Creep Properties Of Cemented Gangue Backfill Column In Constructional Backfill Mining

The cemented gangue backfill material is used for the constructional backfill mining of coal mine,the cemented gangue backfill body is the main supporting component,and thus the stability of the backfill body(including short-term stability during the mining and long-term stability after mining)affects the stability of the goaf.Therefore,it is valuable to study the deformation,damage,and instability properties of the backfill body under the load from the overlying strata.Among them,creep deformation exists in the whole mining process.During the advancing of the coal mining face,the roof of the goaf sinks gradually,and the pressure loads on the backfill body step by step,which causes the short-term creep deformation.Furthermore,when the backfill mining activity is finished,the constant pressure of the roof will load on the backfill body for a long time,which causes the long-term creep deformation of the backfill body.If the creep deformation of the backfill body is over the limit,the bearing capacity of the backfill body will lose due to the creep damage and then affect the safety of the roof.In the thesis,the creep deformation of the backfill body during the mining process and the long-term creep process was studied through the indoor experiment,numerical simulation,and theoretical analysis.Among them,the creep properties of the backfill body under four conditions including under the early curing age,under the high-stress area,under the different step-by-step loading paths,and the long-term creep deformation were studied by the indoor experiment method.The creep deformation of the backfill body under the high-stress area was also researched by the numerical simulation method.Based on the experimental results,the creep models under different conditions were built,which can be used to calculate the creep deformation and forecast the creep instability of the backfill body.During the experiment process,an ultrasonic-Acoustic emission-resistivity monitoring system was used to detect the creep damage,and the long-term stability monitoring method of the backfill body was given according to the experiment results.The main research content and findings are drawn as follows:(1)the experiment of effects of curing under pressure on the deformation and strength properties of cemented gangue backfill body was conducted.The experiment found that curing under pressure(loading at an early age)can improve the uniaxial compressive strength(UCS)and elastic modulus(EM)of the backfill body.The UCS and EM of pressure-cured samples were 1.005 to 1.202 times and 1.098 to 1.523 times higher than those of the control ones,respectively.The curing pressure contributed positively to the cement hydration reaction process,which is positive for the long-term stability of the backfill body.The creep strain increased with the increasing stress-strength ratio,and the earlier the initial loading age is,the higher the total creep strain will be.The early age loading history can influence the long-term creep deformation,and the creep deformation of backfill body can be described by the Burgers model.When the stress-strength ratio loading on the early age backfill body is less than 80%,the final ultrasonic pulse velocity of the backfill body has an exponential relationship with the UCS.Ultrasonic equipment and resistivity equipment were used to detect the damage during the curing creep process and the uniaxial compression process,the experiment result shows that the stability of early age backfill body can be monitored by the ultrasonic pulse velocity and resistivity.(2)The creep properties of cemented gangue backfill body under high stress-area were explored.Under the high-stress area,the backfill body shows obvious rheological characteristics,and the creep strain of the backfill column has nonlinear characteristics.An viscoelastic-plastic creep model(Concatenation of Burgers model and Bingham model)is proposed to express the axial creep strain of the backfill column under the high-stress area.The creep failure strength of the backfill sample under multi-step loading is slightly larger than the average UCS,ranging in ratio from 108.9% to 116.5%.The backfill column has the creep hardening property during the multi-step loading creep process.The instantaneous strain,creep strain,and creep rate increase with increasing stress-strength ratio in the single-step loading creep tests.In the multi-step loading creep tests,the increase in stress-strength ratio interval had a significant influence on the creep process of the backfill column.The creep instability of the backfill column in the high-stress area includes an increase in the creep strain at the non-creep failure stage and the accelerating catastrophe at the creep failure stage.The creep instability of the backfill column under a high-stress area is caused by the accumulation of strain energy under multi-step loading and continuous lateral expansion at the unconstrained middle position during the long-term creep process.The creep instability of the backfill column causes its bearing capacity to be lost immediately.During the multi-step loading creep testing process,the UPV,resistivity,and AE testing equipment were used to detect the creep damage.According to the experimental results,the long-term creep stability of the backfill column under the high-stress area can be monitored based on the variation of electrical resistivity,UPV,and AE signals.(3)The creep properties of cemented gangue backfill columns under different stepby-step loading processes were studied.The instantaneous strain and creep strain of the backfill body at axial and lateral under step-by-step load were analyzed.The deformation and instability properties of cemented gangue backfill body are affected by the step-by-step loading path.As the stress-strength ratio increases,the axial instantaneous strain decreases gradually,the axial creep strain decreases first and then increases.The lateral instantaneous strain decreases first and then increases,while the lateral creep strain increases continuously.When the stressstrength ratio is less than 0.6,the lateral positions of the backfill body shrink during the creeping process;however,when the stress-strength ratio is larger than 0.6,the lateral position of the backfill body expands gradually during the creep process.The backfill body shows the axial creep hardening and lateral creep softening properties during the step-by-step loading process.The instantaneous deformation modulus shows an upward trend on the whole,but it is affected by the step-by-step loading increment and paths.The instantaneous Poisson's ratio exhibits a concave upward trend and is larger than that in the conventional test.The development of cracks of the backfill column under step-by-step load could be divided into 4 stages according to the stress-strength ratio.Under different step-by-step loading paths,the axial creep strain rate is nearly a constant before entering the accelerated creep stage.A non-linear creep constitutive model with a creep strain rate trigger was established to express the axial strain of the backfill column under the step-by-step load.(4)The long-term creep properties of cemented gangue backfill body were studied through the experiment.The long-term creep properties can be affected by the stress-strength ratio and initial loading age.The curing pressure promotes the hydration process of cement and thus the backfill body will be more compacted.Compared with the initial loading age at 28 d,the creep strain of the backfill body which was loaded at early age after 28 d will be smaller.The ultrasonic pulse velocity of the early age backfill body increases linearly with the increase of curing age,and it increases gradually to constant after 28 d.The resistivity of the backfill column increases with the increase of loading time and is related to the environment.The smaller the humidity is,the higher the increasing ratio of pulse velocity will be.Under the condition of the stress-strength ratio at 80%,the backfill body can keep its stability,which means that the design of the cemented gangue backfill body can be loaded with the 80% UCS.Finally,according to the long-term strain-time curve,a creep constitutive model which considers the continuously increasing elastic modulus and the increasing stress at an early age is proposed,which can be used to guide the design of the constructional backfill mining.