Study on Overburden and Surface Deformation Law of Deep Strip Mining Considering Dynamic Response of Overburden
Author:Weng Li Zuo
Supervisor:zuo yuan zhong
With exhausting of coal resources in shallow parts,coal mining gradually extends to deep parts.Mechanical behaviors of coal and rock masses in deep mining tend to be more complicated under geological conditions such as high ground stress and strong mining influences.Corresponding mining subsidence law and time effect of coal pillar reserving are different from those of shallow mining.Studying the dynamic response mechanism of overburden in deep mine and the time effect of pillar stability under high in-situ stress has theoretical significance and application value for developing the numerical simulation method of mining-induced overburden deformation,coal pillar size design,long-term stability analysis and later pillar resource recovery.Taking Gucheng Coal Mine’s large mining depth strip mining as engineering background,this paper studies the subject by means of theoretical analysis,field measurement,numerical simulation and other research methods,combined with the theory of elastic-plastic mechanics.An equivalent numerical model considering the dynamic response of overburden mining is established,which provides a fine numerical simulation method for surface subsidence prediction in deep mining.The mechanism and law of overburden and surface movement in deep mining are analyzed.The time effect of the strip coal pillar stability under high ground stress in deep mining is clarified.The main work and achievements are as follows:(1)Based on the measured data of surface deformation of deep and shallow mining,the parameters of probability integral method are inverted,and the law of surface deformation of deep mining under the geological conditions of this mining area is analyzed:With the increase of mining depth,the surface deformation decreased in a relatively smooth manner;the main influential angle tangent increased;the subsidence coefficient decreased;the synthetical movement angle increased;the synthetical boundary angle increased at first and then decreased,etc.The critical mining depth for dividing deep and shallow mines in mining area is obtained,and the between mining depth and surface movement angle and the probability integral parameters is established,which provides certain basis for the selection of surface movement parameters in deep mining area.(2)Aiming at the problem of difficulty in selection of rock mass mechanical parameters during numerical simulation,the orthogonal experimental method was used to optimize FLAC3D numerical simulation mechanical parameters based on geological conditions in the mining area and measured surface deformation data.It provides reliable numerical mechanical parameters for subsequent research and improves the reliability of numerical simulation calculation analysis.(3)Based on the heterogeneous property and the structural characteristics of overlaying rock“three zones”under mining influences,the compaction course of rock masses in the caving zone under pressure of the overlaying strata as well as the mechanical mechanism concerning how the mechanical natures got weakened due to development of rack fractures in the fracture zone were analyzed.On the basis of strain softening model in FLAC3D software,the secondary development is carried out.The heterogeneous assignment algorithm of rock mass elasticity modulus following Weibull distribution and the elastic modulus weakening algorithm of fractured rock mass are added,and the mechanical parameters of double yield model which can reflect the compaction process of rock mass in caving zone are obtained by inversion.An equivalent numerical model considering the mechanical response of overburden under mining is developed,which realizes the equivalent numerical simulation of the elastic modulus heterogeneous distribution of overlying strata,the compressive strength process of rock mass in caving zone and the elastic modulus weakening of rock mass in fractured zone.The secondary developed equivalent numerical model is used to simulate the deep mining and the results show that the equivalent numerical model are closer to the measured data,which verifies the reliability and correctness of the developed equivalent model,and provides a more realistic and precise numerical simulation method for the surface subsidence prediction.(4)The established equivalent numerical model which could consider mechanical characteristics of overlaying rocks and mechanical responses of overlaying rock mining was used to conduct numerical simulation of subsidence of deep mining.In combination with measured data of the mining area,accuracy and reliability of the numerical simulation method were verified.Weakening effect of mechanical natures of rock masses in the fracture zone were further analyzed.Laws of influences brought by weakening of mechanical natures of rock masses in the fracture zone to mining overlaying rock stress fields,development in the plastic zone and coal pillar stress distribution were obtained:With the increase of the weakening degree of mechanical properties,the peak stress in front of the working face increases,the distribution range of plastic zone expands,and the peak stress concentration in the coal pillar increases,but the concentration range decreases and moves to the depth of the coal pillar.The equivalent numerical model put forward by the present study provides an authentic,fine numerical simulation method which can reflect mechanical responses of overlaying rock mining and is applicable to prediction of surface subsidence.The degree of fracture weakening can be estimated by field measurement or theoretical experience according to the specific engineering geological conditions.It has practical value in mining impact simulation,especially in mining numerical simulation under geological conditions with high fracture degree.(5)The established equivalent numerical model was used to further analyze dynamic movement and deformation law of overlaying strata and surfaces in deep mining.Dynamic movement law of overlaying stress fields and displacement fields during advancing of deep mining panels were obtained.In addition,law about influences brought by mining depth and mining degree to overlaying strata and the ground surface were obtained:including influencing law between mining depth and mining degree and probability integral method parameters and values of surface movement and deformation angles;distribution forms of overlaying stresses at different depths,stress peak locations and changing law of vertical displacement transmission forms of overlaying strata,etc.Dynamic movement and deformation law of overlaying strata and surfaces in deep mining,which were obtained in the equivalent numerical model,were consistent with the law summarized with measured data,so correctness and reliability of the equivalent numerical model proposed were verified.(6)Based on rheological characteristics of coal and rock masses,long-term stability of reserving strip coal pillars in deep mining was researched.The improved Burgers creep model was used for fitting with creep deformation test data of 3 coal samples.Coal creep deformation parameters which could reflect creep deformation characteristics of 3 coal samples were obtained.Simulation computation of long-term stability of strip coal pillars in deep and shallow mining was carried out.Time based changing law of horizontal deformation and stress distribution at different positions in strip coal pillars as well as time-based development depths of the plastic zone in coal pillars were mastered.Results indicate that under these mining geological conditions,the strip coal pillars can remain stable for a long time.Under high stress,creep deformation characteristics of strip coal pillars in deep mining are more obvious than those in shallow mining.With the increase of mining depth,the coal pillars bore heavier overlaying strata;vertical stress in the coal pillars increased;the horizontal deformation was intensified;the plastic zone extended to deeper parts of coal pillars;the bearing capacity of coal rock masses decreased.Hence,it would take longer time for coal pillars to enter the long-term stable state.