Abstract: Hydraulic characteristics of coarse-grained soil for high-speed railway subgrade profoundly influence internal moisture migration and long-term accumulative deformation of the subgrade. Conventional test methods have many disadvantages such as small size of sample and difficulties involved in sample preparation in determining hydraulic characteristics of such coarse-grained soil with high compactness, and thus can hardly be applied. A test device for determining hydraulic parameters of coarse-grained soil is presented thoroughly in this paper. Tensiometers and time domain reflectometers (TDR) are installed in the test device, allowing the measurement of matric suction and dielectric constant of coarse-grained soil at different altitudes during wet and dry stage; and consequently the soil-water characteristic curve (SWCC) of coarse-grained soil of high-speed railway subgrade with high compactness can be obtained. Also, the relationship bewteen unsaturated hydraulic conductivity and matrix suction of such soil can be obtained by transient profile method. The test result indicates that this device can be applied to the coarse-grained soil whose maximum particle size is 20 mm and whose degree of compaction may be up to 0.95. By analysing a group of experiment data and refering to data from Ekblad’s experiments, it is found that ? ( a parameter related to air entry value )decreases and air entry value increases with the increase of fine particle content in such soil, and that the largeness of particle size and low concent of fine particle suggest a low water storage capacity and high value of n. This article will provide a method for measuring hydraulic parameters of coarse-grained soils of subgrade.

Key words: coarse-grained soil, infiltration column, time domain reflectometer (TDR), matric suction, soil water characterisic curve, unsaturated hydraulic conductivity