Study On Water Migration Characteristics Of Unsaturated Expansive Clay

The unsaturated expansive clay is obviously impacted by the change in water content because of its engineering characteristics of swelling and shrinking.The differential settlements of foundation,crack of roadbed or house,and the instability of the slope are often caused by the water migration in soil,which may cause huge economic losses for engineering.Therefore,it has been widely concerned by experts.To ensure the engineering safety of unsaturated expansive clay,it is of important theoretical significance and application value to further discuss the change law of water migration in unsaturated expansive clay.In order to more objectively and effectively explore the variation rules of water migration and the degree of influence of various factors,the author has adopted special test equipment to effectively distinguish the water vapour migration and mixed（vapour and liquid）migration in soil.Comparative experiments were set under different water content gradients,temperatures（constant temperature and variable temperature）,and migration times to explore the changing trend of water migration in clay and the degree of influence of different factors.At the same time,the proportion of water vapour in mixed migration under other migration conditions was studied.The results show that water migration is affected by temperature,time,and water content gradient.For tests with constant water content at one end,the amount of water migration rises gradually with the increase of water content gradient.The migration amount of 18%water content gradient was about 1.5～2 times of 12%and 2～3 times of 6%.Generally,the migration of water was mainly completed within 30 days,and the migration amount of 60 days was about 1.07～1.4 times of that in 30 days.Under the same water content gradient and migration time,the influence of temperature on the water migration was pronounced,and the migration amount of 20°C was about 40%～60%of that at 40°C,and the migration amount of 5°C is about 25%～40%of that at 40°C.When the water content gradient was large,the migration at constant temperature（20°C）had little difference with that at variable temperature（15-25°C）,while when the water content gradient was small,the migration at constant temperature was about 50%-70%of that at variable temperature.The proportion of the vapour water migration to the mixed water migration indicates that the vapour water migration cannot be ignored in the water migration of unsaturated soil.In the samples with a water content gradient of 18%,the proportion of vapour water was above 57%,and the proportion increased with time and temperature.In the samples with a water content gradient of 6%and 12%,the proportion of vapour water was not less than 30%,but with the increase of time,the proportion of vapour water decreased.At the same time,microstructure images of soil samples were quantitatively and qualitatively analyzed.The changes of fabric characteristics,pore structure,and particle characteristics of expansive soil after water migration were discussed.The results show that the soil samples are dominated by small and medium pores,which are conducive to the rise of capillary water in the soil and provide space for bound water.With the decrease of water content,the agglomeration of particles becomes more robust,and the pores between structural units offer channels to water migration.Furthermore,the molecular dynamics microanalysis models of clay minerals were established to simulate the hydration kinetics process of clay minerals with different moisture contents at different temperatures 5°C,20°C,and 40°C.The expansion properties under different water contents and temperatures were analyzed.The characteristics of diffusion coefficient,coordination number,and relative concentration distribution,and conduction evolution of water molecules during the hydration process were discussed.Finally,the diffusion coefficient fitting was applicable to the numerical calculation to further expatiate the macroscopic effect of the microscopic characteristics of clay on the soil water migration.The results showed that with the increase of water content,the interlayer water gradually generated the distribution structure of one layer,two layers,three layers,and even four layers.The interlayer space gradually expanded,which verified the swelling property of clay minerals in the water.At the same time,the peak value of the relative concentration of water molecule decreased by 65%at water content 43%than that at 6%.Still,the distribution space of water molecules more than doubled.That is,the distribution of water molecules was more dispersed.With the increase of water content,the diffusion coefficient increased in power function,and the value ranged from 1.80×10^–11 to 22.0×10^–10 m²/s.Meanwhile,the influence of temperature on the diffusion coefficient was apparent.With the same water content,the diffusivity of 20°C and 40°C were about 1～3 times that at 5°C.It can be seen that the increase in temperature can weaken the hydration ability of cations,resulting in the reduction of bound water molecules.Thus the conduction ability of water molecules was enhanced,and the diffusion coefficient increased.The evolutionary mechanism of expansive clay characteristics caused by water migration was illustrated from the microscopic point of view.Based on Caputo and Conformable’s definition of fractional order,the vapor and liquid migration equation of fractional order of unsaturated soil was established.The numerical solution of fractional vapor-liquid migration equation in polar coordinates was also studied.According to the Stokes-Einstein equation and fractal theory,the calculation formula of diffusion coefficient considering temperature change is discussed.After sensitivity analysis,it was found that the numerical solution with the spatial fractional-orderαof 0.95 had the highest agreement with the measured value.According to the change of water migration in the soil,the parameters were selected by stages,which improved the accuracy of numerical calculation.Compared with the measured values,it was found that the error ratio of the standard integer-order governing equation was 5.20%～28.14%,while the error ratio of this model was1.32%～9.05%,which was about 30%of the former.This provides an effective method to simulate the water migration process in unsaturated soil.