Environmental Engineering Geological Patterns Relating to Mining Activities in Jurassic Coalfield of Northern Shaanxi
Author:Liu Shi Liang
Supervisor:li wen ping
In large-scale and high-intensity mining of coal resources with shallow burial and large thickness in Western China,less attention is paid to the capacity of eco-geological environment,especially the carrying capacity of water resources,resulting in serious problems of eco-geological environment,such as water resources destruction,surface drought,vegetation withering and death,and intensification of desertification,which restricted the energy,economy development and eco0geological protection.Therefore,taking Yushenfu Mining Area of North Shaanxi coal base as an example,the formation of environmental engineering geological patterns(EEGPs)was illustrated using mechanical analysis,on-site measurement,and numerical simulation.Specifically,the research thinking is conducted as follows:the spatial geological structure characteristics of eco-water-coal measures strata→the development height of water-conducting fracture zone of rock-soil overburden structure→division of EEGPs→application of EEGPs→protection and utilization of water resources of different water-retaining mining grades.The main results are as follows.(1)The types of eco-geological environment(oasis typewith phreatic water and bottomland in desert,loess gully type with surface runoff,oasis type with surface water and valley river,and regional deep groundwater enrichment type)were proposed and zoned,and the relationship between eco-geological environment type and normalized vegetation index(NDVI)was determined.Then,13 types of hydro-geological structure were proposed and classified.Finally,taking water resources with limited carrying capacity as the key point,combining with the types of eco-geological environment and hydro-geological structure,the spatial geological structure model of eco-water-coal measures strata was established.(2)The reasons for abnormal changes in the development height of water-conducting fractued zone(WCFZ)in western mining area were expounded from mining intensity,location of main key strata in overburden,sedimentary stability of overburden and structural characteristics of overburden rock mass.The structural characteristics of overburden rock mass in Jurassic coal seam in Western China were divided into two structural types:layered and monolithic structure.On this basis,the theoretical prediction formula for the development height of WCFZ into bedrock was analyzed by comparing the ultimate deflection of each stratum with the free space height of the lower part.Furthermore,the theoretical prediction formula for the development height of WCFZ into soil layer was analyzed based on Pu’s theory and limit equilibrium theory of rock mass.Moreover,when measuring the WCFZ height developed into soil layer,the consumption of fluids,the water level or TV image of boreholes can not be observed accurately in the soil layer due to the mud wall protection of the soil layer,and the true water conductivity of the fracture development can not be judged.Therefore,the micro resistivity scanning imaging logging technology was adopted to measuring the WCFZ height.(3)On the premise that the development height of WCFZ penetrates the bedrock into the soil layer,when the ratio of soil thickness to bedrock thickness(soil-bedrock ratio)is less than 0.5,the development height of WCFZ is inhibited;when the soil-bedrock ratio is greater than 0.5,the development height of WCFZ is promoted.Based on this,the inhibition(promotion)coefficient of WCFZ height was defined,and the relationship between inhibition(promotion)coefficient and soil-bedrock ratio was fitted.It was concluded that the inhibition coefficient is logarithmic to soil-bedrock ratio,and the promotion coefficient is power function to soil-bedrock ratio.(4)According to the affect degree of underground coal mining on phreatic aquifer water table,four types of EEGPs were proposed and defined,i.e.,basically unaffected model,gradually restored model after destruction,gradually deteriorated model,and disaster model.Zoning method for EEGPs was eatablished as follows.Firstly,the thickness of residual aquifuge was calculated based on the height of WCFZ in rock-soil overlying strata.Secondly,according to the relationship between water leakage,recharge and water level change of shallow surface water,the critical values of residual loess,laterite or bedrock thickness between basically unaffected model and gradually restored model after destruction,gradually restored model after destruction and gradually deteriorated model,gradually deteriorated model and disaster model were determined respectively,that is,the thresholds of EEGPs were determined.Thirdly,on the basis of the linear variation law of the threshold values of EEGPs and the combined thickness of residual aquifuge,the zoning standard of EEGPs was formulated.Finally,EEGPs was divided in Yushenfu mining area.(5)The reliability of the zoning results of EEGPs was verified from three aspects:the surface vegetation ecological environment,the phreatic water level of Salawusu Formation,and the zoning standard of EEGPs.Firstly,the zoning results accuracy of EEGPs was confirmed by field investigation on the changes of surface vegetation ecological environment in 12 coalfaces before and after mining.Secondly,the type of EEGPs was identified by monitoring the change of groundwater level in Salawusu Formation of No.108 coalface of Jinjitan Colliery.The monitoring results show that the phreatic water level of the Salawusu Formation shows three stages of decline,stability and rise in the mining process of the working face;the variation range of the phreatic water level before and after mining is 0.4-2.5 m,which is still within the ecological safe water level of northern Shaanxi.It is known that the leakage of the Salawusu Formation phreatic water does not occur,and the corresponding EEGPs of the calface is basically unaffected model.Thirdly,on the basis of measuring the height of WCFZ in No.108 working face of Jinjitan Colliery using distributed optical fiber technology,the types of EEGPs were discriminated based on the zoning standard.The results show that the EEGPs type of No.108 coalface in Jinjitan Colliery is also basically unaffected model.(6)The application method of EEGPs was established,that is,the concept of water-retaining mining grade and its division method were put forward.Firstly,based on the analysis of the unit area storage of groundwater resources in surface water system and Salawusu Formation aquifer layer,the total storage of shallow surface water resources per unit area(1 km~2)in the study area was calculated,which was divided water resource scarcity,medium and rich areas.Then,combining with types of EEGPs,the concept of water-retaining mining grade and its division method were put forward,which is helpful to deepen the connotation of water-retaining mining thought and to facilitate engineering practice and application.(7)The methods of water resources protection and utilization for different types of water-retaining mining were put forward.Taking filling mining as an example,the relationships between filling rate and development height of WCFZ in filling mining and evolution of EEGPs were established.The dissertation contains 69 figures,23 tables,and 259 references.