2015 CSCE Annual Conference Regina - Building on our Growth Opportunities Conference
Dr. Verya Nasri, AECOM
Precast segmental linings are increasingly used in the mechanized tunneling methods using TBM-bored shield tunnels. Tunnel segments are designed following the approach of limit state design recommended by national and international codes and guidelines. Ultimate limit state (ULS) and serviceability limit state (SLS) are specified in these design codes. The former focuses mostly on the ultimate capacity of a structure prior to failure, while the latter is mainly associated with the availability of a structure for users, such as the constraints on deflection or deformation and cracking of a member under loading. The serviceability condition is particularly important for tunnel linings, because cracking can be very detrimental for its consequences in terms of durability, especially if the tunnel is exposed to an aggressive environment or excessive water inflow. In this paper, required serviceability verification for the design life of precast concrete segmental linings is discussed. Necessary design checks for SLS are introduced including stress, deformation and cracking for the main section of precast segment. This paper focuses on cracking serviceability limit states of concrete segments with particular attention to different types of reinforcement including steel bars and fibers. Methods to calculate crack width at SLS are discussed. Maximum allowable crack width for tunnel linings specified in various guidelines and standards are presented. Finally a case of mid-size tunnel is presented to illustrate the applicability of the proposed approach.