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Maintaining the stability of the tunnel face during the construction of the submarine shield tunnel is the prerequisite for safe construction. A series of face instability model tests under saturated strata were carried out. The earth pressure, the evolution of the soil arch area and the amount of soil entering the soil groove caused by the instability of the tunnel face under different face water pressure conditions were studied. A series of numerical simulations were carried out to analyze the effects of tunnel face water pressure. The results show that the vertical earth pressure and horizontal earth pressure at different positions show a gradual decrease with the increase of the baffle backward distance, and the smaller the water pressure ratio, the more obvious this law is. According to the test results, the decrease of the water pressure ratio of the tunnel face can significantly increase the soil intake of the soil tank, which indicates that the collapse range above the tunnel face is large. The trend of instability region obtained by numerical simulation is consistent with the test monitoring results. This provides a reference for the construction of submarine shield tunnels. • Geomechanical model tests are carried out. • The pore water pressure and earth pressure are studied. • The evolution process of the soil arch area and the soil inflow volume of soil tank are investigated. • The failure areas is analyzed. [ABSTRACT FROM AUTHOR]

Abstract It is known that the seepage force has a significant impact on the mobility of bed particles. To assess the effect of seepage force on the sediment transport around a suspended pipeline, a series of numerical tests have been carried out utilising the commercial software Comsol. In the simulation, the shear-stress transport turbulence model was used to simulate the upper turbulence flow field and the porous medium model was adopted to describe the dynamic response of the underlying seabed. Both the fluid flow and the seepage flow within the seabed were modelled. The effect of the vertical seepage force on the Shields number was investigated. The simulation results indicate that seepage plays a vital role in the process of tunnel scour, but becomes quite weak with the form of a scour hole beneath the pipeline. With an increase in scour depth, vortices shed from the pipe lead to a fluctuating fluid pressure, inducing significant upward seepage, and thus influencing the stability of bed particles. If sagging of the pipeline appears, the fluctuation of fluid pressure becomes weak and the seepage effect is minor. Highlights • The seepage effect on the Shields number is studied through numerical simulation. • The vortex shed onto the seabed can induce upward seepage flow due to the negative pressure inside it, which will promote the sediments scour. • Study seepage effect on shields number around fixed and sagging pipeline by coupling numerical simulations with experiments. [ABSTRACT FROM AUTHOR]

Based on the three-dimensional hydraulic head distribution model and the upper bound theorem of the limit analysis, a new model was established to calculate the limit support pressure in shield tunnel faces. A series of numerical simulations were conducted to evaluate the accuracy of the proposed model and the obtained results were compared with those of other classical analytical methods. It is found that the obtained analytical solutions are in good agreement with the numerical simulations and the proposed method outperforms other methods in terms of accuracy. In the present study, the influence of the water level, internal friction angle, burial depth, and water pressure on the stability of the tunnel face is analyzed. The performed analyses reveal that the limit support pressure is negatively correlated with the internal friction angle, while the burial depth does not affect the limit support pressure of the shield tunnel face in subsea tunnels (C/D ≥ 1). Meanwhile, it is found that as the water level and the water pressure on the tunnel face increase, the limit support pressure increases linearly, and the failure area extends to the front of the tunnel face and decreases in height. • A calculation formula is derived to analyze the stability of tunnel face in saturated soil. • The analytical solutions proposed in this paper excels the solutions of other existing methods. • The influence of water level, C/D and internal friction angle on the stability of the tunnel face is investigated. • The effect of the water pressure of the tunnel face on the stability of the tunnel face is explored. [ABSTRACT FROM AUTHOR]

Abstract: The mean net bedload sediment transport rate beneath random waves is predicted taking into account the effect of seepage flow. This is achieved by using wave half-cycle bedload sediment transport formulas valid for regular waves together with a modified Shields parameter including the effect of seepage flow. The Madsen and Grant (1976) bedload sediment transport formula is used to demonstrate the method. An example using data typical to field conditions is included to illustrate the approach. The analytical results can be used to make an assessment of seepage effects on the mean net bedload sediment transport based on available wave statistics. Generally, it is recommended that a stochastic approach should be used rather than using the rms values in an otherwise deterministic approach. [Copyright &y& Elsevier]

Abstract: Hydrodynamic forces exerting on a pipeline partially buried in a permeable seabed subjected to combined oscillatory flow and steady current are investigated numerically. Two-dimensional Reynolds-Averaged Navier–Stokes equations with a k−ω turbulent model closure are solved to simulate the flow around the pipeline. The Laplace equation is solved to calculate the pore pressure below the seabed with the simulated seabed hydrodynamic pressure as boundary conditions. The numerical model is validated against the experimental data of a fully exposed pipeline resting on a plane boundary under various flow conditions. Then the flow with different embedment depths, steady current ratios and KC numbers is simulated. The amplitude of seepage velocity is much smaller than the amplitude of free stream velocity as expected. The normalized Morison inertia, drag and lift coefficients based on the corresponding force coefficients of a fully exposed pipeline are investigated. The normalized Morison force coefficients reduce almost linearly with the increase of embedment depth and that the KC only has minor effect on the normalized Morison coefficients. It is also found that the permeable seabed condition causes a slight increase on the inline force and has a little effect on the lift force, compared with corresponding conditions in an impermeable bed. [Copyright &y& Elsevier]