Study On The Variation Of Overburden Fracture In Coalfiled Area With Temperatures

There is a large area of coal field in Xinjiang,Inner Mongolia and Shanxi,which is not only burn a lot of coal resources,but also seriously damage the fragile ecological environmenta.It is of great significance to understand the evolution characteristics of the overlying fissure in the process of coal fire in the process of coal fire evolution.The change of temperature in the process of coal fire changes the original mechanical characteristics of the overlying rock in the fire area.At present,there are few studies on the variation of rock mechanics parameters with temperatures.Based on this,this paper carried out the variation of fracture development of coalfiled area with temperatures.The main research contents and results are as follows:Different temperatures(25℃ ~900℃)experimental determination of mechanical parameters of sandstone of a coalfield were carried out by MTS815 experimental system.The results show that: With the increase of temperature,the elastic modulus and the brittleness coefficient of the siltstone increase first and then decrease then increase,and the peak strength decreases first and then increases then decreases.The influence of confining pressure on the mechanical parameters of siltstone in fire area was investigated by MTS816 experimental system,and a comparative experiment on the mechanical parameters of siltstone under uniaxial and 2 MPa confining pressure in the range of 25 ℃ ~150 ℃ was carried out.The results show that the mechanical parameters and failure degree of siltstone under 2 MPa confining pressure are all greater than those under uniaxial.Compared with the results of MTS810 test,the elasticity modulus and peak strength of siltstone under uniaxial conditions measured by two instruments in the range of 150℃ are different,but the general trend is consistent.Based on discrete element micro mechanical parameters calibration process,the micromechanical parameters of siltstone and sandstone under different temperatures were determined on the basis of the experimental results.The error between the mechanical parameters obtained by the numerical simulation and the mechanical parameters obtained by the laboratory experiment is compared,and the failure modes of the numerical simulation and laboratory test specimens are compared and analyzed.After several parameters adjustment,the tangential contact stiffness of the siltstone and sandstone under different temperatures is obtained.The micro mechanical parameters such as grain normal contact stiffness,particle stiffness,tangential stiffness,parallel bonding strength,cohesive force and particle friction coefficient.Based on a fire area in Xinjiang and the microscopic mechanical parameters of siltstone and sandstone at different temperatures,a numerical model for fracture development in coalfield fire area is established.The changes of the fracture development and permeability of the overlying rock under the conditions of mechanical parameters no change and change and two different homogeneous lithology(siltstone and sandstone)are analyzed.The results show that: With the increase of the temperature,fracture number and permeability increases.The main factors of controlling the fracture development of overburden rock in different stages of coal fire are different.The fracture development of overlying rock in the initial low temperature oxidation stage and the high temperature burnout stage is mainly controlled by the mechanical parameters,in the stage of coal combustion,the development of fractures was controlled by coal burning and mechanical parameters.At the same time,the fracture development of two different lithology conditions was compared.It was found that temperature has great influence on fracture development of siltstone before 400℃,while after 400℃ the fracture development of sandstone was more affected by the temperature.