Temporary spear bolts in modern concrete segmental linings (CSLs) are inherited features whose current role is unclear to tunnel practitioners. In the absence of specific design guidance and detailed quantitative studies, tunnel designers opt for their deployment on the basis of historic precedence. Understanding the functions delivered by the spear bolts during tunnel construction can help designers in their decision making and lead to project cost savings and a more efficient use of structural materials. In this paper, the functions of temporary spear bolts in gasketed longitudinal joints working under serviceable conditions are thoroughly investigated with the aid of analytical rotational models of isolated longitudinal joints with flat and curved geometries, a simplified calculation method based on two-segment models and 3D numerical models of a ring subjected to sequential loading with radial pressures. It is found that the spear bolts are not effective in holding the joints closed under gasket loads and become redundant features for this particular function in CSLs equipped with dowelled connectors in circumferential joints. The spear bolts can, however, delimit initial longitudinal joint misalignments during ring assembly and mitigate the severity of permanent in-plane angularities and joint misalignments arising near the tail skin due to differential radial rotations between adjacent segments, which in turn can increase the risk of concrete damage around longitudinal joints and gasket underperformance. Temporary spear bolts may be needed at the ring front in tunnel sections where the sealing and grouting pressures are expected to be high or unevenly distributed, e.g. deep tunnels or tunnel sections with curved alignment. In tunnel sections with moderate depth and straight alignment, the use of spear bolts at the front of keystone joints only may be sufficient.
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