Softening Rule And Design Parameters Of Subgrade Material Under Dry-wet Cycling Condition

Recently,highway construction projects undertaken by Chinese enterprises are challenged by special climate and environment in Africa and Southeast Asia.The tropical rainy climate may cause the subgrade to be seasonally immersed,resulting in softening of subgrade material and the quality and safety of engineering consequently.The purpose of this study is to reveal the internal reasons and laws of softening of immersed subgrade materials under dry-wet cycles,put forward a method to calculate its resilient modulus of the seasonal immersed subgrade,and program it for reference in practical engineering problems.Firstly,based on study of unsaturated soil constitutive theory,an elastoplastic constitutive model of unsaturated soil was applied to the Abaqus fluid solid coupling simulation by programing the UMAT subroutine.Then some simulated triaxial tests were carried out,and validated by comparing simulated and measured data.The results show that the model used can describe the effect of hydraulic hysteresis on mechanical properties,and the change rule of the pre-consolidation stress during dry-wet cycle by using plastic saturation as parameter,explaining the accumulation of plastic deformation during dry-wet cycles.In addition,the concept of cumulative plastic saturation increment was proposed to describe the influence of the dry-wet cycle on the cohesive force of soil,which can simultaneously reflect the attenuation law of the cohesive force in the process of water content change and the increase of dry and wet cycles.Secondly,finite element models of immersed subgrade were established,and a moisture dependent resilience modulus model UMAT subroutine is programed to simulate the distribution of resilient modulus and the response of subgrade structure with water table changing.The weighted average method and iterative backcalculation were proposed to calculate the equivalent resilient modulus of each month,meanwhile a comprehensive equivalent modulus and reduction factor calculation method were proposed.The results indicate that water level fluctuation has a hysteresis on the internal part of clay subgrade.The middle of slope has a greatest accumulation of saturation increment,so is affected most by dry-wet cycle.The equivalent modulus obtained by the inverse calculation of the weighted average method and backcalculation with tensile strain under asphalt layer are reasonable,but the one from backcalculation with compressive strain on the top of subgrade is too small.Finally,according to the method proposed above,a subgrade equivalent modulus calculation program was developed based on the Abaqus GUI toolkits and the Python language.The plug-in program can automatically calculate the equivalent resilient modulus with the specific parameters provided by the user,which will benefit the subgrade design.