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Soft soil subjected to cyclic loading typically exhibits an increase in excess pore pressure under undrained condition which brings the soil to an overconsolidated state. Then, under a subsequent large number of cycles (e.g. more than one million) which also takes time, the creep at overconsolidated state influences the cyclic effect and thus results in a complicated long-term undrained behavior. This paper aims to clarify this long-term undrained behavior of overconsolidated clay. The reconstituted samples are prepared from natural samples retrieved in the north of France. First, the shear strength characteristics along monotonic triaxial stress paths are identified. Then load control cyclic tests on overconsolidated specimens are conducted in fully saturated and undrained conditions. Small cyclic deviatoric stresses are applied in order to investigate more particularly the behavior under a very large number of cycles, during which an unusual pore pressure evolution is observed. To explain this, undrained triaxial creep tests are performed on reconstituted specimens with different values of OCRs under some specified stress states. The evolutions of axial strain, excess pore pressure, stress ratio, stress path, plastic strain rates and stress dilatancy during undrained creep are discussed. The additional undrained creep tests also show that two processes are simultaneously acting in a competitive manner: increase in the pore pressure due to the cyclic loading and decrease in the pore pressure because of creep.

The effect of water on the hydromechanical behavior of unsaturated granular materials has been studied with a micromechanical model based on thermodynamic principles. A general framework based on the theory of thermodynamics with internal variables for constructing thermodynamically consistent multiscale constitutive relations for unsaturated granular materials has been developed. Within this framework, the microscopic total Helmholtz free energy has been separated between a mechanical and a hydraulic part, each of which is a function of either the elastic displacement or the capillary bridge volume and the distance between particles at the microscale. The inter-particle dissipation of energy, assumed to be frictional in origin, is a function of the incremental plastic displacements at the microscale. Both the microscale Helmholtz free energy and the dissipative energy have been volumetrically averaged to obtain the homogenized energy functions at the macroscale. In accordance with the suggested multiscale thermomechanical framework, a micromechanical model has been constructed to describe the behavior of partially saturated granular soils. This model has considered the deformation of soil skeleton by applying a Coulomb-type criterion at the inter-particle contacts. The hydraulic potential is made to be dependent on the size of the particles and is derived through use of the expression for the water retention curve by assuming that liquid bridges are isotropically distributed within the specimen. The performance of the suggested model has been demonstrated through numerical simulations of the behavior of sand under various degrees of saturation and a wide range of mechanical loadings.

International audience; This paper studies the dilative behavior of clay during drained creep along triaxial tests under constant mean effective stress, where the volumetric strain is caused by the sole deviatoric stress variation. Tests were conducted on a saturated reconstituted clayey material by the use of a GDS stress path control apparatus. The development of the axial and volumetric strains was studied with respect to time, stress level (SL) and overconsolidation ratio (OCR). Analysis of the results ascertained that both dilatancy and contractancy phenomena could occur during creep. The magnitude of the dilatancy/contractancy during creep was guided by the test conditions (SL and OCR), which specifically governed the direction of the volumetric strain variations. The position of the stress level vis-à-vis the different volumetric domains defined by monotonic triaxial tests in the (p′–q) plane controls the evolution of the volumetric creep strain. The failure of an overconsolidated specimen could be observed for a stress level located under the maximum strength envelope, but above the critical state line in the p′–q plane, accompanied by a significant dilative behavior during creep.

The paper aims to construct an efficient optimization method for identifying creep and destructuration related governing parameters of soft structured clay. An elastic viscoplastic model has been developed and adopted. Different optimization processes, by genetic algorithm or particle swarm optimization with uniform or random samplings initialization methods, are carried out to obtain the material parameters from conventional undrained triaxial tests performed on a K 0-concolidated natural soft clay. All comparisons demonstrate that the uniqueness of the solution is better guaranteed with the genetic algorithm rather than with the particle swarm optimization method. Furthermore, the efficiency of genetic algorithm has been verified by simulating other tests.

Adding refinement and accuracy to constitutive models of soil results in the introduction of complexities along with more model parameters. These parameters (such as hardening-/softening-, dilatancy-/contractancy-related parameters and critical state parameters) are usually not easily obtained in a straightforward way. How to identify these key parameters and estimate their correlations of advanced soil models is a particular issue for geotechnical engineering. This paper was aimed to investigate multi-objective genetic algorithms for identifying parameters of advanced sand models based on standard laboratory tests, followed by the correlation analysis of parameters. A critical-state-based sand model has been developed to simulate three triaxial compression tests performed on loose and dense Hostun sand. Two widely used genetic algorithms with two initialisation methods are examined. The performance of the two genetic algorithms is assessed by comparing their simulation performance using optimal parameters, the convergence speed and the distribution of solutions on the Pareto front. The optimal parameters can then be classified into two factors by their correlation relationship.

This study focuses on the quantitative description of the evolution of creep coefficient (C αe) with both soil density and soil structure under 1D compression. Firstly, conventional consolidation test results on various reconstituted clays are selected in order to investigate the evolution of C αe with void ratio of soils, which can be described by a simple nonlinear creep formulation. Secondly, the contributions of the inter-particle bonding and debonding for soft structured clays to C αe are analyzed based on test results on intact and reconstituted samples of the same clay. A material constant ρ, function of the bonding ratio χ, is introduced in order to quantify the contribution of the soil structure to C αe, and a nonlinear creep formulation accounting for both soil density and soil structure is finally proposed. Furthermore, the parameters used in the formulation are correlated with Atterberg limits, allowing us to suggest a relationship between C αe, Atterberg limits and inter-particle bonding for a given soil. Finally, the validity of the proposed formulation is examined by comparing experimental and predicted C αe values for both reconstituted and intact samples of natural soft clays. The proposed formulation is also validated by comparing the computed and measured void ratio with time on two intact clays.

Cementations formed in geological timescale are observed in various stiff clays. A micromechanical stress strain model is developed for modeling the effect of cementation on the deformation behavior of stiff clay. The proposed approach considers explicitly cementations at intercluster contacts, which is different from conventional model. The concept of inter-cluster bonding is introduced to account for an additional cohesion in shear sliding and a higher yield stress in normal compression. A damage law for inter-cluster bonding is proposed at cluster contacts for the debonding process during mechanical loading. The model is used to simulate numerous stress-path tests on Vallericca stiff clay. The applicability of the present model is evaluated through comparisons between the predicted and the measured results. In order to explain the stress-induced anisotropy arising from externally applied load, the evolution of local stresses and local strains at inter-cluster planes are discussed.