Stability Analysis of a Gravity Anchorage Foundation in Layered Argillaceous Sandstone Subjected to Dry–Wet Cycles and Cyclic Vehicle Loads

This study investigates the dynamic response and local stability of gravity-anchored foundations constructed in layered argillaceous sandstone under the coupled effects of wet–dry cycling degradation and cyclic vehicle loads. Based on in situ direct shear tests and FLAC3D 7.0 numerical simulations, a concrete–rock interface model, a rock mass direct shear model, and a three-dimensional dynamic model of the anchored foundation were developed. The parameters of the interface model were validated using the results of the direct shear tests. Wet–dry cycling degradation was subsequently incorporated to analyze the cyclic shear response of the interface and rock mass under different numbers of cycles. Cyclic vehicle loads were modeled as increments in main cable tension with an equivalent sinusoidal waveform. The results indicate that as the number of wet–dry cycles increases, the cyclic shear hysteresis loops shift overall toward lower shear stress levels. Peak shear stress decreases by approximately 49.26–51.64% compared to the natural state, and the hysteresis loop area decreases significantly. This indicates that wet–dry cyclic degradation weakens the cyclic shear resistance and energy dissipation capacity of the contact surface and rock structural planes. Dynamic analysis results for the anchor foundation indicate that wet–dry cycling degradation significantly increases the displacement response levels of the rock mass near the front toe and rear heel. Specifically, under the n = 20 condition, the displacement at the last peak increased by approximately 109.3–123.9% compared to the undisturbed state; simultaneously, the local plastic zones in the rock mass surrounding the anchorages gradually expanded, and the local safety factors of the rock mass near the toe and heel decreased overall. This study elucidates the degradation mechanisms and dynamic behavior of gravity anchors under the combined action of environmental and operational loads, providing a basis for the design and safety assessment of foundations for long-span suspension bridges.