Recommended deformation limits for the structural design of segmental tunnels built in soft soil

Abstract

This article proposes a new approach to define serviceability deformation limits for the structural design of segmental tunnels built in soft soil. In modern design codes, the structural design of segmental tunnels has been established considering only the ultimate strength of linings; therefore, the used limits for tunnel deformation and distortion were determined empirically. A parametric analysis was performed by means of finite element models in MIDAS FEA© software. The main variables were the diameter of the tunnel, number of segment joints, soil pressure distribution and slenderness ratio. In order to establish the deformation limits, a serviceability limit state was defined. The new approach for defining serviceability limit considered the deformations, crack width and segment joint behavior. The serviceability limits were compared with the deformation limits commonly used in practice. The results indicated that the serviceability limits of segmental tunnels depend on two parameters: slenderness ratio (modified up to 97%) and soil pressure distribution (modified up to 164%). Thus, empirical recommendations are not suitable for every case. The application of serviceability limits showed differences higher than 60%, in contrast with the traditional recommendations for deformation. This can influence the structural design of segmental tunnels, built in soft soils.