(1) Roof Fall and Collapse
1. Roof caving characteristics and its influencing factors
After excavation of underground cavern, the characteristics of surrounding rock often change greatly due to unloading rebound and redistribution of stress and moisture. If the surrounding rock can't bear the action of rebound stress or redistribution stress, it will be deformed or destroyed. The form and characteristics of deformation and failure of surrounding rock are not only related to the initial stress state and cave shape in rock mass, but also mainly depend on the lithology and structure of surrounding rock (Table 92).
Roof fall accident is one of the most common mine geological disasters, which threatens the personal safety of coal miners. According to incomplete statistics, 40% of the deaths caused by work-related injuries in various mines in China are caused by mine roof caving every year, and the death frequency ranks first among all kinds of mine geological disasters.
Table 9-2 Deformation and Failure Forms of Surrounding Rock and Their Relationship with Surrounding Rock and Structure
sequential
(According to Zhang Shuyuan et al., 1994)
The mining practice in Shannan Mine of Hunan Tin Mine shows that the roof of mined-out area may collapse when the pillar without supporting capacity reaches about 60% of the whole stope pillar. The caving of a mined-out area will cause a chain reaction between adjacent mined-out areas, which will lead to a sharp increase in ground pressure in the stope, serious damage to the stope and roadway, and cause casualties. The deaths caused by roof caving accidents in metal mines in the United States, Britain and Japan all account for 1/3 ~ 1/2 of the total number of deaths in underground accidents, with Japan accounting for 40.7%, the United States for 30.2% and Britain, Russia, Poland and Belgium for about 30% ~ 50%.
Roof fall and other ground pressure disasters in metallurgical mines in China account for 25% ~ 27% of all casualties; In recent years, among the major fatal accidents in large and medium-sized coal mines, the roof fall disaster accounts for about 30%.
Signs of roof collapse or side wall collapse include: slag falling from the roof is from small to large, from thin to dense, the number and width of cracks increase, the coal quality of the coal wall softens under high pressure, the support is crushed, the gas emission suddenly increases, and the amount of drenching water increases.
2. Goaf treatment method
The treatment methods to prevent caving in goaf can be summarized as "filling", "caving", "supporting" and "sealing" (Sui Pengcheng, 1998).
1) filling method: after mined-out area, the mined-out area should be filled with gravel, tailings sand, water sand, concrete and other substances in time to support the roof and reduce its pressure on overlying rock and soil. For example, Shannan Mine, a tin mine in Hunan Province, fell behind three times, the ground pressure in the new mining area increased sharply, and the surface continued to sink. In order to ensure safety, the goaf was fully filled, and the filling rate reached 90.6%, which eased the ground pressure activity.
2) Caving method: refers to the method of deep-hole blasting to make the surrounding rock of goaf caving and fill the goaf.
3) Support method: Support the mined-out area with pillars or supports to prevent dangerous deformation.
4) Sealing method: it is often used to deal with isolated small mined-out areas which are far away from the main ore body and will not affect the mining of the main ore body and other ore bodies after the collapse of surrounding rock. The main purpose of sealing these small mined-out areas is to prevent the harm of air shock wave to personnel and equipment when surrounding rock suddenly falls.
In order to effectively prevent roof caving and collapse, we must also adopt a reasonable mining scheme, avoid one-sided pursuit of output and abandon poverty, and resolutely put an end to indiscriminate mining of protective coal pillars; Adopt reasonable design scheme to carry out scientific roof management; According to the stress concentration and distribution form of surrounding rock, the in-situ stress is determined by acoustic emission monitoring technology, and the strength and time of roof weighting are predicted, so as to master the law of ground pressure and take effective measures in time. Formulate scientific and reasonable working face operation rules, support rules and goaf treatment rules.
(2) rockburst
Rockburst, also known as rockburst, refers to the dynamic phenomenon that brittle coal, ore body or rock mass suddenly releases energy into free space when its limit equilibrium state is destroyed under strong ground pressure. It is an earthquake induced by mining or tunnel excavation. It occurs in coal mines, metal mines and various artificial tunnels.
When rock burst occurs, rock fragments or coal blocks suddenly pop out of the surrounding rock, and the thrown rocks vary in size, with the largest diameter reaching several meters or even tens of meters, and the smallest being only a few centimeters or even smaller. Large-scale rock burst is usually accompanied by strong air churning noise, and even makes the surrounding rock mass vibrate. Rock burst will destroy mining equipment and supporting facilities in caves, and sometimes cause casualties.
Types and characteristics of rockburst
Due to the different positions and released energy, rockburst shows many different types and its characteristics are also different (Zhang Zhuoyuan et al., 1994).
1) Rockburst caused by rock fracture on the surface of surrounding rock: Most small and medium-sized rockbursts occurring in deep-buried tunnels or other types of underground caverns belong to this type. Rock burst often sounds like machine gun shooting, so it is called rock shooting. It usually happens near the newly excavated working face. 2 ~ 3 hours after excavation blasting, the surface rock of surrounding rock explodes, and at the same time, irregular flaky rocks with thick middle and thin edges are ejected or peeled off from the surrounding rock of tunnel wall. This kind of rockburst mostly occurs in places with flat surface, hard nodules or weak surface, and mostly occurs parallel to the rock wall without obvious warning beforehand.
2) Rock burst caused by the destruction of pillar surrounding rock: In a mine with deep burial depth, the pillar or surrounding rock is often destroyed due to the large stress of surrounding rock, causing rock burst. This kind of rockburst is usually accompanied by violent air billow and loud noise, even with strong vibration of surrounding rock mass, which has great destructive power and often causes serious harm to underground mining, and is called mine earthquake or rockburst. In coal mines, this kind of rock burst mostly occurs in areas with a certain distance from the roadway wall. This kind of rockburst happened many times in Tianchi Coal Mine, Mianzhu, Sichuan, and the biggest one threw about 20 tons of coal 20 meters away.
3) Rockburst caused by fault dislocation: When the excavated cavern or tunnel intersects with the potential active fault at a small angle, the normal stress acting on the fault plane is small, which reduces the friction resistance on the fault plane and often causes the fault to move suddenly and form rockburst. This kind of rock burst generally occurs in deep wells in active tectonic areas, which is destructive and has a wide range of influence.
2. Conditions and mechanism of rockburst.
Rock burst is a serious brittle failure of surrounding rock of cavern that suddenly releases a lot of potential. From the production conditions, the existence of high energy storage body and its stress close to the ultimate strength of rock mass are the internal conditions of rock burst, while the trigger of some factors is the external cause of rock burst (Zhang Zhuoyuan et al., 1994).
The formation of high energy storage body in surrounding rock must meet two conditions: ① rock mass can store large elastic strain energy; ② The stress in rock mass is highly concentrated. Elastic rock mass has the largest energy storage, and it can store very large elastic strain energy when it is deformed by force, while plastic rock mass has no ability to store elastic strain energy.
From the stress condition, the formation of high stress concentration area of surrounding rock needs high original rock stress first. However, in areas where tectonic stress is highly concentrated, rock burst will also occur in shallow tunnels, even in foundation pits or quarries on the surface.
Rock burst on the surface of surrounding rock of cavern often occurs in the following high pressure concentration areas: the maximum compressive stress concentration area formed by excavation of cavern, the distribution area of high variation stress and residual stress on the surface of surrounding rock, the local stress concentration area determined by lithologic conditions, and the stress concentration area formed near weak structural planes such as faults, weak broken rock walls or dikes.
There are three main forms of rockburst that destroy underground caverns: rock mass expansion, rockburst and vibration-induced caving. Rock mass expansion refers to the phenomenon that the volume of rock mass increases due to rock fracture or structural instability. If the expansion is large and the process is intense, it will do harm to the cavern. When the energy of disturbing seismic wave from far away is high, the surrounding rock fragments of the cavern can be directly injected into the cavern at an extremely fast speed (up to 2 ~ 3m/s), which will cause disasters, which is rock burst in the form of rock burst. Vibration-induced rock caving is a phenomenon that occurs under the action of disturbed seismic waves and huge gravitational potential energy when there is loose rock or weak surface at the top of the cavern.
3. Prediction and prevention of rockburst
(1) monitoring and prediction of rockburst
The prediction of rockburst disaster includes the prediction of rockburst intensity, time and place. Because of the complexity of underground engineering excavation and rockburst itself, the prediction of rockburst needs to consider many factors such as geological conditions, excavation and disturbance. Previous rockburst records are important reference materials for predicting future rockbursts.
The prediction of rockburst can be divided into two aspects: ① The mechanical parameters of coal rock or rock block are measured in the laboratory, and the probability and danger degree of rockburst are judged according to the elastic deformation energy index; (2) On-site observation, that is, by observing the sound and vibration, observe and measure the number of cuttings when drilling in the heading face. , to predict the forecast. At present, the commonly used rock burst prediction methods at home and abroad include drilling cuttings method, geophysical method, displacement test method, humidity method, temperature change method and statistical method (Zhang Bin et al., 1999).
1) drilling cuttings method or core cake rate method: for rocks with high strength, if the cake phenomenon appears on the surface after the drilled core is taken out, it indicates that there is high ground stress underground, which can be judged according to the relative size of the number of rock cakes in the core with a certain thickness. Dynamic responses during drilling, such as cracking sound, friction sound and sticking phenomenon in drilling, can also be used to assist in judgment.
2) Seismic wave prediction method: use the information of rockburst (induced earthquake) to predict the rockburst in the future excavation process, and establish the relationship between the number, size and distribution of rockburst and the change of geostress field, so as to predict the space-time position, number and size of large and medium-sized rockburst. In addition, the single-channel seismograph can also be used to monitor the tunnel face and the rock mass in front, such as testing the elastic wave velocity of the rock point by point every 1 m along the horizontal line, and inferring the possibility of rockburst by using the strength concept.
3) Acoustic emission (A-E) method: Acoustic emission A-E method is acoustic emission method. The establishment of this method is based on the experimental results of acoustic emission before rock failure, and it is the most direct monitoring and prediction method for the process of rock burst. Its basic parameters are the energy rate and frequency of major events, which can reflect the fracture degree and stress growth rate in rock mass to some extent. There is usually a period of energy accumulation before rockburst, which is a quiet period of acoustic emission and can be regarded as a precursor of rockburst. This method is expected to directly and quantitatively monitor rockburst in the field, and it is a monitoring and forecasting method with great development prospects.
Rock burst prediction is one of the important tasks of underground engineering geological exploration. Scholars at home and abroad have established some feasible standards on the basis of summarizing the existing practical experience and research results. In Norway, Barton's method is adopted, and the ratio of uniaxial compressive strength (Re) to in-situ stress (σ 1) (α=Re/σ 1) is taken as the criterion of rockburst:
1) When α = 5 ~ 2.5, moderate rockburst occurs;
2) When α < 2.5, serious rockburst occurs.
Barton method is often used for forecasting in some engineering practices in China. For example, Tianshengqiao Power Station in Guizhou, according to Barton method, it is judged that moderate rockburst may occur in tunnel construction, and the actual situation of engineering excavation proves that the prediction is basically successful (Zhang Zhuoyuan et al., 1994).
In addition, because rockburst is an induced earthquake, the magnitude and occurrence probability of rockburst can be predicted by the prediction method of earthquake magnitude and occurrence time.
(2) Prevention and control of rockburst
Although it is difficult to completely solve the problem of rockburst at present, some effective methods have been explored in practice, and different prevention methods can be adopted according to the actual situation of excavation engineering.
1) preventive measures in the design stage:
Selection of hole axis: generally speaking, the direction of hole axis should be parallel to the direction of maximum principal stress, so as to improve the stress state of cavern structure. The stress conditions that make the cavern relatively stable are that the surrounding rock does not produce tensile stress, the compressive stress is evenly distributed, and the tangential compressive stress is the smallest. When choosing the axis direction, we should compare and choose from many aspects to reduce the unfavorable factors brought by high ground stress.
Selection of section shape of cavern: Generally speaking, the section shapes of cavern are round, oval, rectangular and inverted U-shaped. When the ratio of section width to height is equal to the lateral pressure coefficient, various factors can be comprehensively considered to determine the section shape of the cavern.
2) Preventive measures in the construction stage:
Pre-stress relieving method: In high stress area, ultra-high stress concentration is easy to occur after excavation of cavern. In order to effectively eliminate the phenomenon of stress concentration, pre-cutting method, surface blasting induction method and advanced drilling stress relief method can be used to release in-situ stress in advance. Drilling shallow holes in the dangerous area of rock burst, blasting will cause loose areas on the surface of surrounding rock, which can effectively prevent destructive rock burst. When mining coal seams, firstly, the coal seams without rockburst or general rockburst are mined as pressure relief layers. When mining, the comprehensive caving method is used to manage the roof without leaving coal pillars; For the roof that is not easy to roof caving, deep hole blasting or strong high pressure water injection method should be used to force roof caving.
Water spraying or drilling water injection can promote the softening of surrounding rock: in the cavern area prone to rock burst, water spraying can be applied to the newly exposed surrounding rock in the working face immediately after blasting, which can not only reduce dust but also release the surrounding rock stress. Because water injection reduces the energy at the crack tip, the possibility of crack propagation is reduced, and the efficiency of transforming thermal energy around cracks into seismic energy is reduced. Thereby reducing the risk of violent bursting.
Choose an appropriate excavation method: rockburst is the result of high pressure concentration. Therefore, step-by-step excavation can be used to artificially provide a certain deformation space for the surrounding rock mass, so that the high stress inside it can be slowly reduced, thus achieving the purpose of preventing rock burst.
Reduce the time and area of rock mass exposure: in the process of short footage and multi-cycle construction, support should be provided in time to minimize the time and area of rock mass exposure and prevent or reduce rock burst.
Measures to deal with rockburst: Once rockburst happens, stop immediately and completely avoid it. Observe the rockburst in detail and record it truthfully. Check the working face, side wall or vault carefully, and deal with and reinforce the rockburst area in time.
3) Reasonable selection of surrounding rock support and reinforcement measures: the surrounding rock around the excavation chamber or in front of the tunnel face will change from one-way stress state to three-way stress state, and the surrounding rock reinforcement measures can also prevent the rock mass from ejection and collapse. The main support and reinforcement measures are: ① shotcrete or steel fiber shotcrete reinforcement; (2) Reinforcement with steel mesh and shotcrete; (3) surrounding anchor reinforcement; ④ Grille steel frame reinforcement; ⑤ Advance support can be adopted when necessary.
(3) Coal and gas outburst
In the process of underground coal mining, a large number of coal (rock) powder and gas (CH4, CO2) are suddenly ejected from the coal (rock) wall to the mining face, which is called coal and gas outburst. The phenomenon that a large amount of gas under pressure is ejected from the cracks of coal or surrounding rock at high speed is called gas ejection. Outburst and blowout are both phenomena accompanied by sound and severe stress release effect under the comprehensive action of geostress and gas pressure. Coal and gas outburst will destroy roadway facilities and ventilation system, make roadway full of gas and pulverized coal, cause underground miners to suffocate or be buried, and even cause underground fire or gas explosion. Therefore, coal and gas outburst is a serious mine geological disaster in coal industry.
1. Characteristics and influencing factors of coal and gas outburst
Coal and gas outburst is the result of the combined action of geostress and gas volume expansion force, which is usually dominated by geostress and supplemented by gas expansion force. The basic feature of coal and gas outburst is that solid coal (powder) moves quickly and long distance under the action of gas flow, and coal, fragments and dust are sorted and piled up, and the smaller particles are thrown farther away. In the process of outburst, a large amount of gas (CH4 or CO2) is ejected. Because the gas pressure is much greater than the ventilation pressure in the roadway, the ejected gas is usually against the wind. Coal and gas outburst has obvious dynamic effect, which can carry boulders, overthrow harvesters, destroy equipment and destroy roadway support facilities.
Outburst coal seams are characterized by rapid gas diffusion, low humidity, low mechanical strength of coal, great change and poor permeability, and most of them belong to "structural coal" seriously damaged by tectonic action. The number and intensity of outburst increase with the increase of coal seam thickness, and the most serious outburst coal seam is generally the thickest main coal seam. Outburst time mostly occurs in the process of blasting and coal falling.
Coal and gas outburst disasters increase with the increase of mining depth, and its main influencing factors are geological structure conditions, in-situ stress distribution, coal quality hardness, coal seam occurrence, thickness and buried depth. Generally speaking, the depth of coal seam is large, the outburst times are many, and the strength is also great.
In addition, hydraulic punching and vibration blasting can make high-pressure gas coal outburst under the action of artificial controlled geostress.
2. Preventive measures for coal and gas outburst
There are four main technical measures to prevent coal and gas outburst:
1) First, mine the coal seam with no or less outburst danger. Due to the influence of mining, the ground stress is slowly released with elastic potential, the coal seam expands and deforms due to pressure relief, the permeability increases, or cracks and pores are formed due to the movement of interlayer rocks, and the gas pressure and gas content in the coal seam with outburst danger are obviously reduced, thus avoiding or reducing the danger of coal and gas outburst.
2) In the coal seam with outburst danger, drill holes are evenly arranged, and gas is pumped out at a certain time in advance, so as to reduce gas pressure and gas content, reduce in-situ stress and increase coal seam strength.
3) A certain number of large-diameter boreholes are drilled in the coal body at a certain distance in front of the working face in advance, so that the gas in the coal seam can be released in advance.
4) Comprehensive methods such as plugging, draining and draining are adopted to deal with the gas accumulated in the tunnel.
In order to prevent the serious harm caused by coal and gas outburst, it is necessary to strengthen the management of coal seam roof and in-situ stress monitoring, and strengthen the safety education for employees.