Laboratory loading tests of ductile support systems under isotropic stress considering the effect of the installation position of yielding elements

Ductile support systems (also known as yielding supports) have been used in mountain tunnel constructions in which large radial displacements, such as squeezing rock, are anticipated. Ductile support incorporates multiple slots within the shotcrete layer, into which deformable materials (yielding elements) are installed. The amount of deformation in the shotcrete layer can be reduced by replacing large radial displacements with the deformation of the yielding elements. Although research on the mechanical properties of yielding elements, numerical analyses of ductile supports, and displacement evaluations at actual construction sites have been conducted to date, there are few examples of experimental verification of the mechanical behavior of ductile support systems with yielding elements. In this study, tunnel support loading tests were conducted on a 1/20 scale to study the mechanical properties of a ductile support model including yielding elements. In this experiment, nine jacks were loaded into the tunnel support model with displacement control. The model was prepared using young-aged mortar and ordinary styrofoam with a foam ratio of 30 times as yielding elements. The yielding elements were placed on each side wall of the support model, and isotropic compressive loads were gradually applied, assuming a squeezing rock. The results demonstrated that the deformation of the yielding elements in the circumferential orientation is dominant as a result of loading. It was also confirmed that the axial force of the tunnel support increases and failure occurs at the point when the yielding elements are almost fully compressed (strain level: 80%). The peak loads leading to failure were almost the same for the cases with and without the yielding elements. The effect of the yielding elements in an isotropic stress field was confirmed in a tunnel support model experiment.