Nonlinear seismic response of tall concrete-faced rockfill dams in narrow canyons

The seismic behavior of tall concrete face rockfill dams in narrow canyons is investigated, based on numerical simulation of the staged construction, creep settlements, reservoir impoundment and seismic shaking of the dam. The study takes into account the flexibility of the canyon rock, the hydrodynamic effects and potential dynamic rockfill settlements. The static analysis uses a hyperbolic model for the rockfill, whereas the dynamic analysis uses a nonlinear hysteretic model, which accounts for the initial dynamic stiffness and produces hysteresis loops in agreement with the experimental data regarding the shear modulus and damping ratio. A damage plasticity model is used for the reinforced concrete, whereas frictional contact behavior is considered at the base and vertical walls of the concrete slab panels. An existing 150-m-high dam is used to investigate some key issues on the seismic behavior of such dams subjected to upstream downstream and vertical excitation. Emphasis is placed on the evaluation of the tensile stresses within the slab panels, the compressive stresses at the slab-to-slab vertical interfaces and the opening of the joints. Moreover, the effect of potential dynamic settlements on both the slab stresses and joint openings is investigated. Recommendations for increasing the dam safety and reducing the water leakage through the dam body are given. (C) 2011 Elsevier Ltd. All rights reserved

Longitudinal vibrations of tall concrete faced rockfill dams in narrow canyons

Longitudinal vibrations of concrete-faced rockfill dams may cause significant compressive stresses and joint openings in the slab panels. The behavior of such dams subjected to longitudinal and vertical vibrations is investigated, based on numerical simulation of the staged construction, reservoir impoundment and seismic shaking. The static analysis uses a hyperbolic model for the rockfill, whereas the seismic analysis uses a nonlinear hysteretic model which accounts for the dynamic properties. A damage plasticity model is used for the reinforced concrete and frictional contact behavior is considered at the base and vertical sides of the concrete panels. The seismic analysis takes into account the flexibility of the canyon rock and potential dynamic rockfill settlements. An existing 150 m-high dam is used to investigate the effect of longitudinal vibrations on the compressive stresses near the slab-to-slab vertical interfaces and the opening of the joints. The effect of dynamic settlements is examined and comparisons are made to the response from upstream/downstream and combined vibrations. The effectiveness of introducing 5 cm-wide cuts at selected vertical joints to reduce slab compression in existing CFRDs is demonstrated. The presented results offer useful insight into the effect of longitudinal vibrations on the seismic behavior of CFRDs. (C) 2012 Elsevier Ltd. All rights reserved

Effect of spatial variability of soil properties on the stability and permanent seismic displacements of highway slopes

The effect of spatial variability of soil properties, slope inclination and excitation characteristics on the development of permanent displacements is investigated using random fields, created by the Local Average Subdivision method. These random fields of properties are characterized by specific mean, variance, cross-correlation coefficients and autocorrelation lengths and used for performing stability and seismic analysis of various slopes using the finite difference software FLAC. Statistical analysis of the results of an extensive parametric investigation leads to the development of vulnerability curves, based on the permanent horizontal or resultant displacement for several seismic intensity levels. The results of parametric analysis show that the influence of spatial variability of soil properties is very important. © The authors and IGS: All rights reserved, 2019

Liquefaction performance of shallow foundations in presence of a soil crust

Liquefiable soils are currently categorized by all seismic codes as extreme ground conditions where, following a positive identification of this hazard, the construction of surface foundations is essentially allowed only after proper treatment soil. This article examines to what extent this situation may change in presence of a non-liquefiable soil crust, between the foundation and the liquefiable soil. Means are provided for analytical evaluation of the degraded bearing capacity and the associated seismic settlements for the specific case of strip foundations on a cohesive (clay) crust. Furthermore, the conditions are explored which ensure a viable performancebased design, and the issue of a critical soil crust thickness, beyond which liquefaction effects are minimal, is addressed. © 2007 Springer

Effect of spatial variability of soil properties on permanent seismic displacements of slopes with uniform load

The purpose of this study is to investigate the development of permanent displacements of earth slopes, with spatially varying properties and a uniform surface load, subjected to a strong seismic excitation. The uncertainties associated with the determination of the mechanical properties of the slopes and the intensity of the seismic excitation, make the use of stochastic methods very attractive. As opposed to deterministic methods, stochastic methods allow an acceptable risk level based on the specifications of each project. Recent research described the spatial variability of soil parameters using the Random Field Theory. Apart from the prescribed statistical characteristics and cross-correlation of the various soil properties, the generated random field variables exhibit autocorrelation, a trend in which the soil properties of a point appear to be correlated with the soil properties of neighbouring points. Among the various random field algorithms, a particularly effective one is the Local Average Subdivision (LAS) method by Fenton and Vanmarcke [1]. To this end, a large number of random fields of soil properties is generated for a natural slope, having prescribed mean value µ, standard deviation s, cross-correlation coefficients pij of properties i and j, and spatial autocorrelation lengths lx and ly for the vertical and horizontal directions, respectively. The numerical simulations are achieved using an automated procedure, based on collaboration of Mathematica, the incorporated LAS algorithm, and the finite difference program FLAC. The studied slope is loaded with a uniform load and subjected to a seismic shaking of various intensities using a series of acceleration records obtained from the five different earthquakes. The paper focuses mainly on the development of permanent seismic displacements of the slope at the end of ground shaking. It is demonstrated that the spatial variability of soil properties has a significant effect on the values of permanent displacements, resulting to a wide range of displacement variation. © Published under licence by IOP Publishing Ltd

Effects of spatial variability of soil properties and ground motion characteristics on permanent displacements of slopes

The seismic performance of earth slopes is a problem of great importance in geotechnical earthquake engineering associated with various sources of uncertainty. This research investigates the effects of two important causes of such uncertainty: (a) the spatial variability of soil properties and (b) the variability of the temporal and frequency characteristics of the earthquake excitation. The former is investigated based on a series of detailed dynamic numerical simulations using stationary random fields of both cross-correlated and spatially autocorrelated soil properties. The latter is explored by conducting incremental dynamic analysis at various levels of excitation intensity using (a) a total of forty historic earthquake and synthetic records, modified to match the Eurocode 8 design spectra for rock sites, and (b) thirty original historic earthquake records. The results demonstrate the significant effect of the spatial variability of soil properties on the permanent slope displacements. Moreover, they show that the effect of the uncertainty associated with the variability in the temporal and frequency characteristics of excitations is more important compared to the effect of the spatial variability of soil properties. © 2022 Elsevier Lt

Seismic effective-stress analysis of caisson quay walls: Application to Kobe

Deformation-based seismic design of gravity quay-walls requires realistic computation of residual deformations. This article presents an effective-stress analysis method, which is based on an elasto-plastic constitutive model formulated into a finite-difference algorithm. The model is applicable to cohesionless soils, for a wide range of relative densities and confining pressures. The formulation is applied first to re-analyze one of the failed caisson-type quay-walls of Rokko Island during the 1995 Kobe (Hyogoken-nambu) earthquake (Case 1). Subsequently, it is applied to analyse three closely related case studies of quay-walls, subjected to the same earthquake excitation, to demonstrate the effects of ground improvement on the wall performance. Case 2 considers a quay-wall in which both the foundation and backfill consist of improved, non-liquefiable soils. Case 3 considers a quay-wall in which the backfill soil remains liquefiable, whereas the foundation soil has been improved. Finally, in Case 4 the foundation soil is liquefiable, and the backfill soil improved. The results are consistent with both field observations and earlier independent computer simulations by lai et al. 1998 which were based on the finite-element method and a different constitutive model

Simulation of random fields of soil properties by the local average subdivision method and engineering applications

The seismic performance of oil and natural gas pipelines founded or embedded in earth slopes encompasses great uncertainty related both to the earthquake shaking characteristics and to the natural heterogeneity of geomaterials. Regarding the latter, the parameters of shear strength, stiffness, density, etc., may vary indeed from point to point even within the same soil layer as a result of the natural formation process. Apart from their cross-correlation, such random variables exhibit autocorrelation, in which the soil properties at a given point appear to be spatially correlated with the properties of neighbouring points. Therefore, there is a need to use stochastic methods in the safety evaluation of such systems. Aiming at the reliability assessment of such soil-pipeline systems under seismic shaking, this paper introduces an automated methodology for generating random fields using the Local Average Subdivision (LAS) method by Fenton and Vanmarcke (J Eng Mech 116(8):1733–1749, 1990). Subsequently, it performs rigorous nonlinear dynamic analysis of a given slope using the finite difference method. The automated procedure is used in a Monte-Carlo simulation scheme for computing the probability of exceeding different levels of anticipated permanent slope movement for different levels of shaking intensity. The results demonstrate that the effect of the spatial variability of the soil properties on the permanent displacements of natural slopes is important, leading to a range of variation of about ± 60%. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature

Experimental and numerical modelling of global buckling of underground pipelines due to high pressure and temperature

Pipelines transporting oil and natural gas may develop substantial compressive forces due to high temperature and pressure of the fluid content. Such compressive forces may cause vertical (upheaval) buckling of the pipeline. The development of upheaval buckling due to thermal expansion may pose a major threat to the structural integrity, safety and operability of pipelines. This paper describes a series of physical model tests in order to investigate the uplift behavior of buried pipelines. The experiments investigate the resistance force during vertical pipe pullout for varying pipe geometries, embedment depths and soil properties. Two types of sand are considered, a medium dense sand exhibiting dilative behavior, and a loose sand exhibiting contractive behavior. Furthermore, numerical analyses are conducted to simulate the above experiments and to validate the numerical models. The elastoplastic Mohr-Coulomb constitutive model with hardening-softening behavior, dependent on both the plastic shear strain and the confining stress, is utilized for simulating the behavior of sand. Representative results from the validation process of the numerical model are presented and compared with experimental measurements. Finally, some preliminary analyses of the behavior of buried pipelines in upheaval buckling due to high temperature change are presented. © 2019, National Technical University of Athens. All rights reserved