Study And Application On Stability Of Hard And Massive Overlying Strata In Fully Mechanized Sublevel Caving Face

The theory research and the engineering technology of fully-mechanized sublevel caving face have achieved through many year' s development. But there are a lot of problems which are not solved. Aimed at the stability characteristic of fully-mechanized sublevel caving face, the key problems which this paper will solve are as follows: the first one is the movement rule and key role of hard and massive overlying strata of fully-mechanized sublevel caving face at first weight period; the second one is the relationship between the top coal' s recovery ratio and the stability of roof structure of fully-mechanized sublevel caving face at cyclic weight period. Using the research conclusions mentioned above and the engineering data, the stability of the isolated fully-mechanized sublevel caving face under weighty conglomerate rock of Changcun Coal Mine in Henan province and the mechanism of "Abnormal Pressure" of fully mechanized sublevel caving face are studied.Based on the mechanics model of considering the gravity of fixed beam as the centralizing force, there are three kinds of movement mode of hard and massive overlying strata at the first weight period, namely, the shearing with and without the weak interbedding to directly cause the key falling of strata; rupture resulting from pressure and shearing in the whole strata; and rupture of the strata close to two ends of beam. This paper presents the mechanics judgment modes of the possible movement of the hard and massive strata by analyzing the breaking patterns of the hard roof of the overlying strata of longwall face using the elasticity theory. In this paper, the structure of the main roof before its initial fracture is viewed as the fixed beam model, and the gravity of the hard roof and its overlying soft strata is considered as the distribution force which acts on the fixed beam. By use of the elastic theory and analyzing the stress field of the fixed beam under the influence of the gravity, the author suggests the mechanics judgment model of the overlying strata in three kinds of possiblemovement. The conclusions are that the key role of hard and massive overlying strata after their shearing. According to our study, the analysis of the breaking patterns of the roof of the hard and massive strata based on the consideration of its gravity, compared with the simple uniform stress way which is usually used, is more likely to show the true situation of the strata and the accurate breaking pattern of the key layer in its movement. This can help provide the theoretical basis for controlling the hard and massive strata in mining engineering. Field experiments have also shown clearly that this method is useful for the prediction of the initial breaking and step of the main roof of the hard and massive strata.Based on the roof movement characteristic of fully-mechanized sublevel caving face, two kinds of losing stability mode of roof are brought forward, Namely, the losing stability of the whole deformation of roof structure because of the increasing of the top coal' s recovery ratio; the losing stability of part contact of the main roof because of the increasing of roof span and some fabrics such as faultage in overlying strata. The mechanics models analyzing roof stability at first weight and cyclic weighting period are established. Based on the structure stability theory and the strength theory, two kinds of limited convergence maintaining the stability of the whole deformation of roof structure and the stability of part contact of the main roof are gained. According to the comparison of the two limited subsidence and the subsidence of roof brought about by the movement situation of roof in the course of rotation, the unifying mechanics conditions between the stability of the whole transmutation of roof structure and the stability of part contact of the main roof are analyzed and the judgment rules of roof stability with the top coal' s recovery ratio are established.Based on the movement characteristic of roof structure in fully-mechanized sublevel caving face, the range of movement strata of overlying strata is described particularly, and the possible numbers of rock block composing roof structure are analyzed. The stability judgment curves being relative to strata thickness at first weight and cyclic weighting period are gained. The movementstability judgment methods of key strata are presented according to the thickness of key strata and the top coal' s recovery ratio. The conclusions are that the thickness of key strata is the main factor influencing the stability of the whole transmutation of roof structure, the intensity and thickness of key strata, the friction characteristic of the interface between two blocks rock are the main factors influencing the stability of part contact of the main roof. Based on an example of fully-mechanized sublevel caving face, the feasibility of the methods forecasting the range of movement strata and the stability of roof structure according to adjusting the top coal' s recovery ratio and using mechanics theory is tested.Aimed at phenomena of support' s being pressed to burst or be unable to work in a few fully mechanized sublevel caving longwall faces and based on analyzing the factors of geology, mining, mechanics and so on, the mechanism of "Abnormal Pressure" of fully mechanized sublevel caving face is discussed.