Experimental Investigation of Energy Evolution in Sandstone Failure during Triaxial Unloading Confining Pressure Tests

Table 1

Energy levels at peak stress under different initial unloading confining pressures.

Lithology	Initial unloading confining pressure (MPa)	Unloading pressure rate (MPa/s)	at peak stress (MPa)	Energy at peak stress (kJ/m^3)
				U1	U3	Ue	Ud	U
Sandstone	4	0.05	129.08	351.01	−16.96	298.70	35.35	334.06
	7	0.05	132.39	452.57	−22.23	368.06	62.28	430.34
	10	0.05	136.94	556.68	−55.36	416.70	84.62	501.32
	13	0.05	145.56	581.67	−61.86	430.23	89.58	519.81
	16	0.05	154.78	599.73	−63.37	440.85	95.50	536.36
	19	0.05	175.7	722.39	−88.05	523.95	110.40	634.35