Undrained loading and collapse of unsaturated soils during centrifuge testing. An experimental and numerical study

The paper presents the results of a centrifuge model of a shallow foundation relying of a layer of unsaturated soil and submitted to axial load for different water level. The objective of the work was to represent a foundation of 1.5 m in diameter on a 15 m soil layer. The model of foundation was a circular disk of 30 mm in diameter and the layer was a cylindrical container of 300 mm in diameter and height. In order to maintain similitude between prototype and model the tests were carried out at 50g. The tests were carried out at the LCPC facilities in Nantes (France). The tested material is an eolian silt from Jossigny, East of Paris. In order to decide the initial conditions of the model in terms of water content and void ratio it was performed a preliminary laboratory investigations. As the intention of the study was to examine the behaviour of a collapsible soil therefore it was decided to prepare the model with a low dry density (14.5 kN/m3 ). The evolution of pore water pressure during the tests are compared with the numerical simulation of the prototype with code brig

Analysis of the hydration of a bentonite seal in a deep radioactive waste repository

A deep geological repository for nuclear waste requires the backfilling and sealing of shafts and galleries to block any preferential path for radioactive contaminants. The paper presents the coupled hydromechanical analyses of an in situ test carried out in the HADES underground laboratory in Mol, Belgium. The test examines the effectiveness of an expansive clay seal in a horizontal borehole specifically drilled for this purpose. The analysis covers the phase of seal hydration up to saturation and subsequent pore pressure equilibration. Hydraulic and mechanical constitutive laws suited to expansive clay materials have been chosen for the analyses with all the parameters determined independently of the in situ test. A quite good agreement has been found between test observations and computed results suggesting that the numerical formulation employed is able to reproduce the main features of a real sealing syste

Modelling the mechanical behaviour of expansive clays

A simple formalism is presented to model the behaviour of expansive clays. Two levels of structure are considered. The behaviour of the macrostructure follows the model developed for unsaturated materials by Alonso et al. [Géotechnique 40 (3) (1990) 405–430]. The behaviour of the microstructure is adapted from the work of Gens and Alonso [Can. Geotech. J. 29 (1992) 1013–1032] in order to include the possibility of the micropores being partially saturated. Mechanical coupling between both levels of structure are defined through two functions, one for wetting and the other for drying. They express the change in macrostructural void ratio due to a change in microstructural void ratio, and their value depends on the state of compaction of the macrostructure. The general shape of these curves is discussed on the basis of experimental evidence. Phenomena such as the dependency of strain on stress–suction path, accumulation of expansion strain during suction cycles at low confining stress, accumulation of compression strain during suction cycles at high confining stress, strain fatigue during drying–wetting cycles, macropore invasion by expanded microstructure and development of macroporosity during strong drying can be represented. A mathematical formulation of the model is described and its performance finally assessed by comparison with laboratory tests

Water permeability, water retention and microstructure of unsaturated compacted Boom clay

Three classes of experiments are considered in this paper to provide information for two artificially prepared Boom clay fabrics: mercury intrusion/extrusion tests; main wetting/drying paths; and water inflow/outflow transient (permeability) tests. These tests, which are usually treated separately, are joined in a common reference frame to provide information about the morphology of the porous medium and factors influencing Boom clay unsaturated hydraulic states with reference to water retention curves and relative water permeability values. The main objective is to interpret mercury intrusion porosimetry results in order to define an entrance pore size region at ca 130 to 180 nm separating intra-aggregate and inter-aggregate zones. This pore size region is further associated to a delimiting zone in the retention curve separating regions of ‘intra-aggregate governing suction’ at gravimetric water contents lower than 13–15% (gravimetric water content is not affected by mechanical effects) and ‘inter-aggregate governing suction’ (gravimetric water content is sensible to mechanical actions). This water content is further used to define a threshold zone around a relative water permeability of kw/kws=0.01 delimiting a zone of greater water relative permeability from others that present a restricted flow in a generalised Darcian sense. All these results are consistent with the existence of two main pore size regions: an intra-aggregate porosity with quasi-immobile water that is little affected by loading processes and an inter-aggregate porosity for which the loading mechanism results in a reduction of interconnected macropores affecting free water. Testing results show that intra-aggregate water represents between 54 and 59% of the total volume of water in soil in a low-porosity packing compacted at a dry unit weight of 16.7 kN m-3, whereas it corresponds to ca 28 and 38% in the case of a high-porosity packing compacted at a dry unit weight of 13.7 kN m-3

An elasto-plastic model for unsaturated soil incorporating the effects of suction and degree of saturation on mechanical behaviour

The paper presents an elasto-plastic model for unsaturated soils that takes explicitly into account the mechanisms with which suction affects mechanical behaviour as well as their dependence on degree of saturation. The proposed model is formulated in terms of two constitutive variables directly related to these suction mechanisms: the average skeleton stress, which includes the average fluid pressure acting on the soil pores, and an additional scalar constitutive variable, ξ, related to the magnitude of the bonding effect exerted by meniscus water at the inter-particle contacts. The formulation of the model in terms of variables closely related to specific behaviour mechanisms leads to a remarkable unification of experimental results of tests carried out with different suctions. The analysis of experimental isotropic compression data strongly suggests that the quotient between the void ratio, e, of an unsaturated soil and the void ratio es, corresponding to the saturated state at the same average soil skeleton stress, is a unique function of the bonding effect due to water menisci at the inter-particle contacts. The same result is obtained when examining critical states at different suctions. Based on these observations, an elasto-plastic constitutive model is developed using a single yield surface the size of which is controlled by volumetric hardening. In spite of this simplicity, it is shown that the model reproduces correctly many important features of unsaturated soil behaviour. It is especially remarkable that, although only one yield surface is used in the formulation of the model, the irreversible behaviour in wetting–drying cycles is well captured. Because of the behaviour normalisation achieved by the model, the resulting constitutive law is economical in terms of the number of tests required for parameter determination

Progressive emergence of double porosity in a silt during compaction

The paper deals with an experimental investigation of water retention properties of a statically compacted unsaturated low plasticity silt. The objective is a deeper understanding of the evolution of an aggregate type fabric at different initial conditions in terms of void ratio and water content. A series of Mercury Intrusion Porosimetry tests (MIP) were performed to provide information about factors influencing fabric changes (effect of mechanical stress due to sample compaction) and fabric-properties relationships (water re-tention curve related to porosimetry). The arrangements of aggregation/particles are also investigated with Environmental Scanning Electron Microscopy (ESEM). The experimental data has been used to calibrate a multimodal water retention model for aggregate which is obtained by linear superposition of sub-curves of the van Genuchten type modified. By comparing the WRC obtained by MIP and under suction controlled conditions it has been found a good agreement between the two method for the drying path

Numerical modelling of slope–vegetation–atmosphere interaction: an overview

The behaviour of natural and artificial slopes is controlled by their thermo-hydro-mechanical conditions and by soil–vegetation–atmosphere interaction. Porewater pressure changes within a slope related to variable meteorological settings have been shown to be able to induce soil erosion, shrinkage–swelling and cracking, thus leading to an overall decrease of the available soil strength with depth and, ultimately, to a progressive slope collapse. In terms of numerical modelling, the stability analysis of partially saturated slopes is a complex problem and a wide range of approaches from simple limit equilibrium solutions to advanced numerical analyses have been proposed in the literature. The more advanced approaches, although more rigorous, require input data such as the soil water retention curve and the hydraulic conductivity function, which are difficult to obtain in some cases. The quantification of the effects of future climate scenarios represents an additional challenge in forecasting slope–atmosphere interaction processes. This paper presents a review of real and ideal case histories regarding the numerical analysis of natural and artificial slopes subjected to different types of climatic perturbations. The limits and benefits of the different numerical approaches adopted are discussed and some general modelling recommendations are addressed

Undrained loading and collapse of unsaturated soils during centrifuge testing. An experimental and numerical study

The paper presents the results of a centrifuge model of a shallow foundation relying of a layer of unsaturated soil and submitted to axial load for different water level. The objective of the work was to represent a foundation of 1.5 m in diameter on a 15 m soil layer. The model of foundation was a circular disk of 30 mm in diameter and the layer was a cylindrical container of 300 mm in diameter and height. In order to maintain similitude between prototype and model the tests were carried out at 50g. The tests were carried out at the LCPC facilities in Nantes (France). The tested material is an eolian silt from Jossigny, East of Paris. In order to decide the initial conditions of the model in terms of water content and void ratio it was performed a preliminary laboratory investigations. As the intention of the study was to examine the behaviour of a collapsible soil therefore it was decided to prepare the model with a low dry density (14.5 kN/m3 ). The evolution of pore water pressure during the tests are compared with the numerical simulation of the prototype with code brig