The impact of water inundation on the geotechnical behavior of artificially made soil from waste materials: contrasting results in total and effective stress analysis

The purpose of this research is to observe the modification of mechanical properties of a man-made soil (mixture of colliery spoils and fluidized bed combustion bottom ash) caused by one-time water ingress. Triaxial compression (TRX) and bender element tests (BET) were conducted on compacted samples of spoils-ash mixtures after 60 days of curing, under confining stress of 0-500 kPa. Two water contents were assumed – 'dry' (approx. 14.6%) and 'wet', after 36 h of inundation (20-25%). The purpose is to observe the modification of mechanical properties caused by one-time water ingress. Results are far from unambiguous; the output varies significantly depending on the interpretation approach. Two main ways of testing such materials are commonly assumed. The first approach is typical of rock, concrete, or cement-soil mixtures, with the UC test as a primary tool to define strength. No pore water pressure measurement is done, and the results are based on total stress analysis (TSA). The second method is characteristic of soil mechanics effective stress analysis (ESA), where all the aspects of pore water pressure are considered. This uses TRX with appropriate sample saturation, making comparing strength characteristics of 'dry' versus 'wet' samples not straightforward. This paper attempts a combined approach and compares the results depending on interpretation apporach. With TSA, water inundation seems to significantly affect tested samples, while ESA reveals a much smaller effect.
The observed change in mechanical properties was applied in a numerical analysis of embankment slope stability using the Finite Element Method.