Overhanging rock cliffs may generally become unstable due to toe erosion resulting from the wave action. The critical erosion depth depends upon the height of cliff and the strength (tensile or shear) of the rock, and if the resulting stress state exceeds the strength of rock, it will collapse. Recently, it is reported that overhanging rock slopes may also fail during recent earthquakes. This experimental study was undertaken to investigate the stability of model rock cliffs under dynamic loading conditions in order to clarify the governing factors associated with their failure. The rock mass was modeled as continuum, layered, and blocky model. Shaking table (OA-ST1000X) has a size of 1 m × 1 m and the allowable maximum load is 100 kg under the conditions of maximum horizontal displacement of ±50 mm, maximum velocity of 0.56 m/s and maximum acceleration of 600 gals. The outcomes of the experimental studies are presented and discussed in this study. It is found that the failure modes of overhanging cliffs very much depend upon the number of discontinuity sets, tensile and shear strength of rock mass and their geometrical configuration.
There are many cliffs with toe-erosion along shore-lines of around the world and it may cause stability problems especially to historic structures and lighthouses. Such an example is the Gushikawa castle constructed on Ryukyu limestone cliffs. Sea waves, winds and percolating rain water are the main causes of toe-erosion leading to the failure. To prevent collapses, reinforcement for improving the stability of eroded cliffs using filling materials with the consideration of landscape has been introduced. numerous collapses of the overhanging cliffs have been reported due to earthquakes (Aydan 2013; Aydan and Amini, 2008; Aydan et al. 2012). In the vicinity of Japan, there are the four plates, namely "Eurasia Plate", "North American Plate", "Pacific Plate", "Philippine Sea Plate", interacting with each other in a complex manner (Figure 1). While Japan constitutes less than 1% of the world's area, earthquakes of about 10% of the world occurs in Japan. Considering the Philippine sea plate as an example, which is subducting in the northwest direction at a rate of 3-5 cm per year beneath the south east side of Ryukyu Archipelago. Also, on the continental side, there is a seabed topography called Okinawa trough. Following the formation of fractures the stability of the cliffs declines, and the occurrence of an earth-quake may cause their collapse. But, there are few studies on the estimation of the collapse mechanism of the eroded cliffs during the earthquakes and the effect of discontinuity orientation and erosion on the collapse mode. It is necessary to examine the different failure form and the stability of rock cliffs depending on the discontinuous nature of rock mass at the time of the earthquakes. In this study, the failure modes of eroded cliffs are examined using rock mass models having different number of discontinuity sets, namely, continuum, layered and blocky. Then, the authors classified each failure mode into three categories based on strength and discontinuous nature of rock mass.