Improvement and engineering application of expansive soils: A review

Expansive soils, characterized by high swelling potential due to the dominance of hydrophilic clay minerals, pose significant geotechnical challenges in foundation and subgrade engineering. The pronounced volume change behavior of such soils, manifesting through the effects of swelling, shrinkage, and overconsolidation, often results in structural damage and substantial economic losses. Consequently, the investigation of soil improvement mechanisms is not only a theoretical necessity but also a critical factor in ensuring the long-term stability and sustainability of engineering projects. Current improvement techniques can be broadly categorized into physical modification and chemical stabilization, though these approaches are increasingly integrated in practice to achieve multidimensional performance enhancement. In the field of physical modification, research has undergone a transition from conventional materials to sustainable alternatives, including recycled waste and natural fibers. The utilization of these materials enhances soil stability by improving interparticle friction and reducing swell-shrink potential. Meanwhile, the field of chemical stabilization has evolved from utilizing conventional lime and cement binders to incorporating eco-friendly industrial by-products and bio-mediated techniques, such as microbial-induced carbonate precipitation (MICP) and enzyme-induced carbonate precipitation (EICP). These biological methods leverage biomineralization to form microscale calcite bridges within the soil matrix, thereby enhancing strength and durability. Recent advancements highlight a paradigm shift towards resource circularity, where waste materials transition from secondary additives to primary stabilizing agents. However, despite promising laboratory results, challenges remain in field applications, particularly regarding long-term durability under cyclic wet-dry conditions. This review synthesizes current research trends, identifies key knowledge gaps, and provides critical insights to guide future studies in expansive soil improvement, offering both theoretical and practical values for researchers and practitioners in geotechnical engineering.