Search Results (9)

The segment in an underwater shield tunnel is influenced by the change of earth pressure and water pressure. Therefore, the law of segment deformation should be mastered for the safe operation of the tunnel. To obtain the law of segment deformation under varying earth pressure, loading conditions of constant water pressure and without water pressure were considered. In this study, the numerical simulation and scale model experiment were carried to analyze the strain, curvature, and displacement of the segment. The results show that the strain amplitude of segments is reduced by the water pressure under a range of earth pressure. When the earth pressure ranges from 0 MPa to 2.4 MPa and the water pressure 0.33 MPa, the displacement of the vault and arch waist exhibit a decreased rate of 7.916 mm/MPa and an increased rate of 5.416 mm/MPa, respectively. Under the combined effects of constant water pressure and varying earth pressure, the curvature of the segment tends to stabilize after a rapid change with a maximum of 0.004 m⁻¹. Full article

The construction of underwater shield tunnels under high water pressure conditions and seepage action will seriously impact the stability of the surrounding rock. In this study, an analytical model for the strength of the two-lane shield tunneling construction under anisotropic seepage conditions was established, and a series of simulations were carried out in the engineering background of the underwater section of Line 2 of the Taiyuan Metro in China, which passes through Yingze Lake. The results show that: (1) the surface settlement has a superposition effect, and the late consolidation and settlement of the soil body under seepage will affect the segment deformation and the monitoring should be strengthened; (2) under the influence of the weak permeability of the lining and grouting layers, the pore pressure on both sides of the tunnel arch girdle is reduced by about 72% compared with the initial value, with a larger hydraulic gradient and a 30% reduction at the top of the arch; (3) within a specific range, the tunneling pressure can be increased, and the grouting pressure and the thickness of grouting layer can be reduced to control the segment deformation; (4) the more significant the overlying water level is, the larger the maximum consolidation settlement and the influence range of surface settlement. This study can provide a reliable reference for underwater double-lane shield tunnel design and safety control. Full article

The structural stability of the underwater shield tunnel during operations is affected by temperature variations. The effect of different structure temperatures on the underwater shield tunnel during the operation period was studied. By numerical simulation, the variation in the underwater shield tunnel temperature circle was analyzed. The variation patterns of the top arch, bottom arch, waist arch temperature, maximum principal stress, and settlement of the soil under different temperatures were obtained. The results showed that: (1) The early excavation time of the tunnel was short, and the temperature circle was small. The temperature circle expanded rapidly after 50 days of operating. The diffusion range increased from 1.5 m to 5.35 m: an increase of 256.7%. With the increase in time, the expansion rate of the temperature circle gradually slowed down. (2) The higher the temperature of the soil, the more complex the temperature transfer between the soil and the lining was while generating greater temperature stresses and reducing the safety of the tunnel. (3) When the tunnel was just excavated, the compression settlement of the top arch and the waist arch increased rapidly, reaching 5.43 mm and 0.24 mm, respectively. The bottom arch was squeezed by the soil on both sides, resulting in an uplift and rapid increase, reaching 4.94 mm. The settlement rate increased with the increase in the tunnel structure’s temperature. After the excavation, with the decrease in temperature, the strength of the soil and lining increased. The settlement of the top arch, bottom arch, and waist arch increased slowly with time, and the growth rate decreased gradually. Full article

Underwater shield tunneling will disturb the soil near the river, especially in water-rich soft ground. This may cause a groundwater infiltration hydraulic gradient to exceed the critical value, leading to calamities, such as unexpected flooding or submerged erosion. To ensure the security of construction and the stability of river embankment seepage, it is crucial to assess the safety of the underwater tunnel cover thickness. A shield tunnel project under a river in Hefei is used as an example. The numerical model established by the finite element method is used for calculating and analyzing the changes in the groundwater flow field and the stability state of embankment seepage induced by underwater shield tunneling under different overburden thickness conditions. The results show that the construction disturbance of the shield tunnel through the river is increased, the internal force environment of the embankment slope is destroyed, and the maximum seepage hydraulic gradient is increased. In the case study, the embankment keeps in a stable state of seepage when the cover thickness of the shield tunnel has 2.9 times its outer diameter. The findings of this study can serve as a scientific guide to assure seepage stability in an underwater shield tunneling project and to stop river embankment erosion. Full article

Tunnels are commonly constructed in water-bearing zones, which necessitates stability analyses of saturated tunnels based on the upper bound of the plastic theory. Previous kinematical approaches have the following drawbacks: (1) using an empirical approach to estimate pore-water pressure distributions; (2) using failure mechanisms that are not rigorously kinematically admissible. To overcome these shortcomings, we proposed a rigorously kinematically admissible translational–rotational failure mechanism for an underwater shallow square tunnel where velocity discontinuity surfaces were derived. Then, the pore-water pressure field surrounding the tunnel under the boundary of constant water pressure is analytically generated based on the complex variable method and imported into the kinematically admissible velocity field. Work rates performed by external forces and the internal dissipation rate are numerically computed to formulate the power balance equation, followed by a mixed optimization algorithm to capture the critical states of the surrounding soils of tunnels. The outcomes of pore-water pressure distributions, safety factors, and failure mechanisms are in tandem with those given by the numerical simulation but show higher computational efficiency than the numerical simulation. In the end, we highlight the advantages of the proposed model over the empirical approach, where soil properties and water table elevation effects are analyzed. Full article

In order to improve the endurance of underwater vehicles and make it possible for the underwater vehicle to inspect long-distance water tunnels, a sliding mode control method based on event triggering is proposed for the depth control of underwater vehicles from the perspective of energy saving. Firstly, the kinematics and dynamics models of underwater vehicle dive surface are established. Secondly, an event-triggered sliding mode controller is designed. According to the Lyapunov function, the stability of the designed controller is proved by theoretical analysis, and Zeno phenomena will not appear in the closed-loop control system. Compared with other controllers, the simulation results show that this controller can effectively realize the depth control of AUV, has strong adaptability and robustness to unmodeled nonlinear dynamics and bounded disturbances, and has the effect of saving computing resources. Full article

Reasonable tunnelling parameters for underwater shield tunnels play an important role in maintaining driving efficiency and safety. In this paper, a neural network method was developed to predict tunnelling parameters. Soil properties and geometric parameters were taken as inputs for the neural network, which output the tunnelling parameters, such as advancing thrust, rotation, penetration, torque of the cutter head, and support pressure. In order to improve the stability of the neural network, a genetic algorithm (GA) with a global searching ability was used to optimize the initial weight of the neural network (GA-BPNN). The accuracy of the algorithm, based on GA-BPNN, was studied through an underwater shield tunnel project. The results showed that the integration of GA into the neural network significantly improves the prediction ability for shield tunnelling parameters, especially for adjustable parameters. Later, the developed GA-BPNN model was further utilized to predict and set the range of shield tunnelling parameters in fine sand stratum of high risk. Through a comparative analysis of tunnelling parameters, the reasons leading to ground instability have been found out, and the effectiveness of ground pre-reinforcement has been verified. Full article

Excavation damage zones that occur around tunnels usually lead to a change in rock permeability, which has an impact on the water inflow into tunnels and even induces water inrush disasters. For a better understanding of the effect of the excavation damage zone, analytical solutions that consider the excavation damage zone are developed based on the review and modification of the solutions that consider linings and grouting circles. Then, both analytical solutions and the finite element method are applied to estimate the water inflow, and the results are in good agreement. The effect of the excavation damage zone on water inflow is analyzed based on an impact factor of the excavation damage zone and evaluated in a real engineering case, and the seepage-preventing effects of grouting are discussed. The results reveal that the water inflow increases with increasing permeability and thickness of the excavation damage zone and that there is a limit for the effects of the excavation damage zone. In addition, the effect is stronger for underwater tunnels with small water inflows and stabilizes gradually as the magnitude of water inflow increases. The increase in the impermeability and thickness of the lining and grouting circle can reduce the effect. Full article

Underwater tunnel structures are vital in urban areas, and their use is continuously increasing. The engineering assessments for these impressive phenomena have to be performed from both geotechnical and structural engineering perspectives. The designer should take account of the requirements of the underwater structures to adequately withstand different applied loads. The underwater tunnel might be particularly vulnerable at locations where the geological conditions are uncertain. This study will cover all aspects concerning the stability of the tunnel crossing of the River Nile. The protentional exists for the tunnel failure during construction due to insufficient cover thickness and after construction due to a combination of long-term degradation and local scour. There were no imposed constraints on the alignment and the original design with a cover between the tunnel crown and the riverbed of at least one tunnel diameter. The tunnel stability was analyzed based on the most critical underwater section with a minimum cover thickness of the Greater Cairo Metro, Line 2 as the case study. Then, three-dimensional (3D) numerical analysis based on the Finite Element (F.E.) models was employed to explore soil–tunnel interactions. Comparison between numerical models’ results indicated that the safe cover thickness was equal to the tunnel diameter. The minimum cover thickness can be used to verify the required factor of safety calculated by theoretical analysis. The safety factor of the tunnel stability should not be less than 1.5 for construction and service stages and 1.3 for the exceptional case; scour accrues. Full article