SEISMIC BEHAVIOUR OF FOUNDATIONS AND BUILDINGS ON SLOPES

Abstract

During a seismic event, the buildings on slopes are more vulnerable than the buildings on flat ground due to the synergistic effect of ground shaking and slope instability. The topographic amplification, slope instability, and irregular configuration of buildings with foundations located at different levels, have caused severe damage during the past earthquakes. However, the current seismic design codes, worldwide, are based on a prescriptive force-based design approach and overlook the special needs of these regions. In the past few decades, tremendous development has taken place in simulation of inelastic seismic response of different types of buildings, resulting in a paradigm shift towards performance based design. A number of documents are available for performance based design and seismic fragility analysis of buildings on flat ground. The primary objective of the present study is to extend this framework to the coupled slope-building system. In this thesis, various challenges in simulating the slope-building systems are identified and finite element (FE) models are developed to study their seismic response. Extensive numerical simulations are conducted for better understanding of the seismic behaviour of slopes, foundations on slopes, and coupled slope-building system. To start with, the stability of slopes in terms of factor of safety (FOS), under multiple adjacent foundations/ buildings located on the face of two homogeneous slopes (having stiff clay and dense sand properties) of same height but different inclination angles, is examined using 2D plane-strain finite element limit analysis (FELA) based on strength reduction technique. Further, the effect of different governing parameters, including width of the foundation, offset distance, foundation load intensity, integral frame action, and seismic load intensity, on factor of safety and failure pattern of coupled system, is explored in detail. The study shows that the number of adjacent buildings does not affect the stability of the slopes, significantly, however, the intensity of loading representing the height of buildings influences the slope stability. The bearing capacity of foundations on slopes is comparatively lesser than the identical foundations located on flat ground. The capacity further decreases due to seismic action. Most of the design codes and standards are still lacking in guidelines for estimation of seismic bearing capacity of strip foundations on slopes. In this study, the seismic bearing capacity factors, Ncs and Nγqs for a strip foundation embedded in face of c-ϕ soil slope, are obtained through 2D plane-strain nonlinear FELA. Similar to the observation made in earlier studies for the ground iv inclination factor, G, the results of the present study have revealed that the factor Gc is also a function of angle of internal friction,  in addition to the slope inclination, β. The seismic bearing capacity factors are obtained as functions of horizontal seismic coefficient αh, for varying slope and foundation properties, and a spreadsheet based tool is developed to estimate seismic bearing capacity. An extensive numerical study is performed to evaluate the V-H-M capacity envelopes of strip foundations placed on top and face of slopes and subjected to seismic action using nonlinear 2D FELA. The computed capacity envelopes are compared with their counterpart foundations on flat ground. A comparison of the V-H-M capacity of foundations is also made with that of the RC columns to illustrate the relative hierarchy of strength of columns and foundations of a typical building on slope. The characteristic features of the capacity envelopes are identified and explained considering the failure patterns under different combinations of V, H and M. In addition, influence of effective structural height, offset distance and seismic loading on capacity envelopes and the resulting failure mode, is also explored in detail. Finally, the seismic vulnerability of single and multiple adjacent RC buildings located on face of slopes, is explored through extensive numerical investigation using Incremental Dynamic Analysis (IDA) on 2D plane strain FE models of coupled slope-building systems, subjected to a near field ground motion suite. A holistic approach is developed for this purpose which considers all the consequences of seismic action on slopes, including ground shaking, topographic effect, permanent differential and total ground settlement, integral action of frame-foundation system and slope-building interaction. The effect of various parameters affecting the performance of both fixed base building and coupled slope-building system is investigated in detail. As a final outcome of this study, fragility curves (as a function of the Peak Ground Acceleration, PGA at rock outcrop) are developed for coupled slope-building systems. For the sake of completeness and comparison, the seismic fragility functions for free slopes, and buildings with fixed base, are also presented to bring out the effect of the slope instability, topographic amplification and slope-building interaction, on the seismic fragility of buildings on slopes. Adjacent buildings are found to have insignificant effect on the seismic response of a building located on slope, however, the relative position of the building on slope, has a slight effect.