Impulse waves generated by subaerial landslides of combined block mass and granular material

Experimental results, based on two-dimensional (2-D) laboratory tests, for impulse waves generated by subaerial landslides of combined solid block and granular materials are presented in this study. The results are compared with those of individual models of pure solid block and granular landslides. By considering an identical slide mass and release height, the effect of the mass ratio m* on wave generation was investigated. The mass ratio is defined as m* = MB/M, where MB is the mass of the solid block, M = MB + MG is the total mass and MG is the mass of the granular portion. The experimental results show that the combined landslides with m* = 0.6 and 0.8 in this study generally produce much larger impulse waves in the impact zone compared with those triggered by pure solid block landslides (m* = 1) and pure granular landslides (m* = 0). This suggests that the primary wave amplitudes of impulse waves might have been underestimated in previous laboratory tests with solely solid or granular assemblies when using the same slide mass and release height. The larger primary wave amplitudes of the combined landslides compared with those of the pure solid block landslides are mainly attributed to the relatively large thickness of the combined landslides and the continuous motion of the granular portion, as the inside solid block stops at the hill slope toe. Compared with pure granular landslides, combined landslides generally have a larger Froude number and slide thickness, which account for the larger primary wave amplitudes. The latter plays a quantifiably more important role – nearly two times that of the former. The effects of the hill slope angle α, and the grain size of the granular materials D, were also studied in this work. Four different hill slope angles (α = 22.5°, 30°, 37.5° and 45°) and four different mean grain diameters (D = 5 mm, 10 mm, 20 mm and 30 mm) were tested. By comparing the measured maxima of the wavelet spectra obtained via a wavelet transform method with those reconstructed by means of the phase celerity of the solitary wave cs and the group celerity of linear wave cg(f), it is found that the measured maxima travel at either cs or cg(f), depending on the landslide type and the hill slope angle α. However, the celerity of the measured maxima for the high frequency is lower than cg(f) and cs for the impulse waves generated by combined landslides when α ≤ 37.5° in this study. Further investigations show that the wave amplitude decreases rapidly during wave propagation, following an exponential function. The decrease in the wave amplitude during propagation is mainly attributed to dispersion.