Effects Of Initial Void Ratio Research Articles

Constitutive modelling of idealised granular materials using machine learning method

Predicting the constitutive response of granular soils is a fundamental goal in geomechanics. This paper presents a machine learning (ML) framework for the prediction of the stress-strain behaviour and shear-induced contact fabric evolution of an idealised granular material subject to triaxial shearing. The ML-based framework is comprised of a set of mini-triaxial tests which provide a benchmark for the setup and validation of the discrete element method (DEM) model of the granular materials, a parametric DEM simulation programme of virtual triaxial tests which provides datasets of micro- and macro-mechanical information, as well as a multi-layer perceptron (MLP) neural network which is trained and tested using the DEM-based datasets. The ML model only requires the initial void ratio of the granular sample as the input for predicting its constitutive response. The excellent agreement between the ML model prediction and experimental test and DEM simulation results indicates that the ML–based modelling approach is capable of capturing accurately the effects of initial void ratio on the constitutive behaviour of idealised granular materials, bypassing the need to incorporate the complex micromechanics underlying the macroscopic mechanical behaviour of granular materials. Lastly, a detailed comparison between the used MLP model and long short-term memory (LSTM) model was made from the perspective of technical algorithm, prediction accuracy, and computational efficiency.

Effect of initial void ratio on the tensile strength of unsaturated silty soils

Effect of initial void ratio on the tensile strength of unsaturated silty soils

Modelling of full-scale silo experiments with flow correcting inserts using material point method (MPM) based on hypoplasticity

Flow patterns in bulk solids in silos are of importance for stresses exerted on silo walls. They may be changed by using different inserts. The paper discusses numerical results on confined granular flow of cohesionless sand in a full-scale metal silo with different symmetrically placed inserts, used for correcting a flow pattern (double cone, cone-in-cone and inverted cone). The material point method (MPM) was employed to simulate the dynamic flow of sand in a silo. To describe the flow behaviour of cohesionless sand, a hypoplastic constitutive model was adopted. The emphasis was on flow patterns and pressures on both silo walls and inserts. The numerical outcomes were directly compared with the corresponding experiments in a full-scale silo. A satisfactory agreement was achieved between numerical and experimental outcomes. In MPM calculations, the effect of initial void ratio of sand, wall friction angle, insert position and grid cell size was also studied.

Hydraulic conductivity of reconstituted clays based on intrinsic compression

A series of one-dimensional incremental load consolidation–hydraulic conductivity tests was performed on reconstituted clays. Based on these tests, together with the test data compiled from the literature, the quantitative interrelation between the variation of hydraulic conductivity in the vertical direction with void ratio and the compression behaviour is established. It is found that the hydraulic conductivity in the vertical direction can be expressed as a function of void index, initial void ratio and void ratio at the liquid limit. The effects of initial void ratio and void index on hydraulic conductivity in the vertical direction for a given clay with a given void ratio at the liquid limit can be comprehensively attributed to the effect of void ratio on hydraulic conductivity in the vertical direction. These two findings introduce the point that the hydraulic conductivity in the vertical direction is a function of void ratio and void ratio at the liquid limit. Empirical equations are proposed to determine the hydraulic conductivity in the vertical direction using void ratio and void ratio at the liquid limit or using effective vertical stress, initial void ratio and void ratio at the liquid limit. These two empirical equations are correlated based on the concept of intrinsic compression.

Water retention and characteristic curves representing tropical clay soils from Africa

Soil water retention curves (SWRCs) form an essential component of frameworks coupling the hydromechanical behaviour of unsaturated soils. The curves describe how suction changes with variables such as degree of saturation, void ratio and volumetric/gravimetric water content. SWRCs can be determined from incrementally drying initially saturated reconstituted samples to a final residual state, thus developing the primary drying curve (PDC). The primary wetting curve (PWC) is established from subsequent incremental wetting from residual state and is hysteretic compared with the PDC. SWRCs for reconstituted, high-plasticity, tropical clays from Africa (Sudan, Tanzania and South Africa) will be produced using suction measuring instruments, a tensiometer, filter paper and a dew point potentiometer. The development of SWRCs under various subsequent cycles of drying will be presented and discussed along with details concerning volumetric changes and cracking during drying. In order to investigate the uniqueness of the PDC and PWC and the effect of initial void ratio, SWRCs will be determined for samples formed by reconstituted from slurry under different applied energy levels.

Modeling Static Behavior of Rockfill Materials Based on Generalized Plasticity Model

Rockfill materials are commonly used for dam construction. Establishing an effective model that can reasonably describe the mechanical properties of rockfill material is very important for the calculation of earth-rock dam engineering. Based on the generalized plasticity model of sand, a modified generalized plasticity model suitable for rockfill material is established by modifying the plastic modulus. Focusing on three types of stress paths (e.g., CT test, CP test, and CR test), the stress path adaptability of the modified generalized plasticity is studied. Simulation results show that the proposed model can well predict the strength characteristics while it underestimates the shrinkage characteristics of rockfill materials for constant P test. It is difficult for the generalized plasticity model to predict larger radial shrinkage strain for constant stress ratio tests. This shortcoming can be improved by employing a modified dilatancy equation. Finally, by introducing critical state theory and considering the effect of initial void ratio on plastic modulus, a state-dependent generalized plasticity model is proposed and verified by experiment of granite rockfill materials and TRM with different void ratios. These works covers the most common stress paths related to the construction of earth-rock dams and can capture static behavior of rockfill materials.

Undrained Behavior of Sand–Structure Interfaces Subjected to Cyclic Torsional Shearing

This paper presents results from an experimental study on the undrained behavior of sand–structure interfaces subjected to cyclic undrained torsional shearing. The goals of the study are to develop understanding of the response of sand subjected to cyclic torsional loading and to develop a theoretically sound and scale-sensitive framework for interpretation of results. The results characterize the effects of initial void ratio, confining stress, surface roughness, particle angularity, and imposed loading conditions (i.e., torsional versus axial shearing). The responses of all specimens subjected to torsional shear, including those prepared at high relative density, were dominated by contractive tendencies characterized by significant excess pore-water pressure generation. Mechanisms for excess pore-water pressure generation based on experimental evidence of local volume-change tendencies at various locations within the specimens are presented. Torsional interface shearing consistently generated pore-water pressures at faster rates than axial shearing, making the former more efficient for studying cyclic soil behavior. Implications of the results of this study on the deployment of a proposed soil characterization tool are discussed.

A Simple Fractal-Based Model for Soil-Water Characteristic Curves Incorporating Effects of Initial Void Ratios

In this paper, a simple and efficient fractal-based approach is presented for capturing the effects of initial void ratio on the soil-water characteristic curve (SWCC) in a deformable unsaturated soil. In terms of testing results, the SWCCs (expressed by gravimetric water content) of the unsaturated soils at different initial void ratios were found to be mainly controlled by the air-entry value (Ψa), while the fractal dimension (D) could be assumed to be constant. As a result, in contrast to the complexity of existing models, a simple and efficient model with only two parameters (i.e., D and Ψa) was established for predicting the SWCC considering the effects of initial void ratio. The procedure for determining the model parameters with clear physical meaning were then elaborated. The applicability and accuracy of the proposed model were well demonstrated by comparing its predictions with four sets of independent experimental data from the tests conducted in current work, as well as the literature on a wide range of soils, including Wuhan Clay, Hefei and Guangxi expansive soil, Saskatchewan silt, and loess. Good agreements were obtained between the experimental data and the model predictions in all of the cases considered.

Uniaxial compression behavior of geotextile encased stone columns

The bearing capacity and failure mechanism of encased stone columns are affected by many factors such as encasement length, relative density, strength and stiffness of the encasement material. In soft soils where surrounding soil pressure is low, especially in the top section, the stone columns may be close to a uniaxial compression state, where the uniaxial compression strength controls the bearing capacity of the stone columns. A series of large-scale triaxial tests on ordinary stone columns and uniaxial tests on geotextile encased stone columns have been performed. The stone columns were 300 mm in diameter and 600 mm in height. Samples of four different relative densities, and five types of geotextiles were used in the tests to study the effect of initial void ratio and encasing materials on the uniaxial compression behavior of the stone columns. The results show the uniaxial compressive strength of the encased stone columns is not affected by the initial void ratio but mainly by the tensile strength of the encasing geotextiles. The stress strain curves of the encased stone columns under uniaxial loading condition are nearly liner before failure, which is similar to the tensile behavior of the geotextiles.

Soil water retention curves representing two tropical clay soils from Sudan

Soil water retention curves (SWRCs) form an essential component of frameworks coupling the hydromechanical behaviour of unsaturated soils. The curves describe how suction changes with variables such as degree of saturation. SWRCs can be determined from incrementally drying initially saturated reconstituted samples to a final residual state, thus developing the primary drying curve (PDC). The primary wetting curve (PWC) is established from subsequent incremental wetting and is hysteretic compared with the PDC. SWRCs for two reconstituted, high-plasticity, tropical clays from Sudan have been determined using the filter paper technique, for which the maximum measurable suction is 30 MPa. At this suction the degree of saturation for both soils was greater than 40% and did not reduce much further on full drying under ambient laboratory conditions. Thus the primary SWRCs did not develop into the usual sigmoidal shape expected. The development of SWRCs under various subsequent cycles of wetting and drying are presented and discussed along with details concerning volumetric changes and cracking during drying. In order to investigate the uniqueness of the PDC and PWC and the effect of initial void ratio, SWRCs were determined for samples formed by dynamic and static compaction under different applied energy levels and also for intact samples. These were found to be located to the left of the PDC and often the PWC too.

Evaluating the capability of a critical state constitutive model to predict the collapse potential of loose sand

Many catastrophic flow failures in granular soil slopes are believed to be caused by a rise in pore water pressure associated with substantial loss of soil shear strength. This failure mechanism is known as prefailure instability or static liquefaction. Constant shear (CS) and consolidated undrained (CU) triaxial tests can reproduce stress paths, in which such instability may occur before reaching the failure. In the present study, a previously proposed critical state constitutive model was first used to simulate the behavior of loose saturated sand in CU tests. It was then employed to predict the instability of loose sand subjected to the CS loading. Under such loading, loose dry sand initially experience small volume increase, and then start to contract substantially. In saturated sand, such contractions can lead to the generation of pore water pressure and sudden decrease of shear strength. The capability of the model to predict the onset of the volume contraction and collapse potential of loose dry sand was examined by comparing the model predictions with experimental results of CS tests. The comparison showed that the effect of initial void ratio, consolidation and deviatoric stresses on behavior of loose dry sand can be well predicted by the model.

Discrete simulations of a triaxial compression test for sand by DEM

SUMMARYA quasi‐static homogeneous drained triaxial compression test on cohesionless sand under constant lateral pressure was simulated using a three‐dimensional discrete element method. Grains were modelled by means of particle clusters composed of rigid spheres or spheres with contact moments imitating irregular particle shapes. Attention was paid to the effect of initial void ratio and grain shape mixture on the shear strength, volume changes, force chains, kinetic, elastic and dissipated energies. In addition, the effect of the mean grain size, grain size distribution, grain size range, specimen size and roughness and stiffness of boundaries was numerically analysed in initially dense sand. Some numerical results were compared with available experimental results. Copyright © 2014 John Wiley & Sons, Ltd.

Numerical study of soil collapse behavior by discrete element modelling

In unsaturated granular soil, capillary suction produces an adhesive force that acts perpendicular to the tangential plane at each contact point. In this paper, the collapse behavior of unsaturated granular soil during isotropic compression and biaxial shear are numerically simulated using the distinct element method (DEM) by incorporating this adhesive force initially at all contacts and then releasing it continuously to zero. The simulated results are compared with the results of triaxial compression tests on unsaturated compacted clay and qualitative similarities are obtained. Based on the simulation results, the yield stresses under different interparticle adhesive forces are determined and the collapse processes before and after yielding are investigated in detail. Moreover, the effect of initial void ratio and principal stress ratio on collapse is also investigated and discussed. The paper illustrates the capacity of simulating qualitatively the collapse behavior of unsaturated granular soils through the changes of the interparticle adhesive forces produced by the capillary suction.

A micromechanical study of rolling and sliding contacts in assemblies of oval granules

AbstractThe evolution of the microstructure of an assembly of cohesionless granular materials with associated pores, which carry the overall applied stresses through frictional contacts is a complex phenomenon. The macroscopic flow of such materials take place by the virtue of the relative rolling and sliding of the grains on the micro‐scale. A new discrete element method for biaxial compression simulations of random assemblies of oval particles with mixed sizes is introduced. During the course of deformation, the new positions of the grains are determined by employing the static equilibrium equations. A key aspect of the method is that, it is formulated for ellipse cross‐sectional particles, hence desirable inherent anisotropies are possible. A robust algorithm for the determination of the contact points between neighbouring grains is given. Employing the present methodology, many aspects of the behaviour of two‐dimensional assemblies of oval cross‐sectional rods have been successfully addressed. The effects of initial void ratio, interparticle friction angle, aspect ratio, and bedding angle on the rolling and sliding contacts are examined. The distribution of normals to the rolling and sliding contacts have different patterns and are concentrated along directions, which are approximately perpendicular to one another. On the other hand, the distribution of all contact normals (combined rolling and sliding) are close to that of rolling contacts, which confirm that rolling is the dominant mechanism. This phenomenon becomes more pronounced for higher intergranular friction angle. Characteristics of the rolling and sliding contacts are also discussed in the context of the force angle, which is the inclination of contact force with respect to the contact normal. Copyright © 2003 John Wiley & Sons, Ltd.

Microscopic Particle Crushing of Sand Subjected to High Pressure One-Dimensional Compression

ABSTRACT It is commonly accepted that the yield point during one-dimensional compression is related to the initiation of marked particle crushing. Different materials have clear to very amorphous yield points depending on the compression line curvature. In order to examine the relationship between the curvature and slope of the compression line and the statistics of individual particle crushing taking into account particle size and overall grading, high pressure one-dimensional compression tests on silica sand samples seeded with marked particles were carried out. Five levels of particle damage were obtained and defined from microscopic observations of the particles before and after testing. A statistical analysis was carried out on the data for the observed levels of damage to investigate the frequency variation with increasing applied stress. Additionally, one-dimensional compression tests were conducted to examine the effects of initial void ratio and grain size distribution on soil crushability and consequently the compression behaviour. It was found that even for the same material the yielding characteristics were dependent on the grading curve with much more marked yielding occurring for uniformly graded sands in comparison with well graded sands. This was related to the nature of the microscopic particle crushing during yielding. As the material changed from uniform to well graded, the nature of the particle crushing changed from the sudden catastrophic onset of splitting to the gradual splitting of smaller size particles, breaking of the smaller asperities and grinding of the surface.

1‐ D Strain in Normally Consolidated Cohesionless Soils

A stress‐strain model for one‐dimensional strain in normally consolidated cohesionless soils is compared to test results covering the stress range from 0 to 1,300 atmospheres. The model is called the “1/e versus σν′P model” because the implied relationship between void ratio e and vertical effective stress σν′P is linear in the low‐stress range. The relationship between 1/e and σν′P is nonlinear at higher stresses where the effect of particle crushing becomes significant. The effects of initial void ratio, relative density, particle shape, particle size distribution, and particle material on one‐dimensional strain behavior have been examined in the framework of the model. The model can accurately represent one‐dimensional strain in rockfill‐like materials as well as in sands and gravels. The data and equations presented provide a means of estimating shrinkage during placement of rockfills and estimating settlement of structures on rockfills where it is not practical to conduct one‐dimensional strain or pe...

Behavior of Granular Materials Under High Stresses

Triaxial tests at mean normal stresses up to 1,200 kg per sq cm on a medium-grained, uniform quartz sand show that the nature of sand deformation varies with pressure. As mean normal stress increases, crushing becomes more pronounced and dilatancy effects gradually disappear. Beyond breakdown pressure, at which all effects of initial void ratio disappear, sand behaves essentially as a linearly deformable solid with modulus of deformation E proportional to mean normal stress q. In the same range, sand strength is characterized by a constant angle of internal friction, φ, equal to the angle of interparticle friction from the low pressure tests. Below breakdown pressure, E increases as a power function of q; however the exponent of this function varies with pressure and, to a certain degree, with the initial void ratio of sand. The strength envelopes in this range are all curved and depend on the initial void ratio of sand. Relative compressibility of granular materials generally increases with pressure and may be responsible for scale effects in a number of passive pressure phenomena.