It is found that the shear behavior of hard rock is important for the stability of deep rock engineering. In a deeply-buried hard rock tunnel

the splitting of rock results in the spalling or slabbing in the side wall

which can be evaluated by the uniaxial compression test. However

the spalling or slabbing in the arch shoulder and foot is the result of rock failure under shear boundary condition

which should be estimated by the direct shear test. Although a lot of research work on the spalling or slabbing caused by rock splitting have been conducted

there are few direct shear tests on intact rocks to study the shear stress-induced spalling or slabbing. In order to better understand the mechanism of spalling or slabbing

this paper divides the spalling into two forms: splitting failure and shear stress-induced failure under shear boundary

and establishes the conceptual model of two types of spalling based on the statistical results of spalling at Chinese Jinping underground laboratory and numerical results of the deviatoric stress distribution of laboratory section using the FLAC3D. A series of direct shear tests of intact marbles

which can achieve the shear failure and shear slip continuously

is conducted to verify the proposed conceptual model of spalling or slabbing.