Experimental study on anisotropic unloading mechanical behavior of bedded sandstone.

Sandstone is a kind of bedded rock mass commonly used in engineering. The deformation and strength of bedded sandstone impose some problems during excavation. In this study, triaxial unloading tests were conducted on rock specimens (considering seven different bedding angles (β)). The results revealed the following key points: (1) At a constant confining pressure, the elastic modulus was gradually increasing when the bedding angle increased. Furthermore, after initial decreasing, the deformation modulus was increased and had a U-shaped distribution. (2) During the unloading of the axial compression, the rate of axial strain variations was initially increased and then decreased while the bedding angle increased (it has exhibited an inverted U-shaped distribution). However, the peak strength, cohesion, and angle of internal friction of rock specimens showed an initial decreasing and then increasing trend. (3) During the loading and unloading stages, the confining pressure reduces the anisotropy of bedded rock masses. (4) In the triaxial unloading test, the failure of rock specimens can be classified into four modes. When there was a large intersection angle between the bedding plane and the unloading direction, failure developed at the bedding planes.