Roof type division
According to the position, caving and strength of roof and floor strata relative to coal seam, the roof is divided into three parts from top to bottom: basic roof (main roof), direct roof and false roof. The bottom plate is divided into three parts: false bottom, straight bottom and old bottom. However, for a specific coal seam, these six components of its roof and floor may not be fully developed. A rock formation that may lack one or more components.
1. 1 coal seam roof
1. 1. 1 false roof: it is a thin rock layer close to the coal seam, and it is easy to collapse with the mining of the coal seam. The thickness is generally 0.3~0.5m, which is mainly composed of shale and carbonaceous shale.
1. 1.2 direct roof: it is the rock stratum directly above the false roof or coal seam (if there is no false roof), which often collapses with the withdrawal of the support, and the thickness is generally 1 ~ 2m, and it is mostly composed of mudstone, sandstone, siltstone and other easily collapsed rocks.
1. 1.3 basic roof: also known as the main roof, it is a thick hard rock layer located directly on the roof or directly on the coal seam (there is no direct roof and false roof at this time). Often exposed above the goaf for a period of time, it can only collapse once when it reaches a considerable area, and it is usually composed of hard rocks such as sandstone, conglomerate and limestone.
1.2 coal seam floor
1.2. 1 false bottom: the thin soft rock layer directly below the coal seam, mostly carbonaceous shale or mudstone, with a thickness of 0.2 ~ 0.3m ..
1.2.2 direct floor: the rock stratum with low hardness directly below the coal seam, with a thickness of about several tens of centimeters to 1 m, usually consisting of mudstone, shale or clay rock. If the direct floor is clayey rock, it is easy to swell when it meets water, which may cause floor heave and support to be inserted into the bottom, affect roadway transportation and working face support, and seriously cause roadway damage.
1.2.3 Old floor: refers to the hard rock stratum under the direct floor, mostly sand and limestone.
2 coal mining face roof classification
According to the difficulty of roof caving in working face, the roof is divided into five categories.
2. 1 Soft roof is prone to roof caving. The characteristic of this kind of roof is that the roof of coal seam is a soft rock stratum which is easy to collapse, and it can collapse immediately after prop-pulling, and can fill the goaf. Due to the complete collapse of the roof, the old roof strata located in the broken zone are easy to reach a balance in the mining process, so the cracking, bending and sinking of the old roof have little influence on the working face, the weighting of the working face is relatively moderate, there is no obvious periodic weighting, and the roof subsidence on one side of the goaf is relatively stable and easy to manage.
2.2 medium caving roof. This kind of roof is characterized by direct roof, its thickness is generally less than 6 ~ 8 times of the thickness of coal seam, and its upper part is a hard old roof. Although the roof directly collapses after prop-pulling, the thickness is not enough to fill the goaf, and the main roof is in a suspended state. When the working face advances to a certain distance, the main roof begins to collapse. At this time, due to the large gap in goaf, the working face presents periodic weighting state, which can cause the roof of stope to collapse in serious cases. For this kind of roof, we should pay attention to the activity law of the old roof.
2.3 Hard roof with difficult caving. The feature of this kind of roof is that the main roof is directly above the coal seam, or there is a false roof and no direct roof. Because the goaf gap is too large when the main roof collapses, the working face presents obvious periodic weighting, and the subsidence amount and speed of the working face are small at ordinary times, but the subsidence speed increases sharply when the periodic weighting comes, and the situation of the working face roof deteriorates rapidly. Therefore, measures should be taken in time.
2.4 Hard roof that is extremely difficult to fall down. The characteristic of this roof is that the coal seam slab is an extremely hard whole thick rock stratum, which can expose tens of thousands of square meters in the goaf without caving, and when it collapses, it can form a storm, causing the working face to collapse and serious damage.
2.5 Plastic curved roof. This kind of roof is characterized by a direct roof. Although it is a hard rock stratum with a certain thickness (such as sandstone and limestone), due to the existence of joints and cracks parallel to the working face, the roof sinks after coal mining and breaks into pieces during the movement, but they are still squeezed together and have the ability to transfer levels. Just like a "masonry beam", as the working face advances, the rocks balance with each other and contact the floor with the bending and sinking of the roof.
Classification and prevention of roof accidents in coal mining face
When mining coal in underground coal seam, the rock above the coal seam is called roof, and the roof is divided into false roof, direct roof and basic roof. The direct roof is the rock layer directly above the coal seam, which has certain strength and will fall with the caving of prop-pulling, such as shale and sand shale. The rock stratum with relatively high strength and thickness above 1.5m is called the basic roof, and the common ones are sandstone, gravel and limestone. Sometimes, there is a layer of soft rock with a thickness less than 0.5m between the coal seam and the direct roof (or basic roof), which is called false roof. Common false roofs include carbonaceous shale and argillaceous shale. Roof management is mainly to manage direct roof and basic roof.
When people mine coal in underground coal seam, if the roof is not properly managed, it will fall off, which will affect production and cause casualties. This is a roof accident. Roof accident is one of the major disasters that can't be ignored in coal mines. It is different from gas and coal dust explosion. Serious accidents with group deaths and group injuries are usually rare, but accidental accidents often occur. This kind of accident may happen in any place where people work, and its distribution is particularly wide. It is precisely because the number of deaths at one time is difficult to constitute the evil consequences of mass deaths and injuries, people often pay insufficient attention to it, neglect management and ignore its accumulated evil consequences, which especially arouses the vigilance of the majority of coal mine management cadres and workers.
To realize safe mining in coal mines, according to different geological conditions and production technical conditions, we should not only have targeted safety measures and systems, but also have a strong sense of responsibility and scientific attitude. Scientific and strict management is the key to safe production, and careless and lucky behavior, that is, human factors, are the important reasons for frequent accidents. It can be seen from a large number of accident statistics that the proportion of roof accidents caused by poor management is quite large. According to 1950-2003 coal mine accident casualty statistics, gas and coal dust explosion accidents accounted for 29.5% of the total number of accidents, and casualties accounted for 52.3 1% of the total number of casualties, ranking first among all kinds of accidents; Followed by roof accidents, the number of accidents accounted for 23.5%, and the number of casualties accounted for 8.81%of the total casualties; Maritime accidents account for 12.72% of the total number of accidents, and casualties account for 1 1.28% of the total number of casualties. It can be seen that the prevention and control of roof accidents, like the prevention and control of gas and coal dust explosion and flood accidents, is the key point to reduce the death rate of one million tons in coal mines.
It should be pointed out that if the statistics of roof accidents with 1-2 deaths are included, the roof ranks first in all kinds of accidents, no matter the number of accidents or the number of deaths. According to the statistics of coal mine accidents in 200 1-2003, roof accidents accounted for 49.86% of the total number of coal mines, and deaths accounted for 35.67% of the total number of deaths. It can be seen that roof accidents in coal mines have a great impact on coal mine safety. Strengthening roof management and reducing or eliminating roof accidents is still one of the key tasks to reduce the death rate of one million tons and reverse the situation of safe production in coal mines.
Coal mine roof accidents can be divided into two types according to the location: coal mining face (stope) roof accidents and roadway roof accidents. During the period of 1954- 1985, the number of deaths caused by roof accidents accounted for 45% of the total number of accidents, including 75% in coal mining face and 25% in roadway. During the period of 1986- 1992, the number of deaths from roof accidents accounted for 40% of the total number of deaths, which was lower than that during the period of 1954- 1985, but the proportion changed greatly according to the accident location. Among them, the ratio of roof accidents in coal mining face to the total number of roof accidents decreased, but it still reached 66%, and the roof accidents in roadway rose to 34%. The decline in the proportion of roof accidents in coal mining face is due to the continuous improvement of coal mining technology and operating procedures, the gradual improvement of safety awareness and management level of cadres and workers in coal mining enterprises, and the improvement of supporting equipment and support forms, especially the gradual popularization and application of comprehensive mechanized mining. However, in order to meet the production system of large mines, the roadway section is required to be larger and the service life is longer. Moreover, with the increase of mining depth and the change of lithology, the mine pressure becomes more and more obvious. The rigidity, flexibility, structure and quality of roadway support methods and support materials can not meet the requirements of appeal factors, and the proportion of roadway roof accidents has increased.
There are three basic types of roof accidents in coal mining face: caving, roof leakage and collapse. After the coal mining in the working face, the roof strata with different physical properties will produce more or less roof force perpendicular to the supporting direction of the working face. If the support can't support this roof force, it will collapse and roof fall. According to the strength, layered thickness and cracks in the strata, the roof directly above the coal seam can be divided into three categories: soft, moderately stable and hard. For the direct roof below moderate stability, especially the soft roof, if the support can not protect the broken roof after coal mining, roof leakage will occur. In addition, the direct roof strata may also give the support a thrust along the plane direction, and the stability of the support is not good. If the support is unstable under the action of thrust, push-down and roof fall will occur.
Roof accidents in coal mining face can be divided into local roof caving and large roof caving according to the scope and severity of roof caving. Local roof caving refers to the roof caving with small scope and few casualties (every death 1-2 people). It often occurs near the coal wall, at both ends of the coal mining face and near the caving line. Large-scale roof caving refers to the roof caving with a large range of roof caving and a large number of casualties (three or more people died each time). Including caving from both ends, large-scale caving of thick layer, caving caused by direct caving, caving of composite roof, caving of metal mesh, caving of large free roof, caving of gangue rushing into coal mining face in goaf, etc.
According to the statistical data of coal mine roof accidents, the proportion of accidents caused by human factors is about 9 1.57%, which shows the importance of strengthening roof management. Especially in small and medium-sized coal mines, roof accidents often occur because of small well type, unknown exploration data, complex geological conditions, poor planning, design and construction measures, weak technical force, low mechanization, insufficient safety investment, and some even irregular management. In order to reduce the death rate of one million tons in coal mines, it is necessary to raise awareness and strengthen roof management, which is the most basic requirement for safe production. Coal mining needs a working face, coal transportation needs to pass through the roadway, and intact roof support should be the premise of smooth system and safe production. Therefore, in roof management, attention should be paid to the form and material of the support, the rigidity, strength, flexibility and density of the support, mining equipment and technology, mining height, roof control distance, coal pillar size, methods and means of pressure relief, rationality and scientificity of mining engineering design, and rigor of preparation and approval of operation procedures.