Deduction and use of an analytical expression for the characteristic curve of a support based on yielding steel ribs

The convergence-confinement method (CCM) applies for cylindrical tunnels driven through a rock-mass that is assumed to be subject initially to a hydrostatic stress field. A basic component of the method is the support characteristic curve (SCC), which represents the deformation of the support produced by the load imposed on it; that is, the load that the support must carry for the amount of radial convergence occurred since support installation. The different elements that may constitute the tunnel support (before installing the tunnel lining) are steel arches (also named steel sets or steel ribs), bolts and shotcrete. The corresponding SCC’s have been developed for each of these support elements. However, for steel arches, the expressions proposed assume that the arch behaves as a rigid element, which is certainly the case for some types of arches. An accurate equation of the characteristic curve for yielding steel ribs (YSRs), steel arches with yielding capacity, has not been widely spread. This paper proposes a specific expression for this type of steel arches, capable of explaining the arch behavior which is easily used, as corresponds to the application of the CCM.

► Concept of support characteristic curve for steel ribs.
► Deduction of the characteristic curve of yielding steel ribs.
► Guidelines for support designing using yielding steel sets.
► Analytical expression for the yielding steel set support characteristic curve.