Distribution law and formation mechanism of karst collapses in karst plains of Mashan county, Guangxi

Karst collapse disasters occur frequently and cause huge hazards in the karst dissolution plain of Guangxi. Research on the distribution law and formation mechanism of collapses is the basic premise for disaster early warning, prevention, and control. A total of 97 karst ground collapses have occurred in Xinshitun, Shifeng village, Guling town, Mashan county, Guangxi. The plane morphology of collapses is mostly approximately circular to elliptical, and the cross-sectional morphology is mostly funnel-shaped. The total area of collapse pits is 1,367 m², with a maximum of 254 m² and a minimum of 0.5 m². Ground collapses have damaged buildings, restricted traffic, and severely threatened the safety of nearly 500 people’s lives and property, resulting in direct economic exceeding 10 million yuan. In this paper, the Xinshitun karst collapse in Shifeng Village is taken as the research object. Through a comprehensive analysis of the natural conditions, including topography, geological structure, stratigraphic lithology, and meteorological conditions in the study area, along with the verification of geophysical prospecting and drilling methods, this study discusses the distribution law, influencing factors, and genetic mechanisms of karst collapses in the dissolution plain. Furthermore, it delves deeply into the genetic mechanisms of karst collapses in this area.

The distribution of collapses in this area indicates as follows,(1) Ground collapses are mainly distributed in areas with strong karst development in fault tectonic belts. Of these collapses, 46 occur 0–50 m away from the fault, representing 47.4%; 32 are located 50–100 m away from the fault, accounting for 33%; and 19 are situated more than 100 m away from the fault, comprising 19.6%. Notably, as the distance from the fault increases, the frequency of collapses decreases significantly. (2) Ground collapses are distributed in areas with moderate thickness of overburden layers. Most of these collapses are concentrated in the areas where the thicknesses of overburden layers range from 10 m to 20 m, totaling 73 incidents, accounting for 75.3% of the cases. In contrast, collapses occurring in overburden layers with thicknesses of 0–10 m and 20–30 m are less frequent, representing 18.5% and 6.2% of the total, respectively. (3) Ground collapses are distributed in the areas where there are significant fluctuations in groundwater levels. A rapid decline of groundwater level can cause the roof of a karst cave, which is originally supported by groundwater, to lose its supporting force, thus ultimately leading to a collapse.

In general, the karst ground collapses in the study area are mainly influenced by three factors: geological structure, overburden thickness, and groundwater level. Specifically, these collapses are mainly distributed in areas with strong karst development within fault tectonic zones, areas with moderate overburden thickness, and areas where groundwater levels exhibit substantial fluctuations. Furthermore, the collapses occurring in the area where the three control factors jointly affect are more concentrated. In the study area, collapses are mainly caused by vacuum suction and suffoion. Most of these karst collapses, approximately 88% of the investigated collapse points, are caused by vacuum suction, while a small proportion, around 12%, is attributed to suffosion. The collapse zones created by vacuum suction erosion are mainly found in shallow overburden area characterized by poor permeability. Significant fluctuations in groundwater levels can lead to a drop in water levels below the bedrock, resulting in the loss of buoyancy within the cavities and the generation of negative pressure suction erosion within them. When combined with gravitational effects, this phenomenon may cause the overlying soil above the cavities to exceed the critical support threshold, ultimately resulting in karst collapses at the ground surface. The collapse zones formed by suffosion exhibit a high degree of karst development, development of karst caves, weak adhesion of overburden layers, substantial fluctuations in groundwater levels, and susceptibility of soil particles to erosion. When the roof of an upper soil layer reaches its critical stability, the upper soil becomes unstable, leading to instantaneous collapses. Both factors are the main mechanisms responsible for the formation of karst collapses in karst plains. This study examines the spatial relationship between collapses and fault tectonic belts, fluctuations in groundwater levels, and thicknesses of overburden layers to summarize the distribution laws and formation mechanisms of karst collapses in the Guangxi karst plains. The research findings provide a reference for the prevention and investigation of karst ground collapses in similar karst plain areas.