Formation conditions of the karst collapse and treatment technology at raker’s substrate in a karst mine

The raker’s substrate, situated in a karst mine in southern China, is composed of silty clay, pebbles and limestone. In June 2009 a 3m long and 2m wide collapse took place, causing the raker being buried in a cave. The collapse seriously threatens the safety of the surrounding tailing depot and cement tank. Analysis and simulation show that the collapse of the substrate is mainly led by strongly developed karst fractures, groundwater table fluctuation and long-term water scour that resulting bedrock surface soil particles being brought out and then causing the cave to collapse. The former treatment for collapse is backfill generally, but re-collapse is frequent. In view of the case, the mud hole rotary jet grouting approach in the treatment is adopted this time. That is to say, accelerator of different ratio is added to the cement paste according to the depth firstly, and then, putting the accelerator-added cement paste rapidly into the hole and cutting the surrounding soil layer by high pressure and high speed revolving, which leading the pure cement paste turning into cement clay slurry. Under high pressure, the slurry can split off the soil layer, thus, the cave entrance can by blocked off rapidly and the soil layer be consolidated. Test results show that this method can not only seal the passage of cave and karst fractures and reinforce the soil very well, but also has mature technology and lower cost than grouting. It is suitable for application in karst mine. 