Research On The Instability Mechanism And Prevention Of Residual Coal Pillars In The Composite Re-mining Arers

The residual coal pillars are distributed densely with different shapes and sizes in the composite re-mining Areas,which are interacted each other.The original intention of residual coal pillars is to carry the overburden load and to ensure the long-term stability of the mining area with a form of groups.However,the residual coal pillars in the composite residual mining area would undergo rupture from the surface to the inside under the long-term overburden stress.The bearing capacity would weaken gradually.Instantaneous destruction or the domino" instability of residual coal pillars may be caused,which then led to the dynamic disaster such as the overburden collapse,surface collapse,dynamic shock or gas leaking.The safe and efficient mining in adjacent areas may also be threatened.Based on the interdisciplinary theories of rock mechanics,structural mechanics,systems science,numerical mechanics,mining science,the induced-instability factor is firstly identified in the bearing system of composite re-mining areas.The failure characteristics and mechanism of residual coal pillars in the composite re-mining area are then studied.The basic theory of key coal pillar in the residual pillar mining area is concealed.At last,the prevention technology is developed to control the instability of residual coal pillars in the composite re-mining areas.The main research achievement of the thesis are as follows:(1)The concept of multi-element coal-rock structure body is proposed,which is a bearing system with a coupling relationship between the residual coal pillars and its roof strata,floor strata or interburden strata.The coal element is more likely and easily to be ruptured in the multi-element coal-rock structure body.The damage of rock element is relatively lagging behind,which is mainly caused by the initial rupture of the coal element.Meanwhile,the damage of coal element is more serious than the rock element.The global instability of multi-element coal-rock structure body is the interaction result of initial failure of coal element and the linkage failure of rock element.The coupling relationship between the residual coal pillars and the surrounding rock strata in the composite re-mining areas is reflected as follows:(1)The initial failure of residual coal pillar induced the linkage failure of surrounding rock strata.(2)The decompression and energy-releasement of surrounding rock strata accelerates the instability and damage of residual coal pillars.The induced-instability factor is the coal pillar in the bearing system of composite re-mining areas.(2)According to the difference of distribution level and space,the residual coal pillars in the composite re-mining areas can be divided into series coal pillars and parallel coal pillars.Series residual coal pillars refer to the pillars at the different mining levels.Parallel residual coal pillars refer to the pillars at the certain mining level.Both the uniaxial compressive strength and the elastic modulus are the factors influencing the failure of series and parallel coal pillars.The collapse of serial coal pillar system is occurred along with the local failure of lower UCS or E coal pillar.The damage of parallel coal pillar system is induced by the local failure of lower UCS or higher E coal pillar.And the collapse of parallel coal pillar system is lastly occurred along with the higher UCS or lower E coal pillar.(3)The unbalanced deformation characteristics are revealed in the failure process of serial coal pillars.The domain deformation pillar can be used to represent the unbalanced deformation characteristics.The unbalanced loading characteristics are revealed in the failure process of parallel coal pillars.The domain deformation pillar can be used to represent the unbalanced loading characteristics.The system collapse of serial coal pillars is the evolution result of domain deformation pillar.The critical instability condition of the series coal pillar system is ’ ’()()0u u b bf u +f u =.The system collapse of parallel coal pillars is the evolution result of domain deformation pillar.The critical instability condition of the series coal pillar system is ’ ’()()0l lr rf u +f u =.On this basis,the weakest instability model is then proposed.That is,the overall instability of residual pillars system is firstly occurred in the pillar with the weakest stability factor.Local instability of residual coal pillars with the weakest stability factor is the basic premise for the overall failure of residual coal pillar system.(4)The basic concept of key pillar related with the system instability is proposed.The local instability of key pillar is occurred earliest in the pillar system.Only the local failure of key pillar occurred,the collapsing of adjacent coal pillars with stronger stability can be activated.And the overall residual coal pillar system can occur in the composite re-mining area.The main steps and technical map of determining the key pillar are put forward based on the minimum equivalent width of the key pillar and the maximum distance of load diffusion The KPDS software,which is designed based on the Python language,is developed to identify the position of key pillars.Then,the load relationship between the key pillar and the adjacent coal pillars in the upper and bottom residual mining area is analyzed.It is found that the local instability of the key pillar will cause the load transfer to the nearest residual coal pillars,which will induce the further instability,and eventually trigger the chain collapsing of residual coal pillars.At last,the key pillar theory is preliminarily formed for guiding the instability prevention of residual coal pillars in the composite re-mining areas.And the possible application fields and scope of key pillar theory are prospected.(5)The idea of "homogenization" and "dynamization" should be followed in the prevention and control of the instability of residual coal pillars in the composite re-mining area.The backfilling technology along with the key pillar is developed to prevent and control the overall instability.The core mechanism of backfilling along with the key pillar is revealed.The coupled bearing load of “key pillar-backfilling body” is determined.And the lateral restraint force produced by the pillar-side backfilling body on the key pillar is also determined.The effect of backfilling technology along with the key pillar is evaluated based on the distribution of bearing load and plastic zone of “key pillar-backfilling body”.The main materials,compressive strength,critical width and basic shape of backfilling body at the side of key pillar are determined.The technical measures,such as optimizing the strength of key pillar by grouting modification,weakening the load of key pillar by pressure relief and release,strengthening the restraint of key pillar by FRP wrapping and reducing the exposing of key pillar by thin layer spraying,are put forward to prevent and control instability.