A Study on the Instability Mechanisms of Coal Pillars in Shallow Coal Seams Group Mining

Table 1

Mechanical parameters of rock and coal.

Lithology	Bulk weight (g/cm^3)	Elastic modulus (GPa)	Poisson ratio	Tensile strength (MPa)	Cohesive force (MPa)	Internal friction angle (°)
Loess	1.86	1.8	0.15	0.09	0.92	29.4
Red clay	2.02	2	0.18	0.22	1.28	27.4
Silty fine sand	2.44	2.1	0.19	1.93	6.18	36.2
Medium-grained sandstone	2.28	2.3	0.18	2.18	6.07	37.1
Coal	1.34	0.62	0.26	1.24	4.72	35.2
Fine sandstone	2.29	2.32	0.2	2.29	5.6	37.3
Mud-sandstone	2.3	1.08	0.24	0.29	1.85	28.6