An Experimental Study of the Uniaxial Failure Behaviour of Rock-Coal Composite Samples with Pre-existing Cracks in the Coal

Table 2

Distributions of pre-existing coal cracks [5].

No.	Composite sample photos	Main distribution of pre-existing coal cracks
A-1		Four small vertical surface cracks adjacent to the rock-coal interface (minimum persistence) (intact composite sample)
A-2		Two large vertical surface cracks, one adjacent to the rock-coal interface and the other in the middle and lower part of the coal
A-3		Two vertical penetrative side cracks through the coal near the lateral boundary
A-4		One horizontal penetrative crack and some surface cracks
A-5		One main vertical penetrative crack through the central part of the coal and the other cracks being half penetrative through the coal. One horizontal penetrative crack in the lower part of the coal near the coal-sandstone interface