Numerical analysis of geostructures stabilised with lime-cement columns in soft clays

Doktorsavhandling, 2025

Urban infrastructure is increasingly built on soft natural clays with high compressibility due to the limited availability of land in congested areas, while existing transport infrastructure faces increasing maintenance costs due to excessive displacement. Dry Soil Mixing (DSM) is a widely used soil improvement solution for road embankments and deep excavations especially in Scandinavian countries, where stabilising agents, such as lime, cement, or alternative binders, are mixed into in situ soil as dry powders. Although DSM offers a cost-effective and rapid application, it is associated with high carbon emissions mainly due to the energy-intensive production of lime and cement. Current practices for the numerical analysis of geostructures (i.e. road embankments and excavations) stabilised with DSM rely on simplified semi-empirical methods, which do not realistically capture the time-dependent behaviour of the stabilised system and disregard the inherent three-dimensional (3D) interaction effects as well as installation effects. Consequently, more accurate and computationally efficient methods are needed to optimise designs with DSM for serviceability.

This research investigates the hydromechanical response of soft natural clays stabilised with lime-cement columns for deep excavations and embankments using the finite element method. In the first part of the research, installation effects were back-calculated from the field data following the Observational Method. Advanced soil models were employed to represent the elastoplastic behaviour of natural clay and deep-mixed columns in the 2D and 3D simulations. A rate-dependent advanced soil model for the natural clay enabled the calculation of long-term displacement predictions, which were validated against the field measurements.

In the second part, the stress-strain response of the composite system of a stabilised excavation was examined considering the different hydromechanical responses of natural clay and columns, respectively. A new numerical scheme, utilising Volume Averaging Technique (VAT), was developed specifically for excavations. VAT for excavations was implemented in a 2D finite element framework to model the composite behaviour of regularly spaced deep-mixed columns and in situ clay. This new formulation is a computationally efficient alternative to true 3D analysis, while proving a comparable response. The robustness of VAT, using the existing formulation for embankments, was demonstrated through global sensitivity analyses of stabilised embankments for long-term deformation predictions. System-level analyses explored optimisation scenarios to minimise climate impact. The results highlight that with VAT the responses of the in situ clay and the columns can be accounted for in a plane strain analysis for efficient performance-based design with high fidelity.