Mechanisms and critical criteria of coverd karst collapses under extreme rainfall conditions
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摘要: 近年来，极端降雨条件诱发岩溶塌陷的问题日益凸显，故亟需开展极端降雨条件下岩溶塌陷形成机理的研究。本文通过室内模型试验，分析了不同降雨条件下覆盖层孔隙水压力、土压力与位移量变化规律，探究了极端降雨条件下岩溶塌陷的形成演化过程、塌陷类型与致塌因素。结果表明：（1）降雨过程覆盖层孔隙水压力、土压力和位移量具较好的协同变化规律，且与岩溶塌陷形成演化具较好的相关性。随着降雨强度和循环次数的增加，孔压与土压亦随之增大；位移量的变化表征着岩溶塌陷的形成过程；（2）极端降雨条件下岩溶塌陷类型可分为蠕变破坏型和压剪断裂型两种类型，前者孔隙水压力与土压力为峰丛起伏形态，波动曲线基底不断升高，深层位移量呈先升后降的趋势，浅层和中层位移量则随降雨持续而逐次递增；后者孔压、土压和位移量呈孤峰型，先是在短期内急剧增大然后再迅速降低；（3）蠕变破坏型岩溶塌陷是软化、饱水增荷与损伤等效应循环与累积的结果，压剪断裂型则是以饱水增荷与损伤为主要致塌动力的塌陷类型；（4）覆盖层厚度与岩溶塌陷临界判据条件呈正相关性，厚度越大覆盖层抗塌性越好。Abstract:
In recent years, the increasing occurrence of covered karst collapses triggered by rainfall conditions has posed a threat to human life and property safety. According to field investigations and statistical data, the triggering factors of karst collapses can be categorized into natural and human factors, with 40% of these collapses attributed to natural causes. Natural factors, such as rainfall, earthquake, tsunami, etc., exert different effects on the development of karst collapses. Notably, most karst collapses are associated with extreme rainfall conditions, indicating a correlation between rainfall and the occurrence of collapses. At present, research of the impact of rainfall on karst collapses mainly focuses on pressure changes in groundwater and air, the effects of increased load, vertical permeability deformation, and saturation erosion caused by rainfall infiltration. However, there is a scarcity of studies providing quantitative analyses of the evolution process of karst collapses triggered by rainfall. Therefore, understanding the mechanisms of karst collapses under extreme rainfall conditions has been considered as a key research objective. Based on field investigations, a hydrogeological structural model has been developed, taking into account the hydrogeological conditions and the presence of covered karst collapses in the study area. Subsequently, a physical model of these collapses has been constructed to simulate their formation process under rainfall conditions. To enhance the understanding of the mechanisms of karst collapses and to further refine methods for preventing their occurrence, the dynamic characteristics of pore water pressure, earth pressure, and displacement of overburden materials have been monitored and analyzed through model testing. In addition, the coupling effect among pore water pressure, earth pressure, and displacement of overburden materials has been comprehensively analyzed, along with the formation processes, collapse types, and triggering factors associated with these events. The results show that: (1) The pore water pressure, earth pressure, and displacement of overburden layers under extreme rainfall conditions exhibit synchronous deformation patterns and present as a strong correlation with different evolution stages of karst collapses. As the rainfall intensity and frequency increase, the pore pressure and earth pressure of overburden materials also rise continuously, which is related to water retention capacity of overburden layers. Additionally, variation in displacement of overburden materials reflects the formation process of karst collapses. When a collapse occurs, displacement of overburden materials manifests as a rapid variation curve. (2) Under extreme rainfall conditions, karst collapses can be categorized into two types: creep failure and compression-shear fracture. In the case of creep failure, the pore pressure and the earth pressure of overburden materials exhibit similar form peak-cluster fluctuations. The dynamic curves of pore pressure and earth pressure experience homogeneous variations, while both pressures constantly increase. The displacement of deeper layers initially increases and then decreases, whereas the displacement of the upper and middle layers gradually increases with the frequency of rainfall cycles. Furthermore, in the compression-shear fracture type, the pore pressure, earth pressure, and displacement display isolated peaks, which sharply increase over a short period before decreasing. (3) The formation process of creep failure type of karst collapse involves softening, water loading, and corrosion absorption, leading to soil damage, the increase and enlargement of pores and cracks, and ultimately resulting in forming ground collapses. This type of karst collapses is the result of circulation and accumulation effects of softening, water saturation and increased load, and soil damage. Conversely, the formation process of the compression-shear fracture type of karst collapses consists of softening and water loading, followed by vertical shear failure of the roof and subsequent ground collapse. This suggests that karst collapses are mainly induced by water saturation, increased load, and soil damage. The critical early warning criteria of karst collapses in the study area should take into account the different overburden thicknesses: (1) When the overburden thickness is 0.5 m, the critical pore water pressure in the study area is 17.95 kPa to 19.1 kPa, the critical earth pressure is 15.3 kPa to 17.3 kPa, and the critical displacement is 589.95 μm to 928.4 μm. (2) When the overburden thickness is 1.0 m, the critical pore water pressure is 23.55 kPa to 25.55 kPa, the critical earth pressure is 17.75 kPa to 20.95 kPa, and the critical displacement is 770.7 μm to 988.6 μm. (3) When the overburden thickness is 1.5 m, the critical pore water pressure is 29.15 kPa to 30.4 kPa, the critical earth pressure is 20.25 kPa to 26.5 kPa, and the critical displacement is 967.25 μm to 1,087.5 μm. The overburden thickness is positively correlated with the critical criteria of covered karst collapses, which indicates that a thicker overburden layer provides better anti-collapse properties. This paper focuses on the mechanisms and critical criteria of rainfall-induced karst collapses. However, during rainfall infiltration, a relatively impermeable layer forms within the overburden layer, and fluctuations in the groundwater level will compress the air in the cavity of rock and soil materials, resulting in a complex water-soil-air coupling effect. Understanding this coupling effect and the collapse mechanism is of great significance for improving the mechanism theory of karst collapses, which will be further studied in the following experiments. -
图 3 大雨条件下覆盖层孔隙水压力动态变化特征
Figure 3. Dynamic variations of pore water pressures in overburden layers under heavy rain conditions
图 4 暴雨条件下覆盖层孔隙水压力动态变化特征
Figure 4. Dynamic variations of pore water pressures in overburden layers under rainstorm conditions
图 5 大暴雨条件下覆盖层孔隙水压力动态变化特征
Figure 5. Dynamic variations of pore water pressures in overburden layers under heavy rainstorm conditions
图 6 大雨条件下覆盖层土压力动态变化特征
Figure 6. Dynamic variations of earth pressure in overburden layers under heavy rain conditions
图 7 暴雨条件下覆盖层土压力动态变化特征
Figure 7. Dynamic variations of earth pressure pressure in overburden layers under rainstorm conditions
图 8 大暴雨条件下覆盖层土压力动态变化特征
Figure 8. Dynamic variations of earth pressure in overburden layers under heavy rainstorm conditions
图 9 大雨条件下覆盖层土体位移动态变化特征
Figure 9. Dynamic variations of displacement in overburden layers under heavy rain conditions
图 10 暴雨条件下覆盖层土体位移动态变化特征
Figure 10. Dynamic variations of displacement in overburden layers under rainstorm conditions
图 11 大暴雨条件下覆盖层土体位移动态变化特征
Figure 11. Dynamic variations of displacement in overburden layers under heavy rainstorm conditions
表 1 模型试验参数与相似比一览表
Table 1. List of model testing parameters and similarity ratios
试验参数 相似比 试验参数 相似比 几何尺寸l λl =1∶5 重力加速度g λg =1 降雨强度Q λQ =1∶5 内摩擦角φ λφ=1 黏聚力c λl =1∶5 密度ρ λρ=1 应力σ λσ=1∶5 应变ε λε=1 
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表 2 岩溶塌陷监测预警临界判据条件
Table 2. Critical criteria of monitoring and early warning for karst collapses
降雨
条件厚度
/m临界孔隙水
压力/kPa临界土压
力/kPa临界位移
量/μm降雨
次数大雨 0.5 19.1 15.3 928.4 3次 1 25.25 19.8 908.6 1.5 29.15 26.5 1046.75 暴雨 0.5 18.35 17.3 780.5 2次 1 25.55 17.75 879.85 1.5 29.45 20.25 1087.5 大暴雨 0.5 17.95 16 589.95 1次 1 23.55 20.95 770.7 1.5 30.4 26.05 967.25
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