Appendix D. Hangingwall failure mechanisms. Analysis of the literature review

Transcription

1 Appendix D Hangingwall failure mechanim D1 Analyi of the literature review D1.1 Elatic beam invetigation The freely upported beam and the beam with built in end were compared uing the equation for maximum tenile tre (Chapter 4.1). The analyi howed the maximum beam thickne where the induced tre wa equal to an aumed tenile trength of 10 MPa for variou panel pan. The invetigation reult are provided in Figure D 1 and how that the end at the top of a beam with built-in end will alway fail before the bottom centre of a freely upported beam, auming the ame material propertie throughout the beam. Once failure ha occurred at the end of a beam with built-in end, the beam immediately become a freely upported beam or ome combination of the two mechanim. Thu the analytical olution for a beam with built-in end i not uitable for panel pan deign purpoe, and the freely upported beam theory may be lightly conervative due to the compreive tree generated at the bottom edge once the beam with built-in end ha failed. 1

2 3 2.5 Beam thickne (m) Stable Failed Built in end Freely upported Panel pan (m) Figure D 1 Tenile limit for a beam with built-in end and a freely upported beam. A material tenile trength of 10 MPa wa aumed A econd et of analye were done to explore the effect of material tenile trength on beam behaviour. The invetigation wa done uing the analytical olution for the maximum tenile tre in a freely upported beam and a depth below urface and k of 230 m and 2.0 repectively were aumed. The reult of the invetigation are provided in Figure D 2, which how the maximum beam thickne where tenile failure will occur for variou panel pan. Tenile failure will occur at the bae of a beam decribing the relevant material trength if it i narrower than depicted by the curve. The analye indicate that tenile failure cannot occur at the bae of a beam in a 30 m wide panel if the upport capacity can carry the deadweight of a 1.5 m thick beam and the depth of working i deeper than 230 m below urface, auming a k of 2. Note that tenile failure doe not necearily mean a collape but rather progreion toward ome form of a Vouoir beam. 2

3 4.5 4 Beam thickne (m) Stable Failure T = -2 MPa T = -5 MPa T = -10 MPa T = 0 MPa Panel pan (m) Figure D 2 Tenile limit for a imply upported elatic beam for variou material tenile trength Figure D 3 how the effect of k on the behaviour of a freely upported beam. A material tenile trength of 10 MPa and a depth of 200 m were aumed in the invetigation. The effect of k on beam trength i more ignificant than the material propertie at thi depth, clearly illutrating the detrimental effect of low k on beam behaviour at hallow depth Beam thickne (m) Stable Failed k = 0.5 k = Panel pan (m) Figure D 3 Tenile limit for a imply upported elatic beam for variou k 3

4 Further analyi wa done to how the effect of depth below urface on beam behaviour. An aumed common cenario of k=1.3 wa applied with a material tenile trength of 1 MPa. Under thee condition tenile failure and the development of ome kind of Vouoir Beam i almot guaranteed in a pan of 30 m at hallow depth (Figure D 4). 25 Beam thickne (m) Stable 50 m 100 m 200 m Panel pan (m) Failed Figure D 4 Tenile limit for a imply upported elatic beam for variou depth Finally, the maximum height of a parting plane for Vouoir beam formation wa invetigated uing the analytical olution for a imply upported beam. A typical k of 1.3 wa again elected and the tenile trength of the material wa et to zero. In effect the tenile tre that developed at the bottom centre of the beam wa equal to the applied horizontal compreive tre. The reult of the invetigation are provided in Figure D 5. The analyi how that any parting plane at greater height than indicated by the curve cannot form a Vouoir beam in an environment where k=1.3. 4

5 Parting height (m) Vouoir beam formation impoible Panel pan (m) Vouoir beam formation poible 100 m 200 m 400 m Figure D 5 Parting height limit for Vouoir beam formation The graph above are not meant to be ued a deign curve but rather to illutrate condition under which partial or full Vouoir beam theory may be applicable. It hould be noted that the debilitating and trengthening effect of joint and upport repectively have generally not been conidered in the analyi. D1.2 Vouoir beam analyi Should Vouoir beam form, the analye hown in Figure D 6 are uggeted by the literature for deign purpoe. Cruhing failure i the mot likely mode (Ryder and Jager, 2002), and will occur if the beam thickne i le than the limit indicated in Figure D 6. An admiible train of 3 mm/m in the rock abutment wa aumed for the analyi. A low rock ma modulu i uggeted to account for the dicontinuitie (Ryder and Jager, 2002). Where there are adverely oriented, peritent dicontinuitie at the abutment, hear failure will occur firt on a vertical joint of friction angle φ=30 if the beam thickne exceed the limit hown in Figure D 6. 5

6 Beam thickne (m) Failed if appropriate dicontinuity preent Stable 1 Failed Panel pan (m) Shear 20 GPa 3 mm/m 40 GPa 3 mm/m 60 GPa 3 mm/m Figure D 6 Stability limit in a Vouoir beam for different value of E (after Sofiano, 1996) The analyi hown in Figure D 6 indicate a maximum untable beam thickne of 1 m at a panel pan of 30 m. Auming that the lowet beam i not loaded by other, narrower beam above, current Vouoir beam theory ugget that untable arch formation cannot occur if the upport reitance requirement hown in Figure D 7 are atified. The theory ha potential a a deign tool. However, the majority of collape occur on peritent, hallow-dipping dicontinuitie and the Vouoir beam analye may only be applicable, therefore, if it i aumed that thee dicontinuitie are detected early and appropriately upported. 120 Support reitance (kn/m2) Panel pan (m) Figure D 7 Panel pan and upport requirement deign curve baed on current Vouoir beam theory 6

7 D1.3 Dicuion Pure elatic theory ugget that Vouoir beam only form under limited mining condition. Under normal mining condition deeper than about 230 m below urface, where the upport capacity i able to carry a burden height of about 1.5 m and the panel pan i about 30 m, tenile failure hould not occur at the bae of the beam and therefore a Vouoir beam hould not form. In addition, a Vouoir beam cannot form if the lowet parting occur at a greater height than hown by the curve in Figure D 5 (providing the k-ratio and depth below urface are not le than aumed in the analyi). Should Vouoir beam form, a maximum thickne of 1 m i expected in panel pan of 30 m. Thu a nominal upport reitance of about 30 kn/m 2 i required if it i aumed that the beam i not loaded from above. The analyi how that Vouoir beam appear unlikely to form in 30 m wide panel at depth, particularly a the current pre-treed upport ytem are in exce of 50 kn/m 2. D2 Numerical modelling to determine the effect of vertical and horizontal dicontinuitie on the VTZ D2.1 Introduction Several panel uggeted that parting occurred ome 3.5 m above the working when the panel pan were about 30 m x 100 m. A 2D model wa et up under imilar tre condition to collape to conider the effect of the VTZ on an iolated panel in the preence of vertical and horizontal dicontinuitie. The pan at which the model predicted intabilitie were compared to the analytical olution for the VTZ height of an iolated panel and where cruh pillar have been cut. 7

8 D2.2 Model decription Large platen with applied preure load were modelled to obtain realitic cloure behaviour. The model conited of a blocky hangingwall part and the platen, a illutrated in Figure D 8. 2D plane train condition were applied. Figure D 8 Hangingwall model geometry The platen extended acro the total model dimenion to 460 m X 200 m. The blocky part of the model wa 60 m X 45 m and wa made up of an upper portion, coniting of 1.5 m X 1.5 m quare block, and a lower beam 15 m in height, coniting of 1.5 m wide block. Thi beam contained a ingle horizontal dicontinuity placed at a different height for each analyi. The preure load were applied via platen of the ame material a the hangingwall. A frictionle interface wa introduced between the platen and the hangingwall (and between the platen and reef). For the ret of the model, zero coheion and a contant friction angle were applied throughout. Initially, the friction angle wa et to a typical value of 30. Dicrete contact wa managed through a penalty ytem, whereby bodie were permitted to penetrate each other, with an oppoing load applied which wa proportional to the penalty parameter and the amount of interpenetration. Thi produced a dynamic ytem which relied on an appropriate choice of the penalty parameter. Experience ha hown that penalty value twenty percent greater than the material Young modulu are appropriate for the choen contact algorithm. 8

9 The choen elatic material propertie are aumed generic value for platinum mining: Young modulu 80 GPa Poion ratio 0.2 Denity 3000 kg/m 3 The modelled dicontinuity height were: 1.5 m, 3 m, 5 m, 7.5 m and 10 m. The modelled pan wa increaed to a maximum of 54 m. A depth of 500 m wa modelled with a k of 2.0. The reult were interpreted in term of cloure, the VTZ and the vertical eparation between the two part of the lower beam (at centre-pan). Cloure wa obtained by tracking the diplacement of the centre-pan node uing highreolution hitorie. The VTZ i imply the height of the zone indicating vertical tenile tre, and wa quantified by tracking the vertical tre along a vertical row of element extending from the reef horizon to the top of the beam. The vertical eparation wa obtained by tracking the difference between centre-pan nodal diplacement at the top of the lower portion and the bottom of the upper portion of the beam. D2.3 Model reult The general characteritic of the ytem are bet illutrated by examining the vertical tre ditribution over the coure of a full excavation. The evolution of the vertical tre ditribution for the 7.5 m high layer i preented in Figure D 9 to Figure D 13. The dark blue contour repreent vertical compreion. All other contour band indicate tenion, increaing in magnitude from light blue to red. 9

10 Figure D 9 Vertical tenile tre ditribution for a 7.5 m high dicontinuity at a pan of 12 m Figure D 10 Vertical tenile tre ditribution for a 7.5 m high dicontinuity at a pan of 38 m Figure D 11 Vertical tenile tre ditribution for a 7.5 m high dicontinuity at a pan of 42 m 10

11 Figure D 12 Vertical tenile tre ditribution for a 7.5 m high dicontinuity at a pan of 48 m Figure D 13 Vertical tenile tre ditribution for a 7.5 m high dicontinuity at a pan of 54 m Vertical tenion i initially manifeted at the oppoite lower urface of individual block (Figure D 9). A gradual increae in the extent of the VTZ i oberved at the centre of the pan. The greatet magnitude of vertical tenile tre are concentrated at the bottom of the block, while a zone of lower tenile tre extend upward from centre-pan (Figure D 10). Thi zone eventually reache the height of the horizontal dicontinuity at 7.5 m, whereupon the appearance of the VTZ become chaotic (Figure D 11). After a few more metre of excavation, the behaviour oberved for the lower portion of the beam appear to be reetablihed in the upper portion of the beam (Figure D 12). Thi trend continue with the extenion of the VTZ into the blocky portion of the model. Interetingly, the block within the lower portion of the beam how very high value of vertical 11

12 tenile tre at all four corner, with a ditribution of lower tenile tree within (Figure D 13). Thi analyi how a ignificant behavioural change when the VTZ extend through the dicontinuity. Thi wa oberved in all reult (except the 10 m high layer) and can be demontrated more clearly in the vertical eparation and VTZ ditribution. Figure D 14 how the height of the VTZ a a function of pan for each analyi. The dahed line indicate the height of the dicontinuity. In each analyi (except the 10 m high layer), the greatet increae in the height of the VTZ occur when it croe through the dicontinuity Height [m] Span [m] Layer at 1.5 m Layer at 3.0 m Layer at 5.0 m Layer at 7.5 m Layer at 10.0 m Figure D 14 Height of vertical tenile tre ditribution a a function of pan for each analyi Thi i even more clearly demontrated in Figure D 15, which how the maximum tenile tre along the vertical line at centre-pan. For layer up to 7.5 m high, the pan at which the VTZ croe into the next layer i indicated by a udden jump in the magnitude of the maximum tenile tre. Prior to thi jump, all of the model how a gradual decreae in tenile tre magnitude. For the 10 m high layer, thi trend continue for the duration of the analyi, a the VTZ 12

13 doe not cro into the upper layer for a 54 m pan. Interetingly, the height of the maximum vertical tenile tre at firt remain cloe to the bottom of the lower beam, however, following the croing over of the VTZ into the upper beam, the highet vertical tenile tre i recorded at the top of the lower beam Vertical tre [MPa] Layer at 1.5 m Layer at 3.0 m Layer at 5.0 m Layer at 7.5 m Layer at 10.0 m Span [m] Figure D 15 Maximum vertical tenile tre ditribution a a function of pan for each analyi The abruptne of the change when the VTZ extend into the upper beam i alo dicernible in the eparation of the beam layer. Figure D 16 how the vertical eparation between beam layer a a function of pan. In all cae, the eparation increae at firt and then tend to a contant value, however, where the VTZ ha croed over, a udden increae in eparation i oberved, which how no ign of attaining a contant value with increaing pan. The pan at which thi change occur are the ame a for the change decribed above. Interetingly the ytem itelf how a natural limit in the layer eparation prior to the VTZ croing into the next layer. Thi eparation i upected to be due to a reduction in compreive tre. 13

14 Vertical eparation [mm] Span [m] Layer at 1.5 m Layer at 3.0 m Layer at 5.0 m Layer at 7.5 m Layer at 10.0 m Figure D 16 Vertical layer eparation a a function of pan for each analyi Change are alo oberved in the cloure profile. It i more intructional to examine the rate of cloure (millimetre of cloure per metre of face advance), which i preented in Figure D 17. The general trend prior to the VTZ croing over i toward a lightly increaing cloure rate, which varie from 0.19 mm/m to 0.29 mm/m over the duration of the analyi for the 10 m high layer. Following the VTZ croing over, cloure rate increae dramatically. Note that the increae in cloure rate i only evident after the tranition ha taken place. For example, the VTZ change for the 5 m layer occur between 28 m and 32 m, according to Figure D 14. Thi change i evident in the cloure rate graph (Figure D 17) at around 35 m. 14

15 Cloure rate [mm/m] Layer at 1.5 m Layer at 3.0 m Layer at 5.0 m Layer at 7.5 m Layer at 10.0 m Span [m] Figure D 17 Rate of cloure a a function of pan for each analyi D2.4 Dicuion The numerical modelling reult have revealed a relationhip between hangingwall tructure, the excavated pan and the occurrence of an abrupt change in the deformational characteritic of the hangingwall. Thi change coincide with the croing over of the VTZ from one horizontal layer to another. It appear that the behaviour i cyclic in nature: the VTZ croe into a new layer, which behave imilarly to the layer below it until the VTZ croe into the next layer, etc. Thi appear to be the cae, but more layer hould be modelled to obtain clear evidence of thi behaviour. The pan at which thi change occur, for each modelled layer height, i preented in Figure D 18. A power-law fit i uggeted. 15

16 60 Span at VTZ change [m] y = 7.178x R 2 = Layer height [m] Figure D 18 Span at which the VTZ croing occur for different layer height The reult hown in Figure D 18 depict the behaviour of an iolated panel. The analytical olution for an iolated panel (Ryder and Jager, 2002) and a panel bounded by cruh pillar (auming a pillar reidual trength of 20 MPa with dimenion 6 m X 3 m and a 2 m holing) (Ryder et al, 2005) i hown for comparion in Figure D 19. The model predict a more conervative pan/vtz ratio and therefore untable condition at maller pan than the elatic model for an iolated condition. Collape invetigation at Paardekraal platinum mine (Appendix B) indicated untable condition at pan of 30 m X 100 m and a parting plane at 3.5 m above the tope. The depth of working wa imilar to the model and how a critical pan between the model and the analytical olution. It hould be noted, however, that the tope wa upported and untable condition could have occurred at maller pan in an unupported cenario. In addition, parting above the panel may well have occurred earlier and remained undetected. When the pillar between panel cruh there i a ignificant increae in the height of the tenile zone, commenurate with a decreae in the loadbearing capacity of the pillar. The tenile height then become dependent on the reidual trength of the pillar rather than depth below urface. Furthermore it hould be noted that the VTZ i independent of k. 16

17 60 Panel pan (m) y = x 0.66 R 2 = 1 y = x 0.5 R 2 = Height of vertical tenile zone (m) Cruh pillar Iolated panel Model Collape ite Power (Iolated panel) Power (Cruh pillar) Figure D 19 Analytic olution for the height of the vertical tenile zone a a function of pan. The iolated panel aumed a depth of 500 m below urface While it i tempting to treat the derived relationhip a a deign chart, it mut be noted that the reult of the blocky model are pecific to the choen input. Thi tudy ha not conidered the effect of depth, tre ratio, contact propertie or material trength. One variation wa modelled to demontrate the enitivity of the model to input parameter: the contact friction angle wa changed to 6 for the 1.5 m high layer. The reulting vertical tenile tre ditribution at a pan of 12 m i preented in Figure D 20. Clearly, thi i very different to the reult for a 30 friction angle: the VTZ extend to 16 m for a 12 m pan. At 26 m pan, the VTZ extend for the entire height of the blocky hangingwall portion of the model, and further excavation with thi model i not meaningful. Figure D 20 Tenile tre ditribution for a layer height of 1.5 m and a contact friction angle of 6 (12 m pan) 17

18 The macrocopic behaviour of a dicontinuou beam i imilar to that of an intact beam in that the vertical tenile tre increae with pan, and i dependent on the thickne of the beam. However, the fundamental apect controlling the behaviour of a dicontinuou beam are very different to thoe in the intact cae. Of particular interet i the fact that the highet vertical tenile tree are recorded near the top of the beam. The top of the beam alo how alternating region of compreion and tenion within the VTZ (Figure D 13). It eem that the behaviour of the individual block, and the variation of the tre within thee block, determine the fundamental behaviour of the dicontinuou beam. Further work hould examine how individual block behave, and how the block combine to produce the overall behaviour. Further work in thi area hould alo involve the following tak: The combination of a blocky hangingwall and cruh pillar; Model multiple layer in the dicontinuou beam zone; Invetigate whether the overlying block tructure affect the reult; Calibration with in itu cloure data and tre meaurement; and Senitivity analye including: o Depth; o Stre ratio; o Contact propertie; o Elatic material propertie; and o Material trength. 18

19 D3 Numerical modelling to determine the effect of hallow-dipping dicontinuitie on tability D3.1 Introduction Since mot of the FOG tudied in thi project involved hallow-dipping or curved dicontinuitie a the primary releae urface, it wa neceary to tudy the effect of uch dicontinuitie on tability. A wort-cae cenario wa choen with infinite dip panel pan and infinitely long dicontinuitie (2D plane train condition). Such condition exited in the original tope at Lebowa Platinum Mine, Middelpunt ection (LPM). The colloquial UCB wa a hallow-dipping thrut fault dipping in and out of the UG2 reef horizon. Many of the original tope formed tet cae imilar to the model, which could be ued to check the validity of the model. The joint et were imilar to the common joint oberved on the platinum mine. 2-D UDEC model (with the Barton Bandi joint addition) incorporating the two commonly oberved joint et, the triplet and a hallow-dipping dicontinuity at 20 to the trata (UCB) were et up. The analye were performed uing laboratory-derived input parameter and meaurement made at the mine (Table D 1 and Table D 2). A k value of 1.5 wa aumed in all the aement. 19

20 Table D 1 Parameter ued in the UDEC model for a horizontal tre of 3 MPa Block propertie Joint et I UCB Shear modulu (G) 40 GPa Joint friction angle Bulk modulu 42 GPa Joint (K) coheion Friction angle 48 0 Joint tenile trength Coheion 14.6 MPa Joint et II Tenile 8.9 MPa Joint friction trength angle Joint coheion Joint tenile trength 36 0 Joint friction angle 0.13 MPa Joint coheion 0 MPa Joint tenile trength Triplet 37 0 Joint friction angle 0.27 MPa Joint coheion 0 MPa Joint tenile trength MPa 0 MPa MPa 7.1 MPa Note: The friction angle were determined uing the methodology uggeted by Barton (1997). Table D 2 Orientation and depth of geological dicontinuitie in the model. Dicontinuity Height above tope (m) Dip angle (degree) Triplet I 1.6 m 0 Triplet II 2.3 m 0 Triplet III 3.0 m 0 Weak trata (UCB value) 5.0 m 0 Joint et I 80 Joint et II

21 D3.2 Benchmark model - without a hallow dipping dicontinuity or upport A uite of model wa run without the UCB plane. The model were et up uing the input parameter provided in Table D 1 and Table D 2 o that an initial pan of 3 m could be increaed in tep of 2 m. The model were terminated when the unupported pan reached 40 m. No failure occurred (table condition) although parting took place on the triplet and the plane at 5 m above the tope, creating four beam a hown in Figure D 21. The maximum vertical deformation wa 15 mm and an exaggeration of 35 wa applied to the reult hown in the figure. Figure D 21 Reult of a UDEC model without upport and without the UCB, a pan of 40 m and the vertical deformation magnified by 35 time D3.3 Model with a hallow dipping dicontinuity but no upport The model above wa modified to include the UCB and a uite of tet were run without upport a hown in Figure D 22. Block that formed between the UCB and the two joint et fell out a the pan wa increaed, howing that there i no table, elf upporting pan. At a pan of 5 m the rock ma up to the firt triplet (1.6 m above the tope) became untable. An increae of the pan by 2 m reulted in a migration of the untable zone up to the econd triplet at 2.3 m above the tope. Both the condition at 5 m and 7 m were oberved at LPM in unupported or poorly upported panel. At a pan of 9 m, everything below 21

22 triplet III (at a height of 3 m) became untable. At a pan of 15 m, a collape occurred up to the parting plane at 5 m above the tope. I II UCB 5 m 3 m pan 5m pan 7 m pan 9 m pan 11 m pan 13 m pan 15 m pan Key Triplet Unmined mining tep I Joint et I II Joint et II Figure D 22 Reult of a UDEC model without upport D3.4 Model with a hallow dipping dicontinuity and 50 kn/m 2 upport reitance A upport ytem of 170 mm diameter mine pole paced 2 m x 2 m on dip and trike wa originally ued at the mine. A down-rated upport reitance of 50 kn/m 2 wa aumed for the invetigation and a curve for the elongate upport ytem wa generated from underground meaurement (Figure D 23). The reult of the model are hown in Figure D 24. Stable condition were predicted up to a pan of 9 m. At a pan of 11 m the model howed that parting had occurred on all three triplet a well a at 5 m above the tope. A collape occurred up to triplet III at a pan of 13 m, which i an improvement of 4 m over the unupported cenario. 22

23 60 Support reitance (kn/m 2 ) Deformation (mm) Figure D 23 Support reitance with a 99% confidence, auming large block (upported by at leat three pole) I II UCB 9 m pan 11 m pan 13 m pan Key Triplet Unmined mining tep I Joint et I II Joint et II Figure D 24 Reult of a UDEC model with a upport reitance of 50 kn/m 2 23

24 D3.5 Model with a hallow-dipping dicontinuity and 135 kn/m 2 upport reitance A new upport ytem wa introduced to the mine which included larger diameter mine pole and pack. Thi condition wa modelled auming a down-rated upport reitance of 135 kn/m 2. In the model, only the mine pole were pretreed (to 150 kn) and the profile of upport reitance to cloure wa determined from underground meaurement (Figure D 25). A great improvement, both in table panel pan and rock ma condition, wa achieved by thi upport ytem (Figure D 26). Untable condition occurred at a pan of 19 m and a final collape at 22 m. Note that parting initiated at 5 m above the hangingwall, when the pan reached 17 m. Support reitance (kn/m 2 ) Deformation (mm) Figure D 25 Support reitance profile for the econd upport ytem 24

25 I UCB II K-ratio 0,16 17 m pan 19 m pan 20 m pan 22 m pan Key Triplet Unmined mining tep I Joint et I II Joint et II Figure D 26 Reult of the UDEC model for a pack-elongate upport ytem with a upport reitance of 136 kn/m 2 D3.6 Model with a hallow dipping dicontinuity and 200 kn/m 2 upport reitance The modelled upport ytem wa intalled at a pre-treed of 100 kn/m 2 and provided a peak reitance of 200 kn/m 2 (Figure D 27). Thu the upport capacity wa technically able to upport a rock height of 5 m, with only about 5 mm deformation. To enure wort cae condition, a econd horizontal dicontinuity, with propertie imilar to the UCB, wa et up at 7 m above the tope. Thi wa jut higher than the upport capacity. The reult are hown in Figure D 28. Span were increaed in the model to a maximum of 36 m and table condition were predicted in all run. However it hould be noted that FOG occurred in the face area when the maximum upport to face ditance wa 3.6 m, a hown in Figure D 29. Thee FOG were not oberved when the 25

26 upport to face ditance wa kept to 2.8 m and thi wa alo confirmed by underground obervation. 250 Support reitance (kn/m 2 ) Cloure (mm) Figure D 27 Final upport reitance profile (100kN/m 2 at intallation) I UCB 32 m pan 34 m pan II 36 m pan Key Triplet Unmined mining tep I Joint et I II Joint et II Support Figure D 28 Reult of the UDEC model for a upport reitance of 200 kn/m 2. The upport to face ditance wa kept to a maximum of 2.8 m 26

27 I UCB 32 m pan 34 m pan II 36 m pan Key Triplet Unmined mining tep I Joint et I II Joint et II Support Figure D 29 Reult of UDEC model for a upport reitance of 200 kn/m 2. FOG occurred when the upport to face ditance wa 3.6 m D3.7 Dicuion of the UDEC modelling reult The initial model that wa run without a peritent, hallow-dipping dicontinuity (cutting through the trata) howed table condition even though parting had occurred on the triplet at 3 m and a weak parting plane at 5 m above the tope. The minor panel pan wa increaed to 40 m without incident of intabilitie or FOG in pite of the fact that no upport wa intalled. However, a oon a a hallow-dipping dicontinuity wa introduced, the confinement inhibiting lip on the teeply dipping joint urface wa removed. Block fell out with every blat and collape enued at very mall pan. Interetingly, thi condition wa alo oberved in the early tope at the mine, where attention to proper intallation of upport wa lacking. The introduction of upport greatly improved condition in the model, and tability wa achieved to a pan of 9 m with a upport reitance of 50 kn/m 2. An increae of upport reitance reulted in improved condition and greater table pan a expected. The final upport configuration with an active upport reitance of 100 kn/m 2 at intallation and a total upport reitance of about 200 kn/m 2, topped the collape on the mine and the model alo howed table condition for the modelled pan. Interetingly, a econd 27

28 weak plane that wa introduced into the model at 7 m above the tope, higher than the reach of the upport, did not affect tability even though the UCB cut through the trata to thi height. The modelling eem to ugget that if a ufficiently high active upport reitance can be achieved at intallation, at leat ome of the force clamping the teeply dipping dicontinuitie are retained thu allowing larger pan to be mined afely. The greatet tability i achieved when the coheion acro the hallow-dipping dicontinuity urface i retained by early intallation of uitable upport, cloe to the face. D4 Summary An analyi of Vouoir beam howed that thi condition only form under limited mining condition. Under normal Buhveld mining condition deeper than about 230 m below urface, where the upport capacity i able to carry a burden height of about 1.5 m and the panel pan i about 30 m, Vouoir beam hould not form. In the collape report urvey, only FOG with a maximum height of 0.5 m could be equated to Vouoir beam. Thee FOG occurred initially between upport before rotating the elongate out. The FOG with a fallout thickne greater than the VTZ height all occurred on hallow-dipping or curved tructure howing that the height of the VTZ doe not retrict the fallout height if thee tructure are preent. The ELFEN modelling reult have revealed a relationhip between hangingwall tructure, the excavated pan and the occurrence of an abrupt change in the deformational characteritic of the hangingwall. Thi change coincide with the croing over of the VTZ from one horizontal layer to another. It appear that the behaviour i cyclic in nature: the VTZ croe into a new layer, which behave imilarly to the layer below it until the VTZ croe into the next layer, etc. Thi appear to be the cae, but more layer hould be modelled to obtain clear evidence of thi behaviour. When the pillar between panel cruh there i a ignificant increae in the height of the VTZ, commenurate with a decreae in the load-bearing capacity of the pillar. The VTZ height then become dependent on the reidual trength of the pillar rather than depth below urface. Furthermore it hould be noted that the VTZ i independent of k. The VTZ can 28

29 extend up to about 5 m above a tope where individual panel of 30 m are eparated by cruh pillar. The modelling ugget that if parting occur within thi height that the effective height of the VTZ i increaed. A UDEC model wa run to determine the effect of hallow-dipping or curved dicontinuitie on tability. The model wa calibrated by mining condition affected by a peritent hallow-dipping, curved tructure that dipped in and out of the reef. The model howed that given the jointing condition of the hangingwall with reef-parallel dicontinuitie above the hangingwall, table condition were likely even at pan of 40 m. However, a oon a the hallow-dipping tructure cut through the trata, untable condition developed at very mall pan. The model howed that a upport configuration with a high pre-tre (100 kn/m 2 at intallation) tabilied condition provided it wa intalled cloe to the face and the panel pan were retricted to 30 m. Thi finding wa hown to work in practice. The model uggeted tability even when the parting plane wa higher than the reach of the upport. The finding of the reearch ugget that hangingwall condition with vertical joint can be mined to ignificantly greater pan than the traditional 30 m, even if there are trata-parallel dicontinuitie. Thu beam and Vouoir beam analyi i generally not applicable. However, a oon a a hallow-dipping, peritent dicontinuity cut through the trata, untable condition can occur at relatively mall pan. Thee condition prevail when a o called dome-tructure i interected. (Thee dome tructure have been found to form part of a larger thrut tructure, dipping in and out of the reef (Perritt and Robert, 2007)). A panel with a good rock ma rating can deteriorate when uch a tructure i interected and thee tructure are often the caue of large FOG and collape. D5 Reference Bandi, S.C., Lumden, A.C., Barton, N.R. (1983). Fundamental of rock joint deformation. Int.J.Rock Mech. Min. Sci. and Geomech. Abtr., Vol.20 no.6. 29

30 Barton, N. (1997). Rock ma characteriation workhop. Workhop note. Bieniawki, Z.T. (1967). The effect of pecimen ize on the compreive trength of rock. Int. J. Rock Mech. Min. Sci., Vol.5, no.6, pp Brady, B.H.G. and Brown, E.T. (1993). Rock Mechanic for Underground Mining, 2 nd ed. London: Chapman & Hall, pp 571. Daehnke, A., Anderon, L.M., De Beer, D., Eterhuizen, G.S., Glion, F.J., Grodner, M.W., Hagan, T.O., Jaku, E.P., Kuijper, J.S., Kullmann, D.H., Peake, A.V., Piper, P.S., Quaye, G.B., Reddy, N., Robert, M.K.C., Schweitzer, J.K., Stewart R.D. and Wallmach T. (1998). Stope face upport ytem. SIMRAC Report GAP 330. The afety in mine Reearch Aviory Committee (SIMRAC), Braamfontein, RSA. Diederich, M.S. and Kaier, P.K. (1998). Stability of large excavation in laminated hard rock mae: the vouoir analogue reviited. Int. J. Rock Mech. and Min. Sci., Vol.36, pp Eterhuizen, G.S. (1996). JBlock Uer Manual and Technical Reference, Pretoria. Evan, W.H. (1941). The trength of undermined trata. Tran.IMM. L(50). Fairhurt, C. and Cook, N.G.W. (1966). The phenomenon of rock plitting parallel to a free face under compreive tre. Proc. Au. Int. Metall. Conf., Melbourne, pp Hedley, DGF and Grant, F. (1972). Stope and in-pillar deign for the Elliot Lake Uranium Mine. CIM Bulletin, July. Hutchinon, J.D. and Diederich, M.S. (1996). Cablebolting in Underground Mine, BiTech Publiher Ltd, Richmond, Canada, pp 401. Jaeger, JC and Gay, NC. (1974). Behaviour of lightly confined granular material. Int. J. Rock Mech. Min. Sci. and Geomech. 30

31 Jager, AJ. and Robert, MKC. (1986). Support ytem in productive excavation. GOLD 100. Proc. Int. Conf. Gold. Johanneburg. Jeremic, M.L. (1987). Ground Mechanic in Hard Rock Mining. A.A. Balkema, Rotterdam. Joughin, W.C., Swart, A.H. and Stacey, T.R. (1997). Review of fall of ground problem in underground diamond mine and other mine with maive orebodie and make recommendation on reearch need to reduce fall of ground caualtie, particularly in the face area. SIMRAC Report OTH 411. The afety in mine Reearch Aviory Committee (SIMRAC), Braamfontein, RSA. Kotze, T.J. and Streuder, S.B. (1993). A review of the trategy being followed on chrome mine belonging to the Samancor Group. Rock engineering problem related to hard rock mining at hallow to intermediate depth. Proc. Symp. SANGORM. Rutenburg. pp Lougher, D.R. (1994). An in-itu invetigation into the behaviour of the urrounding rock ma in a hard rock pillar mining environment. MSc Thei ubmitted to the Univerity of the Witwaterrand, Johanneburg, RSA. Özbay, MU & Robert, MKC. (1988). Yield pillar in tope upport. Proc. Rock Mechanic in South Africa, SANGORM. Özbay, MU; Ryder, JA & Jager, AJ. (1994). The deign of pillar ytem a practied in hallow hard-rock tabular mine in South Africa. J. S.A. Int. Min. and Metal., Jan/Feb Peck, R.B. (1969). The obervational method in applied oil mechanic 9 th Rankine Lecture, Geotechnique, Vol.19, pp Perritt, S.H. and Robert, M.K.C. (2007). Flexural-lip tructure in the Buhveld Complex, S.A. J. tr. Geol. Vol.29, pp Robert, D.P., Robert, M.K.C. and Jager, A.J. (2004). Alternative upport ytem for mechanied tope. PlatMine report

32 Ryder, J.A. (2004). Peronal Comm. Ryder, J.A. and Jager, A.J. (2002). A textbook on rock mechanic for tabular hard rock mine. The afety in mine Reearch Adviory Committee (SIMRAC), Braamfontein, RSA., pp Ryder, JA and Özbay, MU. (1990). A methodology for deigning pillar layout for hallow mining. ISRM Symp.: Static and Dynamic Conideration in Rock Engineering, Swaziland. Ryder, JA, Waton, BP & Kataka, MO. (2005). Pillar trength back-analye. PlatMine 1.2, Johanneburg. Salamon, M.D.G. (1968). 2D Treatment of problem ariing from mining tabular depoit in iotropic or tranverely iotropic ground. Int. J. Rock Mech. Min. Sci. Salamon, M.D.G. (1976). The roll of pillar in mining. Rock mechanic in mining practice. Ed. S. Budavari. S.A. Int. Min. Metall., Monograph erie M5, pp Salamon, M.D.G. and Munro, A.H. (1967). A tudy of the trength of coal pillar. J. S.A. Int. Min. Metall., Vol.68, pp Shi, G. and Goodman, R.E. (1987). Block theory and it application to rock engineering, Prentice Hall. Sofiano, A.I. (1996). Analyi and deign of an underground hard rock Vouoir beam roof. Int J. Rock Mech.Min. Sci. and Geomech. Abtr., No.2. Spann, H.P. and Napier J.A.L. (1983). The influence of different bolt pattern and anchor type on the deflection of a laminated beam. C.O.M.R.O. technical report no.:rr25/83, Johanneburg. Stacey, T.R. and Page, C.H. (1986). Practical handbook for underground rock mechanic. Tran. Tech. Publication, pp

33 Swart, A.H., Stacey, T.R., Weeloo, J., Joughin, W.C., le Roux, K., Walker, D. and Butcher, R. (2000). Invetigation of factor governing the tability/intability of tope panel in order to define a uitable deign methodology for near urface and hallow mining operation. SIMRAC Project Report OTH 501. The afety in mine Reearch Adviory Committee (SIMRAC), Braamfontein, RSA. Thruh, P.W. (1968). A dictionary of mining, mineral and related term. Wagner, H and Madden, BJ. (1984). Fifteen year experience with the deign of coal pillar in hallow South African collierie: An evaluation of the deign procedure and recent improvement. Deign and Performance of Underground Excavation. ISRM, Cambridge. Wright, F.D. (1974). Deign of Roof Bolt Pattern for Jointed Rock. Bur.of Mine (NTIS), U.S. Dept. Commerce. York, G., Canbulat, I., Kabeya, K.K., Le Bron, K., Waton, B.P. and William S.B. (1998). Develop guideline for the deign of pillar ytem for hallow and intermediate depth, tabular, hard rock mine and provide a methodology for aeing hangingwall tability and upport requirement for the panel between pillar. SIMRAC Report GAP 334. The afety in mine Reearch Adviory Committee (SIMRAC), Braamfontein, RSA. 33