Undrained Monotonic Tests Research Articles

Undrained cyclic responses of granulated rubber-sand mixtures

Undrained cyclic responses of rubber-sand mixtures are essential for its applications in earthquake-prone areas. Hence, the present study is intended to examine the liquefaction behaviour and modeling the undrained cyclic responses of granulated rubber-sand mixtures with size of sand and granulated rubber less than 2 mm. The stress controlled undrained cyclic triaxial tests followed by undrained monotonic tests were carried out on mixtures with rubber content 0% (pure sand), 10%, 20%, 30%, 50%, 75%, and 100% (pure rubber) to examine liquefaction and post-liquefaction behaviour of the mixtures. The test results show that the mixture possesses higher liquefaction resistance when rubber content is between 30% and 50%. The post-liquefaction behaviour of the mixtures identified as pseudo-elastic-hardening and its corresponding modulus is found to vary marginally with its static modulus. In addition, the post-liquefaction strength of the mixtures is observed to be more than that of sand at the lower axial strain. Further, the undrained responses such as hysteretic behaviour and excess pore water pressure are modeled using Masing’s rule and the relation between degradation index (δ) and excess pore water pressure ratio (ru), respectively. The proposed model effectively predicts the cyclic behaviour of mixtures with rubber content between 10% and 75%.

Effect of the geotextiles reinforcement area on the stabilization of soil deformations in Chlef city (Algeria)

This communication presents a laboratory study on the effect of the geotextiles reinforcement area on the improvement of the mechanical proprieties of a sandy soil, especially on the stabilization of soil deformations that affect road pavement in Chlef city (Algeria), which is a prone area to seismic risk. The study is carried out via a monotonic undrained tests using the triaxial apparatus. Reconstructed soil samples having a relative density corresponding to two soil states: loose (Dr = 20%) and medium dense (Dr = 50%) are tested. The aim of the study is to highlight the improvement that can bring the increase of the area of geotextile reinforcement on the behavior of the studied soil. Results show that the reinforcement area has a great positive effect on the increase of the soil resistance, traduced by the stress deviator and by the friction angle, and on the decrease of the pore pressure that reduce the liquefaction risk.

A state-dependent soil model and its application to principal stress rotation simulations

The plastic strain caused by principal stress rotation is one of the most important factors contributing to substantial deformation under earthquake, wave or traffic loading. The original Pastor–Zienkiewicz Mark III model, a well-known model for the analysis of the dynamic response under cyclic loading, is unable to consider the effects of principal stress orientation as well as state-dependent dilatancy. In this article, a new constitutive model for sand is developed to consider both aforementioned effects based on the original Pastor–Zienkiewicz Mark III model. There are 14 model parameters in total for the static condition and three extra parameters for cyclic loading, and a corresponding calibration method of model parameters is proposed. The predictive capability of the proposed model is verified with the results of a series of experiments on sand, including undrained monotonic tests in different fixed principal stress orientations and undrained cyclic rotational shear tests. The comparisons indicate that the proposed model can effectively incorporate the effects of principal stress orientation and state-dependent dilatancy.

Undrained monotonic and cyclic shear response and particle crushing of silica sand at low and high pressures

A series of undrained monotonic and cyclic triaxial tests were performed on silica sand at two initial densities and different confining pressures from 0.1 to 5 MPa to investigate their shear response and crushing behaviour. The influence of particle crushing on the undrained shear strength and pore-water pressure was examined. To clarify the evolution of particle crushing, undrained monotonic and cyclic tests were terminated at several distinctive stages and sieving analysis tests were subsequently performed on the tested specimens. In the undrained monotonic test, specimens exhibited remarkable dilation behaviour and experienced no apparent particle crushing at low confining pressures. An increase in the mean stress suppressed the dilatancy due to a faster increase of the pore-water pressure, giving rise to the occurrence of particle crushing. In the undrained cyclic test, a higher confining pressure and cyclic stress ratio resulted in a much higher relative breakage. At a specific cyclic stress ratio, the relative breakage increased as the cyclic loading progressed. The confining pressure and shear strain amplitude played a significant role in controlling the evolution of particle breakage. The correlation between the relative breakage and plastic work for specimens under isotropic consolidation, undrained monotonic, and cyclic loadings has been validated experimentally.

Undrained shear strength of sand-silt mixture: Effect of intergranular void ratio and other parameters

The determination of the appropriate undrained residual shear strength of silty sands soils that are prone to liquefaction to be used in the assessment of the post-liquefaction stability of different types of earth structures is becoming a major challenge.The objective of this laboratory investigation is to study the effect of intergranular void ratio on undrained residual shear strength of loose, medium dense and dense (Dr = 12, 50, and 90% before consolidation) sand-silt mixtures under monotonic loading and liquefaction potential under cyclic loading. For this purpose, a series of undrained monotonic triaxial tests were carried out on reconstituted saturated silty sand samples with fine contents ranging from 0 to 50%. The confining pressure was kept constant to 100 kPa in all tests. The results of undrained monotonic tests indicate that the global void ratio does not represent the actual behaviour of the soil and the undrained residual shear strength decreases linearly with the increase of the intergranular void ratio and similar trend is happening with the fine content of the sand-silt mixtures up to 50% as well. Undrained cyclic test results show that the increase of the intergranular void ratio and the fine content accelerate the liquefaction phenomenon for the stress ratio under study and the liquefaction resistance decreases with the increase of the intergranular void ratio and the loading amplitude.

Post-liquefaction undrained monotonic behaviour of sands: experiments and DEM simulations

This paper presents the results of a series of servo-controlled cyclic triaxial tests and numerical simulations using the three-dimensional discrete element method (DEM) on post-liquefaction undrained monotonic strength of granular materials. In a first test series, undrained monotonic tests were carried out after dissipating the excess pore water pressure developed during liquefaction. The influence of different parameters such as amplitude of axial strain, relative density and confining pressure prior to liquefaction on the post-liquefaction undrained response have been investigated. The results obtained highlight an insignificant influence of amplitude of axial strain, confining pressure and a significant influence of relative density on the post-liquefaction undrained monotonic stress–strain response. In the second series, undrained monotonic tests were carried out on similar triaxial samples without dissipating the excess pore water pressure developed during liquefaction. The results highlight that the amplitude of axial strain prior to liquefaction has a significant influence on the post-liquefaction undrained monotonic response. In addition, DEM simulations have been carried out on an assembly of spheres to simulate post-liquefaction behaviour. The simulations were very similar to the experiments with an objective to understand the behaviour of monotonic strength of liquefied samples from the grain scale. The numerical simulations using DEM have captured qualitatively all the features of the post-liquefaction undrained monotonic response in a manner similar to that of the experiments. In addition, a detailed study on the evolution of micromechanical parameters such as the average coordination number and induced anisotropic coefficients has been reported during the post-liquefaction undrained monotonic loading.

Monotonic and Cyclic Liquefaction of Very Loose Sands with High Silt Content

The results from an experimental study on sands with high nonplastic silt content are presented. Drained and undrained triaxial compression tests, undrained cyclic triaxial tests, and drained/undrained instability tests were performed on specimens of loose Nevada sand with 40% silt content. The behavior was observed to be somewhat different from previously published tests with sands at lower silt content. The greater silt content appears to provide a more volumetrically contractive response throughout the entire stress-strain curve. However, some aspects of the response were similar to sands with lower silt content. Monotonic undrained tests indicated “reverse” behavior, i.e., static liquefaction occurred at low confining pressures and increasing dilatant volume-change tendency was observed with increasing confining pressure. Analyzing the results using the concepts of steady state resulted in a unique steady-state line only when undrained tests were sheared from the same isotropic compression line. When specimens of different initial densities were tested at the same initial confining pressures, the resulting steady-state points did not fall on the same steady-state line.