2022, 41(6): 880-894.
doi: 10.11932/karst20220603
Abstract:
The study area is located in the eastern part of Michong village, north Huangqi community, Dali town, Nanhai district, Foshan City. Situated in the southern margin of Guanghua basin and the transitional zone between hilly region and plain region in the northern Pearl River Delta, the area is flat in terrain and convenient by transportation. The study area is characterized by wet spring, long hot summer, warm autumn and winter without frost, with south subtropical monsoon climate. The annual average temperature in this area is 21.9 °C with an annual average precipitation of 1,400-1,900 mm and a multi-year average of 1,613.5 mm. Rainfall is concentrated from April to September, accounting for about 80% of the whole year, and the multi-year average evaporation is 1,153 mm, which is generally less than the rainfall. In recent years, intensive human engineering activities such as the construction of high-speed rail, underground engineering of subway and pile foundation construction in the area have induced a large number of karst ground collapses and ground subsidence. Especially from February 2008 to September 2009, the affected area reached 1.2 km2, and 15 karst ground collapses occurred successively in the area, with the maximum land subsidence amounting to 87 cm. Damage, in different degrees, to Huangqi No.2 Middle School, DHL logistics Center, some factories and residential buildings was caused by the collapse and subsidence, in which more than 2,000 people were directly affected, and the direct economic loss was nearly 200 million yuan. The only exposed stratum in the study area is Holocene Guizhou formation (Qhg), but the hidden strata include lower Carboniferous ShiDengzi formation (C1s) and Ceshui formation (C1c), upper Carboniferous Hutian formation (C2h), lower Jurassic Jinji formation (J1j), upper Cretaceous Dayangshan formation (K2dl), and Quaternary residual layers (Qel). There is one fault zone with 5 groups in the study area, belonging to Shabei fault zone extending from the south-west direction of the fault zone. The fault zone provides a good channel for groundwater migration and favorable conditions for the development of karst caves, and then controls the development trend of karst in the area. The rock and soil mass in the study area is divided into 15 engineering geological layers from top to bottom, and the groundwater can be divided into four types, pore water of loose rock, fissure cave water of overlying carbonate rock, fissure water of overlying bedrock and fissure water of overlying red bed. Quaternary soft soil and sandy soil are relatively developed. Hidden karsts are developed in the study area. Of a total of 266 boreholes collected in the area, soil caves in 16 boreholes were exposed with a cave-spotting rate of about 6.35 %, and karst caves were exposed in 44 boreholes with a cave-spotting rate of 22%. In the study area, there have been 15 ground collapses accompanied by geological disasters such as ground subsidence over a large area. Though the collapse and subsidence did not cause casualties, they brought about damage to Huangqi No.2 Middle School, DHL Logistics Center, some factories and residential buildings. Huangqi No.2 Middle School was relocated on December 3,2008, and 1,300 teachers and students were evacuated. The disaster was then estimated as an oversize one, posing a huge impact on the economy and society of the study area at that time. The development characteristics and causative mechanisms of karst ground collapse in the study area, show that the development of karst caves and soil caves is the prerequisite; the development of water-conducting fractures is the necessary condition; and the construction precipitation is the main trigger factor. The formation mechanism is closely related to the fluctuation of groundwater level, whose types can be divided into "continuous groundwater decline decline", "continuous groundwater decline + rainfall infiltration", "recovery of rapid groundwater rise" and "rainfall infiltration". There is still possibility of karst collapse near F1 fault zone in the study area. Controlling the groundwater level elevation within −3.0 meters and the fluctuation range of groundwater level within 1.5 meters can greatly reduce the probability of karst collapse, which can be used as the warning threshold. The research results can provide a basis for prevention and control of karst ground collapse in urbanization of Foshan City, and also provide a reference for disaster prevention and reduction in other similar areas.
