Research background and significance

China is rich in mineral resources and has a long mining history. After the exploitation of underground mineral resources, a large number of mined-out areas have been left. Especially since the 1980s, China's mining order has been chaotic, and illegal and unplanned indiscriminate mining and excavation have been serious, leaving a large number of unknown mined-out areas around some state-owned mines, which has led to the deterioration of mining conditions and seriously affected mine production and safety. According to the mining time of minerals, goaf can be divided into old goaf, present goaf and future goaf. After the ore body is mined, it collapses, breaks, stratifies, bends and deforms from bottom to top, forming a sinking basin in a certain range of the surface. According to the different damage degree, the overlying strata in goaf can be roughly divided into "three zones" from bottom to top: caving zone, water-conducting fractured zone and bending zone. There are two kinds of subsidence and horizontal movement of the surface above the goaf, and three kinds of deformation are inclined deformation, bending deformation and horizontal deformation. The surface movement of underground mining can be divided into three stages: initial stage, active stage and declining stage [1]. The movement of the old goaf should mainly consider the surface movement after the retreat, which mainly includes two parts: ① the part where the influence of the goaf on the surface has not yet ended; (2) It is a process that the fractured rock mass and overlying strata are gradually compacted and deformed under the gravity of overlying strata. This deformation is influenced by the geological environment conditions of overlying strata, such as the softening and weathering of groundwater and the influence of external load, which reduces the strength of coal and rock mass, leads to structural instability and causes continuous or discontinuous deformation of the surface. This time process is often long, which brings a series of environmental geological disasters, such as creep and deformation of mountain slopes. Even a large area of mined-out area collapses. The subsidence area will lead to the decline and depletion of groundwater level, the destruction of cultivated land, the deterioration of ecological environment, the deformation and destruction of roads and the collapse of houses. , leaving great hidden dangers for the construction of railways, highways and other projects in or through the mining area.

Coal is China's basic energy and important raw materials, accounting for about 70% of disposable energy production and consumption, and has an important strategic position in the development of national economy. "Coal-based and diversified development" is the basic strategy to solve the energy problem in China, but the primary energy structure with coal as the main body will not change for a long time to come. China's coal consumption increased from 18.43× 108t in 2005 to 37.27× 108t in 2006, with an average annual increase of 12.45%. The coal output increased from 21.13×108t in 2005 to 35.20× 108t in 2006, with a compound annual growth rate of 8.88%. It is predicted that the total coal demand in China will continue to increase during the Twelfth Five-Year Plan period. After coal mining, the overlying strata will move and deform, and the strength of rock mass will also decrease. Under the action of building load or train dynamic load, even if there is no building load or train dynamic load, due to other geological reasons, the old goaf may be "activated" again, and the caving rock mass will be compacted again, resulting in new movement and deformation of the surface, and even collapse, forming a collapse pit. The existence of mined-out area will pose a potential safety threat to engineering construction and surface buildings, and it has the following characteristics: it is difficult to understand the characteristics of mined-out area, its "activation" process is influenced by many natural and human factors, and it is difficult to directly observe it. The activation and instability of mined-out area are sudden, and it is difficult to accurately predict the collapse time.

With the large-scale development and utilization of coal resources, it not only brings great economic and social benefits to the society, but also brings serious harm and destruction to the ecological environment around the mine. Sudden geological disasters caused by the sudden collapse of mined-out areas have brought great economic losses and threats to people's property and life safety. On July 28th, 20 1 1, affected by continuous heavy rainfall, the dormitory in Hongshiyazui mining area of Jin Xin Iron Ore Concentrator in Qianxi County, Hebei Province collapsed, resulting in 6 deaths. At 3 o'clock in the morning of August 29, 2007, an earthquake occurred in the goaf of Bianbula Coal Mine in Sunjiacha Town, Shenmu County, Shaanxi Province, and nearby villagers were awakened in their sleep. On February 26th, 2005 13: 05, Li Du Iron Mine of Sanxin Industry and Trade Company in Linzhou City, Li Du Township, Anyang County, Henan Province suddenly collapsed in a large area, forming a large pit about1000 m long, 50m wide and 6 m deep on average ... After investigation, the underground cavity in the surface subsidence area was left over from the mining of 1992 iron mine, which collapsed and collapsed. In this sudden geological disaster, 8 people fell and 3 people were missing. In Jincheng mining area, the railway has been harmed and influenced by mined-out areas since its operation. Due to the instability of the goaf itself and the influence of the dynamic load of locomotives and vehicles, the line is often in an unstable state, which brings great hidden dangers to the driving safety belt. On the morning of April 1999, the inspector of Chengzhuang Line found that the sections DK2+600 and DK2+800 suddenly sank by about 65,438. There are many cracks in the fields on both sides of the line, the widest width is about 10cm, and the geometric dimensions of the track are seriously deformed and the deviation is large. As it is found, it is handled promptly and quickly to prevent the train from derailing and subversion (Figure 1. 1). A large number of lead-zinc mines are distributed on both sides of K309-K3 12 section of chengdu-kunming railway, which is located in Ganluo County, Sichuan Province, forming many mined-out areas, endangering railway safety requirements. After the local government took effective preventive measures in advance, it ensured the operation safety of chengdu-kunming railway. With the continuous exploitation of mineral resources, there are more and more reports that the deformation and destruction of mined-out areas bring hidden dangers to the safe operation of railways.

Figure 1. 1 Collapse of mined-out area near railway

The goaf along the railway is more likely to cause surface deformation and goaf collapse under the action of train dynamic load, which affects the stability of railway subgrade and is a serious problem that must be solved in railway operation. First of all, it threatens the safety of railway transportation. Due to the instability of the mined-out area itself and the influence of the dynamic load of locomotives and vehicles, the subsidence of the mined-out area continues to occur, and the line is in an unstable state, which brings great hidden dangers to driving safety. Secondly, it affects the efficiency of railway transportation. The collapse of mined-out area causes the instability of the line, which makes the traffic unable to travel according to the specified time and speed requirements, and the transportation production efficiency is seriously affected. But also increases the difficulty and workload of railway line maintenance. The line in goaf is extremely unstable, the occurrence of subsidence is uncertain in time and space, and the change of geometric state of the line becomes complicated, which is difficult to control and master, and needs regular monitoring and maintenance. Therefore, the problem of roof stability in goaf is becoming more and more prominent as a bad geological problem in railway construction. This railway line is an extensible building. If a section breaks down, the whole line will be affected. If the underground goaf is not evaluated and reinforced, it may cause surface subsidence, reduce the stability of subgrade and bring unsafe factors to traffic. In the tensile area of the surface, the line also undergoes tensile deformation; In the compression area of the surface, the line is also compressed and deformed, which will produce tensile stress or compressive stress inside the rail. When the internal stress of the rail is greater than the allowable stress, under the dynamic load of the train, the rail line will be unstable and the train will derail [3, 4].

Goaf is a special geotechnical engineering object, which is actually the extension, extension and development of mining subsidence, and the product of combining mining subsidence with geotechnical mechanics, civil engineering, mine geology, mining engineering and underground engineering control. Therefore, analyzing and studying the influencing factors of roof stability in old goaf is not only the premise of goaf safety management and the basis of formulating governance measures, but also has important theoretical and practical significance to ensure the safe operation of railways within the designed service life.