Micromechanical Modelling of Stress Waves in Rock and Rock Fractures

Abstract

The goal of this paper is to simulate the interaction of stress waves and rock fractures in a particle micromechanical model. Stress waves travelling in fractured rock masses are slowed down and attenuated by natural heterogeneities, voids, microcracks and, above all, by faults and fractures. Considerable laboratory and theoretical investigation have uncovered the major aspects of this phenomenon, but models that cover the core mechanisms of the wave propagation in rock masses are necessary to investigate aspects of wave-fracture interaction, which are not completely clear, and in the future simulate full-scale real problems. The micromechanical model is based on the particle discrete element model that reproduces rock through a densely packed non-structured assembly of 2D disks with point contacts. The model of a hard rock core is developed and an irregular rock joint is generated at midheight. A new contact constitutive model is applied to the particles in the joint walls. Numerical static joint compression tests are performed and a typical hyperbolic stress-displacement curve is obtained. Conditions for good quality wave transmission through non-jointed unorganized particulate media are determined, hybrid static-dynamic boundary conditions are established and plane waves are emitted into the compressed joint. The transmitted and reflected waves are extracted and analysed. Joint dynamic stiffness calculated according to the hypotheses of the Displacement Discontinuity Theory shows to increase with the static joint compression until the joint is completely closed. Still in its early stages of application, this rock micromechanical model enables the joint behaviour under static and dynamic loading to be analysed in detail. Its advantages are the reproduction of the real mechanics of contact creation, evolution and destruction and the possibility of visualizing in detail the joint geometry changes, which is hard to accomplish in the laboratory.

Evolution of Joint Roughness Degradation from Cyclic Loading and Its Effect on the Elastic Wave Velocity

“…In particular, the evolution of the joint condition is dramatic on the surface of the weathered rock whose material integrity is altered in a manner similar to the rock joint behavior from weathering effects (Resende et al 2010). As indicated in Kabeya and Legge (1997), a significant change in the weathered rock joint surface can be induced by the modification of grain properties of the joint surface.…”

Ultrasonic method to evaluate the residual properties of thermally damaged sandstone based on time–frequency analysis

“…When ultrasonic pulses transmit in the sandstone sample, a complex damaged body full of holes and fractures and internal fissure surfaces will cause reflex, refraction, diffraction, etc. [29] Therefore, the received wave is somewhat complex compared with the spindle transmitted waveform. The received waveforms of the sandstone, taken from the display window of the supersonic test meter, after different high temperatures are summarised in Figure 4.…”

Vibration propagation in discrete element particle models of rock

“…This test is similar to a laboratory Ultra-Sound test: a wave is generated in the bottom boundary and is received in the opposite wall. The frequency of the wave was picked up so that the wave is preserved while travelling across the model, according to previous work by Resende et al (2010).…”