Swelling Mechanism and Swelling Behaviour of Expansive Clay and Clay Minerals

It is critical to understand the swelling behavior and swelling mechanisms of expansive clays in order to predict geological disasters, minimize infrastructure damage, and improve the design of expansive clay products for various industry applications. However, the complex nature of expansive clay minerals presents significant challenges in characterizing and predicting their behavior, as the swelling behavior is influenced by a range of environmental factors such as water content, external load, temperature, salt solution, interlayer cation species, soil types and treatment materials. Traditional model framework and characterization method is limited to describe the behaviour of expansive clay under complex environmental conditions. A novel method is proposed in this thesis by combining the diffuse double layer theory and molecular dynamics approach to predict swelling pressure of expansive soil. The molecular structure was constructed based on the Na (Ca)-montmorillonite clay, which is the main mineral composition related to the swelling of expansive clay. The effects of electrical potential energy and van der Waal’s energy were considered in this model. The proposed model was validated by comparing the predicted swelling pressure with the experiment results under different cation exchange capacities and dry density conditions. Based on the swelling pressure model, a molecular montmorillonite-salt solution model is proposed to analyse the swelling pressure change of montmorillonite under various non-isothermal and saline conditions. The swelling mechanism of montmorillonite at various salt solutions and temperatures is discussed based on analyses of the water concentration profile, hydration energy of montmorillonite-salt solution system, and hydration state of interlayer cations. The critical temperature that differentiates the negative and positive correlations between swelling pressure of montmorillonite and temperature under different saline environments coincides with the hydration state saltation temperature identified from radial distribution function. To further investigate the effects of treatment materials on the swelling behaviour of expansive clay, four types of cement/lime stabilized montmorillonite (Na-montmorillonite, K-montmorillonite, Ca-montmorillonite and Mg-montmorillonite) molecular models with different interlayer cations and surface charges are established. The results show that volume change of cement/lime stabilized montmorillonite samples is not only related to adsorption energy of montmorillonite, but also controlled by competitive adsorption of cement/lime and interaction between cement/lime and montmorillonite. The interface energy between cement/lime and montmorillonite generated by Ca ions migration from cement/lime to montmorillonite surface, plays a most significant role in governing the swelling behaviour of cement/lime stabilized montmorillonite by providing strong repulsive force to confine the swelling of montmorillonite layers. To further study the effects of mineral composition on swelling behaviour of expansive soil, we proposed a new method to quantify the swelling strain and/or swelling pressure of expansive soil with considering the mineralogical components and their different swelling properties. The proposed multi-component model employs a tandem combination of different sub molecular oedometers for different minerals and the total swelling strain of soil was calculated by integrating the swelling strain of each mineral. A linear relationship between potential energy and swelling strain is identified based on molecular dynamics simulation, which is then incorporated into diffuse double layer theory to calculate the relationship between confining pressure and swelling strain for each mineral. The numerical model was then validated by experimental results from the literature, which shows a good agreement with the experimental data in the literature. This model provides a cost-effective way to estimate the swelling behavior of expansive soil. Finally, we further computed the bulk modulus and shear modulus of montmorillonite via molecular dynamics simulations under various water contents and dry densities. The mechanism that controls the changing of elastic properties subjected to water content and dry density changes was revealed by energy variation and water density field evolution. The results show that the microscopic bulk modulus is highly influenced by dry density and water content, while the shear modulus is less sensitive. The study also found that the microscopic water filling ratio of the interlayer space is the key factor affecting the bulk/shear modulus variation by changing the energies in the clay-water system.