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Deep cement mixing is a typical ground improvement technique, and has recently been introduced in some large-scale reclamation projects in Hong Kong. There is a lack of internationally recognised testing standard for determining the tensile strength of cement stabilised soil. Owing to its low tensile strength and strain at failure, the practical difficulties of these tests are intrinsic. In this study, tensile properties of cement stabilised clay have been investigated using direct tension test and Brazilian test. The interpretation of tensile strength in Brazilian test is not straightforward due to the formation of multiple cracks during loading. As such, focus is given on the validation of the fundamental assumption on crack initiation mechanism. A consolidated database of tensile strength of cement stabilised soil is compiled. A constitutive model in the finite element program PLAXIS, namely Concrete Model, has been studied. This model, originally aiming to simulate the behaviour of concrete and shotcrete, duly considers the cracking and the strain-softening characteristics, and is able to reasonably simulate the fundamental tensile behaviour of cement stabilised soil. Numerical simulations have been conducted using the Concrete Model to assess the stability of seawall founded on cement stabilised clay. Tensile properties are found to have an important bearing in maintaining the seawall stability when column-pattern stabilisation is adopted.

Natural terrain landslides pose a global threat as they often cause casualties and economic losses. Potential impacts of climate change could further aggravate the landslide risk and robust mitigation measures such as rigid debris-resisting barriers are particularly important in protecting lives and properties. Traditionally, rigid barriers are designed based on empirical approaches which generally oversimplify the dynamic nature of debris-barrier interaction. This often results in overlyconservative designs where the barrier structures are not only bulky and environmentally intrusive, but also difficult to construct. There is thus a pressing need to optimise the design approach. In this regard, the Geotechnical Engineering Office has been endeavouring to enhance the process efficiency, in collaboration with top-notch experts, by capitalising on the latest advancement in computational simulations and physical testing, and improving the understanding of the physical process. A technical breakthrough has been achieved with respect to an improved knowledge in the debris flow dynamic and the complex debris-barrier interaction. A novel design method covering geotechnical and structural aspects has been developed for use in Hong Kong. This would bring about more cost-effective barrier designs, with enhanced design reliability and robustness.