Coode Island Silt (CIS) is one of the predominant geological units in Melbourne, Australia. Having high compressibility and low shear strength, CIS is considered a problematic soft soil that challenges the construction of infrastructure in the region. To tackle such challenges, one practical approach is the application of ground improvement techniques such as in situ soil mixing. This PhD study focuses on the application of Cutter Soil Mixing (CSM) for the construction of excavation retaining walls in CIS. Although cement is widely used in most CSM projects, this study investigates the suitability of different lime types available in the Australian market as a potential alternative to cement for the stabilisation of CIS. To investigate the effect of lime stabilisation, a comprehensive geotechnical characterisation of untreated and lime treated CIS is performed. Four different lime types are used: agricultural lime, quicklime, hydrated lime and slag lime. Based on the results obtained from strength tests, slag lime was found to be the most effective among the four types that were tested. The optimum slag lime to CIS ratio is then found for the construction of retaining walls in CIS. Having the geotechnical characterisation of untreated and treated CIS from the laboratory experiments, a series of two-dimensional and three-dimensional finite element method (FEM) analyses were conducted to investigate the applicability and reliability of the selected mixing ratio for the construction of CSM excavation retaining walls in CIS. A nonlinear constitutive soil model was employed, calibrated and verified to be used in FEM analyses to investigate both the stability factor of safety and excavation-induced deformations. The results obtained for both undrained and fully coupled flow deformation analyses prove that CSM panels can be constructed by mixing slag lime and CIS to act as retaining walls to allow for deep excavation in CIS.
A microstructural study of Coode Island Silt (CIS), a soft silty clay from the Melbourne area of Australia, stabilised with slag lime is reported. Slag lime is a blend of 80–85 wt% slag, 15 wt% hydrated lime, Ca(OH)2 and 3–8 wt% gypsum CaSO4.2H2O, and is typically used for soil stabilisation in roading applications. The morphologies of several homogeneous mixtures of slag lime and CIS were studied by Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD), which indicated the formation of the cementitious phases calcium carbonate, calcium silicate hydrate, calcium aluminium silicate hydrate, and calcium aluminium silicate carbonate. The progress of the CIS-slag lime reactions was also investigated by pH measurements. Slag lime was found to very actively promote pozzolanic reactions with CIS, shown by SEM to form crystalline reticular calcium silicate hydrate and other cementitious products. The most effective pozzolanic reactions occurred in a composite containing 12.5 wt% slag lime with CIS and contained the cement mineral ettringite, contributing to its high strength. These composites constitute a new class of materials with excellent potential for construction applications.