Dissipated energy in undrained cyclic triaxial tests

Energy-based methods are an emerging tool for the evaluation of liquefaction potential. These methods relate excess pore water pressure build-up to seismic energy dissipated per unit volume. Further development of these methods require their validation through laboratory testing. In this paper, a comprehensive study of energy dissipated during cyclic triaxial tests is undertaken. Results of undrained cyclic triaxial tests performed on air-pluviated samples of Hostun sand prepared with different initial densities and subjected to several confining pressures and loading amplitudes are presented. The energy dissipated per unit volume is estimated from the experimental results and correlated to the generated excess pore water pressure. The correlation between those quantities appear to be independent of the initial relative density of the sample, isotropic consolidation pressure and cyclic stress ratio used in the tests. Moreover, the relationship between observed doubleamplitude axial strain and the energy dissipated per unit volume is examined. It is found that this relationship is greatly dependent on the relative density of the sample

High Pressure Cyclic Triaxial Tests

Shear modulus and damping from high pressure (up to 500 psi) cyclic triaxial tests of soils are presented. The test results are compared with published models where low confining pressures were used. The apparatus and test setup for the high pressure tests are also discussed

Long-term cyclic triaxial tests with DEM simulations

Modeling the long-term performance of granular materials under cyclic loading conditions is still a challenge and a better understanding could provide a large benefit for the design of foundations. One typical application example are the foundations of wind turbines, for which the evolution of the soil mechanical behavior could lead to irreversible strain accumulation (with tilting and settlement) and dynamic resonance problems [1]. In this framework the Discrete Element Method [2] can provide useful information starting from a micromechanical point of view: it may allow engineers to increase their knowledge on the evolution of the mechanical behavior and to optimize the long-term design of these structures [3]. The present paper presents the capability of DEM to simulate a long-term cyclic drained triaxial test (up to 100,000 cycles). The results regard the progressive accumulation of plastic strain as function of the number of particles and the initial particles rearrangement. The influence of densification and contact orientation (anisotropy) in the evolution of the strength of the soil during the cyclic loading history is investigated

Experimental study of strain accumulation of silica sand in a cyclic triaxial test

The experimental and phenomenological investigation of the elasto-plastic long-term behaviour of soils under dynamic loading is important for the development of risk analysis tools and numerical accumulation models for settlement prediction. Soil parameters, test equipment and loading conditions have a significant influence on strain accumulation, therefore a parameterization of the silica sand and a description of the re-engineered cyclic triaxial test device are performed in this paper. Long term cyclic triaxial tests are performed on a silica sand to investigate the influence of the number of cycles, the initial void ratio, the mean pressure and packages of cycles on the accumulation of residual strains. Empirical formulations of existing strain accumulation models are validated with test results on dry test samples

Experimental study on the cyclic resistance of a natural loess from Northern France

In order to analyze the instability phenomenon observed along the Northern High Speed Line of R\'eseau Ferr\'e de France (RFF), soil blocks were taken at a site near the railway, at four different depths (1.2, 2.2, 3.5 and 4.9 m). Cyclic triaxial tests were carried out on saturated and unsaturated soil specimens. The results from tests on initially saturated specimens showed that the soil taken at 2.2 m depth has the lowest resistance to cyclic loading, in relation to its highest porosity and lowest clay fraction. This soil was then studied at unsaturated state with various initial water contents. Unsaturated soil specimens were first subjected to cyclic loadings to decrease their volume. These cyclic loadings was stopped when the volume decrease was approximately equal to the initial pore air volume, or when the pores filled by air were eliminated and the soil was considered to become saturated. Afterwards, the back-pressure tubing was saturated with de-aired water and cycles were applied under undrained condition. Significant effect of initial water content was evidenced: the lower the initial water content, the higher the cyclic resistance. This can be explained by the densification of the soil during the initial cyclic loadings

A constitutive model for unsaturated cemented soils under cyclic loading

On the basis of plastic bounding surface model, the damage theory for structured soils and unsaturated soil mechanics, an elastoplastic model for unsaturated loessic soils under cyclic loading has been elaborated. Firstly, the description of bond degradation in a damage framework is given, linking the damage of soil's structure to the accumulated strain. The Barcelona Basic Model (BBM) was considered for the suction effects. The elastoplastic model is then integrated into a bounding surface plasticity framework in order to model strain accumulation along cyclic loading, even under small stress levels. The validation of the proposed model is conducted by comparing its predictions with the experimental results from multi-level cyclic triaxial tests performed on a natural loess sampled beside the Northern French railway for high speed train and about 140 km far from Paris. The comparisons show the capabilities of the model to describe the behaviour of unsaturated cemented soils under cyclic loading

Liquefaction Studies on Silty Clays Using Cyclic Triaxial Tests

Liquefaction of saturated soils during earthquake often had been a major cause of damage to structures. Since beginning the liquefaction studies were concentrated on sandy soils, as the sandy soils are known to be more susceptible to liquefaction. However, observations from some sites in China and Loma-Prieta earthquakes show that soils with high plastic fines are also susceptible to liquefaction. Isotropically consolidated undrained cyclic triaxial tests were conducted on undisturbed samples of silty clay soil and results of these tests are used to verify the methods based on SPT data. Slow cyclic tests were performed to investigate the development of pore pressure and cyclic strength, as reliable pore pressure measurement is only possible in slow cyclic triaxial test for clayey soils. The site was than characterized for liquefaction by a computer program developed. The N-value was obtained for the same site by conducting Standard Penetration Test. Test results were verified using methods reported by Tokimatsu & Yoshimi (1983) and Ishihara (1993). The simplicity of the methods and application of the methods to the fine-grained soils are the main criteria for selection of the field methods. The computer program also provides characterization of site using these methods

Some factors affecting the results in cyclic triaxial tests

A major cause of damage to structures and earth embankments during earthquakes or any other such dynamic vibrating loading conditions has been the liquefaction of saturated sands. There have been several investigations to establish a convenient and relatively simple laboratory test procedure and to study the nature of field conditions leading to sand liquefaction. This investigation evaluates the effects of sample size, testing frequency, and the method of sample preparation on the number of cycles to cause initial liquefaction in pulsating triaxial tests using standardized equipment and test procedures. It has been found that sample size does affect test results as larger diameter samples tend to give a lower dynamic strength for sand. Also, higher pore pressures are generated in larger diameter samples for the same number of stress applications. There is an indication that a higher frequency of loading also produces a lower strength, but this is not considered to be conclusive due to very limited data. On the other hand, it is found that the method of sample preparation does not affect the test results if the variables of sample preparation such as relative density, homogenity, grainsize distribution, and the degree of saturation are maintained reasonably constant --Abstract, page ii