Investigation on the macro meso shear mechanical properties of soft hard interbedded rock discontinuity

For the two kinds of typical soft hard interbedded rock discontinuities such a“soft+hard”(the first kind) and“hard+soft+hard”(the second kind),the macro meso shear mechanical properties of those under different moisture contents,normal stress and first order asperity angle were investigated using laboratory direct shear tests and PFC2D meso mechanics simulation (under constant normal load),based on which the shear strength estimation formula was proposed and verified.Research shows that ① the change of shear stress (normal displacement) with shear displacement has remarkable characteristics of stage develop ment.Three stages are found in the two kinds of rock discontinuities respectively.For the first kind of discontinuity,the three stages include initial nonlinear compression shear deformation,increases steadily of stress and increases steeply drops brittlely of stress.For the second kind of discontinuity,the three stages consist of initial nonlinear compression shear deformation,increases steeply slowly of stress and constant stress.② The peak (residual) shear strength and peak shear displacement decrease as the moisture content increases (the maximum normal displacement increases).The peak (residual) shear strength and peak shear (maximum normal) displacement increase as the normal stress increases.When the first order asperity angle increases,the peak shear strength of the first kind discontinuity increases (the residual shear strength increases first and then decreases),while the peak (residual) shear strength of the second kind discontinuity increases,and the peak shear displacement of both kinds of discontinuity decreases generally (the maximum normal displacement increases).③ The change of meso damage crack number and energy with shear displacement can be divided into three stages,i.e.,a slight,an approximately “down concave arc shape”steep and a linear slow increase in the initial,middle and later stage respectively (crack number),and a slight,an approximately“up concave arc shape”slow and a linear steep increase in the initial,middle and later stage respectively (energy).④ The test data are in good agreement with the simulation results,and the macro meso damage degradation mechanism of rock discontinuity is generally described.The typical failure modes of the two kinds of discontinuity can be summarized as three basic types respectively,i.e.,compression shearing cracking failure,dislocating voiding failure and penetrating gnawing failure as well as compression shearing cracking failure,and wearing gnawing failure and penetrating slipping failure.The meso damage particles of the two kinds of discontinuity are distributed in an approximately “inverted U shaped”and“S shaped” near the shear plane respectively,and when the angle of contact force between particles is between 0° and 90°,the number of damaged particles is the most.⑤ According to the limit equilibrium and strength reduction methods,the rationality of shear strength estimation formula for the rock discontinuity (considering the impact of first order asperity angle) is verified by the stability analysis of rock slope examples.