Dynamic fracture and wave propagation in a granular medium: A photoelastic study

Abstract

In our earlier work on the generation mechanisms of earthquakes and related failures, fracture phenomena were treated in the framework of continuum mechanics, and the existence of the universal critical condition for earthquake nucleation as well as the strong dependence of earthquake-induced structural failure patterns on the frequencies and types of incident seismic waves was pointed out. However, other significant seismic phenomena such as landslides and liquefaction may not be simply explained using the theories established for continuum media. For example, in order to clarify the physics of the formation of the geological flame structure, possibly due to liquefaction and ensuing gravitational instability in water-immersed sediments, the mechanical behavior of particles under dynamic load and the influence of waves, if any, on fracture should be understood for granular media beforehand. Here, as an initial investigation into wave and fracture propagation inside granular media, under dry conditions first of all, experimental technique of dynamic photoelasticity is employed. Penny-shaped particles made of birefringent polycarbonate are prepared and placed on a rigid horizontal plane to form two-dimensional model slopes with certain inclination angles. Dynamic impact is given to the top (approximately) horizontal free surface of the slope, and the transient evolution of stress and fracture is recorded by a high-speed digital video camera. It is shown that depending on the profile of energy imparted by the impact, (i) one-dimensional force-chain-like stress transfer or (ii) widely spread multi-dimensional wave propagation can be found. While the case (i) results in mass flow, i.e. total collapse of the slope, in (ii) waves can induce dynamic separation of only slope faces similar to toppling failure. The experimentally observed wave and fracture phenomena in granular media are compared to those in continuum media and the actual slope failure repeatedly caused in Japan, New Zealand and USA.