SESIMIC BEHAVIOUR OF FOUNDATIONS ON ROCK SLOPES

Abstract

During a seismic event, the buildings situated on slopes are more vulnerable than those located on flat ground. The foundations placed on the slopes have reduced bearing capacity as compared to the flat surface. This reduced bearing capacity when coupled with the synergistic effect of a seismic event can lead to a drastic drop in the bearing capacity of the foundations. The slopes are themselves made up of a varied geo-material. Rock-mass being a major geo-material in the slopes, it becomes inevitable to model rock-mass in the simulation of slopes for the seismic analysis of foundations. In general practice, the shallow foundations serve as an economic and reliable solution to support high-rise buildings, bridges and other heavy structures constructed on rock-mass. in flat as well as slope. The seismic bearing capacity of foundations in case of slopes is an important parameter to judge in the overall stability of a structure in the case of a seismic event. In various past earthquakes, even before the failure of a structural element, the failure of foundation due to sliding and overturning has been observed. Various codes of the world are silent on this aspect of seismic bearing capacity of foundation on rock-slope. Thus a need for the robust guidelines for the seismic bearing capacity estimation is required. Past studies have touched upon this aspect but on a limited scale. The current study aims to carry out a rigorous parametric analysis using a Finite Element Limit Analysis FELA with both the Upper Bound (UB) and Lower Bound (LB) estimates of the seismic bearing capacity factors for foundations placed on rock slopes. The objective of the present study is to obtain the seismic bearing capacity factor, Nϕs for a strip footing placed on the top of the slope of rock-mass. The seismic force is considered as pseudo-static force, in terms of horizontal seismic coefficient, αh , applied on the entire rock mass. The earthquake effect on superstructure is considered as additional horizontal force on foundation. The factor Nϕs is obtained and investigated for the different value of horizontal seismic coefficient, αh, β, G.S.I., D, γ, mi, σci (where, β is the slope angle; G.S.I and D are the in-situ parameters and γ, mi, σci are the laboratory parameters required to define the Hoek-Brown failure criteria) iv A rigorous analysis has been carried out to study the effect of all the parameters defining the rock-mass property and the slope geometry. The aim of the thesis is to develop a robust design charts based on the parametric study.