Evaluation of Liquefaction Potential of Soil at a Power Plant Site in Chittagong, Bangladesh

Evaluation of Liquefaction Potential of Soil at a Power Plant Site in Chittagong, Bangladesh

Soumyadeep Sengupta (Vellore Institute of Technology, Vellore, India) and Sreevalsa Kolathayar (National Institute of Technology Karnataka, Mangaluru, India)
Copyright: © 2020 |Pages: 16
DOI: 10.4018/IJGEE.2020010101

Abstract

This study presents an evaluation of liquefaction potential for combined cycle power plant site located in the Chittagong district, Bangladesh, using standard penetration test blow counts (SPT-N values). The peak ground acceleration (PGA) values at a bedrock level were estimated deterministically using both linear and point source models as well as different ground motion prediction equations (GMPEs). The surface level hazard was estimated using amplification factors for the soil conditions present and the response spectrum at the center of the plant site was plotted. The liquefaction potential for the site was arrived at by using the SPT-N values of 33 boreholes in the site and at every 3-meter interval from the ground level to a depth of 30 meters. The results from the LPI contours at successive depths indicate that in the majority of the borehole locations, the soil up to 15 meters depth had a significant hazard of liquefaction. These findings from the present study can prove to be crucial from the structural point of view, for any future construction activities in the area.
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Introduction

Evaluation of liquefaction resistance of soils is of utmost importance when it comes to important sites like power plants due to the presence of critical structures. Soil Liquefaction resistance has become a necessary criterion to be evaluated before any significant construction nowadays (Arango et al., 2000; James et al., 2013). Liquefaction of soils is the conversion of formerly stable cohesionless soils to a fluid mass due to increased pore water pressure. It is majorly observed in areas which have the water table close to the surface and have sandy soils (Marcuson, 1978). The dynamic loading of the earthquake causes pore water pressure to increase, which triggers the soil to behave like a liquid. A great example of liquefaction induced disasters would be the three-month duration of 1964 in which the Good Friday earthquake in Alaska (Mw = 9.2) and the Niigata earthquake in Japan (Ms = 7.5) (Seed and Idriss, 1967; Kawasumi, 1968; Hansen et al., 1966; Kramer, 2007) had occurred. Both the earthquakes caused catastrophic damage to all kinds of structures due to liquefaction. Since then, extensive research has been carried out in this field to prevent any such devastating calamities in the future. In this study, the combined cycle powerplant site at a suburb in Chittagong district, Bangladesh has been taken into consideration. The studies by Ali et al. (1998), and Sharfuddin et al. (2001) suggest that big cities of Bangladesh have been the epicentral site for many past major earthquakes, which is the major factor validating the importance of this study.

In construction sites, the general practice is to conduct in-situ tests like SPT-N value, since the collection of undisturbed soil samples becomes difficult and costly. However, for important structures having good financial resources, the collection of undisturbed soil samples is done. The simplified procedure for the evaluation of liquefaction potential was proposed by Seed and Idriss (1971) and has been validated and modified over the years by many researchers (Seed & Idriss, 1982; Seed et al., 1983; Youd et al., 2001; Idriss & Boulanger, 2004; Juang et al., 2000; Lenz & Baise, 2007; Elton et al., 2003; Moss et al., 2001). The liquefaction potential index was then found as given by Iwasaki et al. (1982) for every 3m depth of soil and the entire 30m depth of the borehole.

The project site is a suburb in Chittagong district that lies beside the Karnaphuli River in the South-Eastern region of Bangladesh. It was found to be majorly composed of fine-grained deposits ranging from fine silty to medium sand in the top layer to non-cohesive deposits of non-plastic silty and silty fine sand in the bottom layer of exploration.

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