| Expansive soil is very sensitive to water,and its destruction often has the characteristics of long-term potential danger,so the research on this "problem soil" has become a topic that cannot be ignored.Based on a mountain airport runway high fill subgrade of expansive soil as the research object,exploring the law of water migration in the case of compaction degree gradient and water content gradient in different layers after compaction due to construction technology or environmental factors,and establishing the water-thermal coupling numerical model to simulate the distribution of temperature field and moisture field and compare with the actual test results,then based on the direct shear test of expansive soil,further exploration in the change law of strength of expansive soil under the action of freeze-thaw cycle.It provides a theoretical basis for preventing the engineering damage of expansive soil subgrade by freeze-thaw action.The main research contents and achievements of this paper are as follows:(1)The research model test system was developed and assembled.According to the test conditions of this paper,researched and assembled the unsaturated soil moisture migration test apparatus,the test device in the study of unsaturated soil subgrade focus the following requirements,controlling the temperature gradient,overlying pressure,filling water conditions,and the data acquisition device,after multiple tests try to form a set of systematic experimental steps,save test time and reduce the test error as much as possible.(2)Water migration model test and rule analysis.Indoor model test simulated 4 kinds of actual engineering situations,including which does not set the pressure on the soil samples,setting the pressure on the soil samples,setting the moisture content gradient on the soil sample and compaction gradient on the soil samples,every sample take three freeze-thaw cyclings.According to the temperature,water content and displacement data of different time and different depth of each measuring point to analyse the change of temperature,water content and deformation with time and space in freezing-thawing process and the influence of freeze-thaw cycle on moisture migration of unsaturated expansive soil were analyzed.(1)The COMSOL Multiphysics was used for numerical simulation,and compared with the results of the model test.According to the law of conservation of energy and combining the Darcy's law to establish the water-heat coupling equation of unsaturated soil.Using the PDE module,after determining the parameters and boundary conditions of the equation,the software simulation is carried out.By comparing and analyzing the calculated data with the indoor model test data,the moisture transfer rule of unsaturated expansive soil under the action of freeze-thaw cycle was obtained.(2)Study on the change of shear strength of unsaturated expansive soil under the action of freeze-thaw cycle.According to the moisture migration patterns of indoor model test of freeze-thaw cycles,along the axis to select five typical cross sections for direct shear test.Through describe the variation trend of the shear strength and shear strength index with the depth and water content of the samples,the effects of load,moisture content gradient and compaction degree gradient on the shear strength and shear strength indexes of unsaturated expansive soil under freeze-thaw cycle were obtained. |
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