<p>Ground improvement technologies such as deep soil mixing and jet grouting have been used for mass rapid transits and underground station works in several countries. For efficient and economic design, the understanding of the strength characteristics and failure behaviour of the improved grounds is of prime importance. With the physical modelling using large geotechnical centrifuge and finite element modelling, it is possible to study the failure mechanisms and develop the preliminary design guidelines to adopt for the design strength and thickness (extent) of the admixture-stabilised soil columns prior to tunnel excavation. This state-of-the-art review paper discusses the in situ ground improvement techniques adopted for tunnelling projects, followed by failure modes and modified stability equation for large circular underground openings in improved soil surrounds. In addition, the effect of cover depth on the failure behaviour of such improved soil surrounds is also examined. The manuscript further discusses the design approach using random finite element analysis for establishing the strength reduction factors that account for the spatial variation in the properties of improved grounds.</p>