LABORATORY LOADING TEST OF TUNNEL SUPPORT WITH YIELDING ELEMENTS IN ISOTROPIC STRESS FIELD

Yielding elements which have a trilinear stress-strain relationship are often used to prevent support deformation in the mountain tunnel construction for a squeezing rock. Yielding elements are usually placed between shotcrete, and it is expected to have the effect of releasing the radial deformation to the circumferential orientation while maintaining the axial force of the support. Although uniaxial compression tests have been conducted on several types of yielding elements and the evaluation of displacement at actual construction sites where this technology is applied, there are few examples of experimental verification of the mechanical behavior of tunnel support with yielding elements. In this study, tunnel support loading tests on a 1/20 scale were conducted to study the mechanical properties of the tunnel model including yield elements. This experiment can be loaded with nine jacks to the tunnel support model with displacement control. The model was prepared using young aged mortar and ordinary use EPS with a foam ratio of 30 times as yielding elements. Note that rock bolts were not modeled here. 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 to 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%). 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 the tunnel support model experiment.