Application of biocementation to improve a well-graded granular material against cyclic loading

Well-graded granular materials are extensively used in the foundation layers of roads and railways. Excessive deformation developed in these layers under traffic-induced cyclic loading represents a major contributor to structural deterioration, but no viable methods are currently available for rehabilitating these layers without causing substantial disruption. In this context, biocementation holds promise as a non-disruptive solution, yet dedicated investigations have been lacking. This study, through a series of multi-stage cyclic triaxial tests, explores the feasibility and effectiveness of biocementation in improving the deformation and shakedown behaviour of a well-graded aggregate representative of typical granular materials used in road and railway foundations. The results show that both uncemented and biocemented aggregates exhibit distinct stable and unstable responses with increasing cyclic stress. Biocementation effectively enhances deformation resistance and elevates the shakedown limit in the stable regime, while it aggravates brittleness in the unstable regime. Further interpretation using the normalised stress ratio (NSR) reveals the existence of a unique critical NSR zone that separates stable and unstable regimes, independent of both cementation level and confining pressure. Microstructural characterisation elucidates multiple precipitation patterns. A cementation mechanism where small aggregate particles and calcium carbonate (CaCO3) crystals merge to form cementing bonds between large particles is postulated.