Abstract
Urban cave-ins may result from a sudden change in local hydrological, hydrogeological and anthropogenic conditions. Monitoring and predicting urban sinkholes is not straightforward, and solving the problem of urban cave-ins involves the incorporation of the fields of geodetics, geophysics and geochemistry. This paper examines the causes and consequences of sinkholes in sand-rich materials by using a small tank model apparatus. Tank model tests were conducted to simulate sinkhole formation, to gain a better understanding of the influencing factors of cavity formation, expansion and upward migration with different cavity water levels. Two commercial materials were used: Jumunjin sand and kaolinite clay. Materials with different grain-size distributions, i.e., sand (100% Jumunjin sand) and sand-clay mixtures (95% sand and 5% kaolinite clay), were considered to examine the roles of clay content in sinkhole risk. The test results show that the sand-rich materials exhibited a typical punching type of sinkhole subsidence and failure, and were very sensitive to changes in groundwater level, regardless of the grain-size distribution. The higher the groundwater level is, the higher the sinkhole risk is in terms of the speed of migration of the underground cavity fluids, cavity size and ground loosening. Therefore, sinkholes in urban areas covered with sand-rich materials are vulnerable to groundwater withdrawal. However, compared with pure sand, materials with small clay contents can reduce the time until collapse and the size of a sinkhole. Variations in pore water pressure may be used as a sinkhole indicator in areas where the ground deformation caused by groundwater depletion is considerable.
Highlights
Urban sinkholes pose considerable threats to people and property because these cavities might develop underground without notice [1,2]
A series of small-scale laboratory tank model experiments was performed to examine the influence of clay content on sinkhole characteristics of sand-rich materials
It is well known that cavity development and surface collapse are dependent on soil density and variations in groundwater levels [20]; the variation in groundwater level is one of the main triggering factors in sinkhole formation
Summary
Urban sinkholes pose considerable threats to people and property because these cavities might develop underground without notice [1,2]. An aged and damaged sewer pipe may cause local subsidence and sudden surface collapse in urban areas, especially along roads [2]. These events may be related to the age of the infrastructure; insufficient proactive management may induce cave-ins in urban areas. Sudden surface collapse in urban areas can occur around construction work due to unreliable groundwater management around soil foundations or civil infrastructures. Sinkhole classification and case histories in karst and cavernous rocks in engineering practice have been examined by numerous studies [3,4,5,6]. Sinkholes can form in volcanic sandy deposits [7]
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