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Title of the thesis: A Reliability Framework for Low-Carbon Column-Stabilised Ground.

Thesis defender: Alejandra Lopez Ramirez
Opponent: Docent Marco D’Ignazio, University of Tampere
Custos: Senior Advisor Leena Korkiala-Tanttu, Aalto University School of Engineering

The purpose of this dissertation was to identify practical pathways to reduce carbon emissions in deep-mixing design for road embankments on soft clay, while maintaining reliable serviceability performance. The work combines laboratory testing, numerical modelling, and probabilistic analysis. It examines both the mechanical performance of low-carbon binders and the role of reliability-based design in reducing unnecessary conservatism in settlement calculations.

The results show, first, that conventional serviceability design based on characteristic values can be highly conservative. For the embankment cases studied, settlement estimates based on characteristic values often corresponded to extreme positions in the probabilistic settlement distributions, typically around 3–5 standard deviations above the probabilistic mean. This means that conventional design may lead to unnecessary deep mixing, higher costs, and higher emissions, especially in the case of floating columns. A reliability-based approach makes the safety margin more explicit and allows more balanced and efficient designs.

Second, the dissertation presents a comprehensive testing programme on low-carbon binders for soft, sensitive, high-water-content clays. The results show that a gypsum–CEM III/A binder system (Terra GTC/GTC3) is a particularly promising solution when low emissions and good mechanical performance are required over a wide dosage range. Likewise, the results also show that the mechanical response of this binder is relatively insensitive to drainage conditions, as similar behaviour was observed in both drained and undrained triaxial tests.

Third, the gypsum-based binder was examined further through numerical simulation and reliability analysis of a full-scale test embankment. The results demonstrate that, by expressing settlement performance in terms of exceedance probability and linking it to column length and binder dosage, it is possible to reduce unnecessary material use and associated emissions. Sensitivity analyses showed that the most influential factors controlling settlement variability are clay compressibility and column stiffness, while uncertainty in column strength has a smaller effect. For floating columns in particular, the compressibility of the underlying soil becomes especially important.

The dissertation contributes to ongoing research on how reliability-based approaches can be integrated into geotechnical design practice.

Keywords: Deep-mixing, soil improvement, road embankment, serviceability limit state, carbon emissions, sustainable design

Thesis available for public display 7 days prior to the defence at . 

Contact information: alejandra.lopezramirez@aalto.fi