Mechanisms and critical criteria of coverd karst collapses under extreme rainfall conditions

PAN Zongyuan; SHU Rui; DAI Jianling; MENG Yan; LEI Mingtang; JIA Long; MA Xiao; BAI Bing

doi:10.11932/karst20250308

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 pm to 928.4 pm.(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 pm to 988.6 pm.(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 pm to 1,087.5 pm.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.