Mine Water and
the Environment (2005) 24: 209-212 © IMWA Springer-Verlag 2005
Technical Communication
Chunlai Yang1
and Zongmin Wang2
1Beijing Research
Inst of Mine Construction. Qingniangou Rd E. Hepingli 10013 Beijing,
China; 2Zhengzhou Coal Corp Engineering Co, 2 Hanghai Rd,
Zhengzhou 450006, Henan, China;
Abstract. Special techniques are used for shaft sinking in aquifer formations. In the unstable aquifer formations encountered in Chinese coal mines, freezing is always adopted for shaft constructions in an aquifer alluvium, while surface pre-grouting is generally adopted in bedrock formations. So far, freezing has been used to construct 500 vertical shafts, comprising a total length of 80 km, and surface pre-grouting has been used in more than 150 vertical shaft sinking projects. The maximum depth of freezing is about 700 m, while the maximum depth of surface pre-grouting exceeds 1000 m.
Key words: Coal mining; shaft freezing; shaft sinking; surface pre-grouting
Shaft Freezing
Freezing in shaft sinking was invented by F.H. Poetsch in 1883. Its essential feature is the solidification, by freezing, of water-bearing ground in which the shaft is sunk. The method was introduced to China from Poland in 1955 and first applied in the Linxi Mine ventilation shaft of the Kailuan Coal Miming Bureau. In the 50 years since then, the method has rapidly developed due to professional and academic research and enhanced equipment technology and construction methods. Today, freezing is one of the most important methods of underground construction in the complex and unstable water-bearing alluvium of Chinese coal mines due to its high reliability. Freezing has also recently become popular in urban construction projects.
To prepare a site for freezing, a series of equally spaced boreholes are drilled on a concentric circle enclosing the site of the shaft. Heat is removed from the ground via probes placed into the boreholes. The probe itself consists of an external pipe, 50-150 mm in diameter, closed at the lower end and containing an open-ended inner tube of slightly shorter length. The inner tube can vary between 20-75 mm in diameter. The freezing tubes are connected with two circulation mains, in such a manner that cold brine may be pumped down the inner tube and allowed to return along the annular space between the two tubes and then through the collection main back to the refrigeration plant. Here it is pumped through a chiller (normally a shell and tube heat-exchanger), where it is cooled down again, and then delivered via a distribution main back to the inner tubes of the probes. The coolant is therefore confined in a closed, recirсulatory flow path. An industrial refrigeration plant is required to cool the brine. Extracted heat is dissipated into a nearby watercourse if available, or into the atmosphere by forced-draught cooling towers or evaporative condensers.
The effect of circulating a coolant through the complete system is to produce long cylinders of frozen strata around each column, which gradually increase in diameter until they merge, forming a circular barrier of frozen ground, known as the ice wall. During excavation, refrigeration is maintained, generally in excess of the level required to offset heat transfer from the warmer strata surrounding the shaft.
Freezing has been used to construct 500 vertical shafts in the northeastern, northern, and eastern part of China, with a total length of 80 km. The auxiliary shaft in Dingji Mine of the Huainan Coal Mining Bureau in Anhui Province has a freezing depth of 570 m, and an alluvium thickness of 524 m. The auxiliary shaft has a designed depth of 880.5 m, a net diameter of 8.0 m, and a throughput of 6.0 Mt/a. The Guotun Mine, which is being constructed in the Juyc mining area in Shandong province, is expected to have the deepest freezing depth (702 m), with an alluvium thickness of 570 m.