Mahdi Taiebat

International Benchmark on Numerical Simulations for 1D, Nonlinear Site Response (PRENOLIN): Verification Phase Based on Canonical Cases

PREdiction of NOn‐LINear soil behavior (PRENOLIN) is an international benchmark aiming to test multiple numerical simulation codes that are capable of predicting nonlinear seismic site response with various constitutive models. One of the objectives of this project is the assessment of the uncertainties associated with nonlinear simulation of 1D site effects. A first verification phase (i.e., comparison between numerical codes on simple idealistic cases) will be followed by a validation phase, comparing the predictions of such numerical estimations with actual strong‐motion recordings obtained at well‐known sites. The benchmark presently involves 21 teams and 23 different computational codes. We present here the main results of the verification phase dealing with simple cases. Three different idealized soil profiles were tested over a wide range of shear strains with different input motions and different boundary conditions at the sediment/bedrock interface. A first iteration focusing on the elastic and viscoelastic cases was proved to be useful to ensure a common understanding and to identify numerical issues before pursuing the nonlinear modeling. Besides minor mistakes in the implementation of input parameters and output units, the initial discrepancies between the numerical results can be attributed to (1) different understanding of the expression “input motion” in different communities, and (2) different implementations of material damping and possible numerical energy dissipation. The second round of computations thus allowed a convergence of all teams to the Haskell–Thomson analytical solution in elastic and viscoelastic cases. For nonlinear computations, we investigate the epistemic uncertainties related only to wave propagation modeling using different nonlinear constitutive models. Such epistemic uncertainties are shown to increase with the strain level and to reach values around 0.2 (log_(10) scale) for a peak ground acceleration of 5  m/s^2 at the base of the soil column, which may be reduced by almost 50% when the various constitutive models used the same shear strength and damping implementation.

Application of an Anisotropic Constitutive Model for Structured Clay to Seismic Slope Stability

The anisotropic nature of response and degradation of shear strength from the undisturbed condition to the remolded state are two fundamental and challenging aspects of response in some clay deposits. This paper presents a comprehensive, yet flexible and practical, version of the SANICLAY model and its application to a seismic slope-stability problem. The model is based on the well-known isotropic modified Cam-Clay model with two additional mechanisms to account for anisotropy and destructuration. The model has been efficiently implemented in a three-dimensional (3D) continuum, coupled, dynamic, finite-difference program. The program has been used to analyze the seismic response of clay slopes to gain better insight into the role of the previously mentioned parameters in real applications. Different aspects of the model, including anisotropy and destructuration, and their effects on the earthquake-induced strains and deformations in the slope have then been explored and presented. By providing a link between the model parameters and the soils undrained shear strength, which is awell-known engineering parameter, a benchmark comparison has been made between the results of the present advanced model and those of an engineering approach. To this end, a modified Newmark sliding-block analysis has been used, in which the yield acceleration is gradually reduced as block sliding progresses during the earthquake. It is observed that although the two analyses display the same trends, the modified Newmark sliding-block method provides conservative results compared with those obtained from the developed simulation model. DOI: 10.1061/(ASCE)GT.1943-5606.0000458.

Simple yield surface expressions appropriate for soil plasticity.

The objective of this paper is to present a number of simple, practical, and useful analytical expressions of a yield surface for geomaterials. In classical plasticity, the analytical expression of a yield surface defines the locus of points in stress space at which plastic flow initiates, and the corresponding function must depend on direct and mixed invariants of stress and tensor-valued internal variables. One single function describes a yield surface in order to avoid singularities and computational difficulties arising from the use of multiple functions representing intersecting surfaces in stress space that are often used for cap-type models in soil plasticity. The presented functions are conveniently subdivided in three main categories depending on the type of analytical expression used, and they all describe properly closed yield surfaces which are continuous and convex. The internal variables in these functions can be used in order to address classical plasticity features such as isotropic and ki...

SANISTEEL: Simple Anisotropic Steel Plasticity Model

A simple constitutive model for the inelastic response of steel under monotonic and random cyclic loading conditions is developed within the framework of bounding surface plasticity. The particular feature that distinguishes this model from other similar ones is the ability of the bounding surface formulation to describe in a very simple way the initial “plateau” type of perfectly plastic response that many kinds of structural steels exhibit upon initial yield in tension or compression, before hardening begins. The key constitutive element is to assume a fixed nonhardening bounding surface during the plateau response until a cumulative plastic strain threshold is reached, while the yield surface softens isotropically and hardens kinematically. In this way not only monotonic but also cyclic loading within the plateau range can be easily described. Three kinematic hardening rules for the bounding surface are explored. The development is focused on uniaxial loading conditions that are typical in many structu...

PRENOLIN: International Benchmark on 1D Nonlinear Site‐Response Analysis—Validation Phase Exercise

This article presents the main results of the validation phase of the PRENOLIN project. PRENOLIN is an international benchmark on 1D nonlinear (NL) site‐response analysis. This project involved 19 teams with 23 different codes tested. It was divided into two phases; with the first phase verifying the numerical solution of these codes on idealized soil profiles using simple signals and real seismic records. The second phase described in this article referred to code validation for the analysis of real instrumented sites. This validation phase was performed on two sites (KSRH10 and Sendai) of the Japanese strong‐motion networks KiK‐net and Port and Airport Research Institute (PARI), respectively, with a pair of accelerometers at surface and depth. Extensive additional site characterizations were performed at both sites involving in situ and laboratory measurements of the soil properties. At each site, sets of input motions were selected to represent different peak ground acceleration (PGA) and frequency content. It was found that the code‐to‐code variability given by the standard deviation of the computed surface‐response spectra is around 0.1 (in log10 scale) regardless of the site and input motions. This indicates a quite large influence of the numerical methods on site‐effect assessment and more generally on seismic hazard. Besides, it was observed that site‐specific measurements are of primary importance for defining the input data in site‐response analysis. The NL parameters obtained from the laboratory measurements should be compared with curves coming from the literature. Finally, the lessons learned from this exercise are synthesized, resulting also in a few recommendations for future benchmarking studies, and the use of 1D NL, total stress site‐response analysis.

Seismic Evaluation of Existing Basement Walls

Current state of practice for seismic design of basement walls is using the Mononobe-Okabe (M-O) method that is based on the Peak Ground Acceleration (PGA). The National Building Code of Canada (NBCC) has considerably changed the seismic hazard level from 10 % in 50 years in NBCC1995 to 2 % in 50 years in NBCC2010, which leads to doubling the PGA in Vancouver from 0.24g to 0.46g. The current design PGA leads to very large seismic forces that make the resulting basement walls very expensive. Because there is a little evidence of any significant damage to basement walls during major earthquakes, the Structural Engineers Association of British Columbia (SEABC) initiated a task force to review current seismic design procedures for deep basement walls. Presented in this paper are some preliminary results of the work conducted by this committee. A series of dynamic numerical analyses have been carried out on a typical basement wall designed using the M-O earth pressures with the NBCC1995 PGA for Vancouver. This wall is then subjected to three ground motions spectrally matched to the Uniform Hazard Spectrum prescribed by the NBCC2010 and the seismic performance of the wall under this level of demand has been presented and discussed. Particular attention has been given to the resulting drift ratio in the walls.

Modeling and Simulation of Saturated Geomaterials

Fully coupled, porous solid-fluid formulation and related modeling and simula- tion issues are presented in this work. In particular, coupled dynamic field equations withuipiU formulation are used to simulate pore fluid and soil skeleton responses. Present formulation allows, among other features, for water accelerations to be taken into account. This proves useful in model- ing dynamic interaction of media of different stiffness (as in soil-foundation-structure interaction). Fluid compressibility is also explicitly taken into account, thus allowing excursions into modeling of limited cases of non-saturated porous media. One of the main challenges in modeling saturated soils is the appropriate modeling of elastic- plastic behavior of soil skeleton. This challenge is met with the use of an advanced material model (Dafalias and Manzari 2004) and is discussed at some length. Illustrative examples describing dynamical behavior of porous media (saturated soils) are pre- sented. Of particular interest is the verification and validation (V&V) of current models and simu- lations, and examples will be used for that purpose. Background

Validation of a New Elastoplastic Constitutive Model Dedicated to the Cyclic Behaviour of Brittle Rock Materials

AbstractOld mines or caverns may be used as reservoirs for fuel/gas storage or in the context of large-scale energy storage. In the first case, oil or gas is stored on annual basis. In the second case pressure due to water or compressed air varies on a daily basis or even faster. In both cases a cyclic loading on the cavern’s/mine’s walls must be considered for the design. The complexity of rockwork geometries or coupling with water flow requires finite element modelling and then a suitable constitutive law for the rock behaviour modelling. This paper presents and validates the formulation of a new constitutive law able to represent the inherently cyclic behaviour of rocks at low confinement. The main features of the behaviour evidenced by experiments in the literature depict a progressive degradation and strain of the material with the number of cycles. A constitutive law based on a boundary surface concept is developed. It represents the brittle failure of the material as well as its progressive degradation. Kinematic hardening of the yield surface allows the modelling of cycles. Isotropic softening on the cohesion variable leads to the progressive degradation of the rock strength. A limit surface is introduced and has a lower opening than the bounding surface. This surface describes the peak strength of the material and allows the modelling of a brittle behaviour. In addition a fatigue limit is introduced such that no cohesion degradation occurs if the stress state lies inside this surface. The model is validated against three different rock materials and types of experiments. Parameters of the constitutive laws are calibrated against uniaxial tests on Lorano marble, triaxial test on a sandstone and damage-controlled test on Lac du Bonnet granite. The model is shown to reproduce correctly experimental results, especially the evolution of strain with number of cycles.

Evaluation of p-y Curves Used in Practice for Seismic Analysis of Soil-Pile Interaction

A widely used approach in practice for modeling the seismic response of pile supported structures is Beam on Nonlinear Winkler Foundation (BNWF). In this approach, the nonlinear p‐y curves, recommended by the American Petroleum Institute (API), are used as the backbone curves for the lateral load‐deformation relationships. In this paper, the efficiency of these curves for seismic analysis of soil‐pile interaction is investigated. For this purpose, recorded results of four centrifuge tests conducted on single pile foundations are compared with the computed results based on the BNWF approach employing the API p‐y curves. The prediction of the seismic response based on the API p‐y curves was found to be poor in the cases studied here.

Numerical Simulation of Seismic Ground Motion Isolation Using Fully Coupled Nonlinear Response in Saturated Sands

Af ully coupled nonlinear dynamic numerical approach and an advanced constitutive model have been implemented in a three dimensional finite element computer code to predict theresponse ofsaturated porous mediaunder different types of loading condi- tions including earthquake loading. The formulation and implementation are already verified and validate, and were then applied to the problem of sand liquefaction and cyclic mobility phenomena for investigation on a specific configuration of layered soil column, where liq- uefaction of the deeper loose elements prevents transmission of earthquake induced shear stresses to the upper layers.