Mining Subsidence Risk Assessment Of Guangxi Heshan Coalmineb

Our country has more coal and less oil, coal is an important energy minerals. There are widespread damage to the geological environment in the development of coal resources, such as mining subsidence, ground fissures, vegetation of land destruction resources, water waste and destruction, water and soil pollution and so on. The mining subsidence has the greatest effect in these problems. There are abundant coal resources in Heshan city. The area of goaf is 32.9% of the city which formed in hundred years. And mining subsidence can be found in everywhere. Because of land subsidence, the total length of the sinking of highway and railway are 30 kilometers, farmland inclined cracking up 40,000 acres, irrigation canal destroyed more than 20 km in Heshan. As the resource-exhausted cities, Heshan city needs re-planning and bulid a lot of infrastructure in the transition process. So it can not avoid to construct new buildings and re-use land on the goaf. How to evaluate the construction risks and safety extent is first problem in the construction process. Therefore widespread goaf seriously hampered the urban construction and development. Explore the coal mined risk assessment theory and system will be important basis theoretical of urban transformation and construction.The coal mining subsidence area has general characteristics of mining subsidence of typical resource-exhausted cities at the respects of formation time, stratum structure, mining methods, collapse features and hazards object in Heshan, Guangxi Province. It is Very representative. Based on national mine geological environment treatment special project (coal mine geological environment management project in Heshan, Guangxi), and the goaf JK07 of the east coalmine as a case, the paper has made research on the risk assessment of the mining subsidence of resource-exhausted cities. Four main research contents are as follows:hazard assessment, vulnerability assessment, the determination of risk allow standard and the risk assessment.For the hazard assessment, based on the critical layer theory, the initial breaking and the breaking mining of subsidence the cycle critical layer of limit equilibrium model have been established.And then by using the Monte Carlo method, the corresponding program have been wrote to solve the reliability and failure probability. Finally hazard value of the old goaf mining subsidence has been got. For the vulnerability assessment, the buildings vulnerability assessment model and personnel vulnerability assessment model have been proposed in coal mining subsidence. For the determination of risk allow standard, the paper has collected a lot of statistics date of the number of geological disasters and the mining subsidence, casualties, property damage. And screening and analysis the data, mine subsidence risk standard value of Guangxi Heshan coal has been obtained.Finally, with analysis the characteristics of the typical resource-exhausted cities Heshan and collection the r disasters classification information, economic data, the risk of old goaf has been comprehensive assessed.Through the above analysis and research, the results are as follows:(1) Research on the characteristics of mining subsidence of resource-exhausted citiesWith the collection of background data and mining subsidence site surveys of coal mining in Heshan, Guangxi, the results showed that there are two sets coal seams of Heshan city:steeply inclined coal seam and gentle sloping coal seam. There are a lot of funnel-shaped collapse pits in steeply inclined coal seam area. It is very devastating. Mobile basin and small-scale collapse pits formed in gently inclined seam. Mobile basin caused the tension fracture of houses and the formation of ground fissures.Based on the life-cycle theory of resource-exhausted cities, the mining subsidence characteristics of resource-exhausted cities have been proposed which are different from general collapse disaster. And accordingly, the mining subsidence features of Heshan city have been organize summarized. The mining subsidence characteristics of resource-exhausted cities manifested as irreversible destructive, the term stability and the force activation. The mining subsidence features of Heshan city are a lot of continuous collapse and a small amount of non-continuous collapse. The paper put research focus on the continuity collapse.(2)Research on the risk evaluation system of mining subsidence of resource-exhausted citiesWith internationally accepted for disaster risk management processes and the actual characteristics of resource-exhausted cities mining subsidence disaster, the risk evaluation system of mining subsidence of resource-exhausted cities has been proposed. The risk assessment process of mining subsidence is based on risk analysis, vulnerability analysis, risk calculation and risk assessment.(3) Hazard assessment of Mining subsidence1) Research on the distribution model of geotechnical layer parameterBased on the study of the distribution model of random variables, a lot of physical and mechanical properties of the mined overlying rock layer have been tested. And the distribution models for different mechanical parameters have been statistical studied on the basis of tests. The results show that rock parameters tensile strength, elastic modulus, density and structural surface parameters inclination, orientation, length of the half-track, spacing all fit to the normal distribution, gap width of structural surface fit to the negative exponential distribution. Finally, the distribution models have been tested by using the KS test method. It is show that rock parameters conform to the original hypothesis, which can be accepted.2) Establishment of limit balance equationOn the basis of obtaining the rock mechanical parameters, combining with the distribution of the mined overlying rock layer and coal mining conditions, the deformation of gob area has been simulated by using FLAC3D. The results showed that the maximum amount of subsidence in gob area is 203mm. The maximum surface horizontal displacement along the tendency is 100mm. The maximum surface horizontal displacement along the strike is 80mm.3) Hazard assessmentThe common methods of reliability calculation are simulation method and analytical method. For the shortcomings of manual calculation complex of the analytical method, the paper calculates reliability by using Monte Carlo of the analytical method. The programs corresponding to the initial breaking and the cycle breaking of the limit equilibrium equations have been established in Matlab software. Meanwhile, a different order of random rock mechanics parameters have been imported. Tesults show that the reliability of the initial breaking is 1.938. And the failure probability is 0.0262. The reliability of the cycle breaking is 1.936. And the failure probability is 0.0263. Because of the overburden will occur the initial breaking and the cycle breaking at different time points, so the hazard of mining subsidence is 0.0525.(4) Vulnerability assessment of Mining subsidence1) Vulnerability assessment model of buildingsLogistic Vulnerability model of buildings is related to disasters intensity and buildings fragility. When disaster intensity remains constant, the lower of the fragility value the smaller of the Logistic Vulnerability curve result. It is consistent with the actual.2) Disasters intensity modelDisasters intensity model has been established by defining the surface displacement index and the deformation speed index. Finite element model of frame structure and brick structure in gob area have been built by using Midas/Gen structural finite element software. With applying the surface horizontal displacement, the internal force has been analyzed and the surface displacement index has been calculated. The simulation results that the surface displacement index of the frame building repair workshop No.9, No.10, No.12, brick building houses No.13 respectively are 0.27,0.27,0.144,0.56 in old gob area.By introducing time series model of the mining subsidence science, the deformation speed index is established. And Knothe time series model has been corrected. Combined with the actual gob area monitoring data, the modified model has been validated and the model parameters have been solved. The results show that the modified model has better fitting effect. And the calculated strain rate index is 0.3) Buildings fragilityBased on the building fragility analysis, building structure type, the extent of damage, used g age are defined as fragility assessment index. And building fragility assessment model has been set up. Then combined with field survey data of buildings and related construction industry standards, the fragility value of the repair workshop No.9, No.10, No.12, private houses No.13 respectively are 1,0.866,0.798,0.904 in old gob area.4) Personnel vulnerability assessmentFor the continuity of mining subsidence effects, an indirect way can be applied to evaluate the personnel vulnerability based on buildings vulnerability. According to each vulnerability assessment model of hazard bearing body, the vulnerability value of the repair workshop No.9, No.10, No.12, private houses No.13 respectively are 0.388,0.354,0.100,0.949 in goaf JK07. The personnel vulnerability in them are 0.010,0.008,0.002,0.297.(5) Research on the risk level and allow standards of mining subsidence of resource-exhausted citiesComprehensive consideration the risk expressing results of a large number of scholars, from the risk expression of subjectivity and objectivity two angles, the paper use of risk degree and risk expectation to evaluate mining collapse risk of resource-exhausted cities in two ways. 1) Mining subsidence risk levelRisk level corresponding to the subjectivity of risk, quantitative risk results are expressed by risk degree. Risk level is the criterion of the risk value. On the basis of the risk matrix method which is often used in the international arena, and combining with the numerical grading methods (regional analysis of Bradford Law) which currently is often used in the classification of danger and vulnerability, the paper divides the risk level into five range:0.00-0.04,0.04-0.16,0.16-0.36, 0.64-1.00, which respectively represent very low-risk areas, low risk areas, medium-risk areas, high-risk areas and very high-risk areas.2) Risk allow standards of mining subsidenceRisk allow standards corresponding to the objectivity of risk, quantitative risk results are expressed by risk expectation. Risk allow standards is the criterion of risk expectation, which is socially acceptable or acceptable economic losses expectation. The paper describes the meanings of risk allow standards according to ALARP principle. And by using international FN law, risk allow standards of mining subsidence have been determined.Risk allowable standards are divided into two aspects:life risk allow standard and economic risk allow standards. By collecting the data of casualties and the direct economic losses caused by national geological disasters from 2004 to 2014, and simultaneous analyzing the proportion of mining subsidence disasters to geological disasters, the acceptable life risk standard of the geological disasters is determined which is 1×10-7. The tolerable life risk standard of the geological disasters is 1×10-6. The acceptable life risk standard of the mining subsidence is 2.3×10-9. The tolerable life risk standard of the mining subsidence is 2.3×10-8. The acceptable economy risk standard of national geological disasters is 1×10-4. The tolerable economy risk standard of the geological disasters is 1×10-3. The acceptable economy risk standard of the mining subsidence is 2.3×10-6. The tolerable economy risk standard of the mining subsidence is 2.3 × 10-5. Then combined with local codes, life and economic risks FN curve have been drawn by seting risk consequences limit lines.(6) Mining subsidence risk assessment of resource-exhausted cities1) Mining subsidence risk degree assessmentBy using the expressions of risk degree to calculate the old buildings in goaf JK07, the building risk of the repair workshop No.9, No.10, No.12, private houses No.13 respectively are 0.020543,0.018757,0.005311,0.050292. The personnel risk in them are 0.000541,0.000445, 0.000095,0.015736. According to the risk classification, the risk of repair workshop No.9, No.10, No.12 all less than 0.04, which is very low risk level. The risk of the private houses No.13 is 0.05, which is low risk level.2) Mining subsidence risk expectation assessmentThrough the investigation of hazard bearing body, and the collection the denominated prices way of hazard bearing body and the existing unit price, the hazard bearing body’s own economic value have been calculated. Finally, comprehensive the risk degree of each hazard bearing body, GDP of Heshan city, expectation economic losses has been calculated caused by mining subsidence for 74,200 yuan. And economic loss ratio is 2.11×10-5. According to the division of national allow economy risk standard of the mining collapse, mining subsidence economic risk is tolerable risk. However, it is slightly stringent making national allow economy risk standard as the criterion of local risk values. Maybe in the Heshan city which geological disasters frequent occur and the economy is lagging behind, the risk expectation is more acceptable at an acceptable risk level.