Study of the effect of traffic and static loads on the stability and deformation of shallow urban utility tunnels

In this study, the effect of tunnel overburden under static loading, along with traffic loading caused by the passage of vehicles, on the rate of deformation and the ground surface settlement has been fully evaluated. This study is based on 3D numerical modeling using Plaxis 3D Tunnel finite element software and comparing the results with instrumentation data. First, the initial conditions of excavation and execution of the tunnel below the street surface were investigated with the application of all geometric parameters and geotechnical characteristics of the tunnel route. Then, the effect of different depths of the tunnel placement under the street surface was evaluated with traffic load on land settlement rate by validating the numerical model based on field settlement data recorded by instrumentation data. Among the various overburden, a 7 m overburden was introduced as a suitable option for constructing a surface water collection tunnel. Because with increasing the depth of the tunnel and, in fact, the tunnel overburden, in addition to suitable conditions for creating the phenomenon of arching around the excavation, the effect of deformation in the soil layer at the ground level decreased. Also, based on the optimal tunnel depth model, a parametric analysis was performed to evaluate and achieve an optimal tunnel support system. Then, the effect of different tunnel placement depths under the street surface was evaluated with traffic load on the surface settlement rate by validating the numerical model based on field settlement extensometer data recorded by precision instruments. Among the various overburdens, a 7 m overburden was introduced as a suitable option for constructing a surface water collection tunnel. Because with increasing the depth of the tunnel and, in fact, the tunnel overburden, in addition to achieving a suitable condition for creating the phenomenon of arching around the excavation, the effect of deformation in the soil layer at the ground level decreased. Also, a parametric analysis was performed based on the optimal tunnel depth model to evaluate and achieve an optimal tunnel support system. Accordingly, the longitudinal distance between the tunnel retaining frames was increased, and the shotcrete thickness was somewhat reduced for tunnel support. As a result of increasing the longitudinal distance of the frames and reducing the thickness of the shotcrete, in addition to saving maintenance costs, the speed of the tunnel construction also increases with increasing the length of the drilling step.