Effect of cyclic stress ratio on pore pressure and shear modulus response of coal ash

Coal ash from the thermal power plant is conveyed to the disposal site, where it can exist in loose to medium-dense conditions. Coal ash is re-used for several engineering applications as fill material for highway and railway embankments. This material is extremely prone to liquefaction when subjected to earthquake loading conditions. Therefore, a detailed investigation is required on the cyclic response of coal ash under different magnitudes (cyclic stress ratios, CSR) of earthquake loading conditions. The three different initial states were chosen to be 86%, 90% and 95% of MDD. These specimens were consolidated under the normal effective overburden stress of 100 kPa. To analyse the effect of overburden stresses, the specimen was consolidated at three different effective vertical overburden stresses (50, 100 and 150 kPa) on a specimen prepared at 95% MDD. The frequency was employed to be 1 Hz for all the cyclic simple shear (CSS) tests. The abrupt increase in the double amplitude shear strain and pore pressure was obtained for the coal ash specimens at higher CSR as compared to lower CSR values. The liquefaction was initiated when the double amplitude shear strain reached 7.5% or the pore water pressure ratio (ru) reached 0.9, whichever occurred first. With the increase in CSR value, loading cycles at liquefaction were observed to be reducing. A rapid decay in the shear modulus was also observed for the coal specimens subjected to the higher CSR value. The evaluation of damping ratio indicated a slight increase with the increase in CSR value.