope stability conditions are evaluated taking into account the inertial effect of seismic forces and the earthquake induced pore pressure which reduce soil effective stress state. Displacement analysis has been performed using an extension of Newmark’s sliding block model for rotational failure mechanism and taking into account the reduction of slope critical acceleration due to changes in pore pressure Applying the proposed model a numerical analysis has been performed in order to point out those parameters which mostly affect seismic slope response and some useful stability charts are provided

Generation of fully non-stationary random processes consistent with target seismic accelerograms

Abstract In this paper, a method for generating samples of a fully non-stationary zero-mean Gaussian process, having a target acceleration time-history as one of its own samples, is presented. The proposed method requires the following steps: i) divide the time axis of the target accelerogram in contiguous time intervals in which a uniformly modulated process is introduced as the product of a deterministic modulating function per a stationary zero-mean Gaussian sub-process, whose power spectral density (PSD) function is filtered by two Butterworth filters; ii) estimate, in the various time intervals, the parameters of modulating functions by least-square fitting the expected energy of the proposed model to the energy of the target accelerogram; iii) estimate the parameters of the PSD function of the stationary sub-process, once the occurrences of maxima and of zero-level up-crossings of the target accelerogram, in the various intervals, are counted; iv) obtain the evolutionary spectral representation of the fully non-stationary process by adding the various contribution evaluated in the various intervals

Dynamic Model Tests on Gravity Retaining Walls with Various Surcharge Conditions

Seismic design of retaining walls is traditionally based on the Mononobe-Okabe method of analysis. In recent years a number of theoretical analyses have been presented to predict the seismic behaviour of gravity retaining walls. In this paper some shaking table tests performed on a small prototype of gravity wall retaining dry sand are described and the experimental results are presented with the aim to provide, though qualitatively, an insight into some important aspects of the dynamic behaviour of retaining structures resting on rigid foundation soil. The M-O theory do not consider the particular boundary condition that in the practical design of retaining structures are often in use like backfill geometries or loading condition. Shaking table studies were carried out in order to study the dynamic behaviour of gravity retaining walls resting on rigid foundation soil. Two different system have been taken into consideration namely, a wall retaining a horizontal backfill on which uniform surcharge was placed and a wall on which the uniform surcharge was placed to a distance «d» to the head of the wall

Screening-level analyses for the evaluation of the seismic performance of a zoned earth dam

Many existing earth dams have been designed and built worldwide before the establishment of a seismic code, so that it is of relevant interest to evaluate their seismic performance and post-seismic operational conditions. This requires an accurate geotechnical characterisation of the dam and foundation soils, a proper definition of the seismic scenarios at the site of the dam, the use of simplified procedures for screening-level seismic analyses and advanced nonlinear dynamic analyses to study the most critical seismic scenarios. This process has been used for the evaluation of the seismic performance of a zoned earth dam located in a high seismic hazard area of Southern Italy. In this paper the available data of historical seismicity at the site of the dam and the results of a probabilistic seismic hazard analysis are first discussed and input ground motions are selected using compatibility criteria with the energy and frequency content of the expected target motion in a range of vibration period relevant for the non-linear response of the dam. Seismic performance of the dam is then evaluated through procedures based on Newmarktype computations, in which permanent displacements are related to ground motion characteristics and to the seismic resistance of the dam, the latter evaluated detecting the earthquake-induced plastic mechanisms and the corresponding critical accelerations. Also, an original improvement of the well-known Makdisi & Seed procedure, was proposed to better capture the actual influence of non-linear soil behaviour in the evaluation of horizontal acceleration and permanent displacements of the crest of the dam. The analysis results pointed out the relevant role of the earthquake-induced shear strength reduction on the dam permanent displacements

Decoupled seismic analysis of an earth dam

The seismic stability of an earth dam is evaluated via the decoupled displacement analysis using the accelerograms obtained by ground response analysis to compute the earthquake-induced displacements. The response analysis of the dam is carried out under both 1D and 2D conditions, incorporating the non-linear soil behaviour through the equivalent linear method. Ten artificial and five real accelerograms were used as input motions and four different depths were assumed for the bedrock. 1D and 2D response analyses were in a fair agreement with the exception of the top third of the dam where only a 2D modelling of the problem could ensure that the acceleration field is properly described. The acceleration amplification ratio obtained in the 2D analyses was equal to about 2 in all the cases considered, consistently with data from real case histories. The maximum permanent displacements computed by the sliding block analysis were small, being less than 10% of the service freeboard; a satisfactory performance of the dam can then be envisaged for any of the seismic scenarios considered in the analyses. © 2003 Elsevier Science Ltd. All rights reserved

The Static and Seismic Bearing Capacity Factor Nγ for Footings Adjacent to Slopes

AbstractFoundations resting adjacent to the crest of a slope exhibit a smaller bearing capacity respect to the case of horizontal ground. In this paper the static and seismic values of the bearing capacity factor Nγg for shallow strip foundations adjacent to slopes were evaluated using the method of characteristics, extended to the seismic case by means of the pseudo-static approach. Nγg was evaluated for different values of the slope angle and, under seismic conditions, accounting only for the effect of horizontal and vertical inertia forces arising in the foundation soil. The results, for both smooth and rough foundations, are presented and checked against those obtained through finite element analyses

Effect of vertical input motion and excess pore pressures on the seismic performance of a zoned dam

This paper investigates the combined effect of the vertical component of the input motion and of the weakening effect associated to pore-pressure build-up with reference to a zoned earth dam for which the most recent probabilistic seismic hazard analyses promoted the need of evaluating its seismic performance. The two effects were studied through advanced plane-strain non-linear dynamic analyses using a finite difference numerical model calibrated on an accurate geotechnical characterisation. The occurrence of ultimate limit states in the dam embankment is checked using sets of horizontal and vertical input motions properly selected to account for possible frequency coupling with the dam. The analysis results are presented and discussed in the paper focusing on (i) the main features of the plastic mechanisms temporarily induced by the seismic actions, (ii) the amplification of the horizontal and vertical accelerations acting in the dam body, (iii) the role of the energy and frequency content of the input motion on the magnitude of earthquake-induced permanent displacements, (iv) the combined effect of frequency coupling and non-linear soil behaviour on the overall dam response

Dynamic characterization of fine-grained soils for the seismic microzonation of Central Italy

An accurate measurement of dynamic soil properties is essential to predict the nonlinear soil behavior under seismic loading conditions. This paper presents a database of cyclic and dynamic laboratory tests carried out after the 2016-2017 Central Italy Earthquake sequence, as part of the seismic microzonation studies in the area. The database consists of experimental results obtained on 79 samples investigated by means of dynamic resonant column tests, cyclic torsional shear tests or cyclic double specimen direct simple shear tests. The dynamic soil behavior is analyzed with reference to the small-strain wave velocity and damping ratio and to the modulus reduction and damping ratio curves. Experimental data are compared with the most widely used predictive models to underline the peculiarities of the investigated soils. Finally, a predictive model is calibrated to capture the nonlinear behavior of typical fine-grained soils from Central Italy