1、英文原文：Numerical simulation of the top coal caving process using the discrete element methodAbstractHow to understand the top coal caving process and increase the top coal recovery ratio is one of the urgent problems to resolve for longwall top coal caving technology. This paper discusses a new theory
2、 and technology of top coal caving with vibration. The vibration device is mounted on the shield beam of the top coal caving support. The vibrator destroys the arch structure formed during the top coal caving process, which results in the smooth top coal caving process, and in this way the top coal
3、recovery ratio is increased. The distinct element method (DEM) numerical software particle flow code in 2 dimensions (PFC2D) is used for the numerical simulation. It is proven that the vibration can easily destroy the arch structure which is formed during the top coal caving process. The parameters
4、about when the arch structure would be formed during the top coal caving process have been obtained. The movement of the top coal caving process under the vibration condition makes a more stable layer uniform caving performance. The aim of the top coal caving with vibration technology to increase th
5、e top coal recovery ratio and to lower down the waste content rate has been achieved.Keywords： Longwall Top coal caving Vibration DEM simulation Recovery ratio1. Introduction Since the mid 1980s, the longwall top coal caving(LTCC) technique has been widely used in China as an advanced under-ground c
6、oal mining method with high output and high efficiency 113. According to field experience in China, the top coal caving technique can be successfully applied to thick coal seams with moderate and low hardness. But as a new technology for the thick coal seam mining, longwall top coal caving technolog
7、y also brings about some new problems for the longwall mining system： rock controlling and mine safety. It is an essential issue in sustainable development of coal industry to keep higher recovery ratio. However, losses of coal are inevitable in LTCC technique since the top coal is caved from the up
8、 and rear of the supports. Therefore, it is one of the key problems in LTCC technique that how to obtain or even control the moving law of the top coal caving in order to increase the recovery ratio 1,4,5,716. In general, the LTCC technique is a conventional retreat longwall face with a second armor
9、ed face conveyor (AFC) towed behind the shields (as shown in Fig. 1) to recover the top coal that would otherwise fall into the goaf gob and be lost. There is a hydraulically operated tail-canopy at the rear of the LTCC support which can be moved up and down to allow the broken top coal in the goaf
10、area to spill onto the second AFC. This process is allowed to continue until all of the broken top coal is recovered and the waste rock appears. At this time, the tail canopies would be lowered and shut the gates.Unfortunately, there are some obvious weaknesses and short- comings in LTCC technique a
11、nd its support, which result in a lower recovery ratio and a higher waste rock ratio, especially at both ends of the working face because of the beaming capacity of the immediate roof. The fundamental reason for such an unfavorable feature of LTCC technique has been found. The LTCC technique is main
12、ly dependent on the influence of the beaming capacity, abutment stresses, bending strength, etc., which makes the top coal broken and fall naturally under the gravity condition. If the coal strength is low or the front abutment stress is high, the top coal can be broken easily into small pieces or b
13、locks, resulting in successive top coal caving and higher recovery ratio. But when the block of the broken top coal is big enough or the top coal is hard, the top coal caving process would be disturbed. The arch structure would be formed during the top coal caving process, one foot of which lies on
14、the shield beam, the other on the waste. The arch structure cannot be destroyed easily with the existing support structure automatically. The arch structure prevents successive upper top coal caving and lets the upper top coal stay in the gob, leading to a lower recovery ratio. That is the fundament
15、al reason for the low recovery rate.2. PrincipleThe technology of top coal caving method with vibration is proposed to solve the above problems 17,18. The fundamental principle for this technology can be demonstrated by Fig. 2. A vibrator is mounted on the shield beam of the longwall top coal caving
16、 support, which can produce low frequency vibration with high amplitude of force along the shield beam inclination. Based on the mechanical analysis, it is found that the vibration wave can decrease the loose material (top coal) inner and outer friction coefficient, decrease the top coal shear strength. As the function of the vibration, the internal friction angle is usually reduced by1度到3度while as the cohesive force c is decreased by 50% or more 19,20
