When mountain tunnels are constructed in weak ground or under high ground pressure, the surrounding ground may deform significantly inward into the tunnel, a phenomenon known as squeezing ground. Under such conditions, conventional tunnel supports may crack or fail as ground deformation progresses. To address this challenge, deformation-accommodating support systems have been developed. These systems incorporate deformable materials called yielding elements into part of the shotcrete layer, allowing the support system itself to absorb a portion of the ground-induced displacement.
To date, yielding elements have been studied through compression tests of the materials themselves, numerical simulations, and displacement measurements at actual construction sites. However, experimental studies that directly examine how an entire tunnel support system incorporating yielding elements deforms as a structural system have been limited. In particular, it has not been sufficiently verified how the deformation behavior of the entire support system changes depending on where the yielding elements are installed within the tunnel support. Understanding this behavior is important for advancing tunnel design and construction under challenging ground conditions.
A research team led by Associate Professor Yota Togashi of the Graduate School of Science and Engineering, Saitama University, in collaboration with Taisei Corporation, aimed to experimentally clarify how the installation position of yielding elements affects the deformation behavior of deformation-accommodating tunnel supports. The team prepared 1/20-scale tunnel support models using young-aged mortar to simulate concrete immediately after shotcreting and ordinary styrofoam used as model yielding elements. They then conducted loading experiments under conditions simulating squeezing ground by applying isotropic compressive loads using nine independently controlled jacks. By comparing models in which the yielding elements were installed at different positions, the team revealed that the yielding elements absorb displacement mainly through compression in the tunnel circumferential direction, that the support fails after the yielding elements are almost fully compressed, and that the deformation pattern of the entire support system changes depending on the installation angle of the yielding elements. The study was published online on June 4, 2026, in Tunnelling and Underground Space Technology, and clarified for the first time at laboratory scale the mechanical behavior of deformation-accommodating supports with yielding elements.
Key findings of the study include:
- The support models incorporating yielding elements allowed larger displacement than the mortar-only support model, demonstrating their ability to absorb tunnel deformation.
- When the yielding elements entered the strain region in which their compressive deformation increased rapidly, the deformation of the entire support system also increased sharply.
- The support failed after the yielding elements were almost fully compressed, and the maximum load at failure was comparable to that of the mortar-only support model.
- The deformation pattern changed depending on the installation position of the yielding elements: a nearly vertical arrangement mainly allowed vertical displacement, whereas an inclined arrangement also allowed horizontal displacement.
- A simplified beam–spring model reproduced the deformation patterns observed in the experiments, while also showing that further refinement is needed to predict stress distributions with higher accuracy.
"This study is significant because it experimentally shows how yielding elements function not merely as individual materials, but as part of an entire tunnel support system," says Associate Professor Togashi of Saitama University. "By demonstrating that the deformation behavior of the support changes depending on the installation position of the yielding elements, our findings provide practical insight for designing deformation-accommodating supports more rationally in tunnels where large ground deformation is expected."
The study also highlights the importance of combining laboratory loading experiments with structural analysis. "For tunnel engineering, it is not enough to perform numerical simulations of ductile support alone, because there are limitations, such as the difficulty of representing large strains in yielding elements ," Associate Professor Togashi explains. "We need to understand how that material interacts with the shotcrete layer and how the entire support system behaves under realistic loading conditions. Our experiment helps bridge that gap."
Looking ahead, the researchers expect the findings to contribute to safer and more efficient tunnel construction in mountainous regions, urban underground development, and infrastructure projects that pass through weak or squeezing ground. In the not-too-distant future, advances in the design methods for deformation-accommodating supports could help tunnel design and construction, while enabling more reliable underground infrastructure for transportation, energy, and lifeline systems.
"In the future, we hope to conduct laboratory experiments using actual yielding elements in the market while minimizing the gap between actual construction and experiment, and to link those results to design methods that can be applied more directly to construction sites," Associate Professor Togashi says of the team's future plans. "If deformation-accommodating supports can be optimized according to ground conditions, they could become an important technology for making underground infrastructure more resilient."
