One of the most common problems in pressurized tunnels is the generation of cracks in the lining of the tunnel and the surrounding rocks as a result of high inner water flow pressures. By increasing the size of the cracks exceeding the allowable value, leakage would be happened from the lining towards the surrounding rock. In tunnel engineering, such phenomenon is a serious hazard, so called as hydraulic jacking which could be resulted in structural failure. There are several hydraulic structures such as water conveying tunnels, penstocks, pipelines and shafts which happened likely the hydraulic jacking phenomenon. In some cases, due to the effects of an earthquake or fast operations valves and gates, transient flow creates inside the above mentioned hydraulics structures, thereby the water flow inside the tunnel imposes high fluctuating forces on the rock surrounding the tunnel, causing high displacements in the rock, leading to rock fracture. Therefore, the effects of transient flow as well as the pressure fluctuations should be considered by design engineers. In the present study a two-dimensional pressurized water tunnel with a concrete lining was modeled, using the finite element method by applying the ABAQUS software. The lining was considered just as a roughening coverage to decrease roughness coefficient and to increase the flow discharge. The mechanical parameters of surrounding rock were assumed as weak as possible compared to the inner tunnel concrete lining. The water flow inside the tunnel was modeled, using acoustic elements that are capable of simulating the hydrodynamic forces. Accordingly, the mechanism of developing the cracks in the surrounding rock as the most probable place to prone this destructive phenomenon is studied applying both steady state and transient pressure conditions considering fluid-structure and rock interactions. Steady state and transient flow analysis were performed using the HAMMER software. The hydraulic loadings resulted from the hydraulic analysis by HAMMER software was transmitted to ABAQUS for structural analyses. The stress analysis of cracked elements of the surrounding rock was carried out based on Mohr-Coulomb fracture criterion. The least values of overburden rock to prevent the failure due to hydraulic jacking were evaluated by imposing the water flow pressure in both steady and transient flow states. Finally, the effects of increasing the height of the overburden on hydraulic jacking phenomenon were investigated considering transient flow conditions. Results indicated that, because of coordinate of surrounding rocks elements, extra overburden on tunnel is not a safe Solution for the eradication of hydraulic jacking phenomenon in rock around tunnel. Keywords: Pressurized tunnel, hydraulic jacking, transient flow pressure, Fluid-structure interaction, overburden.
