Shotcrete lining shows high resistance but extremely low deformability. The utilization of yielding elements in shotcrete lining, which leads to the so-called ductile lining, provides a good solution to cope with tunnel squeezing deformations. Although ductile lining exhibits great advantages regarding tunnel squeezing deformation control, little information has been comprehensively and systematically available for its mechanism and design. This is a review paper for the purpose of summarizing the development history and discussing the state of the art of ductile lining. It begins by providing a brief introduction of ductile lining and an explanation of the importance of studying this issue. A following summary of supporting mechanism and benefits of ductile lining used in tunnels excavated in squeezing ground conditions is provided. Then, it summarizes the four main types of yielding elements applied in shotcrete lining and introduces their basic structures and mechanical performances. The influences of parameters of yielding elements on the supporting effect are discussed and the design methods for ductile lining are reviewed as well. Furthermore, recommendations for further research in ductile lining are proposed. Finally, a brief summary is presented.

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The application of tunnel-slag-improved high liquid limit soil as filling materials in subgrade is a green environmental technology. This study explored the influence of tunnel slag mixing on the physical and mechanical properties of improved soils, based on the engineering background of Liyu highway, Guangxi Province, China. Firstly, the optimal moisture content, maximum dry density, shear strength parameters, California bearing ratio (CBR) and resilience modulus of plain and tunnel-slag-improved high liquid limit soils were experimentally determined. Results showed that the direct utilization of untreated soil was unacceptable in subgrade practice. A significant enhancement of integrity of high liquid limit soils could be obtained by tunnel slag mixing, and the value of 15% was determined as the optimal tunnel slag content in soils, leading to improved soil performance meeting the specification requirements. Then, numerical simulation on the stability of subgrade slope of tunnel-slag-improved soils at the content of 15% was conducted. It also reported the long-term subgrade settlements. The feasibility of utilization of tunnel slag in improving properties of high liquid limit soils was further validated. Finally, a good application of tunnel-slag-improved high liquid limit soil as subgrade filling materials in Liyu highway was achieved. The findings in this study could provide useful guidance for similar engineering.

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