Spatiotemporal Evolution Of Bed Separation Due To Mining And The Dynamic Hazards Identification For Groundwater Inrush

This paper presents an investigation of spatiotemporal evolution of bed separation and the mechanism of bed separation water inrush hazards,proposes the dynamic hazards identification for groundwater inrush,which is a typical roof water hazards in coal mines,and has an important implication for theory and practice.In this study,field measurements,mechanical analysis,numerical simulation and scale model testing are used.The main achievements are as follows:(1)According to the analysis of the geological,mining conditions and the groundwater inrush characteristics in the Cuimu Coal Mine of the Yonglong Mining Area in the souther Ordos Basin,it is concluded that the groundwater inrush from the panels of the Cuimu Coal Mine is the types including the static water release of the overlying aquifers,bed separation water inrush and re-inrush,or both of them.The main reason for the frequent occurrence of bed separation water inrush in the area is that there prone to generate a bed separation between the aquifer of the Yijun formation of the Cretaceous system and the aquiclude of the Anding formation of the Jurassic system.(2)A mechanical model of initial and periodic bed separation development are established,which is fixed supported by four sides;or fixed by three sides and simply supported by one side,respectively.The formulas for the calculation of the amount and size of the bed separation space are derived and the criterion for the development position of the bed separation is put forward.(3)The spatiotemporal evolution process of bed separation development is investigated based on mechanical theory,numerical simulation and scale model testing.The bed separation space developed in overlying strata above caving zone experiences the process of development-expansion-continuation-closure with the characteristics of similarity and periodicity.The results show that the bed separation development successively appears the caving and fracturing zone,the plastic deformation transition zone and the unbroken zone.The bed separation space development is lagging behind the panel advancing.With the different inclined length of the panel,the bed separation space development in the overburden can be divided into two parts:bed separation space under the subcritical mining and bed separation space under the critical mining.The cross-sectional shape of the bed separation space is similar to an inverted triangle and an inverted trapezoid.In addition,the three-dimensional bed separation shape is similar to a bowl and a disk,respectively.(4)The vertical transfer coefficient(K_s)and horizontal development coefficient(K_l)are been proposed to evaluate the rate of bed separation development in vertical and horizontal with different panel advancing distances.The results show that the ultimate fracture distance in the overburden with the strong strenghth and large thickness is obviously larger than that in the lower strata,the speed of bed separation propagation upward may slow down with the panel advancing,the period of bed separation development along the direction of the panel advacing will increase,the expansion-continuation stage of bed separation development will increase,the space and speed of horizontal dimension development of the bed separation will also increase correspondingly.The vertical distribution area of bed separation development is a trapezoidal one.(5)A mechanical model of bed separation space filled water is established.A calculation formula of vertical displacement and deflection of strata on the condition of the bottom strata of bed separation breaking pre and post breaking of rockmass is deduced.The expansion mechanism of bed separation with the hydrostatic pressure is proposed,and the prediction formula of internal clearance and the expansion volume increment of bed separation are deduced.(6)The occurrence conditions and the distribution characteristics of bed separation water inrush hazards are analyzed based on the geological and mining conditions in Chinese coal mines.And three types of bed separation water inrush hazards are classified:propagating type,fracturing type and regenerating type.Among these three types,the propagating type is a definite hazard of bed separation water inrush hazards with high risk level.The fracturing type and regenerating type are the potential hazards of bed separation water inrush hazards,which has a lower risk level than the propagating does.(7)The dynamic mechanism and dynamic evolution process of the fracturing type,propagating type and periodic type of bed sepration water inrushes are investigated.After the formation of the bed separation space with filling water in the overburden,the dynamic deformation area of the overburden disturbed by panel advacing appears the caving and fracturing zone,the expansion zone and the non-collapsed zone on the condition of the bed separation space filled with water and the different water accumulation ratio.The water-filled bed separation in the overburden increases the overburden deformation to a certain extent,which makes the area of the caving and fracturing zone expand to the plastic deformation transition area at the bottom of the bed separation filled with water.And height of the water conducting fractured zone reaches bed separation with water filled resulting in water inrushes.(8)Based on the dynamic mechanism and the influencing factors of bed water inrushes,a fault tree model and a dynamic identification model are established.The subjective and objective factors of bed separation water inrushes are analyzed based on analytic hierarchy processing(AHP)and entropy weight method(EWM),respectively.A comprehensive identification model based on AHP and EWM is established,and an identification criteria and classification threshold of identification of bed separation water inrushes are proposed.(9)A fault tree model and a hazard prevention model are established of bed separation water inrushes.The fuzzy analytic hierarchy process(FAHP)is used to analyze the prevention of bed separation water inrushes and determine the index weights of different influence factors.Then,five stage measures to prevent bed separation water inrushes based on the two aspects of exploring the possibility of bed separation water inrush formation and detwatering bed separation water in advance are put forward.This paper includes 77 figures,25 tables and 212 references.