More recently, the demand for application of segmental tunnel lining has been raised for both long & deep tunnel in rock and shallow tunnel in soft ground, particularly in urban areas. From technical and economical points of view, a proper dimensioning of the segmental lining has always attracted the interest of many designers in the field of tunneling. This paper will focus on a critical comparison between the doubleconvex and flat joints of radial joints of a segmental lining in terms of induced bearing and splitting stresses in concrete and finally the steel ratio to be used to satisfy the structural verifications in Ultimate and Serviceability States. The results of this study have revealed that the contact length of a convex joint shape is independent from geometrical load eccentricity whereas the contact length of a flat joint decrease considerably due to load eccentricity. Therefore, for a given condition, the ultimate resistance of concrete against bursting force (in compression) for a convex joint is higher than that for a flat joint. However, splitting tensile stresses in concrete of radial joints are higher in double convex joints; consequently, a more steel ratio is needed for the double convex radial joint configuration. In practice, double convex may allow for reduced segmental lining thickness, while flat joints, generally, requires a lower steel reinforcement. Taking into consideration of above-mentioned critical comparison and technical-economical requirements of a given project, a more practical radial joint feature might be chosen.
The idea of application of double-convex radial joint, instead of flat radial joint, in designing segmental tunnel lining could be attributed to the need for the optimization of the deduced bearing stresses in concrete segment since it exceeds, in most cases, the allowable bearing compressive strength of concrete in long term state. The definition of radial (longitudinal) joint is presented in Figure 1.