Study On Creep Failure Mechanism And Stability Evaluation Of Strip Coal Pillar Under Water Immersion Condition

Strip mining is one of the important ways to promote and achieve green coal mining.It has played an important role in the fields of "coal mining under buildings,railways and water-bodies" mining and ecological environmental protection in mining areas for many years.The long-term stability of strip coal pillars is a core issue of concern in strip mining practice,which is of great significance for the engineering construction and utilization of existing strip mining subsidence areas and guiding the optimization design for future strip mining.In order to explore the creep failure mechanism and stability of strip coal pillars under the combined effects of internal factors(such as the creep effect of long-term coal bearing capacity)and external disturbance factors(such as water immersion softening of coal pillars),this study taking the 7300 mining area as the engineering background,by using various methods such as similarity simulation,theoretical analysis,numerical simulation,and on-site measurement were used,The research focuses on the creep failure mechanism and stability evaluation method of strip coal pillars under immersion conditions,and the main results and conclusions are as follows:(1)Comprehensive exploration of the old goaf with an average mining thickness of 2.60 m by combining high-density 3D seismic,transient electromagnetic method,borehole exploration,and other methods.The exploration results showed that the strip goaf was completely filled with water,and the average heights of the water conducting fracture zone and the collapse zone were48.60 m and 6.06 m,respectively.The cumulative mining height conversion average porosity of the water conducting fracture zone was 57.12%,and the plastic failure width of the strip coal pillar near the goaf was 18.50 m.(2)Conducting similar simulation experiments under submerged conditions in goaf areas has solved technical problems such as similar mechanical strength softening of simulated materials soaked in water and sealing of goaf water.Physical simulation of the deformation and failure characteristics of overlying rocks and the stress changes of coal pillars during strip mining has been achieved,and the "activation" failure characteristics and movement deformation evolution laws of overlying rocks in goaf under immersion conditions,as well as the stress distribution characteristics and evolution laws of coal pillars has been mastered.(3)Based on the multi-level creep experiment of immersed coal samples,the evolution law of creep performance of coal samples under immersion conditions was studied,and a fractional order creep constitutive model of coal rock under immersion conditions was constructed based on elastic,nonlinear viscoelastic,and ideal viscoplastic elements;The creep model parameters were obtained through the immersion coal pillar creep experiment,and the reliability of the creep model and parameters was verified by comparing numerical and similar simulations.(4)We constructed two-dimensional and three-dimensional finite element numerical models of different scales in the study area,implanted the creep time relationship in the proposed creep constitutive model,and evaluated the long-term stability of the strip coal pillars in the study area;The mechanism of various factors affecting the creep failure of strip coal pillars was simulated and analyzed from the perspectives of maximum principal stress,distribution of plastic zone,and creep strain,revealing the creep failure mechanism of strip coal pillars under immersion conditions.(5)A stability evaluation index system for submerged strip coal pillars was constructed based on 13 main influencing factors such as mining depth and stopping time,revealing the mechanism of each index’s impact on coal pillar stability;A TFAHP cloud model based stability evaluation method for submerged strip coal pillars was proposed and applied to the stability evaluation of 5 coal pillars in the7300 mining area.The reliability of the evaluation method was verified by comparing it with surface and borehole measured data.