Constitutive Theory Of Clay (Bentonite) Saturated With Salt Solution And Its Finite Element Analysis

Geotechnical materials saturated with salt solution are commonly encountered in various fields involving the interactions between geotechnica and environment,such as nuclear waste management,petroleum and natural gas storage,coastal foundation engineering,and ocean engineering.The chemical activity of ions in the salt solution can significantly impact the hydraulic and deformation properties of these materials,which can cause degradation of engineering performance and even failure,resulting in severe engineering disasters and environmental pollution issues.Therefore,it is crucial to accurately analyze the effect of chemical activity of salt solution ions on the deformation and seepage properties of saturated soils,predict the deformation behavior of salt-bearing geotechnical bodies,and prevent geotechnical and environmental disasters.To address these challenges,this paper presented a constitutive theoretical framework for geotechnical materials saturated with salt solution based on the principles of mixture theory and solution thermodynamics.The constitutive model of clay(bentonite)saturated with NaCl solution was established under the guidance of this framework,and the consolidation characteristics of the clay and the seepage-diffusion law were analyzed.First,the constitutive model for clay saturated with NaCl solutionis was established.This model adopted the mass fraction of solute as the chemical state variable to reflect the influence of chemical activity.It also utilized the free energy potential function and dissipative potential function to establish the elastic and plastic constitutive relationships of the materials,respectively.The model further decomposed the solid phase deformation into skeletal strain caused by porosity change,matrix strain caused by solid material deformation,and mass exchange strain caused by material exchange such as chemical reaction,to highlight the key role of porosity in the hydraulic-mechanical-chemical multi-field coupling mechanism.Based on the established theoretical framework,the solid-phase and fluid-phase constitutive relationships and the seepage-diffusion equations of solutes in clay saturated with NaCl solution were established,and the conditions of applicability of the classical osmotic pressure equation were analyzed.The proposed model was compared with experimental data,and the results showed that it can effectively simulate the isotropic compressive mechanical behavior of clay saturated with salt solution,demonstrating that the theoretical framework can guide the modeling of geotechnical materials saturated with salt solution considering chemical activity.Second,the paper presented a swelling constitutive model of bentonite saturated with NaCl solution.It is often used as a buffer material in repositories for nuclear waste disposal due to its high swelling and low permeability.The developed one-dimensional constitutive model considered the increase in the ionic mass fraction of the salt solution as a source of chemical plastic strain that inhibited the swelling deformation of bentonite and makes it less compressible.The model was compared with experimental data,and the results indicated that it can better simulate the swelling and compressive deformation behaviors of bentonite saturated with salt solution.Furthermore,the proposed theory was combined with the modified Cambridge model to establish a three-dimensional constitutive model of bentonite saturated with salt solution under triaxial stress state considering the effect of shear.Third,finite element analysis was employed.The solidification control equation and seepage-diffusion equation of clay saturated with NaCl solution were analyzed by finite element analysis.A computational program was prepared in Fortran language to analyze the solidification characteristics of clay saturated with NaCl solution and diffusion law,and the analysis was conducted under different parameters.The results showed that the mass fraction of ions decreases and then increases during consolidation until it reaches the fixed mass fraction,and the larger the permeability coefficient,the more obvious the decrease is.Pore pressure gradually decreases until the consolidation process is completed.