Numerical Modeling of Nonhomogeneous Behavior of Structured Soils during Triaxial Tests

Abstract

Abstract:The nonhomogeneous behavior of structured soils during triaxial tests has been studied using a finite element model based on the Structured Cam Clay constitutive model with Biot-type consolidation. The effect of inhomogeneities caused by the end restraint is studied by simulating drained triaxial tests for samples with a height to diameter ratio of 2. It was discovered that with the increase in degree of soil structure with respect to the same soil at the reconstituted state, the inhomogeineities caused by the end restraint will increase. By loading the sample at different strain rates and assuming different hydraulic boundary conditions, inhomogeneities caused by partial drainage were investigated. It was found that if drainage is allowed from all faces of the specimen, fully drained tests can be carried out at strain rates about ten times higher than those required when the drainage is allowed only in the vertical direction at the top and bottom of the specimen, confirming the findings of previous studies. Both end restraint and partial drainage can cause bulging of the triaxial specimen around mid-height. Inhomogeneities due to partial drainage influence the stress-strain behavior during destructuring, a characteristic feature of a structured soil. With an increase in the strain rate, the change in voids ratio during destructuration reduces, but, in contrast, the mean effective stress at which destructuration commences was found to increase. It is shown that the stress-strain behavior of the soil calculated for a triaxial specimen with inhomogeneities, based on global measurements of the triaxial response, does not represent the true constitutive behavior of the soil inside the test specimen. For most soils analyzed, the deviatoric stress based on the global measurements is about 25% less than that for the soil inside the test specimen, when the applied axial strain is about 30%. Therefore it can be concluded that the conventional global measurements of the sample response may not accurately reflect the true stress-strain behavior of a structured soil. This finding has major implications for the interpretation of laboratory triaxial tests on structured soils.

Influence of sample slenderness and boundary conditions in triaxial test - a review

“…More non-uniformities in the axial and radial strains were found when rough ends were simulated instead of smooth ones. The importance of the drainage and the kind of end on the sample shape is stressed also by Liyanapathirana et al [56]. The authors analyse tests with H/D=2 ratio and compare stresses and strains in the mid zone and next to the plate.…”

Sensitive bounding surface constitutive model for structured clay

“…Lagioia and Nova reported interesting data from isotropic compression testing for natural calcarenite; in these tests, the effective stress, p , was observed to decrease, while the sample is volumetrically compressed, which might be expected for highly structured sensitive soils. Constitutive model parameters for the present study were mainly based on recommendations from Liyanapathirana et al (Table ). The softening rate parameters were set at 3.2.…”

Pore-size distribution of a compacted silty soil after compaction, saturation, and loading

“…Structured soils are involved in many applications in geotechnical engineering. It concerns natural soils for which the structure can arise from sedimentary depositions or from chemical bondings or dissolutions (Leroueil and Vaughan 1990;Liyanapathirana et al 2005). Also, earthen constructions are generally made of structured soils due to their particular modes of construction by soil compaction (Alonso et al 2010).…”