Three-Dimensional Fundamental Solution for Unsaturated Poroelastic Media under Dynamic Loadings

International audienceThis paper aims at obtaining a 3D fundamental solution for unsaturated soils under dynamic loadings in Laplace transform domain using the method of Hörmander. These solutions can be used, afterwards, in a convolution quadrature method (CQM)-based boundary element formulations in order to model the wave propagation phenomena in such media in time domain

Thermo-Hydro-Mechanical Modeling of Damage in Unsaturated Porous Media: Theoretical Framework and Numerical Study of the EDZ

Copyright © 2011 John Wiley & Sons, Ltd.DOI: dx.doi.org/10.1002/nag.1005The damage model presented in this article (named ‘THHMD’ model) is dedicated to non-isothermal unsaturated porous media. It is formulated by means of three independent strain state variables, which are the thermodynamic conjugates of net stress, suction and thermal stress. The damage variable is a second-order tensor. Stress/strain relationships are derived from Helmholtz free energy, which is assumed to be the sum of damaged elastic potentials and ‘crack-closure energies’. Damage is assumed to grow with tensile strains due to net stress, with pore shrinkage due to suction and with thermal dilatation. Specific conductivities are introduced to account for the effects of cracking on the intensification and on the orientation of liquid water and vapor flows. These conductivities depend on damage and internal length parameters. The mechanical aspects of the THHMD model are validated by comparing the results of a triaxial compression test with experimental measurements found in the literature. Parametric studies of damage are performed on three different heating problems related to nuclear waste disposals. Several types of loading and boundary conditions are investigated. The thermal damage potential is thoroughly studied. The THHMD model is expected to be a useful tool in the assessment of the Excavation Damaged Zone, especially in the vicinity of nuclear waste repositories

Influence of soil properties and geometrical characteristics of sediment-filled valleys on earthquake response spectra.

International audienceIt is well-known that the response of a site to seismic excitation depends on both its local topography and its soil properties. Although the recent building codes (i.e. Eurocode8) incorporate these site effects, they only take into account unidimensional ones and ignore complex cases due to two-dimensional irregular configurations. Recent work has been mainly focusing either on the development of numerical methods allowing always more precise results or on the elaboration of simplified approaches usable for engineering purposes. The aim of this work is to propose a simple criterion, combining soil properties and geometrical characteristics of valleys to estimate the amplification of earthquake response spectra in sediment-filled valleys. We will first study the response of unidimensional soil layers to obtain a criterion that can be extended to bidimensional configurations. The seismic input is a synthetic SV Ricker wave with vertical incidence. Horizontal displacements at surface points are computed by using the HYBRID code which combines finite elements in the near field and boundary elements in the far field (FEM/BEM)

Retrofitting of the Compressor Foundation by Cement Grouting

In this paper, the dynamic behavior and foundation improvement of a K10-type compressor with maximum horizontal load of 1800 KN, operating at 3.3 Hz (200 rpm) located at top of a concrete foundation is studied. The vibration measurements (acceleration, velocity, and displacement) of the compressor and its foundation have been recorded before and after cement pressure grouting. In brief, the maximum displacement before cement injection exceeded the specified values by the manufacturer. Therefore, the existing pile-soil-foundation system needed to be improved. As the result, a retrofit scheme using cement pressure grouting by injection was recommended. In order to predict the dynamic behavior of the foundation after cement pressure grouting, a finite element model was constructed and then calibrated with the measurements that were taken before cement injection. Second set of measurements for compressor were carried out and results were compared with the model results after cement injection and curing procedure. It was concluded that the model has the ability to simulate and predict the behavior of the compressor accurately. The results show that the cement injection technique can be a reasonable and cost- effective remedial alternative to reduce the dynamic vibrations of the compressor. Moreover, the dynamic behavior of the compressor can be modeled using FEM

A multi-scale seismic response of two-dimensional sedimentary valleys due to the combined effects of topography and geology.

International audienceIt is well known that the response of a site to seismic excitation depends on the local topographical and geological conditions. The current building codes already take into account unidimensional site effects but ignore complex site effects due to two-dimensional irregular configurations. The aim of this work is to contribute to the establishment of a simple predictive method to estimate site effects. The horizontal ground movements at the surface of sedimentary valleys subjected to SV waves with vertical incidence are calculated by using the HYBRID program, combining finite elements in the near field and boundary elements in the far field (FEM/BEM). A parametric study is conducted to examine the combined effects of topography and geology on the amplification of the response spectrum at various points across the valley. The influence of different parameters is considered, such as filling ratios (from empty to full valleys), impedance contrasts between bedrock and sediments, and dimensions

A Boundary Element Formulation for the Wave Propagation in Unsaturated soils

International audienceThis paper aims at obtaining an advanced formulation of the time -domain Boundary Element Method (BEM) for two-dimensional dynamic analysis of unsaturated soil. Unlike the usual time-domain BEM the present formulation applies a Convolution Quadrature which requires only the Laplace-domain instead of the time-domain fundamental solutions. The coupled equations governing the dynamic behavior of unsaturated soils ignoring contributions of the inertia effects of the fluids (water and air) are derived based on the poromechanics theory within the framework of the suction-based mathematical model. In this formulation, the solid skeleton displacements, water pressure and air pressure are presumed to be independent variables. As there is no analytical solution for the 2D wave propagation in unsaturated soils in the literature, to verify the accuracy of this implementation, the displacement response obtained by the boundary element formulation is partially verified by comparison with the elastodynamics problem

3-D Dynamic Analysis for Stability of Geotechnical Structures

This paper presents the interests of using a three-dimensional finite element model with an adequate elasto-plastic constitutive law for analyzing problems related to the construction of a complex geotechnical structure. As displacements and ground surface settlements are the key factors, which cause damages to the structure, these parameters have been carefully investigated. The stability of the whole structure is studied as well as the stability of each component. Both static and dynamic analyses (modal and spectral analysis) are performed

Experimental study on the swelling behaviour of bentonite/claystone mixture

International audienceA mixture of the MX80 bentonite and the Callovo-Oxfordian (COx) claystone were investigated by carrying out a series of experiments including determination of the swelling pressure of compacted samples by constant-volume method, pre-swell method, zero-swell method and swell-consolidation method. Distilled water, synthetic water and humidity controlled vapour were employed for hydration. Results show that upon wetting the swelling pressure increases with decreasing suction; however, there are no obvious effects of synthetic water chemistry and hydration procedure on the swelling behaviour in both short and long terms. For the same initial dry density, the swelling pressure decreases with increasing pre-swell strain; whereas there is a well defined logarithmic relation between the swelling pressure and final dry density of the sample regardless of the initial dry densities and the experimental methods. It was also found that swelling pressure depends on the loading-wetting conditions as a consequence of the different microstructure changes occurred in different conditions. Furthermore, it was attempted to elaborate a general relationship between the swelling pressure and the final dry density for various reference bentonites

Oedometric compression and swelling behaviour of the Callovo-Oxfordia, argillite

International audienceThe Callovo-Oxfordian (COx) argillite is a possible host rock for radioactive waste disposal in which the ANDRA underground laboratory of Bure (East of France) has been excavated. In this paper some aspects of the volume change behaviour of the COx argillite are investigated. To do so, high pressure oedometers with a maximum capacity of 113 MPa have been used. In a first stage, swelling tests were carried out on samples initially compressed at constant initial water content (unsaturated) that were afterwards soaked under vertical loads, respectively, smaller and higher than the in-situ vertical stress. All samples exhibited swelling, even at stress higher than the in-situ one. In a second stage, standard step loading compression tests were carried out on samples previously saturated under the in-situ vertical load, so as to investigate the volume change behaviour under load cycles. The strain-stress curves obtained appear to be different to what is currently observed in overconsolidated or cemented clays with no clear appearance of yield and pre-yield reversible behaviour. The volumetric behaviour during both compaction and swelling is interpreted in terms of damage created by the collapse of pores within a fragile matrix. The amount of swelling observed is related to the extent of damage