Fractal Characteristics of Coal and Sandstone Failure under Different Unloading Confining Pressure Tests

Table 4

Fractal dimensions of surface cracks with different unloading conditions.

Lithology	σ3 (MPa)	(MPa/s)	σ1–σ3 (MPa)	Fractal dimension	Correlation coefficient
Coal	4	0.02	30.82	1.0605	0.9427
	4	0.05	29.15	1.077	0.9562
	4	0.08	26.20	1.0981	0.9631
	7	0.02	36.15	1.0624	0.9554
	7	0.05	34.99	1.0726	0.9231
	7	0.08	35.20	1.0774	0.9635
	10	0.02	50.54	1.044	0.9523
	10	0.05	48.75	1.0587	0.9442
	10	0.08	43.15	1.058	0.9522
	10	0.11	44.10	1.074	0.9633
	10	0.14	42.24	1.076	0.9325
Sandstone	4	0.02	131.33	1.024	0.0952
	4	0.05	129.08	1.034	0.9421
	4	0.08	120.40	1.059	0.9365
	7	0.02	134.84	0.9976	0.9365
	7	0.05	132.39	1.019	0.9521
	7	0.08	122.09	1.0621	0.9456
	10	0.02	147.18	0.9206	0.9471
	10	0.05	136.94	1.0108	0.9369
	10	0.08	134.59	1.014	0.9354
	10	0.11	128.90	1.054	0.9512
	10	0.14	128.36	1.0626	0.9432
	13	0.05	145.56	0.9875	0.9366
	16	0.05	154.78	0.9371	0.9412
	19	0.05	175.70	0.8712	0.9356