Abstract

Rockfalls are very common along the Himalayan roadways because of the highly jointed nature of rock mass. Discontinuities in the rock mass render them extremely anisotropic, reduces the strength and create avenues for different failure mechanisms like planar, wedge and toppling in destabilizing the slopes. The formation of overhangs due to excavation, coupled with the high density of joints make them a highly susceptible zone for the initiation of rockfall activity. The present study area is a part of seismically active Himalayas (Luhri, Himachal Pradesh), where the occurrence of mild to major tremors are quite rampant. Thus, the study concentrates on the possibility of initiation of rockfall activities due to seismic activities using distinct element modelling (DEM) approach. The results shows contrasting difference in the magnitudes between displacement and velocity in static as well as in dynamic case, causing rockfall to initiate in the latter case.

1 Introduction

Rockfall is a major concern along transportation corridors in hilly areas, usually prevalent in the jointed rock slopes (Ferlisi et al. 2012 and Budetta 2004). It is a two-stage phenomenon where initial stage is the detachment of blocks while the second stage relates to the motion of the falling body, post failure. The main triggering factors for the detachment of blocks are erosion and weathering along the structural discontinuities, rainfall, earthquakes and others (Dorren 2003, Singh et al. 2010, Asteriou et al. 2012). Studies have shown a good correlation between the landslides density and the areas of strongest ground motion, while their frequency declines on moving away from the epicenter (Meunier et al. 2007). Sepulveda et al. (2005) studied the rock slope failure due to topographic amplification of strong ground motion in case of Pacoima Canyon, California and observed that the most frequent failure was planar and wedge type derived from rockfalls in many cases. The dominant mode of failure can be established from the relationship between the orientation of structural discontinuities and surface topography at any particular site, but the analysis of mechanism behind the initiation of rockfall due to seismic shaking requires the application of rigorous numerical models that have the capability to model large strain of a discontinuous media. Seismically induced rockfall occurs as a combined effect of gravity and seismic acceleration producing short-lived stress, which exceeds the cohesive and frictional strength of the earth materials (Newmark 1965). As a result, slope failure can take place due to slight disturbance in slopes that may have been stable under static loading time. Earthquakes produce two types of ground accelerations. Out of the vertical and horizontal accelerations, the later one is known to cause greater impact on slopes (Romeo 2000). Past studies have shown that even an earthquake of magnitude 4.0 can trigger a rockfall activity (Keefer 1984). The threshold displacement responsible for causing landslides ranges from 2–5 cm (Wilson & Keefer 1985).

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