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Thermal power plants are the major sources of ash generation in India. Since low ash high-grade coal (Lignite) is reserved for metallurgical and other industries, thermal power plants have no choice but to use low-grade coal (sub-bituminous and bituminous) having ash content ranging from 35% to 50%. Current existing 85 thermal power plants in India produces about 112 million tons of coal ash annually where as its utilization is only about 42 million tons (37.5%) per year, mainly in the areas of cement as well as concrete manufacturing, building products, and to some extent in earth fills (Dhadse et al., 2008). Routinely, the unutilized volumes of ash are mixed with large amounts of water and then disposed off in the ash ponds. Typically, an ash pond spreads over an area up to 10 km2 for a 500-MW power plant and is filled with ash up to 10 m in height within a period of 5 years (Gandhi et al., 1999). About 265 km2 of valuable area is already occupied in the vicinity of the power plants and by 2015, it is estimated to require 1,000 km2 for its disposal (Das & Yudhbir, 2005).
Once the designed life is reached, these ash ponds are to be closed in compliance with statutory regulations. Besides using the ash in geotechnical works, it would be ideal if these abandoned ash ponds can be reclaimed to accept engineered structures like gas based power plants, light weight structures, or parking lots, which will have significant economic value from the view of exponential increase in the cost of urban land from day to day (Ramaiah & Ramana, 2008). However, it is well reported in the literature that as the coal ash deposit will be in loose state (hydraulic deposition); ash particles are predominantly of silt size, non-plastic and less permeable compared to silt, the ash deposit will have the problems like poor bearing capacity, large settlements, liquefaction susceptibility (Gandhi et al., 1999; Pandian, 2004). Hence, an assessment of shear strength, compressibility and liquefaction susceptibility of coal ash is a prerequisite for constructing any proposed engineered structure on these abandoned ash ponds particularly in seismic prone areas.
A significant body of research can be found in the literature for shear strength as well as compressibility behavior of coal ash (Indraratna et al., 1991; Jakka et al., 2010b; Kim et al., 2005; Kumar & Stewart, 2003; Seals et al., 1972; Sridharan et al., 1998; Trivedi & Sud, 2002). However, there are very limited studies reported on the ground response analysis, liquefaction susceptibility assessment and engineering measures to improve the use of in-situ ash as a foundation material (Jakka et al., 2010a; Singh et al., 2010). There are numerous examples of failures of tailings ponds worldwide (some of them are ash containments), due to liquefaction and other related phenomenon (Bross, 1981).
It is widely accepted that shear wave velocity (Vs) is a fundamental parameter to estimate the site-specific amplification factor of the subsurface materials as well as to assess the liquefaction susceptibility of the coal ash deposit. Measurement of shear wave velocity of coal ash ponds in the field is difficult due to several reasons such as: (i) difficulty in accessibility due to loose state of deposition, (ii) presence of water, and (iii) type of the source required to generate the surface waves. In the present study shear wave velocities measured at three slurry deposited ash ponds in Delhi, India by SASW technique are presented and an attempt has also been made to develop a correlation between SPT-N value and measured shear wave velocity for ash ponds in Delhi. Based on the measured shear wave velocities, the ash ponds are also classified as per the NEHRP (BSSC, 2003) site classification.