Influence of Valleys Terrain on Pressure of Fully Mechanized Working Faces in Shallow Coal Seams

Table 5

Effect of absent/present strata on the stress of the main key stratum.

Coal seam	Main pressure index of the stope	Missing the main key stratum	Not missing the main key stratum
1^−2 upper coal seam	Initial pressure step/m	22	26
	Periodic mean pressure step/m	9.1	16.2
	Maximum height of ground steps/m	3	0
	Maximum height of ground crack/m	3.7	0.2
	Main key stratum instability	Yes	No
	Times of dynamic strata manifestation/times	3	0
1^−2 coal seam	Initial pressure step/m	30	29
	Periodic mean pressure step/m	7.8	8.7
	Maximum height of ground steps/m	3.4	0
	Maximum height of ground crack/m	5.1	0.5
	Main key stratum instability	Yes	No
	Subcritical strata instability	Yes	No
	Times of dynamic strata manifestation/times	1	0