| In the last century,China adopted more backward coal mining methods,resulting in a large number of coal pillars left in the old mining area,and a serious waste of resources.Most of the old residual coal resources are scarce coal with good mining conditions and high mining value.Therefore,the realization of large-scale residual coal recovery is of great significance to the sustainable and healthy development of China's coal industry.The stability of the remaining coal pillars in the old mining area is one of the discriminant indicators for the feasibility demonstration of residual coal re-mining,and it is also a key factor affecting the stability of the surrounding rock in the mining face.As a kind of creeping body,in many cases,the destruction of coal body is not due to its insufficient strength,but it is caused by the instability caused by long-term cumulative deformation of creep deformation.And a lot of engineering practice shows that the instability of coal pillars is closely related to factors such as in-situ stress,water,coal quality and temperature,and water is an important factor that affects the stability of coal and rock masses.In this paper,the anthracite coal left in Qinshui Coalfield No.3 is taken as the research object,and the creep characteristics of anthracite coal under different water contents are studied.The following conclusions were obtained:1)An anthracite samples with water contents of 0%,0.74%,1.62%and3.21%were prepared by the immersion method.During the immersion process,it was found that the anthracite's water absorption changes can be divided into four stages:accelerated growth stage,stable Growth phase,deceleration growth phase and stable phase.2)Carry out high-precision micro-CT scanning of anthracite samples.Based on the scan results,the parameters such as CT number,porosity,porosity and over-permeability of anthracite samples are obtained.The cluster analysis method was used to classify 24 anthracite samples into 3 types.The cluster centers of the first type:CT number(0.0299052),permeability(10.2200),probability of overseeing(0.34),and number of pore clusters(186677),Cluster center of type 2:CT number(0.1526576),permeability(7.8481),probability of percolation(0.08),number of pore clusters(25974),cluster center of type 3:CT number(0.1526576),penetration Rate(17.6059),probability of percolation(7.21),number of pore clusters(3281150),the number of anthracite samples for each classification is 7,16,and 1,the proportions are 29%,67%,and 5%,respectively.3)Using the upgraded and improved rock triaxial rheological test system,the uniaxial grading loading creep characteristics test of anthracite under different water content conditions was carried out.The results showed that:(1)In the uniaxial creep test of anthracite under different water content conditions,Anthracite coals have successively experienced three stages of attenuation creep,stable creep and accelerated creep;(2)When the water content is the same and the load level is different,take the water content as 0%as an example,when the load level increases from 1.010 MPa to 5.204 MPa,The instantaneous strain is increased from 0.642×10-3 to 5.086×10-3,the time to reach the stable creep is increased from 150 min to 360 min,and the rate of the stable creep stage is increased from-1.299×10-4/h to 0.0169×10-3/h.Other load levels with the same water content also have similar laws;(3)Taking the first load level(1.002MPa)as an example,when the water content increases from 0%to 3.21%,the instantaneous strain is from 0.336×10-3 increased to 2.286×10-3,the time to reach stable creep increased from 84min to 258min,the rate of stable creep stage increased from-1.299×10-4/h to 1.428×10-4/h;(4)It can be seen from the isochronous stress-strain curve of anthracite that anthracite has obvious nonlinear creep characteristics.4)According to the creep law of anthracite coal under different water content,the classic Burgers model is improved,and a nonlinear viscous element describing accelerated creep is connected in series with the improved Burgers model to establish an accurate description of anthracite coal under different water content.The six-element non-linear viscoelastic-plastic rheological model of the whole creep process,and the rheological constitutive equation of the model is derived.Then,using the custom function function in Origin software,based on the Levenberg-Marquardt(LM)algorithm,the"strain-time"curves of grading loading of anthracite under different water content are fitted to obtain the creep of anthracite under different water content The parameters and test results are in good agreement with the theoretical curve of the established creep model,and the correlation coefficients of multiple groups are all above 0.9.5)On the basis of damage mechanics,considering the water degradation effect,introducing the water damage factor,and deriving the damage evolution equation,establishing a six-element nonlinear viscoelastic-plastic creep equation considering the water degradation effect. |
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