Mechanical behaviour of biocemented sands at various treatment levels and relative densities

Previous studies have shown that biocement, or microbially induced calcite precipitation, can improve the mechanical behaviour of clean sand. However, the behaviour of biocemented sand is affected by several factors. In this paper, triaxial consolidated drained tests and K-0 consolidation tests were carried out on sands (Ottawa sand, ASTM graded) with varying biocement treatment passes and relative densities to study the failure and drained stress-strain behaviour and compressibility of biocemented sand. It is found that for loose and medium dense sands, the slight biocement treatment on sand can be as good as or better than the densification treatment in terms of the strength improvement and the deformation control. In the triaxial tests, the shear strength, the slope of failure line in p'-q plane and the peak dilation rate increase with the increase in treatment passes at various levels of relative density. For the loose sand (D-r=30%), 2-pass biocement treatments (1.0% calcite content) are sufficient to achieve a shear strength, a slope of failure line and a peak dilation rate higher than or similar to that of untreated dense sand (D-r=90%), and for the medium dense sand (D-r=50%), 1-pass biocement treatment (0.79% calcite content) is sufficient. In the K-0 consolidation tests, the axial strain of the sand decreases with the increasing treatment passes. For medium dense sand (D-r=50%), 1-pass treatment can control the axial strain to a level similar to that of untreated dense sand (D-r=90%). The variation of K-0 value versus axial strain during K-0 consolidation for the biocemented sand shows a different pattern compared with the untreated sand, due to the presence of biocementation effect. Biocemented sand shows a smaller K-0 value than the corresponding untreated sand at the final state of the K-0 consolidation tests. Scanning electron microscopy was also conducted on the sand samples to investigate the particle-level structure of the biocemented sand and its correlations to the mechanical behaviour.