Unsaturated Granular Materials Research Articles

Effect of principal stress axis rotation on cyclic plastic deformation characteristics of unsaturated base course material

It is generally observed that the stress state induced through traffic loads inside the substructure of pavements cannot be exactly reproduced in laboratory element tests. Moreover, there are not sufficient studies that examine the behavior of granular material under unsaturated conditions, although such conditions are very prevalent in pavement structures. In this research, an attempt is made to determine the cyclic plastic deformation characteristics of unsaturated granular base course material, and to examine the effect of the rotation of the principal stress axis (experienced by pavement elements under traffic loads) on the mechanical behavior of unsaturated granular base course material by applying various loading methods. A series of laboratory element tests using a multi-ring shear apparatus, which can take into account the rotation of the principal stress axis, were carried out on granular base course material under different moisture contents. The experimental results show that the moisture content has an obvious effect on both the shear behavior and the cyclic plastic deformation of granular base course material. However, the moisture content for the maximum deformation of granular base course material varies according to the loading method. It is also explained that cyclic plastic deformation considerably increases due to the rotation of the principal stress axis under repeated axial and shear loading tests. Therefore, the principal stress axis rotation has a significant influence on the cyclic plastic deformation of unsaturated granular base course material.

DEM simulation of collapse behaviours of unsaturated granular materials under general stress states

In this study, a numerical simulation of true triaxial tests was conducted using the three-dimensional distinct element method (DEM) in order to examine how unsaturated granular materials collapse under general stress states. The collapse process was simulated by reducing the intergranular adhesive forces corresponding to the effect of the capillary suction during the isotropic compression and the shearing processes under general stress states. Based on the relationship between the void ratio and the mean principal stress after collapsing, it was found that the initially soaked compression line obtained with an inundation test may be used to predict the collapse of granular materials under a general stress state. From the analysis for the fabric tensor in the particle aggregate after collapsing, the skeleton structures became identical to those in which no intergranular adhesive force was applied. Furthermore, even though the collapse process was simulated under a plane strain condition, the shear band inside the sample did not occur clearly, and the slippage between particles was instead induced randomly during collapsing.

Compressive strength of an unsaturated granular material during cementation

The cohesive behavior of unsaturated granular materials is due to the presence of cohesive bonds between grains. These bonds can have various physico-chemical characteristics and may evolve with environmental conditions. We study the case of a granular material partially saturated by an aqueous solution. The bonds are thus initially of capillary type and the mechanical strength is weak. At low relative humidity, the phase change of water involves crystallization of the solute at the contact points between grains, generating thus solid bonds. The mechanical strength of the material is then enhanced. An experimental study of the evolution of the mechanical strength during crystallization of the solute shows clearly the transition from capillary regime to cemented regime. This transition is not correlated with the mass of the crystallized solute, but rather with the residual degree of saturation. This behavior is analyzed here in the light of discrete element simulations. We introduce a local cohesion law that accounts for transition from capillary to cemented bonding. This law is formulated in terms of the degree of crystallization as a result of the evaporation of water at the boundary of the sample. The cohesion of the packing is initially of capillary type. A crystallization front then spreads from the sample boundaries to the center of the sample, and the strength increases as a result. Uniaxial compression allows us to determine the strength at different times. The numerical strength agrees well with the experimental data, and reveals strength enhancement as the solute crystallizes, as well as the transition from capillary to cementation regime.

Testing Methods for Hydro-Mechanical Characteristics of Unsaturated Soils Subjected to One-Dimensional Freeze-Thaw Action

This paper proposes new testing methods for evaluating the effects of freeze-thaw action on the deformation-strength characteristics and the water retention-permeability characteristics of granular materials under unsaturated conditions. A triaxial apparatus and a permeability apparatus for unsaturated soils, with cooling systems to control the temperature of soil specimens and use the pressure membrane method instead of the pressure plate method, were newly developed in order to examine the hydro-mechanical behavior of unsaturated soils subjected to the freeze-thaw sequence, as experienced by in-situ soils in cold regions. Results indicate that the proposed testing methods with these newly developed test apparatuses are highly useful in the evaluation of the effect of freeze-thaw action on the hydro-mechanical behavior of unsaturated granular materials and in the reduction of total testing time.

Micromechanics of granular materials with capillary effects

Based on discrete element simulations and micromechanical calculations, this paper presents studies on the shear strength properties of unsaturated granular materials with capillary effects. Capillary forces are described on the local scale, depending on the water content, and are superimposed on the standard dry particle interaction that is currently described through an elastic–plastic relation. Simulations of triaxial compression tests at several water-content levels in the pendular regime are used to investigate the macroscopic consequences of this local description. The two methods are in rather good agreement, both reproducing the dependency of shear strength on water content, as classically observed in laboratory experiments. For the water-content levels considered, a nonlinear evolution of the cohesion with the water content is obtained, resulting in shear-strength saturation with increasing water content. Moreover, the computations allow a capillary stress tensor to be exhibited from capillary forces, directly related to the cohesion of the material. Finally, emphasising this capillary stress contribution, the generalised effective stress concept is reviewed, with a conclusion on the limitations of the classical macroscopic description of water effects in unsaturated granular materials.

A microstructural elastoplastic model for unsaturated granular materials

The homogenization technique is used to obtain an elastoplastic stress–strain relationship for dry, saturated and unsaturated granular materials. Deformation of a representative volume of material is generated by mobilizing particle contacts in all orientations. In this way, the stress–strain relationship can be derived as an average of the mobilization behavior of these local contact planes. The local behavior is assumed to follow a Hertz–Mindlin’s elastic law and a Mohr–Coulomb’s plastic law. For the non-saturated state, capillary forces at the grain contacts are added to the contact forces created by an external load. They are calculated as a function of the degree of saturation, depending on the grain size distribution and on the void ratio of the granular assembly. Numerical simulations show that the model is capable of reproducing the major trends of a partially saturated granular assembly under various stress and water content conditions. The model predictions are compared to experimental results on saturated and unsaturated samples of silty sands under undrained triaxial loading condition. This comparison shows that the model is able to account for the influence of capillary forces on the stress–strain response of the granular materials and therefore, to reproduce the overall mechanical behavior of unsaturated granular materials.

Normalizing Behavior of Unsaturated Granular Pavement Materials

One of the important components of a flexible pavement structure is granular material layers. Unsaturated granular pavement materials (UGPMs) in these layers influence stresses and strains throughout the pavement structure, and have a large effect on asphalt concrete fatigue and pavement rutting (two of the primary failure mechanisms for flexible pavements). The behavior of UGPMs is dependent on water content, but this effect has been traditionally difficult to quantify using either empirical or mechanistic methods. This paper presents a practical mechanistic framework for determining the behavior of UGPMs within the range of water contents, densities, and stress states likely to be encountered under field conditions. Both soil suction and generated pore pressures are determined and compared to confinement under typical field loading conditions. The framework utilizes a simple soil suction model that has three density-independent parameters, and can be determined using conventional triaxial equipment that is available in many pavement engineering laboratories.

The work input to an unsaturated granular material

The work input to an unsaturated granular material

Effective media formation and conduction through unsaturated granular materials

A theory for the effective thermal conductivity (ETC) of an unsaturated and saturated medium containing three phases is presented using the effective continuous media (ECM) approximation. The boundary condition for ECM through a successive dispersion technique is first derived. Small dispersions are then allowed in the proposed ECM. The method is an analytical one and shows the validity and emergence of ECM for large dispersions. The theory is applicable for all saturating fluids including gas and is useful for determining the content of petroleum products in underground soil. A comparison of calculated values of ETC with experimental ones when the saturating fluids are air and water shows a good agreement.

Notes on water movement in the unsaturated zone

Recent assessment of the resource potential of the Yazor Gravel Aquifer near Hereford ( Liddament & Clark 1980 ) has provided evidence which supports the concept of physical controls proposed by Wellings & Bell (1980, 1982) and suggests that the same controls govern the movement of water through unsaturated granular material as well as Chalk. The Yazor Brook drains an area of about 50 km 2 west of Hereford and north of the River Wye. The lower and central parts of the Yazor Valley are infilled by superficial glacial and post-glacial sediments up to about 15 m in thickness. A significant proportion of these sediments are ill-sorted glacial-outwash sands and gravels which make up the local, but important, Yazor Gravel Aquifer System. A mathematical model of the Yazor Gravel Aquifer was constructed but there were great problems in modelling the water balance of the system. The major problem was in quantifying the recharge to the aquifer, because the traditional Meteorological Office approach, using the soil moisture deficit (SMD) concept, did not appear to work. The behaviour of the water table in two observation wells equipped with continuous water level recorders help to show quite clearly that, in the Yazor Valley, the traditional SMD concept cannot be applied entirely. Recharge can take place where an SMD exists and considerable evapotranspirative losses can occur by upward flux from quite deep water tables. Recharge Through each summer all, or almost all, rain should be soaked up, removing the SMD. In

A Rapid Substitute for Textural Analysis

ABSTRACT A rapid technique is presented which is based on the relationship between the texture and the equilibrium moisture content of all unsaturated fine-textured granular material. For a group of mineralogically similar samples, the technique makes it possible to calculate with sufficient accuracy one or more textural parameters from the results of a simple moisture content determination. This can be performed much more rapidly than the usual particle-size analysis. Limitations of the method are discussed.