Biocementation of Road Foundations: Element Testing, Physical Modelling, and Micro-Scale Characterisation

The foundation of a road, composed by layers of granular materials overlying the subgrade, plays a key role in ensuring the overall performance of the road. Excessive deformation in these layers represents the root cause of structural deterioration, leading to various surface defects that impair road safety and serviceability. This problem is particularly pressing in countries like the UK, where the majority of existing roads were built decades ago with their foundations falling short of meeting modern standards. However, current road maintenance primarily focuses on surface repairs without rectifying structural deterioration, due largely to the lack of an efficient, cost-effective, and sustainable method for foundation rehabilitation. In this context, biocementation through microbially induced carbonate precipitation (MICP), a bio-inspired technique leveraging bio-mineralised CaCO₃ to stabilise granular materials and soils, offers a promising solution. This research therefore explores the application of this technique to improve road foundations. A crushed granite aggregate, representative of typical granular materials used in road foundations, was consistently used throughout the experimental work. Chemical analyses and permeability measurements monitored the progression of MICP during treatment. The reaction kinetics were found to be regulated by numerous interdependent factors related to bacterial activity and solution chemistry. A mutual feedback mechanism emerged: CaCO₃ precipitation altered the pore structure and reduced the permeability, which impacted back the treatment process by mediating reagent transport and ion exchange. Characterisation of the biocemented aggregate elucidated the spatial distribution of precipitated CaCO₃ and its microstructural features. Notably, it was observed that fine aggregate particles and CaCO₃ crystals merged to form composite cementing bonds between coarse particles, presenting a cementation mechanism unique to well-graded granular materials. A series of monotonic triaxial tests were conducted to examine the shear behaviour of uncemented and biocemented aggregates. The observed changes in stress-strain responses, strength and stiffness evolution, and stress dilatancy are consistent with those reported for biocemented and conventionally cemented soils. The relatively low cementation levels used effectively rectified the deficient performance in a poorly compacted state to optimal levels. Further comparison with biocemented sands also showed equal or even better effectiveness of biocementation in improving the aggregate. The effects of biocementation at different stages of shearing were interpreted from a micromechanical perspective using the concepts of soil structure and force-chain evolution, revealing the decisive roles of cementing bonds and their breakage in causing the observed behavioural changes. An analytical approach capable of quantifying the evolution of different strength components was employed for macromechanical characterisation. The results revealed the commonality of changing the cementation level, initial density, and confining pressure in altering the macromechanical strength composition, which is uniquely correlated with the peak stress ratio. This research then moved forward to investigating the effects of biocementation on the deformation behaviour of the aggregate under traffic-type cyclic loading through extensive multi-stage and single-stage cyclic triaxial tests. As the applied cyclic stress increased, both uncemented and biocemented aggregates exhibited a behavioural transition from a stable shakedown state to an unstable one, with biocementation found to exert opposite effects: improving deformation resistance and elevating the shakedown limit in the stable state but aggravating brittleness in the unstable state. Through the use of cyclic stress ratio (CSR) to normalise the cyclic stress by the peak monotonic strength, a unique critical CSR zone of 0.4–0.5 was identified, independent of changes in confining pressure or cementation level. The theoretical framework established for monotonic loading was extended to interpret the micromechanical processes occurring under cyclic loading, attributing the transition from stable to unstable behaviour to the competition between structural self-stabilisation and load-induced destabilisation. Based on the analysis of strain rate, an empirical model was derived to describe strain accumulation at varying CSR and cementation levels, yielding a close match with experimental data. Comparison of multi-stage and single-stage test results showed distinct influences of stress history in uncemented and biocemented aggregates. Finally, accelerated pavement testing (APT) was carried out using the Cambridge APT facility to assess the practicality of applying biocementation to improve road foundations. Two three-layer foundation models, one uncemented and one biocemented, were subjected to incrementally increasing wheel loads, with laser scanning and particle image velocimetry (PIV) coupled to monitor surface and subsurface deformation. Despite its rather low cementation level, the biocemented model showed significantly improved performance compared with the uncemented one, sustaining higher loads while exhibiting slower rutting development. PIV analysis further revealed the deformation mechanisms at play: the uncemented model experienced localised failure in the base, while the biocemented model involved an active, displacing zone beneath the wheel and a passive, confining zone on the side. The rotation of principal stresses in the plane perpendicular to whee movements, which caused surface upheaving on the uncemented model, was suppressed in the biocemented model.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2025

Author dc:creator

• Fu, Tianzheng

Advisor dc:contributor.advisor

• Haigh, Stuart

• Biogeotechnics
• Microbially Induced Carbonate Precipitation (MICP)
• Biocementation
• Road Foundations
• Granular Materials
• Shear Strength
• Cyclic Loading
• Deformation Behaviour
• Shakedown
• Laboratory Tests

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/c3127c5c-1c3e-44cc-a987-ea9bf98c7844/download
• http://purl.org/NET/rdflicense/allrightsreserved

Author Identifier

0000-0002-8658-8743

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/387192