Contrary to the firmly established mechanics of continuum media, the mechanical behavior of particles in granular media, especially that under dynamic load, has not been fully understood. Here, as an initial investigation into wave and fracture propagation inside granular media, experimental technique of dynamic photoelasticity is utilized. Penny-shaped birefringent particles are prepared and placed on a rigid horizontal plane and two-dimensional dry model slopes with some inclination are formed. Using a high-speed digital video camera, the transient stress and fracture development due to dynamic impact on the top free surface of the model slope is recorded. It is found that there exist at least two failure patterns depending on the energy profile associated with the impact: (i) total collapse of the slope or mass flow resulting from one-dimensional force-chain-like stress transfer; and (ii) dynamic, toppling failure-like separation of the slope face induced by widely spread multi-dimensional wave propagation.

1 Introduction

Besides the disastrous power of Tsunamis, one of the puzzling phenomena noticed on the occasion of the devastating 2011 off the Pacific coast of Tohoku (Great East Japan) earthquake (moment magnitude Mw 9.0) is the unique dynamic failure in slopes in the city of Sendai where open cracks, located some meters away from the upper edge (crest) in the top surface, extended parallel to that edge. It should be noted that similar tensile cracks were generated precisely in the identical slopes by the 1978 Miyagi-ken-oki earthquake (Mw 7.5; Fig. 1a), and using the conventional countermeasures that consider the influence of body waves, the slopes were reinforced with steel pipe piles after the 1978 event. This earthquake-induced failure pattern, not widely recognized, has been found also at other rockier places including the South Island, New Zealand, in 2011 ( Mw 6.2) (Hancox et al. 2011) and California in 1906 (Mw 7.8), 1957 (Mw 5.7) (Sitar et al. 1980) and 1989 (Mw 6.9) (Ashford & Sitar 1997). However, the mechanical analyses of body wave interaction with continuum model slopes (e.g. Sitar et al. 1980, Ashford & Sitar 1997, Ashford et al. 1997) as well as dynamic study of granular slopes, often assumed also in the field of rock mechanics, does not seem to be able to straightforwardly reproduce the failure pattern. In typical granular models (see e.g. Cleary & Prakash (2004)), usually, the effect of wave propagation on dynamic fracture is neglected, and only granular mass flow, namely, nearly simultaneous, total collapse of the slope face, edge and top surface, is depicted.

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