Mechanism of Secondary Breakage in the Overlying Strata during Repetitious Mining of an Ultrathick Coal Seam in Design Stage

Table 1

Mechanical performance parameters of overlying strata (OS) in the B_M coal seam.

No.	Lithology	Thickness (m)	Elasticity modulus (GPa)	Density (10^3 kg·m^−3)	Compressive strength (MPa)	Cohesion (MPa)	Internal friction angle (°)
#1	Weathered sandstone	40	5.0	1.7	/	0.5	/
#2	Sandstone layer	6.7	10.0	2.3	5	1.1	28
#3	Interbedded mudstone-sandstone	16	16.5	1.9	32.8	8.3	38
#4	Gritstone	4.2	39.7	2.7	54.8	23.6	36.5
#5	Mudstone	10	10.8	1.7	25.7	1.6	38.5
#6	Interbedded mudstone-sandstone	7	17.6	1.9	34.9	12.6	39
#7	Sandy mudstone	9	38	2.5	23	2.5	32
#8	Siltstone	3	16	2.1	39	10.5	35
#9	Medium sandstone	4	19.2	2.5	43.2	14.8	40
#10	Sandy mudstone	4	38	2.5	23	3.5	32
#11	Fine sandstone	9.5	19.2	2.4	50	4.5	40
#12	Gritstone	3.6	42.1	2.7	56.3	23.9	36.5
#13	Fine sandstone	3	35	2.6	56.6	23.1	37.5
#14	Coal	44	13	1.5	18.3	1.54	30