Abstract:

[Objective] With the advancement of rainwater and sewage diversion projects in cities along the river, trenchless pipe jacking technology has been widely adopted due to its efficiency and environmental advantages. However, pipe jacking in the river-crossing sections faces challenges posed by complex hydrogeological conditions, which can cause riverbed surface displacement and even lead to engineering accidents such as water inrush or blowout. There is still a lack of systematic analysis of the mechanisms influencing surface displacement in the river-crossing sections during pipe jacking construction. Most existing studies are based on assumptions of semi-infinite elastic bodies or static stratum conditions, making it difficult to accurately reflect the disturbance patterns under dynamic changes of parameters such as bulk density, cohesion, and internal friction angle of geomaterials in river-crossing sections. This study focuses on a pipe jacking project in the river-crossing section of a city along the Yangtze River, investigating the displacement patterns of the riverbed surface under fluid-solid interaction. It aims to reveal the influence mechanisms of key parameters, thereby providing theoretical support for safe construction. [Methods] A combination of theoretical analysis, numerical simulation, and empirical formula comparison was adopted. First, the strength reduction method was applied to reflect the soil weakening effect under fluid-solid interaction by reducing the geotechnical mechanical parameters (cohesion c and internal friction angle φ). Then, a 3D numerical model was established using COMSOL Multiphysics. The model simulated actual operating conditions through roller supports and fixed boundary conditions. Considering soil elastoplasticity, the grouting layer, and hydrostatic pressure boundary conditions, this model simulated stress redistribution and surface deformation during the pipe jacking process. In addition, Peck’s empirical formula was introduced to predict settlement, and the results were compared with the numerical simulation to verify the reliability of the model. Finally, the single-factor analysis method was used to systematically study the influence patterns of pipe diameter, grouting pressure, and soil elastic modulus on riverbed surface displacement. [Results] (1) Characteristics of soil stress distribution: During the pipe jacking process, the stress in the soil around the pipe exhibited a near “M”-shaped distribution, with the minimum stress at the pipe axis and the stress on both sides increasing first and then decreasing. The closer to the pipe, the narrower the “M”-shaped trough became. At 38 meters of jacking distance, the maximum stress value increased by about 60% compared to the initial state, concentrated mainly at the pipe bottom and pipe crown. (2) Riverbed surface subsidence pattern: The surface subsidence trough followed a “U”-shaped normal distribution, with the maximum subsidence located directly above the pipe axis. At 38 meters of jacking, the maximum subsidence reached 1.352 mm, closely matching the prediction of 1.313 mm by the Peck formula. However, due to the influence of high water pressure and soil parameter weakening, the simulation result was slightly conservative. (3) Influence of parameters: Increasing pipe diameter from 1.8 m to 2.4 m raised the maximum settlement by approximately 40% and widened the subsidence trough by 23%, indicating that large pipe diameters significantly intensified soil disturbance. Raising grouting pressure from 0.1 MPa to 0.3 MPa reduced the maximum subsidence by 35%, and the support and lubrication effect of the slurry sleeve effectively inhibited soil loss. Increasing the soil elastic modulus from 7.2 MPa to 14.4 MPa reduced the maximum subsidence by 46%, indicating that hard soil had significantly stronger deformation resistance compared to soft soils. [Conclusion] Under the disturbance caused by pipe jacking construction, the soil stress redistribution exhibits an “M”-shaped pattern, and the surface subsidence trend is consistent with the predictions of the Peck empirical formula, validating the applicability of the numerical model. (1) Pipe diameter, grouting pressure, and soil elastic modulus are key parameters influencing surface displacement. In engineering practice, it is necessary to balance the selection of pipe diameter (large diameters improve jacking efficiency but increase subsidence risk), optimize grouting pressure (to suppress subsidence and avoid excessive uplift), and improve the disturbance resistance of soft soil through reinforcement (such as pre-grouting). (2) This study is the first to construct a 3D fluid-solid interaction model for pipe jacking in the river-crossing section, combining the strength reduction method and parameter sensitivity analysis to provide a theoretical basis for similar projects. Its limitation lies in the lack of field monitoring data for validation. In the future, site monitoring should be incorporated to further improve model accuracy. The findings can provide guidance for design optimization and risk control of pipe jacking projects along the Yangtze River Economic Belt and under similar hydrogeological conditions, contributing to the implementation of the “joint efforts for environmental protection” strategy.

Key words: pipe jacking construction, fluid-structure interaction, strength reduction, surface subsidence, numerical simulation