First, the analysis of the direct top The completeness of the direct roof depends on two factors: one is the mechanical properties of the rock formation itself, and the other is the development of the bedding and fissures caused by various reasons in the direct top rock stratum. The initial slump step L0 of the direct top is used as the engineering index of the direct top classification, and the top plate with the initial slump step L0 ≥16 ~ 20 m is called the stable top plate, and the top plate with L0 ≤8 m is called the unstable top plate. The top plate between L0 = 9 and 15 m is called a medium stable top plate. The West Germany Essen Mining Research Center proposed. The degree of end face fracture refers to the degree of fracture of the top plate between the front end of the bracket and the coal wall, that is, the degree of fragmentation is expressed by FA/F%. According to a large number of measurements in the West Deir area, the top plates of the area can be divided into three categories: E = 0 to 10% for the first category and E = 11% to 30% for the second category. The three categories are E > 30%. For the third category, the end face spacing should be as small as possible from the bracket design and production management to prevent the roof from falling. The relationship between these three types of end face distance and top plate breakage is shown in Figure 5-1. Figure 5-1 Relationship between the face distance and the degree of roof fracture Second, the analysis of the basic top The ratio of the direct top thickness hi to the coal seam height hm is introduced in the basic top classification. N, N = hi / hm. Classification considers: I. N > 5, at this time, the basic top of the slump and the misalignment have little effect on the working face bracket, which is called the top plate without periodic pressure or periodic pressure. II. 2 < N < 5, at which time the instability of the basic top has a more serious effect on the working face support, which is called a top plate with periodic pressure. III. N < 2, not even direct top. At this time, the suspension and slump of the basic top will have a serious impact on the working face support. It is called the cycle to press the severe top plate. IV. The basic top is particularly hard and has no direct top. At this time, the roof plate often hangs over 10,000 square meters in the goaf without falling. When it falls, a storm is formed, and the roof is often cut along the work surface, causing an accident. This type of roof is called a very hard top. Since large-area hard roofs are difficult to handle, for a long period of time, coal pillar support can still be used to manage such roofs. That is, after a certain distance, a coal pillar is left in the goaf to prevent the roof from falling. Obviously, this method will cause a large coal loss and is not conducive to the use of comprehensive mechanized coal mining. However, through years of research and practice, using the blasting to drop part of the roof, or injecting high-pressure water to soften the roof, etc., the roof can be basically controlled to seriously control the serious threat to the working surface caused by large-scale roof collapse. These will be further described in Chapter 10. V. A plastically curved top plate. The top plate above the coal seam is slowly sinking as the working surface advances, and then gradually contacts the coal floor. Formed in this case, obviously the nature of the top layer of the slate, rock mining and high thickness. It is generally only possible to appear in thin coal seams or limestone roofs of medium-thick coal seams of small thickness. Third, the roof classification scheme and its indicators Table 5-1 Direct top classification indicators and reference elements category Class 1 Unstable roof Class 2 Medium stable roof Category 3 Stable roof 4 types Very stable roof 1a 1b 2a 2b Basic indicator Τr≤4 4<Ï„r≤8 8<Ï„r≤12 12<Ï„r≤18 18<Ï„r≤28 28<Ï„r≤50 Lithology and structural features Mudstone, mud shale , joint fissure development or soft Mudstone, carbonaceous mudstone, joint fissures are more developed Dense mudstone, siltstone , sandy mudstone, joint fissures are not developed Sandstone, limestone, joint cracks are rare Tight sandstone, limestone, and joint fissures Main mechanical parameter reference interval Comprehensive weakening constant CZ=0.163±0.064 CZ=0.273±0.09 CZ=0.30±0.12 CZC=0.43±0.157 CZC=0.48±0.11 Unidirectional compressive strength RC=27.94±10.75 RC=36±25.75 RC=46.3±20 RC=65.3±33.7 RC=89.4±32.6 Layer thickness H0=0.26±0.125 H0=0.285±0.13 H0=0.51±0.355 H0=0.675±0.34 H0=0.72±0.34 Equivalent bending resistance RC h0<7.52 RC h0=2.9~11.4 RC h0=7.8~29.1 RC h0=33~104 RC h0=45.5~139.4 Note: Among the reference indicators, CZ, RC, and h0 are the mean value plus or minus mean square error of the corresponding parameters of each coal seam in the top plate of this type. Table 5-2 Basic top grading indicators Basic top rating I II III IV Basic top pressure Not obvious obvious strong Very strong IVa IVb Grading indicator ≤895 895<≤975 975<≤1075 1075<≤1145 >1145 Fourth, the bottom plate features The floor rock layer involves two types of problems in mine pressure control: one is the damage of the floor caused by coal seam mining, and its range will be related to the mining range and the distribution of bearing pressure around the goaf. The damage of the groundwater may be caused by the damage of the floor. For example, the bottom plate of many coal seams in North China is Ordovician limestone, which is rich in water. The deformation and destruction of the bottom plate after coal seam mining may cause water inrush and other accidents. Therefore, it is necessary to study the damage law of the bottom plate after mining; In terms of the protection system, the stiffness of the support system is composed of “base plate-bracket-top plateâ€. Therefore, the rigidity of the floor rock layer will directly affect the performance of the support. Since the bottom area of ​​the single-pillar is only 100cm2, the comparison is made in the bottom plate. In the case of softness, the struts are easily inserted into the bottom plate, which affects the control of the top plate. Figure 5-2 Relationship between measured strut load and pillar penetration in working face It should be pointed out here that the bottom shoes should not be made of wood, because the lateral compressive strength of the wood is very small, only about 3 MPa, which is similar to the case of the soft bottom plate, so the anti-insertion ability is poor and the effect is not obvious. According to the influence of the floor of the coal mining face in China on the pillars, the bottom plates are classified, as shown in Table 5-3. The bottom area that the struts should have can be selected according to this table. Table 5-3 Classification scheme of the floor of the gently inclined coal seam working face in China Base plate category Basic indicator Auxiliary indicator reference indicator General lithology name Code Allowable specific pressure qc / MPa Allowable stiffness Kc / MPa•mm-1 Allowable penetration βc/ mm-1 Allowable uniaxial compressive strength Rc / MPa Extremely soft I < 3.0 < 0.035 < 0. 20 < 7.22 Filling sand, mudstone, soft coal Soft II 3.0~6.0 0.035~0.32 0.20~0.40 7.22~10.80 Mud shale, coal Softer IIIa 6.0~9.7 0.32~0.67 0.40~0.65 10.80~15.21 Medium hard coal, thin layered shale IIIb 9.7~16.1 0.67~1.27 0.65~1.08 15.21~22.84 Hard coal, dense shale Medium hard IV 16.1~32 1.27~2.76 1.08~2.16 22.84~41.79 Compact shale, sandy mudstone hard V > 32 > 2.76 > 2.16 > 41.79 Thick sandy shale, siltstone, sandstone Da Series Pressure Valves,Oem Pilot Unloading Valve,Hydraulic Pilot Unloading Valve,Hydraulic Pilot Unloader Valve Shanghai Jinhai Hydraulic Co.,Ltd , https://www.shanghaijinhai.com
Where FA is the area of ​​the fracture and F is the area of ​​the entire beam end to the coal wall of the working face. However, the area where the height is more than 5 ~ 10 cm is generally counted in the broken area. 