**3. Experimental Materials and Methods**

#### *3.1. Crushed Rock Samples*

The crushed rock samples used in the tests were the crushed mudstones obtained from the gob roof of 12201 working face in the Haragou Coal Mine (Shenmu, China). The natural density and the uniaxial compressive strength of the crushed rock samples were 2.65 g/cm3 and 19.8 MPa respectively. In accordance with the need to comply with acceptable size of the particles of crushed rocks in reliable testing, a ratio of 1/5 between the inscribed circle diameter (*D*) of cubic container [21], the largest crushed rock particles had to be maintained. Therefore, the maximum particle size of samples used in tests was limited to 80 mm. Three types of crushed mudstone samples with different ranges of particle size categories (10~30, 30~60, 60~80 mm) were prepared for the tests, as shown in Figure 4.

**Figure 4.** Crushed mudstone samples for the tests.
