| The caving mechanism of loose top-coal is one of the basic problems in the field of longwall top-coal caving(LTCC)mining research.Its essence is the flow of non-ideal loose media and it is very difficult to give a strict mathematical expression or mechanical solution theoretically,so the similar material simulation test and numerical simulation were employed to study the caving mechanism of loose top-coal in this dissertation.Firstly,in-depth study was carried out on the effect of the presence of caving shield on the drawing body's shape by three-dimensional similar material simulation test and numerical simulation.Secondly,effects of dip angle and upward/downward angle of working face on the caving mechanism of loose top-coal in complex coal seam conditions were analyzed.Finally,technology of improving top-coal recovery in longwall top-coal caving mining based on BBR research system was studied.The main conclusions are as follows:The Boundary-Body-Ratio(BBR)based research system,in which the boundary of top-coal,drawing body,recovery ratio and rock mixed ratio of top-coal are studied as a whole,was established by systematic research on the top-coal caving mechanism in longwall top-coal caving mining.The shape of the boundary of top-coal affected by the existence and movement of shied support was analyzed and a simplified parabolic model is proposed to describe the development process of the boundary of top-coal in normal top-coal drawing cycles.The developing process of drawing body can be divided into three stages: incomplete development stage,basically mature stage and mature stage.The front part of the drawing body develops faster than any other part because of the influence of shield beam and it is always beyond the ellipsoid area.The drawing body of top-coal is a non-standard ellipsoid cut by the shield beam,which can be defined as Cut-Variation Ellipsoid(CVE).Three are three features of CVE: “oblique cut”,"differential flow" and "advanced development".And they are caused by the geometric properties of the shield itself,the localization of loose media flow and the difference between internal and external friction angle of loose coal respectively.During the initial stage of top-coal drawing,the drawing body is composed of pure coal;the rock is then gradually mixed Rock mixing first appears in the middle part of the drawing body next to the gob and subsequently expands upward and downward.The main trend is an upward expansion.To improve the recovery ratio and reduce the rock mixed ratio of top-coal,the overlapping area between the drawing body and the boundary of the top-coal should be expanded to the greatest extent possible,which can be realized by selecting reasonable drawing technique parameters and controlling the shape of the boundary of the top-coal.This is also the main idea behind the BBR research system.Top-coals to be drawn are in the distressed zone under the pressure arch between the rock in gob and the canopy of shield support,and it is easy for the top-coals around the drawing opening to form arches.In longwall top-coal caving,the shields will cause the decrease in the drawing volume.By modifying the formula for calculating the drawing volume in classical ellipsoid theory of metal mines,the formula to calculate the drawing volume in longwall top-coal caving is proposed.The phenomenon of drawing body advanced development is observed in 3D loose top-coal drawing test,which meets one of the three features described in the BBR system.The controlled test verifies that the "advanced development" of drawing body is caused by shield pressence.The deflection angle decreases with the increasing caving height.The method of classifying the degree of influence of the shield based on the parameter ? is proposed for the first time.The existence of the "advanced development" of drawing body and correctness of the proposed formula to calculate the drawing volume in longwall top-coal caving are also verified in numerical simulations by using particle flow code PFC3 D.The results from numerical simulation and similar simulation test have a good consistency with each other in different caving heights.The initial drawing volume increases with increasing face dip angle and it increases faster when the dip angle is larger than the natural resting angle of loose coal.The section of the drawing body on the vertical palne along the face line approximate an ellipse with vertical major axis.With the same top-coal initial boundary.The length of the major and minor axes increases with increasing face dip angle and the value of eccentricity roughly fluctuates within the range of 0.76-0.81.The boundary of top-coal near the upper end of the face is gradually close to the gravity line,and its shape gradually becomes gentle and long.The top-coal recovery ratio increasing at first and then decreasing with increasing face dip angle.This means that top-coal recovey in inclined or steep condition is lower than that in flat condition,so the targeted optimization measures should be used to incease the top-coal recovery ratio in inclined or steep top-coal caving mining.The caving volume of top-coal caving face in closely horizontal coal seam with multi-gangue increases from upper face end to lower face end,while the refuse content decreases accordingly,and the lower face end has a higher caving efficiency;the top-coal recovery ratio zoning presents dynamic evolution process along the coal seam's strike direction and periodic distribution along the dip direction;the relationship between volume of coal-gangue drawing body and drawing height presents the a power function;the top-coal recovery ratio and refuse content show stages of change with the drawing time,and the interval caving method can improve the top-coal recovery ratio of multi-gangue coal seam.The top-coal recovery ratio shows "?" shape distribution along the dip direction longwall top-coal caving mining in steeply inclined thick coal seam.The 3D loose top-coal drawing experiment shows that the drawing body has an obvious developing tendency towards the upper end of the working face and the difference value of top-coal lose at the upper and lower end of working face,which is resulting from the "jujube core shape" of drawing body and the asymmetry of interface between coal and rock,is the root cause of the "?" shape distribution of top-coal recovery ratio.Such distribution law is also verified by the numerical calculation and the filed test.According to the "?" shape distribution of top-coal recovery ratio,some targeted measures,such as reduce the number of the undrawing sheields in the lower end and increase the length of working face appropriately,are recommended to increase top-coal recovery in top-coal caving face in steep seam.Among the factors that affect the drawing body's shape in upward/downward longwall top-coal caving mining,gravity is dominant and the supporting action is secondary,while the upward/downward angle has little influence.With the same top-coal initial boundary,the drawing volume increases with increasing upward angle but decreases with increasing downward angle,while the basic shape of the drawing body remains the CVE.The shape of the top-coal terminate boundary becomes steeper with increasing upward angle.The increasing of the gravity component of top-coal along the strike direction leads to a substantial possibility of top-coal flowing into the gob.Due to the upward angle,the top-coal located in front of the working face was easily drawn out ahead.These two factors make the top-coal recovery ratio fluctuate(decreasing at first and then increasing)with increasing upward angle.The convex point of the boundary of the top-coal moves toward the goaf with increasing downward angle,which would speed up the mixing of rock from gob and result in the incomplete development of the top-coal drawing body.The top-coal recovery ratio decreases gradually with increasing downward angle.The drawing weight is negatively correlated with the rock mixed ratio in the top-coal drawing process;compared to the horizontal LTCC,the complete drawing zone of the top-coal recovery ratio tends to develop towards the working face's advancement in ULTCC.The upward angle has minimal influence on the top-coal recovery ratio along the dip direction of the coal seam.With increasing distance to the floor,the top-coal recovery ratio decreases along the vertical direction of the coal seam.The field observations on the top-coal drawing process in ULTCC mining were conducted by a self-developed top-coal tracking device(TTD).The results show that the drawing sequence of different heights of top-coal along the vertical direction of the coal seam is top ? low ? middle.The top part of top-coal has a lower recovery ratio than the low and middle parts because of the higher possibility of top-coal flowing into the gob.The top-coal recovery ratio in mutil-openings condition is larger than that in single-opening condition.The increased volume comes from the middle and upper part of the top-coal above the central face.With the increasing of opening length,the volume of drawing body increases linearly and the rock mixed position in drawing body moves to directly above the opening;the shape of the boundary of the top coal appears the bulging characteristic toward the goaf gradually.The partially reverse drawing technique increases the top-coal recovery ratio in the lower end of the face by 13.98% compared to normal drawing technique.The shape of the boundary of the top-coal near the junction of reversed segment and normal segment is beneficial to the development of drawing body,which decreases the residual coal in the goaf.With increasing reversed segment's length,the growth rate of the drawing volume in reversed segment decreases after an initial increasing phase.The top-coal recovery ratio in both the middle and the lower end of the face will be increased by using the partially reverse drawing technique with double-openings. |
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