SIMULATION AND MECHANISM ANALYSIS OF WATER INRUSH FROM KARSTIC COLLAPSE COLUMNS IN COAL FLOOR

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1 (15) Chinese Journal of Rock Mechanics and Engineering Aug ( ) ( ) FLAC 3D - FLAC 3D TD 82 A (2004) SIMULATION AND MECHANISM ANALYSIS OF WATER INRUSH FROM KARSTIC COLLAPSE COLUMNS IN COAL FLOOR Yin Shangxian 1 Wu Qiang 2 ( 1 Facility of Safe Sciences and Technology North China Institute of Sciences and Technology Beijing China) ( 2 Dept. of Res. Expl. Eng. China Univ. of Mining and Tech. Beijing China) Abstract Karstic collapse columns are a kind of vertical structures typically found in Carboniferous-Permian coalfields in north China and are widely distributed in 45 coal mine areas of 20 coalfields. Because of its hidden characteristics of the outburst and natural relationship with karstic groundwater the water inrush caused by karstic collapse columns is more harmful to mining safety. In order to study the failure characteristics of surrounding rock masses and mechanism of water inrush caused by karstic collapse columns in coal seam floor several situations of movements of mining work face are simulated with FLAC 3D when a collapse column exists in the coal floor. Numerical simulations and experiments show that the rock failure mechanism under hydraulic pressure is of a shear-tension-pressure complex with shear as the control factor. Because of the karstic collapse column the geological environment and rock structure in the coal floor will change and the effective thickness of the protective layers as well as the strength of rock mass will be reduced. In addition the stress and strain distributions are asymmetrical and local stress concentration coefficients become larger so that minor principal stresses of key stratum decrease further. When hydraulic pressure is larger than that of the minor principal stress of the key stratum mining pressure and seepage as well as dilation will cause fractures to expand along the weaker directions. Fractures eventually transfix and water inrush takes place. When the border cliff of the collapse column and * ( ) 38 yinshx@21cn.com

2 division line of compressive and expansive section of coal floor are coincident on the same line the shear yield and water inrush from coal floor are most likely to take place. Key words mining engineering mechanism of water inrush FLAC 3D simulation karstic collapse columns rock failure 1 coal floor 20 m FLAC 3D (Li Jinkai ) [1] m 3 /min 3 [2] 1 A. A. BOPÈCOB Fig.1 A numerical model [3] [4 6] [7] 20 m [8] [9] [10] 604 m 6.04 MPa 490 m Hoek- (y ) (x ) (z ) m 800 m σ m 3.5 m( 1 ) 50 m( 1(b)) 20 m [11 12] z x y (a) (b) (x = 570 m) 5 1 Brown 2 1 s σ 3 + ( m σ cσ 3 + sσ c ) = (1) 1 2 [2]

3 Table 1 Rock mechanics parameters at Nansi anticline /GPa /MPa /( ) /MPa (a) σ 1s σ 3 m s σ c 10% 2.2 (1) m 20 m 1 cm (2) 40 m 1 cm 60 m 2 cm 28 m 40 m 2 22 m Table 2 Displacement and its depth without collapse 2 cm 2 3 column in floor (3) 300 m ( 2) (b) m 160 m Fig.3 z-displacement with or without a collapse column at first roof collapsing with working face length of 160 m and advance of 160 m /m /m /cm (a) (b) m ( Fig.2 z-displacement with or without a collapse column at first roof collapsing with working face length of 160 m 40 m [13] 200 m ( 4) (4) 160 m ( 2) 5 m ( 4) 40 m 5 m 80 m 22 m ) 160 m

4 S zz 100 m S zz ( 3) (a) (b) m 160 m 5 m Fig.4 Yield state at 5 m depth with or without a collapse column with working face length of 160 m and advance of 160 m (a) 200 m 230 m 34 m 266 m 300 m 500 m 800 m 5 80 m 500 m S zz Fig.5 Vertical stress with or without a collapse column at first roof collapsing with working face length of 80 m m 100 m [13 14] (5) S zz S yy ( 5 7) S zz S yy 6 80 m S yy (b) (a) (b) (6) Fig.6 Horizontal stress with or without a collapse column at first roof collapsing with working face length of 80 m

5 (a) 8 Fig.8 Sketch of influence of a karstic collapse column on yield of coal floor at working face (b) ) m 80 m S zz Fig.7 Vertical stress with or without a collapse column with working face length of 160 m and advance of 80 m) 3 S zz Table 3 Vertical stress with influence of a collapse column /m /MPa [9] [15] [7] [16] - ( 8) (1) - - ( ) ( ( ) ( ) 3 (3) ( 8) 4 - ( 8) (2) -

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