Study of anisotropic shear strength of granular materials using DEM simulation (original) (raw)
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Transportation Geotechnics, 2020
The noncoaxiality of the principal stress direction and plastic principal strain increment has been broadly recognized as an influencing parameter for design of soil structures. Here we performed a series of systematic hollow cylinder experiments to study the effects of stress anisotropy on the noncoaxiality of dense Babolsar and Toyoura sands. A total of 25 undrained torsional shear tests were carried out under constant mean confining pressure, and fixed principal stress directions, α. We investigated the stress-strain behavior of dense sands for different α-directions, and cyclic stress ratio, CSR, under monotonic and cyclic loading conditions. The results show that the noncoaxiality value depends on the CSR, the level of plastic strain, and α-direction. Independent of the principal stress direction, maximum noncoaxiality value was observed at peak shear strength when there is most interlock between sand particles. Minimum noncoaxiality was recorded in α = 45°tests, in which the direction of maximum shear stress coincided with the horizontal bedding plane direction (weakest plane).
FRAMEWORK FOR ASSESSMENT OF SHEAR STRENGTH PARAMETERS OF
Failure of soil may cause collapse of structures resulting in loss of lives and economic damage. Most geotechnical instability problems including failure of soil are associated with shear failure. Shear strength is one of the most important properties for design of engineering structures and also one of the most difficult to evaluate. In order to determine the shear strength parameters that govern shear strength, such as angle of internal friction and cohesion, typical laboratory tests such as the direct shear test and triaxial test are used. However, these laboratory tests have some shortcomings regarding sample collection such as lack of in-situ conditions and difficulties for obtaining undisturbed soil samples. In-situ testing methods are also used to determine the shear strength of soil such as the Vane Shear Test, the Standard Penetration Test and the Cone Penetration Test. However, these tests estimate the shear strength of the soil with appropriate empirical correlations that have a wide margin of error. Traditional testing methods to acquire the shear strength parameters are expensive, complicated, time consuming, and require extreme care during the process of collecting, storing, transporting and preserving samples. The objective of this paper is to develop a phenomenological model that could be used to predict the shear strength parameters from their index properties (liquid limit) and other engineering properties (specific gravity, void ratio, maximum dry density), which are relatively easy to determine. The validity of the method was proven by determining shear strength parameters for various types of soils and by comparing them with the results taken from a conventional testing method. This could be used to rapidly estimate cohesion and friction angle in situations where either the good quality samples or the equipment needed to conduct such tests are not available.
2010
The effects of confining pressure, relative density and sample preparation methods on the shearing strength of Chlef sand were studied in this article. For this purpose, the results of drained and undrained monotonic triaxial compression tests performed on samples with initial density of 0.29 and 0.80 under initial confining pressures ranged from 50 to 200 kPa are presented. The specimens were prepared by two depositional methods namely dry funnel pluviation and wet deposition. It was found that there was a marked difference in the undrained behavior even though the density and stress conditions were identical. The conclusion was that the soil fabric was responsible for this result. The results indicated also that at low confining pressures, the specimens reconstituted by the wet deposition method exhibited complete static liquefaction (zero effective confining pressure and zero stress difference). As confining pressures and densities were increased, the effective stress paths indicated increasing resistance to liquefaction by showing increasing dilatant tendencies. The same trends were observed in drained tests results in the form of an increase in the volumetric strain and the rapid transition from the contractancy phase to the dilatancy phase.
Strength anisotropy of fibre-reinforced sands under multiaxial loading
Géotechnique, 2019
The strength anisotropy of fibre-reinforced sands in the multiaxial stress space has been investigated using a hollow cylinder torsional apparatus. Probing stress paths under constant cell pressure have been performed on both unreinforced and reinforced sand specimens to assess the influence of the orientation of the principal stress directions on the fibre strengthening contribution. For the first time, a deviatoric strength envelope for fibre-reinforced soils in the multiaxial stress space was identified. The addition of fibres produces an anisotropic increase and a distortion of the deviatoric strength envelope if compared to the unreinforced soil matrix. The fibre strengthening contribution is governed by the tensile strain domain developed and the fibre orientation distribution. Further observations on the effect of the addition of fibres on the volumetric response, principal stress and strain rate non-coaxiality, as well as the shear bands formation of the composite, are prese...
International Journal of Geosynthetics and Ground Engineering, 2023
Geosynthetics are extensible reinforcements used to enhance the engineering performance of a soil. The transfer of stresses from the soil to the reinforcement is achieved through soil-geosynthetic interaction. The proposals from the literature to assess the shear strength of the reinforced soil under triaxial conditions use three different approaches. These involve analysing the reinforced soil: (i) as a homogeneous composite material (Approach A), (ii) as two different materials (Approach B), or (iii) as soil having the same fundamental shear strength, with the effect of the reinforcement represented as an additional lateral or confining stress (Approach C). In this paper, triaxial tests data of a soil reinforced with a geosynthetic, and specimens with different dimensions (diameters 70 and 150 mm) were used to assess changes in shear strength and to carry out a statistical analysis. The increases in shear strength of the reinforced soil and of the soil-geosynthetic interface were analysed using equations from the literature. The difference between the triaxial results obtained from specimens of different sizes was assessed using an Analysis of Covariance (ANCOVA). When the joint term of the regression model was not statistically significant, the characterisation from different specimen sizes was used to generate soil failure envelopes. Thus, a new methodology to obtain a failure envelope with different specimen sizes is presented.
Effect of fabric anisotropy on shear localization in sand during plane strain compression
Acta Mechanica, 2007
The paper focuses on the effect of fabric anisotropy on shear localization in cohesionless granular materials. For the numerical simulation, a hypoplastic constitutive model was used. In order to take into account a characteristic length of the micro-structure, the constitutive model was extended to include the second gradient of the Euclidian norm of the deformation rate. The hypoplastic model captures the salient features of granular bodies in a wide range of density and pressure with a single set of parameters. Transversal isotropy is described by the dyadic product of the normal vector of the space orientation of the plane of symmetry. FE-simulations of plane strain compression under constant lateral pressure were carried out with a medium dense specimen for both uniform and stochastic distribution of the initial void ratio. The effect of the direction of the bedding plane and the initial void ratio distribution on the load-deformation behavior was investigated. Moreover, the location, thickness and inclination of the shear zone were also analyzed.
Computers and Geotechnics, 2016
Sand response changes with intensity of cementation bonds between sand grains, magnitude of intermediate principal stress and with fabric anisotropy. First a critical state bounding surface plasticity model is presented in this paper. In this constitutive model, the loading surface always passes through the current stress state regardless of location or position of the stress path. Second to simulate hollow cylinder tests which represent different modes of shearing including triaxial compression and triaxial extension, the fabric anisotropy and b-parameter are incorporated in the model. Simultaneous integration of cohesion, non-associated flow rule, fabric anisotropy, kinematic hardening, critical state and state parameter makes the proposed model unique compared to previous proposed bounding surface models. Comparison of model outcomes and hollow cylinder experimental tests shows great predictive capability of the proposed model. Sensitivity analysis also suggests that triaxial compression and triaxial extension are respectively strongest and weakest modes of shearing.
2018
This study aimed to examine the shear strength and stress-unit deformation behaviors of dry sands under static loads with the help of shear box experiments. For that purpose, sea sand taken from Trabzon, Sinop, and Zonguldak Provinces and river sand taken from Aydin Cine District were used. We attempted to evaluate geological and mineralogical properties of natural sand of different origins. For the classification of samples, specific gravity, sieve analysis, and maximum and minimum dry density tests were performed in the first stage. During the shear box experiments, the samples prepared at different empirical (varying) compaction varying between 45% and 85% were consolidated under vertical stresses varying between 30 and 400 kPa and then loaded up to failure. On the basis of the test results, Mohr-Coulomb failure parameters were determined according to relative compaction and effective stress. Semi-empirical relationships specific to the sands were developed due to relative compac...
EFFECT OF PARTICLE SIZE ON THE SHEAR MODULUS OF GRANULAR SOIL
Granular materials such as soil are inherently discrete and their behavior is very complex. Understanding behavior of such materials under cyclic loading is intricate. Traditionally, continuum principles are employed to study the deformation of soil by geotechnical engineers. But, discontinuous nature of soil is required to be considered to understand micro features such as anisotropy, dilatancy, shear localization etc. Also the strength loss when subjected to cyclic loading is affected by particle size, shape and its distribution. In this paper 3D discrete element modeling of cyclic triaxial test was carried out with respect to different particle size distribution and void ratios under drained condition. This study evaluates the dependence of shear modulus on particle distribution.