Abstract: Moisture migration inside unsaturated silty subgrades under dynamic loading is of great significance for investigating the stability of heavy haul railway subgrades in water immersion conditions. Based on soil electrical properties, a measurement system combined with a GDS cyclic triaxial instrument was developed relying on the van der Pauw principle to measure the water content distribution properties of unsaturated soil specimens. With the use of the aforementioned measurement system, water-supply dynamic triaxial tests were carried out. Moreover, the stratified water content of the unsaturated specimen was continuously measured during the test to explore the influence of the initial compaction degree of the unsaturated specimen and dynamic stress amplitude of dynamic loading on water migration inside the unsaturated specimen. This measurement system which can be used to determine the stratified water content of unsaturated specimens in real time, is able to achieve nondestructive and consecutive testing during dynamic triaxial tests, and the maximum error is only 0.7%. As the water content of the unsaturated soil specimen gradually increases under dynamic loading, the influence of the soil specimen initial compaction degree on resistivity decreases, and the influence can be ignored when the water content is close to saturated water content. A function exists to correlate soil resistivity with soil specimen initial compaction degree and water content, and the function has a high level of correlation and uniqueness. According to the analysis of the testing results, under the combined action of rainfall and cyclic loading, the water inside the subgrade accumulates to a certain depth inside the soil, and the pore water pressure in this area increases. The increase of pore water pressure decreases the strength and leads to the occurrence of mud-pumping disease. Increasing the subgrade compaction degree and the axle weight of the train will inhibit the downward water migration inside the subgrade and contribute to subgrade stability. However, an excessive increase in axle load may result in subgrade instability. When the axle load is greater than the critical dynamic stress, the subgrade will be destroyed.

Key words: van der Pauw principle, unsaturated soil specimens, cyclic triaxial test, moisture migration