تحلیل دگرگونی شکل خاک غیر اشباع منبسط شونده در طول چرخه مرطوب سازی و خشک کردن Limit Deformation Analysis Of Unsaturated Expansive Soils During Wetting And Drying Cycles

Introduction Unsaturated expansive soil is susceptible to the effect of environmental conditions because an increase in water content causes the soil to swell, while water removal causes the shrinkage of soil. Many engineered structures built on this material are significantly damaged and cracked, such as shallow foundations, drainage channels, and buffers of radioactive waste disposal. Determination of possible swell and settlement of expansive soils due to suction changes are generally considered as a necessary part of the construction design. Furthermore, the hydromechanical response of expansive soils affected by wetting and drying cycles is essential information for understanding its constitutive behavior. Laboratory tests on these materials during wetting and drying cycles reported by several researchers [1-4] show that a limit equilibrium state can be reached after several wetting and drying cycles. In this context, the limit deformation at the equilibrium state after wetting and drying cycles should be studied to better understand the hydromechanical behavior of unsaturated expansive soils. Many models were proposed to predict the complex hydromechanical behavior of unsaturated expansive soils with the application of suction cycles [5-8]. The widely accepted model is the Barcelona expansive model (BExM) developed by Alonso et al. [2, 3, 8], able to simulate the basic behavior of unsaturated expansive soil, including the strain fatigue phenomenon during wetting-drying cycles and the prediction of the final equilibrium state at the end of the suction cycles. However, this model presents a large number of parameters such as the coupling functions for micro- and macrostructural strains. The calibration of these parameters needs several experimental tests, which lead to a time-consuming procedure to characterize their mechanical behavior. The Zarka limit analysis method is a simplified theory, initially developed for kinematic hardening materials, such as metals [9, 10]. It defines the accumulated plastic strains at limit equilibrium states with Melan’s static theorem by introducing less model parameters. Sharp et al. [11] applied a similar limit analysis concept for the first time to the unbound granular materials, which defined the limit load as the key design load. Chazallon et al. [12, 13] have developed an elastoplastic model based on the Zarka limit state theory with a nonassociated flow rule for unbound granular materials when they are subjected to repeated load triaxial tests in order to determine the limit accumulation of plastic strains. This paper presents a limit approach based on the Zarka analysis method to model the mechanical behavior of unsaturated expansive soils subjected to wetting and drying cycles. The required parameters of the proposed method are calibrated by the odometric test results carried out on different initial states of an expansive soil. The model is validated for the intermediate samples to compare model calculations with experimental results. These comparisons show that the proposed model is able to replace the step-by-step method to estimate limit accumulated plastic strains of expansive soils during suction cycles and the subsequent mechanical behavior after suction cycles.