Assessing enzyme-based soil stabilisation for unpaved road construction

Unpaved or unsealed roads make up the majority of the global road network. Yet, it is often not designed to appropriate standards which affect its structural integrity and compromise the life span of these pavements. The repercussions of this improper designing can incur a significant financial burden on the road governing agencies for its repair. While stabilisation of pavement soil layers using traditional additives such as cement and lime are recommended and extensively used in the current practices to mitigate this issue, these additives cause significant environmental concerns, especially during the additive production. This research investigates the effective use of enzymes as an eco-friendly, non-traditional alternative to cement-based stabilisers for problematic soils. The primary objective of this research is to understand the efficacy of enzyme-based additives in the design and construction of the unsealed pavements. This work is a combination of experimental, numerical and field trial works conducted to understand the fundamentals and application efficiency relating to enzyme-based soil stabilisation. The methodology of this study facilitates understanding of the influential parameters and stabilisation mechanism of enzymes, its optimisation, effects of combining with secondary additives, and the durability and performance of the treated pavements. The comprehensive laboratory testing program followed within the study includes the evaluation of mechanical behaviour of the stabilised soil using physio-chemical, microstructural and pore structural techniques. The study highlights the importance of understanding the fundamentals of the stabilisation mechanism to facilitate effective enzyme-based stabilisation. Firstly, tests were conducted to identify suitable parameters and conditions for enzyme stabilisation, which includes soil type, sample preparation, drying, curing, and testing processes. Secondly, mechanical tests were conducted to determine optimal contents for effective stabilisation. Lastly, chemical and imaging techniques were utilised to identify the stabilisation mechanism of the additive. The combination of the enzyme with fly ash (secondary additive) is also investigated in this study using a similar testing approach as a means to further improve the mechanical and durability benefits of enzyme-based soil as well as a means to improve the sustainability of the treatment by promoting higher fly ash utilisation rates in construction practices. As highlighted in the study, the combination of the two additives could not only offer a cost-effective road stabilisation method but also assist in waste mitigation for countries such as Australia, which face issues regarding the disposal of coal ash. The detailed testing conducted within the study provides optimal values of these combined additives as well as offers insight into their mechanism. The findings from the laboratory tests for enzyme-based stabilisation were applied in trial road constructions which showed sound performance of the treatment. Mechanistic analysis of pavement is also conducted within the study to understand the immediate effect of the treatment on pavement design. However, the long-term assessment of the enzyme stabilised pavement from the trial road construction highlight the importance of combining enzymes with secondary additives. Durability assessment of the treatment is further explored experimentally by using novel experimental techniques, which include a recompaction test as well as a modified wetting and drying tests. Additionally, three-dimensional finite element analyses were conducted to further evaluate the efficiency of the stabilisation under pavement operational loads. The findings from this thesis show that the use of enzymatic fly ash is a sustainable treatment method which offers substantial benefits in altering the mechanical behaviour of weak fine-grained subgrade soils. This additive can be used in the road construction industry instead of calcium-based approaches to effectively stabilise unsealed pavement. The results and findings from this study have been published in a number of international journals.