Microstructural and hydraulic properties of compacted high-speed rail subgrade coarse soils

The hydraulic characteristics of coarse-grained soils of high-speed railway subgrade profoundly affect the internal moisture migration and long-term accumulative deformation of the subgrade. A new TDR sensor and large-diameter preamble column testing devices are developed to study the hydraulic properties of the compacted coarse materials used in high-speed railways. In addition, the hydraulic properties is highly relate to the corresponding microstructure, and a new soil-water characteristic curve model which can consider the initial density of soils with dual-pore structure is proposed. The hydraulic properties are compared with those of the microscopic tests (scanning electron microscopy test and mercury injection test). The experimental results show that with the increase of compaction degree, the volumetric water content of subgrade filler with fine particle content of 15% decreases under low suction, however, it tends to be consistent under high suction. The microscopic characteristics of the soils show that the roadbed filler with a fine particle content of 15% exhibits a dual-pore structure. As the degree of compaction increases, the macroporous structure is gradually compressed, while the micropore structure is difficult to be compressed. The microscopic test results are found to be consistent with the hydraulic ones, indicating the water transport laws of the roadbed filler under different compaction degrees are still valid.