Investigation Of Elastic Wave Propagation In Jointed Rockmass Under Stress Conditions

Elastic wave testing technique is one of the widely used methods in such fields as mining engineering, geological exploration, hydraulic engineering and tunnel engineering, and seismic exploration. Elastic wave propagation in rock is not only affected by the internal structure, but also related to the stress state in rock mass. Therefore, it is necessary to consider the effects of the stress field on elastic wave velocity and attenuation in the actual engineerings. The relationships between elastic wave and the boundary stress of defects in the rock and the loading outside the rock are studied by theoretical and experimental methods.In the present paper, acoustic tests of three complex materials, i.e. EPS concrete, steel fiber reinforced cement mortar (SFRCM), and aluminum foam, under compression is conducted in three frequencies (50kHz,300kHz, and 1MHz) to reveal the elastic wave propagation in the media containing different internal structures. The influences of pore size and pore content on elastic wave velocityare investigated by application of EPS concrete with different EPS diameters. The propagation of acoustic waves in media containing both holes and inclusions is explored by sonic experiment of steel fiber reinforced cement mortar. A relative speed method is presented to analyze the relationship of wave speed to loads and frequencies. Weibull distribution function is applied to fit the strain field of aluminum foam, which is obtained by digital image correlation method (DIC) based on the fact that deformation of the holes in aluminum foam can be easily observed. The relationship among holes, stress and velocity is established by linking weibull parameters with wave speed changes.Based on the single elliptical crack model, boundary stress condition of defects is introduced to simulate the rocks containing different inclusions. The boundary stress condition can be divided into constant boundary stress condition and constant stiffness boundary condition depending on whether stress changes with crack opening displacement or not, and then the model calculation is applied to study the rules of elastic wave speed and attenuation under different stress boundary condition. We find that constant stress only affects the velocity and attenuation of the low frequency domain, which produces evident velocity dispersion; The velocity and attenuation in low frequency domain, Rayleigh scattering domain and resonance scattering domain are affected, when constant stiffness boundary condition is applied.A second crack with particular direction and position has been set up near the main crack. Then a double elliptical cracks model which takes the interactions among cracks into consideration has been presented to analyze the partial multiple scattering. It finds that the effect of multiple scattering is stronger and dispersion is weaker when the two cracks become closer; when the distance between the cracks is very far, calculation of "double defects" model is the same as single scattering. It indicates that multiple scattering can be neglected when porosity is very low. Meanwhile, we study influences of rock porosity, defect size and other parameters related to the actual engineering on the rule of elastic wave propagation. Finally, the relation of elastic wave to different inclusions is studied by introducing boundary stress condition of defects into in the "double defects" model.Finally, the acoustic velocity of steel fiber reinforced cement mortar (SFRCM) under compression is simulated by the two methods. At the microscopic scale, considering the interaction of waves and holes, the combination of double-elliptical crack model with B-B model is conducted to simulate wave propagation rule in SFRCM specimens. The results well reflects the relationship between wave speeds and loads qualitatively. On the quantitative estimation, the simulation results of 50 kHz are consistent well with the actual measurement results, but the wave speed of 300kHz has certain deviation. An appropriate correction method is then proposed to minimize this deviation. However, it needs further study to consider how to apply it. At the macro scale, the effective combining cementation model with unconsolidated model is introduced with a semi-theoretical formula on the effective media method. Then we use the effective model to analyze the relationship of wave speed change to load. The results can reveal the evolution rule of wave speed to load and frequency, although it has slight deviation in quantification.