Behavior of deformation joints of RC utility tunnels considering multi-hazard conditions

Utility tunnels have been widely constructed worldwide to accommodate the lifeline infrastructure and ever-expanding municipal needs caused by fast population concentration and urbanization in recent years. Typical reinforced concrete utility tunnels consist of tunnel segments no longer than 30 m in length and deformation joints between adjacent segments. Although extensive research has been carried out on the seismic behavior of utility tunnels using shaking table testing and numerical method, little progress has been made in the understanding of the mechanical behavior of deformation joints under differential settlement and earthquake ground motion. Engineering practice has demonstrated that the deformation joints also play an important role in ensuring the functionability and durability of utility tunnels. In this research program, two groups of eight test models of utility tunnel deformation joints with shear dowel bars were constructed and tested under monotonic and reversed cyclic loading respectively, aiming to simulate the responses of the deformation joints subjected to differential settlement and earthquake excitation. Both ends of each shear dowel bar were inserted into adjacent segments for certain length, where one end is bonded with the segment concrete while the other debonded to allow for potential longitudinal movement due to temperature change. Concrete strength, dowel bar amount, insert length and layout were considered as the test variables. Test results indicated that concrete strength has little influence on the shear capacity of deformation joints. Placing dowel bars only at two vertical sides of the tunnel box-section can improve the deformation capacity and avoid concrete cracking at the bottom slab of the deformation joints. The reversed cyclic loading test results showed great seismic performance of the test models with great load bearing capacity, ductility, energy dissipation and stiffness. Although arranging all dowel bars at side walls can reduce the cracking of bottom slabs, the ductility and energy dissipation capacity are less favorable compared with placing dowel bars all around the box-section. In addition, although 10d is sufficient for the dowel bar insert length, it is suggested that use of 15d can help prevent severe concrete damages. The numerical analysis verified the test results and can be used for further study of the deformation joints of utility tunnels.