Ageing effects on the small-strain stiffness of a bio-treated compacted soil

The effect of ageing on the small-strain shear stiffness of compacted silty-clayey sand following a soft biological treatment is discussed. The samples were prepared by static compaction adding urea-degrading bacteria in the compaction water. No nutrients were artificially added, relying on the natural availability of urea and Ca2+ in the humic soil and in the compaction water for the bacteria to precipitate calcium carbonate. After compaction, part of the samples were cured in a system open to vapour transfer, and part of them in a closed system, to replicate different environmental boundary conditions. The small-strain shear stiffness was periodically tracked with bender elements during the ageing period. Tests were run in parallel to investigate the unconfined compression strength, the water retention properties and the soil pore size distribution changes during ageing. The tests results revealed a small but consistent increase in the small-strain shear stiffness during the ageing period due to the microbiological treatment, in both the closed and the open to vapour transfer systems. In the latter case, the contribution of the microbiological treatment to the increase in stiffness could be estimated after correcting the data for the suction increase due to evaporation.