Numerical Simulations On Boundary Effects In Cyclic Triaxial Tests Of Natural Soft Clay

Subjected to cyclic loading,the behaviour of soft soils such as clay can be affected.Its stiffness is reduced leading to possible large deformations and sometimes failure.To better understand the dynamic behaviour of natural soft clay under cyclic loading it is necessary to establish a reliable constitutive model able to describe it.The ‘Cyclic Mobility Model' is an elasto-plastic constitutive model able to describe the mechanical behaviour of soils under both monotonic and cyclic loadings with only one set of parameters.It combines the influence of the stress-induced anisotropy,the density and the structure of the soil.If the model works well for the sand,inconsistencies have been noticed in the description of clayey soils.In the finite element method,the quality of the results is directly linked to the soil constitutive model,the integration scheme and the numerical resolution.In order to verify the reliability of the model,numerical simulations on cyclic triaxial tests of natural soft clay are conducted using a soil-water coupled finite element-finite difference(FE-FD)algorithm named as DBLEAVES.The results are compared to experimental triaxial tests and the performance discussed.Though not perfect,the Cyclic Mobility Model associated with DBLEAVES is able to describe the mechanical behaviour of the clay under cyclic loading.The main dynamic characteristics are reproduced.In a second phase,several simulation parameters,boundary and loading condition and their influence are examined to investigate the features of this model and its limits.This will allow to explain the origin of the inconsistencies.It is shown that factors such as the end restrictions and the loading frequency can have a significant impact on the soil dynamic response contributing to its non-uniformity.End restrictions,and therefore friction,create unevenness and induce a non-uniform mechanical behaviour of the clay by creating strain localization areas.They have to be considered when it comes to the pore water pressure build-up,and hence the effective stress.Higher loading frequency causes a stronger response and increases the resistance of the soil.Comparisons between undisturbed and remoulded clay samples are also done and compared to experimental results.It is shown that this model has difficulties to reproduced correctly the difference in shear resistance of those two types of samples.Finally,new experiments will be done based on the conclusion made from the numerical simulations and the relevance of the use of a mid plane probe in a triaxial test to measure the pore water pressure will be investigated.As it turns out,by measuring directly the pore water pressure in the middle of the specimen where the end effects are not present and the soil response more uniform,this can be a solution to reduce the problems due to them and to get a more accurate measures of the pore water pressure within the soil.However,using a mid plane probe has also some drawbacks like a risk for the probe to move during the loadings and a risk of leakage.