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Top1. Introduction
In India, cost efficient and durable methods of construction are habituated in day to day life irrespective of type of construction activity. Several methods are available for improving the performance of free running type of soil i.e. cohesionless soil. Chance of liquefaction is prevalent in partially or fully saturated granular deposits when excited by dynamic loads. Need for improvement in performance of saturated soil deposit is essential and its performance can be enhanced by the provision of coal ash layer. Out of those, provision of sandwich layer of material (other than parent material of soil deposit) over cohesionless soil is prevalent. Abundance, adoptability of coal ash facilitates its usage in several applications such as reclamation of landfills, roads and embankments, stabilization of soil etc. Productive utilization of coal ash as a sandwich layer on or between soil layers has been experienced from past years as it exhibits similar properties to some kinds of soils. In this study, coal ash has been considered as a sandwich material. It was adopted considering the possibility of providing good resistance to external loads through bonding between interparticles of soil and pond ash. Coal ash material shows morphology similar to that of soil (Das & Yudhbir, 2005) and grain size smaller than that of soil which facilitates to settle between the soil particles. In addition, due to industrial waste and abundance availability, coal ash was used to minimize the problem of its deposition and reduce pollution of natural resources. Coal ash has variety of applications both on land and under water. In Japan, coal ash disposal under sea and land reclamation using fly ash slurry gained effective results for its utilization as studied for a long period time by Horiuchi et al. (2000). Behaviour of ash and ash mixtures was investigated in laboratory by Kim et al. (2005) and concluded that properties of ash mixtures were as favourable as conventional fill materials for utilization in highway embankments. Field performance of coal ash constructed embankment was monitored for one year and concluded that coal ash mixture (fly ash and bottom ash) is serviceable alternative to conventional fill materials (Yoon et al., 2009).
Response of coal ash embankments under various moderate seismic loads was observed to be satisfactory by many researchers (Mohanty & Patra, 2012, 2016a, 2016b). It is vital to know the dynamic behaviour of soil deposit, as local effects are prevalent on response of soil-structure before and after the construction. Hence, dynamic properties of soil need to be determined prior to ground response analysis. It was observed from the laboratory shake table tests and analytical study of Anjali et al. (2015) that Ganga sand has experienced amplification during shaking. Similarly, amplification of acceleration value was observed in Renusagar pond ash embankment, seismic zone-III, India when excited under various earthquakes (Vijayasri et al., 2017). Usually most of the laboratory investigations of seismic response of soil involve sinusoidal motion which is simulated as earthquake motion to apply as an excitation to the soil. But, liquefaction of saturated cohesionless (sand) material subjected to irregular seismic loads was studied in laboratory by Kumar et al. (2018) and observed a significant variation in response depending on strong motion parameters.
Parametric study on static performance of pond ash deposit was performed in past and the performance was better when reinforced with optimum geogrid layers (Reddy et al., 2017). Numerical studies on liquefaction analysis of pond ash deposits were performed in the past to understand their response under dynamic loading conditions (Mohanty & Patra, 2014; Khanna & Mohanty, 2017; Mohanty & Patra, 2018) and observed that pond ash was susceptible to liquefaction. Centrifuge testing of composite system (homogeneous cohesionless deposits overlain by shallow foundation) for evaluation of seismic performance was performed in the past by Liu and Dobry (1997). 2D dynamic response of foundation (strip footing) on layered soil deposit performed by Mohanty and Reddy (2016) and Reddy et al. (2019a) revealed the necessity of dynamic analysis of composite soil–foundation system where composite deposit (layered soil and foundation) experiences near liquefaction (i.e. ru = 0.85) under the influence of Nepal earthquake, which clearly shows the effect of water table.