De’an Sun

Semi-analytical solution to one-dimensional consolidation in unsaturated soils

This paper presents an analytical solution of the one-dimensional consolidation in unsaturated soil with a finite thickness under vertical loading and confinements in the lateral directions. The boundary contains the top surface permeable to water and air and the bottom impermeable to water and air. The analytical solution is for Fredlund’s one-dimensional consolidation equation in unsaturated soils. The transfer relationship between the state vectors at top surface and any depth is obtained by using the Laplace transform and Cayley-Hamilton mathematical methods to the governing equations of water and air, Darcy’s law and Fick’s law. Excess pore-air pressure, excess pore-water pressure and settlement in the Laplace-transformed domain are obtained by using the Laplace transform with the initial conditions and boundary conditions. By performing inverse Laplace transforms, the analytical solutions are obtained in the time domain. A typical example illustrates the consolidation characteristics of unsaturated soil from analytical results. Finally, comparisons between the analytical solutions and results of the finite difference method indicate that the analytical solution is correct.

Numerical study of soil collapse behavior by discrete element modelling

Abstract 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.

Swelling characteristics of fractal-textured bentonite and its mixtures

Swelling deformation and swelling pressure tests of Tsukinuno bentonite and its mixtures with Toyoura sand were performed on a modified odometer. The swelling strain and swelling pressure of bentonite and its mixtures are dependent on the dry density, the increment of water content and the surface fractality of bentonite particle aggregates in voids. The water volume absorbed by bentonite is related to the surface fractal dimension (Ds) of bentonite. The correlation of the water volume to vertical overburden pressure (p) is obtained as Vw/Vm=Kp Ds� 3 for Tsukinuno bentonite with fractal-textured surface. The maximum swelling strain is predicted according to the correlation of the water volume to vertical overburden pressure. The predictions of the maximum swelling strain are in satisfactory agreement with the experimental data. Furthermore, the good relationship between swelling strain and elapsed time is also proposed in this paper. D 2003 Elsevier Science B.V. All rights reserved.

Collapse Behavior of Compacted Clays in Suction-Controlled Triaxial Tests

Using a suction-controlled triaxial apparatus for unsaturated soils, a series of tests on a compacted clay was performed to investigate the influences of stress states, water content, void ratio, and matric suction on the collapse behavior. The triaxial tests were carried out under the conditions of (1) different stress ratios and mean stresses, (2) triaxial compression and extension, (3) different initial void ratios of specimens with the same water content, (4) different initial water contents with the same degree of compaction, and (5) different controlled matric suctions. The main conclusions were obtained as follows: (1) The volume change induced by the collapse mainly depends on the initial void ratio and mean net stress under which the collapse occurs, irrespective of imposed matric suction; (2) the amount of collapse is small at both low and high confining stresses, and there exists a maximum value of collapse at a particular mean stress; (3) the shear strain increment induced by collapse depends on stress ratio, triaxial compression, or extension stress; (4) when imposed suction is decreased, large collapse deformation takes place in the samples compacted dry-of-optimum, while little collapse deformation takes place in the samples compacted wet-of-optimum; and (5) the collapse behavior can be explained by the elastoplastic theory for unsaturated soils.

DEVELOPMENT OF A NEW IN-SITU DIRECT SHEAR TEST

An in-situ direct shear test apparatus, in both small and large sizes, and its testing techniques have been developed. This test is performed simply by pulling horizontally a latticed shearing frame, embedded in the ground, with a flexible rope or chain under the application of a constant vertical load on the sample. In the new test, the real normal and shear stresses acting on the shear plane can be exactly measured. A number of the large-sized in-situ direct shear tests have been performed on 8 kinds of coarse-grained granular materials including rockfills at various construction sites of embankments. The measured shear strengths approximate those of large-sized triaxial compression tests on samples with parallel grain-size distributions. The smaller version of this new test has been used successfully for testing sands and clays. The extreme simplicity and high accuracy of this newly developed test are emphasized.

A fractal model for soil pores and its application to determination of water permeability

Soils are generally penetrated by a pore system. Most transport processes usually occur in pore space of soils. The description of pore space shape is necessary to quantify how the geometry influences the percolation of substances such as water and nutrients. In this paper, the pores in soils are described by fractal geometry, and the fractal dimension is obtained from porosimetric measurements. The water permeability function is studied to correlate with the pore-size distribution of unsaturated soils, and is derived from the fractal model of pore-size distribution. The comparisons between the collected data of permeability and the proposed permeability function show that the calculations of the proposed permeability function are in satisfactory agreements with the measurements.

An Elastoplastic Hydro-mechanical Model for Unsaturated Soils

This paper presents a coupled elastoplastic constitutive model for predicting the hydraulic and stress-strain-strength behaviour of unsaturated soils. Hydraulic hysteresis in the water-retention behaviour is modeled as an elastoplastic process with the elastic region of the saturation degree. The effect of change in degree of saturation on the stress-strain-strength behaviour and the effect of change in void ratio on the water-retention behaviour are taken into consideration in the model, in addition to the effect of suction on the hydraulic and mechanical behaviour. Model predictions of the stress-strain and water-retention behaviour are compared with those obtained from triaxial tests on unsaturated soil along isotropic compression, triaxial stress paths with or without variation in suction.

An Elastoplastic Model for Granular Materials Exhibiting Particle Crushing

A practical elastoplastic constitutive model for granular materials is presented. And the model is suitable for description of the material behaviour for a wide range of stresses, including those sufficient to cause particle crushing. With a limited number of model parameters, the model can predict the confining-pressure dependent stress-strain relation and shear strength of granular materials in three-dimensional stresses, especially of variation of shear strength and dilatancy characteristics due to particle crushing under high confining pressure. The model parameters, which have clear physical meanings, can be determined from the results of isotropic compression test and conventional triaxial compression tests. The model performance is demonstrated for triaxial compression tests of a sand for a wide range of the confining-pressure from 0.2MPa to 8.0MPa.

Hydro-mechanical behaviours of highly compacted sand-bentonite mixture

This paper presents the results of laboratory testing on a heavily compacted sand-bentonite mixture. To measure the soil-water retention curve (SWRC) of the mixture over a large range of suction, a pressure plate apparatus and filter papers were used. The obtained SWRC shows that the measurements via the two methods consistently agree with each other. By using a suction-controlled oedometer for unsaturated soils, a series of one-dimensional compression tests were performed on the unsaturated compacted sand-bentonite mixture at different constant suctions. The testing results indicate that the yield stress increases and compression index decreases with the increase of imposed suction. The results also demonstrate that the mixture wetted to saturation and subsequently dried to a certain suction level has a lower yield stress than that wetted directly to the same suction.

Effect of Density on the Soil-Water-Retention Behaviour of Compacted Soil

This paper presents a series of experimental results obtained from wetting and/or drying cyclic tests and constant suction triaxial tests on unsaturated compacted clay with different initial densities using two suction-controllable triaxial apparatus. The primary objective of this research is to investigate the influence of the soil density on the soil-water-retention behaviour. The initial density and initial degree of saturation were controlled by changing the compaction energy using clay powders with almost the same water content. The test results contain the water-retention curves and deformation behavior at different initial densities using different specimens and one specimen before and after collapse. The test data indicate that the measured water-retention curves vary with the current specimen density. A denser specimen results in a higher degree of saturation at the same imposed suction, which implies that the water-retention curve shifts to the right in the ln s-S r plane.