Physical Model Test Study on Karst Collapse under Cyclic Negative Pressure

Karst collapse ranks among the most pervasive geological hazards in carbonate-bearing regions worldwide, posing a formidable threat to urban infrastructure and the safety of local residents. In southwest China, periodic groundwater level fluctuations triggered by mine dewatering, urban groundwater exploitation, and reservoir operation have been identified as the dominant triggering factor for karst collapse. Among various collapse-inducing mechanisms, negative pressure suction erosion occurring at the soil-rock interface during water level drawdown is widely acknowledged as a critical driver of rapid instability and failure in clay overburdens. Previous studies have predominantly focused on the collapse process induced by a single episode of water level drawdown, elucidating the fundamental laws governing seepage deformation and mechanical instability of soil layers under hydraulic loading. Nevertheless, in practical engineering contexts, groundwater levels typically undergo recurrent cyclic fluctuations rather than unidirectional monotonic changes. The resultant cyclic negative pressure action induces progressive cumulative damage to the overburden soil, an evolutionary process that cannot be accurately characterized by results from conventional single-cycle tests. In particular, the duration of effective negative pressure within a single fluctuation cycle, governed by the rates of water level rise and fall, represents a key factor shaping the evolution of cumulative soil damage and the final collapse morphology. To date, no quantitative relationship has been established between negative pressure duration, cycle number, and critical collapse conditions, which impedes precise risk assessment of collapse under long-term periodic water level variations. To address this research gap, this paper presents physical model experiments to investigate the cumulative damage and collapse process of a homogeneous clay overburden under cyclic water level fluctuations, with particular emphasis on quantifying the influence of single-cycle negative pressure duration on the evolutionary trajectory of soil failure.The experiments were performed using a self-developed karst collapse model test system consisting of a transparent plexiglass model box, a karst cavity simulation unit, a circulating water level control system, and a multi-parameter monitoring subsystem. The test soil was typical red clay collected from a karst collapse-prone area in Guizhou Province. A uniform overburden was fabricated by layer-by-layer compaction, with the initial dry density and moisture content strictly controlled. Three test scenarios were designed by adjusting the opening of the water injection valve to regulate the water level rise rate, corresponding to three distinct durations of effective negative pressure per cycle. Each cycle comprised a rapid water level drawdown phase and a gradual water level recovery phase, with the fluctuation amplitude maintained consistent across all scenarios. Throughout the tests, high-definition cameras were deployed to continuously record the initiation, propagation, and final morphology of surface cracks. Meanwhile, multiple embedded sensors monitored vertical soil displacement, pore water pressure, earth pressure, and air pressure at the soil-rock interface at a constant sampling frequency to ensure data synchronization.The experimental results demonstrate that overburden failure under cyclic negative pressure exhibits pronounced cumulative damage characteristics, and the failure process can be divided into four distinct stages: negative pressure accumulation, soil damage progression, crack propagation, and collapse occurrence. In the initial stage, the soil undergoes slight vertical compression under repeated hydraulic loading, with negligible surface deformation, while pore water pressure and earth pressure remain generally stable. As the number of cycles increases, tensile cracks first emerge on the surface above the cavity edge and propagate progressively in both vertical and horizontal directions. During this stage, the surface deformation rate increases markedly; pore water pressure dissipates rapidly with crack generation; vertical earth pressure at the overburden base gradually rises due to stress redistribution; and the negative air pressure within the cavity attenuates continuously as air infiltrates through the cracks. These coupled variations collectively reflect the progressive deterioration of the soil structure.Comparative analysis reveals that the duration of single effective negative pressure fundamentally alters the cumulative damage rate and the ultimate failure mode of the overburden. Under the long-duration scenario, sustained high-intensity negative pressure generates greater seepage gradients and tensile stresses within the soil matrix, leading to earlier crack initiation, faster propagation rates, and critical collapse occurring at a smaller number of cycles. The resulting collapse pit is relatively deep with a concentrated failure zone. Under the short-duration scenario, the intensity of negative pressure per cycle is comparatively low, so the initiation of visible cracks and the onset of rapid deformation are significantly delayed. Nevertheless, the subtle damage induced by each cycle accumulates continuously over repeated fluctuations, giving rise to wider lateral crack expansion and larger cumulative vertical displacement prior to final collapse, and ultimately forming a shallower collapse pit with a broader influence range. Nonlinear fitting indicates that the critical number of cycles to collapse decreases with increasing single negative pressure duration in a negative power-law fashion (R² = 0.955). Nevertheless, this empirical relationship requires further validation across a wider spectrum of test conditions.From a mechanistic perspective, the cumulative damage under cyclic negative pressure is essentially a fatigue failure process induced by hydromechanical coupling. Each cycle of negative pressure action induces irreversible microcrack generation and particle rearrangement within the soil; the superposition and accumulation of micro-damage ultimately result in macrocrack coalescence and overall instability and collapse of the overburden. This study preliminarily elucidates the complete evolutionary process of damage accumulation in clay overburdens under cyclic negative pressure and quantifies the influence law of single negative pressure duration on critical collapse conditions. The findings can provide theoretical support for risk assessment and early warning of karst collapse triggered by periodic pumping and drainage in karst regions.