Abstract:

Current theoretical studies on the trench stability of diaphragm walls seldom consider the coupled action of rainfall and water table fluctuations. Quantitative understanding of the evolution of soil mechanical properties under intermittent rainfall infiltration remains limited. First, on the basis of the Green-Ampt model combined with the Bodman-Colman layered hypothesis, a modified Green-Ampt infiltration framework is developed to simultaneously capture the degradation of saturated hydraulic conductivity caused by wetting-drying cycles and evaporative losses during rainfall interruptions. In this framework, a fractal pore-structure analysis is employed to derive a predictive equation for the saturated hydraulic conductivity of unsaturated soils. This equation, in turn, governs the dynamic propagation of the wetting front. The Penman-Wilson equation is then used to quantify evaporation during hiatus periods and to update the initial volumetric water content in the transition zone. Second, by coupling Darcy’s law with the Mohr-Coulomb criterion, a piecewise formulation for apparent cohesion is established as a function of rainfall intensity and duration. Third, an upper-bound limit-analysis procedure is developed that incorporates a three-dimensional rotational (log-spiral) failure mechanism derived from a spatial discretization scheme. The mechanical work associated with apparent cohesion is introduced into the virtual-power equation, yielding an explicit upper-bound factor of safety for trench walls subjected to intermittent rainfall infiltration and water-table oscillations. Finally, comparisons with numerical simulations, previously reported full-scale experiments, and field monitoring data show excellent agreement, thereby verifying the predictive capability of the proposed model. In addition, a sensitivity analysis was conducted on key intermittent-rainfall parameters, including rainfall intensity, number of rainfall events, and average temperature, as well as key soil and geometric parameters relevant to diaphragm-wall stability, including pore-size distribution characteristics, wall aspect ratio, effective cohesion, effective internal friction angle, and groundwater depth. Parametric studies reveal that higher rainfall intensity, more frequent rainfall events, and elevated ambient temperature deepen the wetting front and reduce the factor of safety. A shallower water table markedly decreases stability, whereas a sufficiently high slurry level provides partial support. A larger pore-distribution parameter λ, a smaller fractal dimension D, and a trench width-to-depth ratio of L/H<1.5 exacerbate 3D edge effects and reduce stability. Conversely, increases in effective cohesion  c' and effective friction angle φ'  markedly enhance shear resistance, and edge effects become negligible when L/H>10. These findings provide a rigorous theoretical basis and practical guidance for risk assessment and support-parameter optimization in diaphragm-wall construction under coupled intermittent rainfall and water-table fluctuations.

Key words: intermittent rainfall, Green-Ampt model, fractal theory, limit analysis, trench wall stability