Chemical grouting: a critical review of materials, mechanisms, and performance evaluation

Chemical grouting has been widely used to reduce permeability, increase strength and stiffness, and mitigate hazards such as seepage-induced instability and liquefaction. However, its engineering performance remains difficult to predict due to the coupled influence of grout chemistry, soil fabric heterogeneity, groundwater conditions, and injection strategy. This review aims to clarify the governing mechanisms of chemical grouting and to support rational, performance-based selection of grout systems for complex geotechnical applications. To this end, foundational literature and recent experimental, analytical, and field studies are synthesised to critically assess major chemical grout families, including sodium silicate, colloidal silica, acrylate-based systems, polyurethane grouts, and resin-based grouts. Emphasis is placed on the relationships between formulation, rheology, gelation behaviour, transport characteristics, and resulting microstructural changes. Beyond the traditional family-based classification, this study proposes a mechanism-based classification framework that categorises chemical grouts according to their dominant in situ improvement action: pore filling, particle cementation, foaming-induced filling, or strong structural bonding. This framework highlights key sources of performance variability and current limitations in predictability. By explicitly linking material behaviour, improvement mechanisms, and engineering performance, this review facilitates informed grout selection and advances chemical grouting towards a predictable, performance-driven technology for sustainable and resilient geotechnical infrastructure.