December 2022
Meegada V Ravi Kishore Reddy received his doctorate in Civil Engineering (CE). His research work was supervised by Dr. Supriya Mohanty, IIT BHU and co-supervised by Dr. Shaik Rehana. Here’s a summary of his research work on Dynamic Response Analysis of Shallow Foundation Resting on Coal Ash Deposit:
SYNOPSIS The current research presents an experimental investigation of coal ash samples (fly ash, bottom ash, and pond ash) collected from NALCO thermal power plant and Talcher Thermal Power station, Odisha, India, and a numerical investigation of the dynamic response of shallow foundation on soil and pond ash deposit. Morphological, mineralogical, and geotechnical properties of coal ash (fly ash, bottom ash, and pond ash) samples were extensively investigated. The pond ash sample was chosen for further study because of its unique gradation of all the samples collected. It was intended to determine the dynamic characteristics of pond ash subjected to dynamic excitation through laboratory tests and numerical analysis. A set of multiple cyclic triaxial tests were planned and performed on pond ash examining the influence of test parameters like relative compaction, effective confining pressure, frequency, and shear strain. The properties obtained through the experimental tests were applied in the numerical analysis of pond ash subjected to different seismic excitations i.e., Nepal earthquake (Mw: 7.8) and Northeast India earthquake (Mw: 7.5). Liquefaction-induced deformation of pond ash was evaluated from the numerical analysis.
Initially, morphological, mineralogical, and geotechnical properties of coal ash (fly ash, bottom ash, and pond ash) were investigated. Then pond ash was subjected to dynamic loads using cyclic triaxial testing equipment. Factors influencing the liquefaction potential of pond ash were evaluated. It was noticed that at low frequency (0.3 Hz) the pond ash specimen takes 87 cycles to liquefy at its most densified state. Also, the maximum dynamic shear modulus was noticed at this stage i.e., 6534.80 kPa, and the decay was observed at 87 cycles. In addition, the decay in dynamic shear modulus was rapid with the enhancement of cyclic shear strain. The presence of 50% fines and the high compacted state of pond ash specimens caused close packing of particles which offered more resistance to shear strain; this resulted in possession of the high shear modulus as observed in this study.
The maximum value of damping ratio was obtained for the highly compacted specimen confined at high pressure subjected to the high frequency at medium cyclic shear strain considered in this study. The amplification of shear strain from medium shear strain to large shear strain caused a decrement in the damping ratio of pond ash.
The presence of 50% fines in the present pond ash specimen made the specimen exhibit less Gdyn (i.e., 33% less) than that of past studies by Mohanty and Patra (2014). The presence of predominant silt range particles (51%) made the specimen offer less resistance against the applied high strains despite better interlocking of silt and sand range particles of compacted pond ash specimens in the present study. 50% degradation of the dynamic shear modulus occurred within 4-28 cycles of loading. The dissipation of energy over the first few cycles of loading was noticed between 12% and 94%, and it continued with a decreasing trend afterward.
The observed values of damping ratio were in contrast with that of the past studies on pond ash tested for similar adopted parameters (2014) and contrary to the studies on different kinds of sandy soils (Kokusho, 1980; Govindarajulu, 2005). The damping ratio of fly ash with a predominant range of silt particles (~80%) was found to be increased with an increase in shear strain (Chattaraj and Sengupta, 2016); this observation contrasts with the present study. Trend of damping ratio beyond 0.5% cyclic shear strain is in good agreement with the past studies of Kumar et al. (2017).
A regression analysis has been carried out for the results obtained and out of various analyses, i.e., linear, polynomial, logarithmic relationships, the relationships that provide the best fitting to the results have been presented. The percentage variation of observed and predicted results is 4%.
The 1D, 2D, and 3D ground response analysis of soil, and pond ash and seismic response analysis of soil, and pond ash with the foundation were subjected to Nepal and Northeast India earthquake excitations. No case of liquefaction was observed for 1D, 2D ground response analysis of soil, pond ash under the excitation of Northeast India earthquake & Nepal earthquake. However, the soil and pond ash were found to be liquefiable under the excitation of Nepal earthquake only in case of 3D ground response analysis. Both soil-foundation and pond ash-foundation systems were found to be liquefiable under the excitation of Nepal (most of the cases) and Northeast India (few cases) earthquake in the case of 3D response analysis. A state of near liquefaction was noticed for soil-foundation and pond ash-foundation systems under the excitation of Nepal (most of the cases) and Northeast India (few cases) earthquake in the case of 3D response analysis. A state of liquefaction has been noticed when subjected to Nepal earthquake motion in both the cases of 3D response analysis of soil–foundation, and pond ash– foundation system.