Karst collapse and land subsidence

Ground karst collapse in coal mines is very common, and the main collapse forms are loose layer collapse, surface cracking and collapsed holes. In karst areas covered by loose sediments, when mines drain a lot of water and cities exploit a lot of karst groundwater, the groundwater level in karst areas will inevitably drop greatly, and the buoyancy of cave fillings and overburden below the original groundwater level and above the current groundwater level will decrease, which is equivalent to increasing the additional load on the soil. At the same time, it will induce groundwater in overlying loose pore aquifer that is in hydraulic contact with it to flow downwards, increase the hydraulic gradient and water flow velocity, and the groundwater flow will produce osmotic pressure on the skeleton and cave fillings in the medium. The excavation starts with the filling of the karst cave, and then a soil hole is formed at the mouth of the karst cave at the bottom of the overburden, and then it gradually develops upwards. Due to the expansion of soil hole, the roof soil is unstable, which leads to surface cracking, subsidence and collapse.

Karst collapse is caused by the discharge of groundwater in karst water-filled deposits. Collapse occurs not only in coal mines, but also in metals and other mines. Geographically, it is almost all over southern provinces, especially Hunan, Guangdong, Hubei, Guangxi, Jiangxi and other provinces. More than 600 collapses 1600 were found in Fankou lead-zinc mine, Guangdong Province, covering an area of 5 km2. Hunan Enkou Coal Mine 1972 began to collapse. Up to now, there are more than 6,000 subsidence pits, with the subsidence area of 25km2, 9,500mu of farmland washed away, 8 small reservoirs relocated and houses18,300m2. According to the statistics of 1982, the cost of agricultural compensation and comprehensive treatment of collapse in this mine is as high as 9.8 million yuan. From 1967 to 1983, there were 2200 collapse pits in Doulishan Coal Mine, Hunan Province. The collapse not only destroyed farmland and buildings, but also caused heavy losses and four deaths.

Karst collapse in northern China is mainly distributed in the eastern coastal provinces and east of Taihang Mountain. Modern karst collapse occurred in Kailuan, Tangshan as early as 1920s, but the number was very small. The larger one occurred in Laiwu Iron Mine in Shandong Province in 1970s. During the pumping test from 1973 to 1974 in Gujiatai mining area, karst collapse occurred, including 3 collapse pits in Yezhuang mine and 1974 in Zhaozhuang mine. In the drainage test of Yezhuang Mine 1975, there are 16 collapse pits in the skylight of Guoniangquan, 2000m away from Ye Zhuang Mining Area, and 1 1 collapse pits in the skylight of Mengjiazhuang, 2000 m away from it.

Mine water inrush can also cause karst collapse, and mine water inrush has occurred many times in Kailuan mining area, which is caused by the decline of karst water level. As early as1965438+June 7, 2004, the water inrush from the northwest well of Tangshan Mine caused the river bed of Douhe River in Zhuang Lei to collapse, and the water inrush from 1920 mine repeatedly collapsed. 1954 65438+On February 5th, Linxi mine collapsed twice due to water inrush. On June 2nd, a rare water inrush occurred in 1984 Fangezhuang Mine, and a 17 ground collapse pit appeared in Pengjiatatuo-Linxi Machinery Factory, a hidden outcrop area of Ordovician limestone, within 3 ~ 7 kilometers north of the water inrush point. See table 2- 1 for karst collapse in the main mining areas in the north.

Table 2- 1 Karst Collapse List of Main Mining Areas in Northern China

The continuous drainage of the mine leads to the decline of the regional groundwater level, and the groundwater in the overlying Quaternary loose layer permeates downward in the form of "skylight" or "overflow". The groundwater level in the unconsolidated layer decreases, the pore water pressure decreases, the effective stress increases, the particles are displaced and arranged more closely, the contact area of particles increases, the porosity decreases, and the unconsolidated layer is compressed, thus causing land subsidence. For example, in Gushan open pit of Gushan Iron Mine, the Quaternary aquifer is drained through drainage roadway. In the process of drainage, the ground around the stope sinks, and the center of the stope sinks by 30 ~ 60 cm, forming a ground subsidence funnel centered on the mine pit, with an influence range of 2km.