Undrained Behavior Of Sand Research Articles

Influences of low fines content and fines mixing ratio on the undrained static shear strength of sand–silt–clay mixtures

Clay and silt have complicated effects on the undrained behaviour of sands. To study the influences of low clay and silt contents on the undrained behaviour of sands in the clay–silt–sand mixtures, sands with three different fine contents (0%, 5% and 10%) and three different fines mixing ratios were investigated based on the consolidated-undrained test by a strain-controlled triaxial testing system. The test results show that the static liquefaction takes place in clean sands under static loading; the undrained behaviours of the specimens are found to differ for the same fine content and different fine mixing ratios. The fines can simultaneously influence filling, bonding and soil skeleton characteristics of sand particles, and the extent of the influence on each characteristic varies depending on the clay and silt contents.

Effect of Grain Size and Shape on Undrained Behaviour of Sands

The stress–strain and stress path characteristics of sands are influenced by their grain size, shape, and packing. Morphological characteristics and size of particles play important role on the undrained shear strength of sands. Often, effects of these parameters are complex and cannot be easily distinguished. This study advances the knowledge of the role of particle size and shape on the undrained shear strength of sands. To eliminate the consequence of morphological characteristics, two sands with different particle sizes but similar angularity, and another sand with different roundness were selected for the study. These morphological characteristics for all three sands were determined from the analysis of scanning electron microscope images. F131 sand with higher median grain size and lower shape factors (rr and rs) had highest undrained peak shear strength and phase transformation value. Undrained strength (qpt) and effective principal stress (P′pt) in phase transformation point had direct relationship with grain median grain size (D50) and inversely effect of shape factor (rr and rs). F131 and F161 sands represented highest peak and ultimate steady-state strengths, respectively. Flow potential appeared to be directly proportional with (rr and rs) and inversely with D50. The peak index decreased with increasing shape factors (rr and rs).

Undrained behavior of sand under cyclic paths that match storm-wave loading conditions

ABSTRACTIn ocean engineering, storm-wave loading imposes irregular cycles in the magnitudes acting on the coastal or seabed cohesionless soils, giving rise to more complex consequences on the their liquefaction susceptibility, compared with those under uniform cyclic loadings that were considered in most routine engineering designs. This paper presents a systematic experimental investigation into the undrained behavior of saturated loose sand subjected to irregular cyclic triaxial paths that match the storm-wave loading conditions, with the presence of static shear stress that mimics the initial condition of sand deposits in the field, along with the conventional uniform cyclic triaxial tests for comparison. The results indicate that both the initial static stress and the cyclic irregularity influence the pore pressure development and cyclic resistance of sand. When the static shear stress presents, the sand samples under storm-wave loading show a monotonic increase in cyclic resistances to liquefaction. In addition, the concept of strain energy is used to delineate the accumulation of pore pressure during cyclic loading. A unique relationship between the normalized pore pressure and the strain energy is observed, irrespective of the static stress level, cyclic stress amplitude, and applied cyclic loading patterns.

Undrained behavior and post-liquefaction behavior of sand containing fines

The influence of fines on undrained behavior of sand was evaluated through a series of tests conducted in silty sand retrieved from Tokyo Bay Area after the 2011 Tohoku Earthquake and an artificial mix of low-plasticity fines with sand. Since the use of density measures as void ratio, relative density or sand skeleton void ratio has led to divergent results, a different approach was taken in this research, by using compaction energy as the parameter of comparison. Stress-strain curves and stress paths for non-plastic and low-plastic fines are given. Excess pore pressure development and liquefaction curves are provided and compared. It can be seen that the addition of fines decreases liquefaction resistance either if they are of non-plastic or low-plastic nature. After conducting cyclic shearing, drainage was opened to measure volumetric deformation and results are also reported. Relevant conclusions regarding the role of fines in liquefaction potential and post-liquefaction behavior are provided

Undrained Behavior of Sand-Fines Mixtures and Their State Parameter

AbstractThe influence of fines on the undrained behavior of sand was investigated experimentally over a range of fines content and effective mean stress. The role of fines on compressibility and formation of meta-stable soil fabric was analyzed. For a fines content in the range of 5–30%, most of the specimens manifested reverse behavior opposed to the normal behavior of clean sand. The addition of fines led to shifting of both the isotropic consolidation line and the steady-state line, increased compressibility, and occurrence of reverse behavior. All these behavior attributes are linked when the test results are interpreted in the context of the critical-state soil mechanics framework. In particular, reverse behavior is associated with an isotropic consolidation line that locates above the steady-state line. Several quantitative measures for characterizing the undrained deviator stress-strain responses were proposed, and these measures manifested correlation with the state parameter at the start of undra...

Prediction of Undrained Monotonic and Cyclic Liquefaction Behavior of Sand with Fines Based on the Equivalent Granular State Parameter

AbstractThe undrained behavior of sand with fines under both monotonic and cyclic loading was investigated experimentally by triaxial testing. The testing program covers fines content ranging from 0 to 30%, and a wide range of initial void ratios and confining stresses. The spectrum of cyclic loading includes one- and two-way cyclic loading, and in the latter case, the cyclic loading may be symmetrical or nonsymmetrical. The influence of fines content can be captured by using the equivalent granular void ratio, e∗, in lieu of the void ratio, e. All steady-state data points can be described by a single relationship referred to as the equivalent granular steady-state line. This study proposes the concept of the equivalent granular state parameter, ψ∗, for capturing the combined effects of the stress state, density state, and fines content. The observed behavior can then be related to e∗ and/or ψ∗. Irrespective of fines contents, the behavior under monotonic loading can be classified into three types, and ψ∗...

Influence of specimen-reconstituting method on the undrained response of loose granular soil under static loading

This paper describes the results of an experimen- tal study on the undrained shear behaviour of loose sand col- lected from the location close to the epicenter of the recent Chlef (Algeria) Earthquake (October 10, 1980). The study focuses on the effects of the mode of the soil deposition on the liquefaction resistance of the Chlef sand. For this purpose, the results of undrained monotonic triaxial com- pression tests performed on samples with initial density of 0.29 under initial confining pressures ranged from 50 kPa 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 behaviour of sand in terms of maximal devi- atoric stress, peak strength, residual strength and excess pore water pressure, 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).

Effect of grain shape and angularity on the undrained response of fine sands

The objective of this paper is to discuss the effect of grain shape and angularity on the undrained behaviour of sands. Four fine to medium (D50= 0.15–0.29 mm) and uniform sands were examined. These sands have similar and (or) identical grading curves, but differ with respect to the angularity of their grains. Under the electron scanning microscope two sands have rounded grains and the other two have subangular to angular grains. For sands with rounded grains, evidence of undrained behaviour undergoing systematic weakening (shear stress reduction after a transient peak deviator stress) is provided by torsional hollow cylinder tests. However, sands with the same grading curve and void ratio, but angular grains, show stable response with a continuous increase in strength after a transient peak. A small variation in the sand gradation, i.e., D50= 0.22 ± 0.07 mm, does not appear to alter the pattern of undrained response described earlier, with the angular sand (D50= 0.15) exhibiting stable response to shear and the rounded sands (D50= 0.22 and 0.29) having unstable response to shear. The stiffness characteristics of the sands are also presented. Triaxial compression tests were performed to confirm the pattern of response observed under torsional testing.

Undrained behaviour of Changi sand in triaxial and plane-strain compression

The results of an experimental study of the undrained behaviour of Changi sand under axisymmetric and plane-strain conditions are presented. K0 consolidated undrained plane-strain tests and K0 or isotropically consolidated triaxial tests on very loose and medium dense specimens were conducted. The undrained behaviour of sand at very loose and medium dense states under plane-strain conditions was characterised and compared with that under axisymmetric conditions. It was observed that the undrained behaviour of very loose and medium dense sand under plane strain is similar to that under axisymmetric conditions. However, because of the formation of shear bands in plane-strain tests, the post-peak behaviour of medium dense sand in plane strain is different from that in triaxial tests. It was also established that an instability line for plane-strain conditions can be defined in a way similar to that for axisymmetric conditions. Using the state parameter, a unified relationship between the normalised slope of instability line and the state parameters can be established for both axisymmetric and plane-strain conditions. Using this relationship, the instability conditions established under axisymmetric conditions can be used for plane-strain conditions.

Static liquefaction of sand in plane strain

Experimental results on the static liquefaction behaviour of sand under plane-strain conditions are presented in this paper. Undrained tests on very loose sand under both plane-strain and axisymmetric conditions were conducted and the results compared. The test data show that the undrained behaviour of sand under plane strain is similar to that under axisymmetric conditions. However, the critical-state line (CSL) on both the q–p′ and the e–p′ planes determined under plane-strain conditions is different from that under axisymmetric conditions. The slope of the CSL is different as a result of the influence of the intermediate principal stress. The state parameter (ψ), which is measured with reference to the CSL in the e – log p′ plane, is also different: the ψ value for plane strain is about 0.05 less than that for axisymmetric conditions for the sand tested. The instability behaviour of very loose sand under undrained plane-strain conditions is also studied. Based on the testing data, a unique relationship between the stress ratio of the instability line and ψ is established to enable the triaxial results to be used for plane-strain conditions.Key words: failure, sands, laboratory tests, liquefaction, shear strength.

Fabric Behaviour of Sand in Post-liquefaction

An anisotropic plasticity model for post-liquefaction of the undrained behavior of sand is presented. The model incorporates the critical/steady state concept that postulates the existence of a state where sand continuously deforms at a certain constant effective stress depending two main parameters of both initial bulk parameters (void ratio or relative density) and the stress level (mean stress). The local instability of saturated sand within post-liquefaction is highly dependent on the residual inherent/induced anisotropy, bedding plane effects and stress/strain path. Most of the models developed using stress/strain invariants are not capable of identifying the parameters depending on orientation such as fabric. This is mainly because stress/strain invariants are quantities similar to scalar quantities and not capable of carrying directional information with them. The constitutive equations of the model are derived within the context of non-linear elastic behavior of the whole medium and plastic sliding of interfaces of predefined multi-planes. The proposed multi-plane based model is capable of predicting the behavior of soils on the basis of plastic sliding mechanisms, elastic behavior of particles and possibilities to see the micro-fabric effects as natural anisotropy as well as induced anisotropy in plasticity. The model is capable of predicting the behavior of soil under different orientation of bedding plane, history of strain progression during the application of any stress/strain paths. The influences of rotation of the direction of principal stress and strain axes and induced anisotropy are included in a rational way without any additional hypotheses. The spatial strength distribution at a location as an approximation of probable mobilized sliding mechanism is proposed as an ellipsoid function built up on bedding plane.

Undrained Shear Characteristics of Saturated Sand Under Anisotropic Consolidation

A series of undrained triaxial compression tests was performed on Toyoura sand in order to investigate the behavior of sand under large deformation. The present study focuses on the effects of anisotropic consolidation on the undrained behavior of sand. A wide range of initial states of sand is covered and taken into account with the behavior of sand varying from contractive to dilative. Different states of consolidation stress were shown to affect the stress-strain behavior of sand and the development of excess pore water pressure up to an axial strain of 5%. Beyond a strain in excess of 10%, the behavior of sand was shown to become independent of the stress state at consolidation. Consequently, the relation between void ratio and confining stress at steady state and quasi-steady state are independent of the extent of anisotropic consolidation. Moreover, the initial dividing curve between dilative and contractive behaviors in an e ~ p' diagram was shown to move down as the sand is more anisotropically consolidated.

Effect of microstructure on undrained behaviour of sands

This paper presents a mathematical modelling of the effects of initial fabric on the mechanical behaviour of sand. A stress-dilatancy model that incorporates microstructural aspects of sand is hereby obtained while writing energy conservation for an ensemble of particles over a representative elementary volume at micro- and macro-scales. The resulting stress-dilatancy model, when used within an elastoplastic framework, successfully reproduces certain aspects of sand behaviour that are reflective of its microstructure under both drained and undrained conditions. The role of microstructure in relation to the characterization of steady, quasi-steady, and phase-transformation states is discussed within the framework of the model. Numerical simulations obtained from the proposed model are generally very consistent with experimental observations and provide insightful information.Key words: sand, liquefaction, fabric, dilatancy, constitutive laws, granular materials, plasticity.

Effect of microstructure on undrained behaviour of sands

Effect of microstructure on undrained behaviour of sands

Effects of Principal Stress Direction and Intermediate Principal Stress on Undrained Shear Behavior of Sand

Undrained monotonic loading triaxial compression and extension tests were conducted on Toyoura sand. It was found that the shear behavior was more contractive and softer in triaxial extension than in triaxial compression. This difference suggests that the stress conditions, such as the direction of the principal stress and the magnitude of the intermediate principal stress have some effects on the undrained behavior of sand. To clarify these effects, a series of tests was performed by means of an automated hollow cylindrical torsional shear apparatus. Dry-deposited Toyoura sand was used in these tests. The angle of the maximum principal stress from the vertical to the bedding plane, α , and the intermediate principal stress coefficient, b, were fixed in each test. The intermediate principal stress was fixed to horizontal. For any density, tests with a larger α -value, namely, a larger inclination of σ 1 from the vertical, and a larger intermediate principal stress coefficient b were shown to generate greater excess pore water pressure. In addition, the undrained simple shear behavior of sand under initial isotropic and anisotropic stress conditions was studied. It was made clear that triaxial compression ( α = 0°, b = 0) gives the highest resistance with lowest contrac- tancy, while triaxial extension ( α = 90°, b = 1 ) gives the opposite extreme in the assessment of flow failure. Simple shear mode of deformation was shown to exhibit an intermediate stress-strain behavior between triaxial compression and extension, which is closer to most of the field conditions.

New nested adaptive neural networks (NANN) for constitutive modeling

Abstract The concept of using neural networks in constitutive modeling has been proposed by the first author and his co-workers. In this methodology, neural networks are trained directly with the results of material tests, and the trained neural networks can be used in analysis of boundary value problems similar to any other material model. In this paper, we introduce nested adaptive neural networks, a new type of neural network developed by Ghaboussi and his co-workers, and apply this neural network in modeling of the constitutive behavior of geomaterials. Nested adaptive neural networks take advantage of the nested structure of the material test data, and reflect it in the architecture of the neural network. This new neural network is applied in modeling of the drained and undrained behavior of sand in triaxial tests.

Undrained Strength of Sand Undergoing Cyclic Rotation of Principal Stress Axes

This study concerns the effects of continuous rotation of principal stress axes on excess pore water pressure development during cyclic loading. Continuous rotation of principal stress axes has been known to exert some influence on the development of excess pore water pressure and failure of sand during cyclic loading. In an effort to investigate this aspect of the problem, several series of cyclic undrained tests were carried out in a static manner on saturated sampes of sand using a triaxial torsion shear apparatus. In this test apparatus, a hollow cylindrical soil specimen is subjected to a simultaneous application of both triaxial and torsional modes of shear stresses, which brings about the continuous rotation of principal stress axes. The test results indicated that, while the angle of internal friction remains unaffected, the continuous rotation of principal stress axes substantially reduces the resistance of sand to liquefaction by generating a greater amount of excess pore water pressure than in the case without the rotation. Some special tests were also performed to examine the effects of previous liquefaction on the undrained behavior of sand during the subsequent cyclic loading. The results show that the sand having previously experienced large deformations due to liquefaction exhibit a highly anisotropic stress-strain characteristics, whereby reducing the resistance of the sand drastically when sheared in certain directions.

Discussion of “Prediction of Undrained Behavior of Sand”

Discussion of “Prediction of Undrained Behavior of Sand”