Several geotechnical infrastructures are being built throughout the world in soils such as collapsible and expansive soils, which are typically in an unsaturated state, and are widely referred to as problematic soils in the literature. There is increasing demand all over the world for constructing infrastructures founded on or installed in problematic and unsaturated soils due to rapid urbanization.

Although previous studies have substantially improved our understanding of problematic and unsaturated soil behaviour and its interaction with structures, there is still an urgent need to address the challenging scenarios that are encountered with problematic and unsaturated soils. The mechanical behaviour of problematic and unsaturated soil needs to be constantly upgraded by developing multi-phase/scale analysis approaches. Such studies vastly contribute to enhance the safety and reliability of infrastructures and encourage the construction of civil infrastructure in problematic soils that pose challenges to geotechnical engineers. The key scientific issues that are closely linked with problematic and unsaturated soil mechanics include micro-macro behaviour, shear strength theory, stress-strain relationships, constitutive models, and numerical analysis that can be used as tools.

For this reason, in this Special Issue, we aim to encourage original submissions that provide innovative solutions to problematic and unsaturated soils. Particularly, theoretical and experimental studies from micro to macro scales are welcome. In addition, novel geotechnical techniques and outstanding engineering practices of problematic and unsaturated soils are encouraged. The problematic soils include loess soil, frozen soil, saline soils, expansive soils, soft soil, marine soil, artificial fill, municipal solid waste, etc. Original research and review articles are welcome.

Potential topics include but are not limited to the following:

• Novel geotechnical test techniques in laboratory and in situ
• Multi-phase/scale behaviour and numerical modelling
• Constitutive models
• Chemical, mineralogical, electrical, rheological, magnetic, and biological properties of geomaterials
• Characterization, treatment, and valorization of industrial by-products and municipal solid waste
• Sustainable development solutions for environmental protection
• Engineering practice applications and case studies