Study On The Effect Of Dry-wet Cycle On Water Migration In Subgrade Soil

Subgrade is the foundation of pavement structure.The solid,uniform and stable subgrade provides an important guarantee for the pavement structure to bear the vehicle load for a long time.The strength and stability of subgrade are largely related to the humidity of subgrade.Affected by the external environmental factors,the moisture inside the subgrade will appear migration phenomenon,which will cause the change of humidity inside the subgrade.When the moisture content in the subgrade is large,the bearing capacity of the subgrade soil structure will be greatly reduced.At the same time,the water in the subgrade soil will have a scouring effect on the internal movement of the soil,which will affect the stability of the soil structure.In this paper,the typical subgrade soil in Nanjing area is taken as the research object,and the effect of dry wet cycle is mainly considered.The indoor soil column model test,water heat coupling numerical analysis and other research methods are adopted.The indoor soil column dry and wet cycle test system is designed and manufactured by ourselves.The moisture transfer test in subgrade soil under the dry and wet cycle is carried out.A one-dimensional water heat coupling numerical model is constructed to analyze the change rule of temperature field and water field under the dry and wet cycle.It has important theoretical significance and practical value for optimizing subgrade design,improving subgrade drainage and improving subgrade service life.The main research results of this paper are as follows:Based on the basic theory of water transfer in unsaturated soil,the main sources of water in subgrade soil,the theory of soil water potential and the soil water characteristic curve are analyzed.Based on Darcy’s law,saturated unsaturated seepage theory,differential equation and definite solution condition,the problem of saturated unsaturated seepage of subgrade soil is deeply analyzed.In order to simulate the desiccation and dehumidification process of Subgrade in laboratory,the indoor soil column desiccation and humidification cycle test system is designed and manufactured,taking temperature and humidity into account.The test system consists of three parts: the test cylinder,the dry wet cycle simulation system and the data acquisition system,which realizes the real-time monitoring of the changes of the internal temperature and moisture content of the soil column.At the same time,the calibration of capacitive soil moisture sensor has been completed through many experiments.The indoor soil column dry and wet cycle test is carried out to simulate the water transfer in the vertical direction when the groundwater intrudes into the subgrade upward.Three dry and wet cycles are completed for the soil column model body,and the change rules of water content,temperature with time and soil column height in the two stages of heating drying and water supplement and humidification are analyzed.The numerical model of water heat coupling of subgrade soil is established to analyze the law of water transfer under the action of dry wet cycle.According to the conservation of mass and energy,the coupling equation of water and heat under the condition of this test is put forward.The model is solved by PDE module in COMSOL software,and the reliability of the model is verified by the data of the previous indoor test.In addition,the geometric model of highway filling subgrade is established,and the effect of dry and wet cycle inside the subgrade is different under different bottom humidity boundary conditions.