Influence of the scale effect on the parameters of damage plasticity model for mechanical behaviour of rock mass

Rock masses are inhomogeneous, fractured, anisotropic, and initially stressed in their natural state. They consist of intact rock or rock monolith and macro-damages, which include fractures, bedding planes, faults and other forms of discontinuity. It has been experimentally confirmed that with the increase of the rock mass scale, the mechanical properties of the rock mass decrease as a consequence of discontinuities. The subject of this paper is the analysis and determination of the scale effect on the damage plasticity model parameters, which describe the mechanical behaviour of the rock mass for different loads and scales. The scale effect is analysed on a constitutive model that can simulate the most complex mechanical behaviours in rock masses (elasto-plastic with damage in stiffness and strength). The analysis is based on the results of experimental tests of rock mass, which were carried out for different scales (scale of the rock mass sample in laboratory and scale of the in situ test – shear test), while the tests were performed at the same microlocation. For the considered experimental tests, appropriate finite element models (FEMs) were formed with boundary conditions that correspond to the ones from the experiments. By simulating the performed experiments with a series of FEM analyses with complex optimization algorithms, the parameters of damage plasticity model were determined for both values of the rock mass scale, proving the scale effect numerically. Based on the obtained results, analysis and discussion of the scale effect for a given rock mass were performed.