13. Pillar supported mining methods

Transcription

1 13. Pillar suorted mining methods

2 13.1 Comonents of a suorted mine structure Economic design of a suort system - Minimizing illar suort while assuring the stability of the mine structure Pillars - Panel illars and barrier illars

3 13.2 Field observation of illar erformance Stress distribution in a illar - Concentration of stress on the surface of illars and host rock Resonse of illars deends on - Rock material roerties, geological structure, illar dimension etc.

4 13.2 Field observation of illar erformance Three main modes of illar behavior under stress close to its strength (massive rock) (a) Salling (necking or fretting) (b) Shear failure (esecially at high illar height/width ratio) (c) Lateral bulging (barrelling) with internal slitting when transverse weak lanes exist between the illar and adjacent country rock

5 13.2 Field observation of illar erformance Pillars with a set of natural transgressive fractures or foliation (schistosity) (d) Sli along the fractures when the fracture di angle exceeds the friction angle (e) Buckling failure

6 13.2 Field observation of illar erformance Evolution of fracture and failure in a illar in massive rock (a) Local shear failure (b) Surface salling (c) Network of cracks making extensive fractures (d) Failure

7 Long rib illars 13.3 Elementary analysis of illar suort Tributary area method - Showing an average axial illar stress (σ ) - The same formula of illar stress is alied to both of the long rib illars and column illars ( ) σ ( ) ( ) 1 ( ) σ w = w + w = w + w w zz o zz o r = w w + w r = w w + w σ o o o ( r) = zz 1 1

8 13.3 Elementary analysis of illar suort ( )( ) σ ( )( ) σ ab = a + c b + c = a + c b + c ab zz zz ( 2 ( ) when ) σ = zz wo w w + a = b = wo and c = w ( )( ) ( )( ) 1 ( )( ) 1 ( 1 r) r = a+ c b+ c ab a+ c b+ c r = ab a+ c b+ c σ = zz Column illars

9 13.3 Elementary analysis of illar suort - Pillar stress soars at a certain high level of extraction ratio. - Extraction ratios greater than 0.75 are rare in natural illar suort. - Limitation: only the average axial illar stress is obtained; only the re-mining normal stress comonent is considered. - Pillar volume and shae affect its strength: ( ) b β o o o a a b α S = S v w h = S v R or S = S h w ( a, α < 0 and b, β > 0 refer to Table 13.1) ε a b b R S = Sov R o 1 + 1, R> Ro 5 = 2.5 ε R o ( ε ) ( r) σ zz = 1 1

10 13.3 Elementary analysis of illar suort - Failure starts at illar boundary and migrates towards the center. - Effective width is useful for illars of irregular shae: e w = 4A C e ( w = 2 w for long rib illar )

11 13.3 Elementary analysis of illar suort - Width of aralleleied illars (Galvin et al., 1999): For R < 3 w = w= w sin θ (min. width) e For R > 6 1 ( ) w = w =Θ w, Θ = 2 w w + w, 1 Θ < 2 e eo o o o For 3 R 6 w =Θ w=θw e ( R /3 1) o S = S h w S h w Θ α β α β β o o ε a b b b R S = Sov RoΘ ε R o

12 13.3 Elementary analysis of illar suort - Pillar strength in hard rock mines (Lunder and Pakalnis, 1997) 1 2 c ( c ) ( ) 0.44 ( ) S = Kσ C + C κ S = σ + κ 1 2 S: illar strength K: scale factor relating illar strength to laboratory scale strength C, C : emirical constants κ : factor of friction mobilized in the illar core under confining stress 1 C av κ = tan a cos 1 C + av C av ( w h) = 0.46 log / : average illar confinement

13 13.3 Elementary analysis of illar suort

14 13.4 Design of a stoe-and-illar layout Ex) Thickness and deth of an orebody: 2.5 m and 80 m Unit weight of rock cover: 25kNm -3 San of a room and square illar: 6 m, and 5 m Formula of illar strength: 3 (a) Pre-mining stress: S = 7.18h w P = 80 m 25 knm = 2.0MPa zz (b) Average axial illar stress: [ ] (c) Pillar strength: σ = 2.0 MPa (6m + 5 m) 5m = 9.68MPa S = = 8.22MPa (d) Safety factor: F = = 0.85 To increase the safety factor ( 1.6, refer to Fig.13.14) (i) to reduce the room san and therefore illar axial stress (ii) to increase illar width (iii) to reduce the illar height

15 13.4 Design of a stoe-and-illar layout 11.0m (a) w = 3.0 m, w = 5.0 m, h = 2.5 m o (b) w = 6.0 m, w = 7.75 m, h = 2.5 m o (c) w = 6.0 m, w = 5.0 m, h = 0.96 m o

16 13.4 Design of a stoe-and-illar layout Extraction volume and equivalent working height (square illar) ( ) V h w w w 2 2 e = o + ( ( )) ( ) ( ) e = 1 o + e o + = e = o + h h w w w h w w V h w w w - Increased h e indicates an increased orebody recovery

17 13.4 Design of a stoe-and-illar layout - Increase of w o or h increases h e (and therefore orebody extraction). To maximize the orebody recovery (S.F. remains constant) (a) The comlete thickness of orebody (M) is mined. (b) The maximum room san is mined.

18 13.4 Design of a stoe-and-illar layout - Volumetric extraction ratio, R = he M - The maximum extraction ratio decreases with increasing deth of the orebody and with increasing thickness of the orebody - General conclusions in illar design: (1) With singe hase of mining, the stoes must have the largest stable sans. (2) Fully suorted methods using illars are limited to low stress or hard rock conditions. (3) Thick orebody in weak rock masses may be mined in successive hases.

19 13.5 Bearing caacity of roof and floor rocks - Roof or floor rocks can be unched by illars Caacity and F.S. - Long rib illars: 1 qb = γ wnγ + cnc 2 N = N 1 cot, N = 1.5 N 1 tan φ (bearing caacity factors) ( ) φ γ ( ) c q q π tanφ 2 π φ Nq = e tan where γ = unit weight, c = cohesion, φ = friction angle - Panel illars: 1 qb = γwnγsγ + ccotφnqsq ccotφ 2 S = w l, S = 1+ sin w l (shae factors) γ FS.. = q b σ ( ) q φ ( )

20 13.6 The Elliot Lake room-and-illar mines North South - Uranium-bearing orebodies: 3 m ~ 8 m thick, di south 15 ~20, 1,050m dee at max. - Transort drift: along strike, at 47 m vertical interval ( 76 m of stoe length)

21 13.6 The Elliot Lake room-and-illar mines - Rib illar: Strength, S = 133 h w 0.5, 23 m aart on strike (S.F.=1.5 adoted) - Extraction ratio: 70~85% until Pillar failure: when 9 level stoes were in rogress

22 13.6 The Elliot Lake room-and-illar mines - Rib illar: Strength, S = 133 h w 0.5, 23 m aart on strike (S.F.=1.5 adoted) - Extraction ratio: 70~85% until Pillar failure: in trackless area when 9 level stoes were in rogress

23 13.6 The Elliot Lake room-and-illar mines