Investigation of peculiarities fracture and dilatancy processes during constrained shear deformation of block-structured geological media

The peculiarities of dilatancy processes in block-structured media that experience nonequiaxial compression under shear deformation are investigated using movable cellular automaton (MCA) method. For a characteristic of compression nonequiaxiality (also termed the degree of constraint) a dimensionless parameter - the lateral to normal pressure ratio in the deformation plane - used. The main objective of the work is to trace the sequence in which various dilatancy mechanisms are involved in deformation depending on the level of shear stress and degree of constraint. It is shown that in the block-structured medium an increase in the degree of constraint causes the dominating dilatation mechanism to change from slip of discontinuity surfaces to opening and expansion of pores. The dominating dilatancy mechanism changing because increasing the degree of constraint increases the threshold shear stress at which the slip is activated. Beginning with certain lateral pressures, the slip is impeded giving way to expansion of the pore space; however, the latter fails to provide so considerable volume change as the slip of contact surfaces does, and this decrease critical dilatation characteristics of the medium and, in particular, its dilatation coefficient and volume changing.