One-Dimensional Effective Stress Non-Masing Nonlinear Ground Response Analysis of IIT Guwahati

One-Dimensional Effective Stress Non-Masing Nonlinear Ground Response Analysis of IIT Guwahati

Devdeep Basu (Indian Institute of Technology, Guwahati, India), Arindam Dey (Indian Institute of Technology, Guwahati, India) and Shiv Shankar Kumar (Indian Institute of Technology, Guwahati, India)
Copyright: © 2017 |Pages: 27
DOI: 10.4018/IJGEE.2017010101
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Abstract

Ground response analysis (GRA) helps to assess the influence of the soil medium on the propagating shear waves and indicates about the characteristics of the waves reaching the ground surface from the bedrock level. Such a study becomes imperative for the urbanized alluvial banks of North-Eastern region of India, which is located in the highest seismic zone of the country. Conventionally, GRA is carried out based equivalent linear approach, which being a simplistic approach is unable to capture the nonlinear characteristics of saturated silty sands subjected to seismic shaking. This article presents the outcome of seismic one-dimensional nonlinear GRA of IIT Guwahati (located on a varying geology in the saturated alluvial banks of River Brahmaputra) considering pore-water pressure dissipation characteristics and non-Masing unload-reload criteria. Various ground response parameters obtained from the study helps in the accurate identification of the earthquake intensity based site amplification of the region expressed through a 2-D mapping.
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Introduction

Several major seismic events around the world, namely the 1960 Chile earthquake, the 1985 Mexico City earthquake, the 1989 Loma Prieta earthquake and the 1995 Kobe earthquake, demonstrated the importance of local site geology in governing the ground response. At any particular site, the safety of the structures is guided by the response of the underlying soil stratum subjected to the propagating seismic waves. Soil geomorphology and site topography play an important role in altering the intensity and frequency content of a seismic motion. The localized damage potential of an earthquake largely depends on the local soil properties and, thus, performing seismic ground response analysis becomes a task of utmost necessity. 1D ground response analysis (GRA) methods are widely used for seismic characterization of a site, which can be performed either in frequency domain (linear or equivalent linear total stress approaches) or in time domain (nonlinear total and effective stress approaches).

In frequency domain approach, the obtained Fourier amplitude spectra give an idea about the distribution of the strong motion energy over different frequencies. Subsequently, at different frequencies, transfer functions are utilized to obtain the amplification or attenuation of the seismic motion. Frequency domain methods consider strain-compatible equivalent soil stiffness and damping ratio for different layers of the propagating medium. These methods are easy to use and require less computational time. However, owing to their inherent approximation of the actual nonlinear soil behavior with a linear or equivalent linear soil model, these methods are not rigorous enough to provide reasonable results under the circumstances when the soil deposit is subjected to high seismic intensity and/or high strains are developed in the soil layers. In these cases, a nonlinear time domain solution is more appropriate, as it can account for the degradation of soil stiffness and change of soil damping during the seismic excitation.

Guwahati is the largest city in North-East India, on the banks of river Brahmaputra, is located at N latitude and E longitude (Figure 1, Google Maps). North-eastern regions of India have been subjected to numerous moderate and large magnitude earthquakes in the past, and the entire region has been designated as seismic zone V (IS: 1893-Part 1 2002), the zone of highest seismic activity. Widespread damages resulting in settlement, structural failures, and soil liquefaction have been reported (Oldham, 1882; Oldham, 1899; Poddar, 1959) during the event of the past major earthquake occurrences in this region (1869 Cachar earthquake, the 1897 Shillong plateau earthquake and the 1950 Assam earthquake). The regional geology being largely favorable to ground shaking, seismic amplification and possible liquefaction, it is highly imperative that a detailed GRA should be carried out for the city with increasing urbanization. Such GRA studies for Guwahati city using frequency-domain equivalent linear approach have been reported in past (Raghukanth et al., 2008; Kumar & Murali Krishna, 2013; Kumar & Dey, 2015).

Figure 1.

Map revealing Guwahati City and the study area (https://www.google.co.in/maps/@26.2005404,91.6242819,12z?hl=en)

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