Study On The Coupled-crack Base And Application For Coal-rock Under Complicated Environment

The key to mine very thick coal seam under complicated conditions is to improve the topcoal caving and reduce stress concentration. Injecting water and blasting are widely used as aneffective means of fracturing. Because of the complex coal-rock structure and material, thecoupling fracturing problem is the complicated action of multi-media includedcoal/rock-crack-water-explosive gases. the study on the mechanism of coupling fracturing,optimization effect and microscopic parameters for discrete coal, have important scientificsignificance and application value.Centered on the coupled-crack for steeply inclined coal-rock under the complicatedenvironment, the combination of theoretical analysis, rock mechanics experiments, numericalsimulation, neura network, field monitoring and industrial test are used in this study. Themain achievement are shown as below:(1)The mining environment and stress conditions of mining steeply dipping coal seamare complicated. The fracture in roof rock has not fully developed, the roof is nearly intact,which easily leads to the gravity collapse and other dynamic disaster. Working faceencounters puzzle of effectively weaken thick coal seam and pressure relief of hard roof rock.They are related to stress concentration caused by the encounter of mining and tunneling workface, the unknown mining history by small coal mines and H2S gas and frequent dynamicdisasters, which further complicate mining environment and stress conditions.(2)The improving caving effect of coal is mainly related to the macro fracture zoneproduced by blasting, the micro crack disturbance should be considered for some strictlycontrolled blasting engineering. The crushed zone and fractured zone induced by explosionare related to the explosive performance and physical and mechanical properties of blastedbody. The crushed zone radius and crack area radius respectively divided by explosive radius does not increase with the explosive radius, but the radius of fracture zone and cracked zoneincreases with the increase of explosive weigh.(3)The strength deterioration evaluation index is proposed to evaluate hydraulicfracturing. The specimen failure pressure has an increasing intend by the action of size effect,failure pressure showes a good correlation with the valued of specimen radius (R)/boreholeradius (r), the hydraulic pressure increases with the high demand of damage range. Therequired damage load is reduced under the effect of seepage pressure, the larger seepagepressure is, the earlier the damage of top specimen occurs. The relationship of degradationindex Y and seepage pressure of inject water is derived, the expressions of specimen strengthand peak load with seepage pressure are obtained.(4)The meaning and fracturing mechanism of coupled-crack are put forward and defined.The essence of coupled-crack is that the detonation gas and shock wave formed by explosionspread in soften coal-rock. The coupled-crack effect is greater than the simple superpositionof blasting and injecting water. The strength deterioration rate f is introduced as the evaluationindex of coupled-crack effect, and the crack growth criterion in the implementation ofcoupled-crack is given, the relationship between the main coupled-crack parameters andstrength deterioration degree is obtained. The equivalent conversion of coal from integral togranular is achieved, obtaining the micromechanical parameters for equivalent coupled-crackeffect based on discrete element, the relationships of release rate with different strength coaland coupled-crack parameters are established. The caving and releasing tests showed that thegranular medium flow theory is suitable for explaining the low-position top-coal cavingmining. Coupled-crack can not only reduce the strength of coal-rock, but also can reduce theoverall stress level, reducing the stress gradient.(5)The quantitative evaluation of coal-rock coupling-fracturing effect is achieved byusing BP neural network. Taking the steeply dipping extra-thick coal-rock coupling-fracturingin complex environment as the background, the coupled fracturing scheme of improvingcaving for coal and unloading for rock mass is designed, neural network is used to evaluatethe coupled-crack effect, and the caving effect of top-coal in industrial experiment isconsidered and analysed. The microseismic events is monitored to reflect relief degree ofcoupled-crack. Field results show that the problems of top coal caving and power disasterhave been better controlled and solved by the measures of coal-rock coupled-crack.The research results have better scientific and practical value in promoting the basicresearch of solid-liquid coupling and its effect evaluation, which provides guidelines for theformation of coupling-crack technology used in top coal caving faces with very thick coal seam under the complicated conditions.