Effect Of Micro-Defects And Macro-Joints On Stress Wave Propagation In Rock Mass

The effects of discontinuities on stress wave propagation in a sedimentary rock mass is investigated. The discontinuities in the rock mass are divided into two groups. The primary discontinuity set is the one with relatively large or the same order spacing to the wavelength, known as "macro-joint", while the secondary discontinuity set, characterized by high density and relatively small spacing to the wavelength, termed to be "micro-defect". The sedimentary rock with micro-defects is modeled as an equivalent continuum. The effect of micro-defect on stress wave propagation is evaluated by a transient wave propagation method which is calibrated by longitudinal impact tests of a sedimentary rock bar using a pendulum. Measured stress waves show that the defected rock behaves visco-elastically under dynamic conditions. The effect of micro-defect on the viscoelastic response of rock is further investigated using the Numerical Manifold Method (NMM). Complex viscoelastic modulus of the sedimentary rock is obtained and the effect of micro-defect quantity, stiffness and length are discussed. The macro-joint in the rock mass is modeled explicitly as physical discontinuities. An Extended Displacement Discontinuity Method (EDDM) is introduced to investigate the effect of macro-joint on the wave propagation through viscoelastic rock mass. The transmission coefficient and reflection coefficient for the stress wave propagation across the macro-joint in the viscoelastic rock mass are analytically deduced. The results of a numerical example show that the overall mechanical response of the rock mass is viscoelastic, both the micro-defects and macro-joints have significant effects on the stress wave propagation through the rock mass, the NMM can be used to analyze the effect of micro-defect on the wave propagation efficiently, and the approach using equivalent viscoelastic medium with explicit macro-joint can be used in the dynamic analysis of complex jointed rock mass.