Study On The Swelling Mechanism And Mesomechanical Swelling Model Of Expansive Soils

The study with the experimental, theoretical and numerical methods on the expansive soils mainly concentrated in the macroscopic swelling characteristics. The mesoscopic swelling mechanism hasn't, however, been investigated deeply and systematically. Hence, the utilization of the model simulating the mesoscopic swelling mechanism is usually not proper. So, in this paper, the main research is to reveal the swelling mechanism systematically and objectively based on the research of related disciplines. On the other hand, after the swelling mechanism is been grasped, how to transform the mesoscopic theoretical results to the macroscopic quantities is the other research emphasis. The main researches are introduced in detail as follows:1. The crystalline swelling model is presented, which comprise four types of repulsive forces, i.e. hydration force of cations, hydration force of layer surface, electrostatic repulsive force between two layers, osmotic force, and two types of attractive forces, i.e. electrostatic attractive force between cations and clay layers and van der Waals'force. Based on the electrostatic field theory, the formulas for the electrostatic repulsive force between two layers and electrostatic attractive force between cations and clay layers are proposed. The expression for concentration between clay layers is derived from charge conservation principle, and then the osmotic pressure formula is presented. The hydration forces of cations and layer surfaces are renewedly calculated on the basis of MSA equation. Several factors affecting the crystalline swelling, such as surface charge density, the location of the charge in the clay layers, the size of the cations between clay layers and solution concentrations are investigated with the new model. The investigations show that the theoretical results conform to the experimental results, which approve the validity of the proposed model in this paper.2. The feasibility of the applying of Gouy-Chapman double layer model to the long-range swelling is discussed. Firstly, the Gouy-Chapman model is compared with its simplified models. The comparisons indicate that the simplified model with the constant surface potential is closest to the original model, and other simplified models have their own application conditions. Also the Gouy-Chapman double layer model under the conditions of distilled water is developed. Finally, the Gouy-Chapman model is used to fit the experimental data of the long-range swelling pressure between clay layers. Although the theoretical data for swelling pressures are close to the observed data, the Gouy-Chapman model cannot explain the existing of the Stern layer at the layer surface. So it is not conformable to reality if only the double layer model describes the long-range swelling.3. The mechanism of the long-range swelling is researched and then the long-range swelling model is proposed. The double layer repulsive model is presented based on the Gouy-Chapman-Stern-Grahame (GCSG) model, which could describe the existing of the Stern layer due to the cations adsorbing onto the layer surface. The calculated repulsive forces, however, are a small part of the long-range swelling pressures, even negligible. It is concluded that the hydration forces play a dominant role in the long-range swelling. We consider the increasing of the free energy, which originates from the disruption of water hydrogen bond network due to the electrostatic field of clay layer charge, as the reason for the hydration force. The ratio of electric field intensity of the diffuse layer charge to interlayer distance is believed to determine the value of the hydration forces, which ratio is defined as the incremental linear density of the quasi free energy. The relationship between the ratios and hydration forces is linear in the dual-logarithm coordinates system. Finally, the linear relation, the GCSG double layer repulsive model and the van der Waals'force are combined to establish the long-range swelling model. The theoretical results are in agreement with the observed data, which verify the presented model and then the reasonability of the hypothesis that the incremental linear density of the quasi free energy determinies the hydration forces values.4. The mesomechanical swelling model is established. Based on the principle of the mesomechanics of composite materials, the mesomechanical swelling model is developed to transform the swelling forces between clay layers into the swelling forces of the macroscopic soil specimens, in which model the clay particles in the same orientation is regarded as the same phase, and the probability density function for exponential distribution is introduced to describe the distribution of the orientation of particles. The lateral swelling pressure could be obtained with this model. For the swelling strain tests, the state when the swelling deformation is completed is regarded as the research object, which is described by the dry density, so the swelling strain could be taken as the function of the dry density, hence the simplified swelling strain model is proposed from the points of swelling pressure. Based on the same idea, the one-dimensional and two-dimensional mesomechanical swelling constitutive model are established. With these two models, the Dinnik condition is examined.5. The effects of the solution concentration on the swelling characteristics of expansive soils. The experimental studies are carried out to analysis the effects of the solution concentrations on the swelling pressures and swelling strains. When the concentrations of the solutions to prepare and to inundate the specimens are not identical, the effects on the swelling characteristics are studied emphatically.