As a waste material generated during coal mining and washing

coal gangue is often used for filling roadbeds. The compacted coal gangue has a dual-porosity structure

which usually leads to bimodal soil-water characteristic curves (SWCC). SWCC is closely related to the permeability coefficient of unsaturated soils

which has an important influence on the water migration inside the roadbed and the long-term serviceability of the roadbed. In order to investigate the influence of compaction degree

silt content and maximum particle size on bimodal SWCCs

a series of pressure plate tests was conducted on compacted coal gangue samples. The experimental data was fitted by a bimodal SWCC function model

and the variation of fitted parameters was analyzed. The test results show that when the silt content is 5%

samples with different compaction degrees have bimodal SWCCs. When the silt content is 50%

the SWCCs of samples change from bimodal to unimodal with the increase of compaction degree

so it can be inferred that there exists a critical value of compactness between 0.90 and 0.95. Increasing compaction degree or silt content

or decreasing maximum particle size will reduce the difference of step height in SWCCs. With the increase of compaction degree or silt content

the values of model parameters

(parameters related to the air-entry value) and

(parameters related to drainage degree) decrease gradually

that is

the air-entry values and the water holding capacity of both macro-pore part and micro-pore part increase.