Liquefaction Behaviour of Pond Ash under Monotonic and Cyclic Loading

Abstract

The liquefaction behaviour of pond ash was investigated experimentally by monotonic and cyclic triaxial tests. In order to investigate the effect of specimen preparation method, moist tamping (MT) and paste deposition (PD) specimens were employed. The latter was developed to simulate wet disposal of pond ash in ash ponds. Both original and re-used materials were used to re-constitute specimens. Re-used pond ash experienced varying extent of particle breakage during previous tests. Thus this was quantified by the increase in the specimen surface area (ΔSSA) relative to the original material, noting that SSA was measured by a laser particle size analyser. Attempts were made to synthesise the test results under the critical state soil mechanics (CSSM) framework. The position of critical state line (CSL) in e-log(p’) space for MT specimens was found to be different from that of PD specimens. CSL of re-used pond ash was also found to be located lower than that of original material in e-log(p’) space. However, the CSL in q-p’ space was unique irrespective of specimen preparation method and ΔSSA. Initial states of tests manifesting different liquefaction patterns, i.e. flow liquefaction and cyclic mobility, could be approximately divided by CSL in e-log(p’) space. The effects of initial state and ΔSSA on liquefaction behaviour of pond ash could be unified under the CSSM framework. The occurrence of flow liquefaction under cyclic loading was linked to monotonic behaviour. MT specimens were found to have a higher cyclic strength than their PD counterparts. A special technique was developed to enable the taking of scanning electron microscope (SEM) photos of specimens in as-formed state and critical state so that the associated microstructure can be investigated. The SEM photos showed the influence of specimen preparation method on the as-formed microstructure, which could be used to explain some of the unexpected differences in stress-strain behaviour between MT and PD specimens. Furthermore, the SEM photos revealed that microstructure of a PD specimen at CS was also different from that of MT specimens. This is in line with the difference in the CSL obtained from MT specimens and PD specimens.