Shinya Inazumi

Buried gas pipelines crossing irrigation canals are critical infrastructure that may be affected by canal rehabilitation, embankment instability, ground deformation, and fluctuating hydraulic conditions. This study presents a sequential numerical assessment framework integrating slope stability, ground deformation, soil–pipeline interaction, and hydraulic erosion analysis for an existing buried gas pipeline beneath the Rapeephat Canal in Thailand, which was recently widened to increase irrigation capacity. Two- and three-dimensional finite element analyses were conducted under three hydraulic conditions: maximum water level, minimum water level, and rapid drawdown. The existing embankment exhibited inadequate stability, with factors of safety ranging from 0.51 to 1.42. Following stabilization with a concrete face slab, the factors of safety increased to between 1.50 and 1.73, exceeding the adopted criterion of 1.50 under the investigated conditions. The maximum calculated ground settlement was 13.58 mm, while the maximum pipeline displacement under the normal operating pressure of 4.52 MPa was 10.10 mm. The maximum PLAXIS-derived bending stress was 0.088 MPa, whereas a supplementary hoop-plus-bending stress check indicated an equivalent combined stress range of approximately 116–145 MPa under the normal operating pressure, remaining below the adopted stress criterion based on 90% of the specified minimum yield strength. The maximum calculated boundary shear stress was 6.41 N/m², below the adopted permissible value of approximately 12 N/m² for stiff clay. The results demonstrate the influence of canal modification and hydraulic conditions on embankment stability, ground deformation, and pipeline response. The framework provides a practical approach for assessing existing pipeline–canal crossings, while the results should be interpreted within the adopted modeling assumptions and the limitations of the available field data and sequential coupling approach.

Road collapses caused by aging sewer pipes are an escalating urban hazard, as exemplified by the January 2025 sinkhole in Yashio City, Japan. Model experiments have described the macroscopic growth of subsurface cavities fed by soil loss through pipe defects, but the particle-scale role of suction has remained unresolved. This study constructs a theoretical, particle-scale account of cavity formation using a coupled Moving Particle Semi-implicit (MPS) and Discrete Element Method (DEM) analysis, in which pore water is modeled by the MPS method, the granular skeleton by the DEM, and suction is introduced as an inter-particle liquid-bridge force whose surface-tension coefficient is fixed a priori by a coarse-graining scaling law. Because the study is a numerical experiment, its findings are verified against established geotechnical theory, namely Terzaghi’s effective-stress principle, the Bishop effective stress, trapdoor-arching theory, and capillary liquid-bridge theory, rather than against experimental data. The results show that a self-supporting arch forms only above a strength threshold consistent with a limit-equilibrium balance between apparent cohesion and self-weight, that raising the groundwater level drives the cavity ceiling upward to the water level itself, and that full saturation eliminates suction and produces unbounded outflow with no stable cavity. Under a fluctuating water table the mechanism operates as a ratchet: each peak exceeding the current ceiling converts a reversible hydraulic oscillation into an irreversible upward step, because collapsed soil is lost through the defect while the water level recovers. These findings provide a microscopic basis for groundwater-integrated sinkhole-risk management.