Study On Strength Damage Mechanism Of Coal Pillar Dam In Coal Mine Groundwater Reservoir

The natural environment,ecological environment and engineering geological environment of the ecological fragile mining area in the west of China have their particularity.The high intensity of coal resources and large-scale mining inevitably have different disturbance effects on its environmental conditions.When the impact of coal mining exceeds the carrying capacity of the environment,it will destroy or change the environmental conditions of regional balance.Among them,the destruction of roof overburden structure of coal seam leads to the occurrence and recharge of groundwater,the change of circulation mode and the decrease of groundwater level,which results in the change of new water circulation model,which is dominated by interlayer runoff and vertical runoff.As a result,most roof sandstone weak aquifers and quaternary groundwater aquifers infiltrate into mine goaf and form new aquifers?underground reservoirs?.One of the key factors affecting the safe operation of underground reservoir is the weakening characteristics of the strength of coal pillar dam under the action of overlying rock pressure and water rock.The weakening of body strength is one of the key factors affecting the safe operation of underground reservoirs.Based on the sampling of Daliuta 5-2 coal and the combination of laboratory test and numerical simulation,the time-space evolution of water content,the evolution of cracks before and after failure and the failure mechanism of coal pillar dam in underground reservoir of coal mine are studied in this paper.Firstly,through natural water absorption test and nuclear magnetic resonance test,the spatial-temporal evolution law of water content?including repeated immersion?of water-bearing coal samples is obtained:with the increase of water content,the process of water absorption of coal samples is divided into accelerated water absorption,slow water absorption and stable water absorption.The relationship between water content and immersion time and times of repeated immersion increased exponentially.The distribution of water content along the direction of coal height was not uniform,and the spatial water distribution changed from non-uniformity to uniformity.The time-space evolution of the fracture before and after the failure was obtained by NMR,3-D CT,and uniaxial compression.The three times of water immersion were2.40%higher than that of the two native structures.After loading,the low water content coal-like internal pores,the fracture development is serious,the degree of fragmentation is severe,the high water content coal sample exhibits through the fracture,the micro-structure development is less,and the degree of damage is relatively mild.Based on the analysis of macro-and micro-destruction forms and acoustic emission characteristics of coal samples,the failure types of coal samples are analyzed:with the increase of water content of coal samples,the failure forms of coal samples change from tensile failure to shear failure.As a result,the degree of failure presents the characteristics of mass caving-local shedding-penetrating crack.By studying the water content distribution,acoustic emission location and high amplitude signal extraction before the coal sample failure,the spatio-temporal evolution law of the internal fractures of the water-bearing coal sample under loading is obtained:the acoustic emission localization event is consistent with its macroscopic failure form.Finally,FLAC^3D numerical simulation is used to study the damage evolution characteristics of the dam body of the underground reservoir.In this paper,the development and change of stress environment,displacement and plastic zone of coal pillar dam during repeated flooding are analyzed,and the development and failure characteristics of"X"shape and bottom plastic zone of coal pillar are put forward.