DESIGN CONCEPT OF TUNNEL SUPPORT STRUCTURE

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In the design of tunnel support structure, the method called "Characteristic Curve Method" is used. The method consists of two curves ; one is convergence curve for surrounding ground and the other is confinement curve for support (CCS) based on the thin or thick ring theory. This study is concerned with the CCS. The purpose of the study is to present the newly confinement curve for support (namely stiffness of support). To this end, the tunnel is idealized as a perfectly cylindrical thin shell with two stiffening steel rings at ends. The principal conclusions of this study are summarized as follows.(1)A simple closed-form analytical expression for estimating the tunnel support (stiffness of support) is presented. In addition, this equation is useful for a better understanding of the characteristics of the tunnel support.(2)The results for the stiffness of support obtained by the present equation are less than the CCS result ; the maximum error is 20 percent.(3)The effect of stiffening steel ring may be presented in terms of an effective width L^* (=αL), where L is the length between two rings and the coefficient α is greater than or equal to 0.35 within the range of geometric and material properties considered. Thus, in the case of α≦1,a critical state may be occur in tunnel support.(4)The elasticity modulus and thickness of the shotcrete has a major effect on the value ofα.