A parametric study on friction-loss in water conveyance tunnels considering misalignment of precast concrete segments

In many tunneling projects, it is common that the lining concrete segments misaligned during Tunnel Boring Machine (TBM) operation or after excavation process. Misalignment of concrete segment, especially in water conveyance tunnels, can affect fluid transfer capacity and causes local head-loss. Present study is composed of a series of Three Dimensional (3D) numerical simulations which explores the effects of the steps occurrence the tunnels created by segment off-sets in subcritical free surface flow. Results show that recirculating flows occur after the steps. This strong vortical motion after the steps causes considerable head-losses especially for higher steps which should not be neglected in the estimation of total head-loss. The results of simulations also show that the effects of the wall roughness and segment length on the head-loss could be ignored for practical estimations. Moreover, it is found that for a specified high expansion ratio, local head-loss coefficient of the bottom step is less than coefficient of the side step. Exploring presence of a sequence of misaligned concrete segments shows that the head-loss coefficient of sequenced steps is less than summation of independent coefficients of the same steps.