Measurement and prediction of internal stresses in an underground opening during its filling with cemented fill

Transcription

1 Measureent an preiction of internal stresses in an unergroun opening uring its filling with ceente fill T. Bele Université u Québec en Abitibi-Téiscaingue, Dépt. es Sciences appliqués, Rouyn-Norana, Canaa A. Harvey Cabior Inc., Mine Doyon, Rouyn-Norana, Québec, Canaa R. Sion & M. Aubertin École Polytechnique e Montréal, Montréal, Québec, Canaa To cite this paper Bele T., Harvey A., Sion R., Aubertin M. 24. Measureent an preiction of internal stresses in an unergroun opening uring its filling with ceente fill. Proceeings of the 5 th Int. Syp. on Groun support in Mining an Unergroun Construction. Villaescusa & Potvin (es.), 28-3 Septeber 24, Perth, Western Australia, Australia, Tayler & Francis Group, Lonon, pp ABSTRACT: Knowlege of the internal stresses of ceente fill use in unergroun ines is very iportant with respect to groun control. The stope of a Canaian gol ine was instruente with earth pressure cells to easure the stresses within the pastefill uring an after filling. The results inicate that the stresses increase uring backfilling but ten to ecrease after the copletion of backfilling. The Longituinal stresses were observe to be the highest, while the vertical an transverse stresses were lower an roughly equivalent. Two new 3D oels are propose to preict the internal stresses of pastefill an the pressure on barricaes as a function of the fille height. These 3D oels have been etene to take into account the tie factor in the evelopent of stresses within pastefill. Coparison of the oel responses with eperiental ata inicates that the oels preict the stresses within the pastefill an the pressure on the barricae with reasonable accuracy. 1 INTRODUCTION The recovery of unergroun har rock ore boies often involves the use of ine fills. The type of ine fill epens on the ining ethos an sequences. Where later recovery is epenent on stability of eposures of earlier place fill, ceent an/or ceentitious aterials are ae to the fill. Once such fill type is pastefill which is in fact becoing a stanar practice in Canaa (e.g. Lanriault 1995, Lanriault & Tenbergen 1995, Naylor et al. 1997). However, the use of pastefill to aintain groun stability involves soe ifficulties relate to the copleities of its behaviour. These copleities are ue to the continuous evolution of the properties of ceente fill uring placeent, consoliation, an harening ue to the hyration of biner agents. Despite recent work conucte on ceente fills (e.g. Hassani & Archibal 1998, Benzaazoua et al. 22, Bernier et al. 1999, Benzaazoua & Bele 2, Bele et al. 22) any questions reain concerning the stability analysis of a stope fille with pastefill. Inee, after backfilling with ceente fill the structural integrity of a stope can be threatene by several acroscopic factors (eclusive of the hyration process) which influence the echanical strength of the pastefill. These factors inclue: copression an consoliation of the pastefill, the volue an geoetry of the stope, the stress istribution within the backfill an between the backfill an the stope, wall convergence, shrinkage, an the effect of arching within the pastefill. Consequently, an unerstaning of these various factors of influence is necessary to provie ore efficient eans of groun control. Inee, knowlege of the agnitue of pressures on barricaes will allow for better planning of ining sequences. Aitionally, the knowlege of the stress fiels within pastefill will facilitate analysis of its stability when one of its faces will be epose or when an access gallery to a new stope is ecavate through the pastefill ass. The objective of this stuy was to follow the evolution of pressures evelope in ceente fill uring placeent an consoliation. There is very little ata or ocuentation regaring in situ easureent of the echanical properties of pastefills. However, soe work has carrie out in this irection (Gay et al. 1988, Ouellet & Servant 2, Been et al. 22, Le Rou 24). Work concerning the instruentation of pastefill has been conucte by (Corson 1971, Hassani et al. 1998, Hassani 1999, Rankine et al. 21, Revell 23). A coon point of these eperiental stuies is that the results are liite to the subject ines. In this paper, we first briefly escribe the Doyon Gol Mine where a selecte trial stope was instruente an backfille. The influence of the curing tie of the pastefill an the height of fill on the easure pressures will be iscusse. Base on the results, analytical oels will be propose to preict the pressures evelope in pastefill as well as the pressure on barricaes uring an after backfilling.

2 2 DOYON GOLD MINE The Doyon Gol Mine, property of Cabior Inc., is locate approiately 4 k east of the city of Rouyn-Norana an has been in operation since the beginning of the 198's. To-ate, about 25,6, etric tons of gol ore have been etracte fro the ine. Doyon Gol Mine uses the open stoping ining etho in conjunction with post place paste backfill since the late 199's. The stoping etens to a aiu epth of 8 over an area of The long-hole etho is the only ining etho use at the site. Since the ore boy consists of narrow veins of quartz-pyrite-touraline of ifferent with (.1 to 1.2 ), the stope iensions are not the sae an vary fro 3 23 in plan an 22 high to in plan an 3 high. Currently, up to approiately 14 stopes are backfille per year at the ine (roughly 17 tons of pastefill are place per ay). The pastefill is transporte by gravity using 15-c iaeter pipes in a staircase network (Harvey 24). The stope backfilling etho consists of pouring first the "plug" (high ceent content) up to 3 behin the raw point followe by a 1-ay curing tie. After that the rest of the stope is fille ("resiual fill"). 3 DESCRIPTION OF THE INSTRUMENTATION 3.1 Location of easureent points To follow the evolution of the pressures evelope in the pastefill, two trial stopes were selecte an instruente each with eight pressure cells but only the Stope 8-1 FW of the Doyon Gol Mine is concerne in this paper. The pressure cells were place at four locations within the stope: the floor of the stope, at the plug/resiual fill interface, on the lower wall, an on the barricae. Figure 1 shows the geoetry an the iensions of the stope, which has an average with, B, of 11 (along the transverse ais, y), a length, L, of 21 (along the longituinal ais, ) an a height, H, of 29 (along the vertical ais, z). The trial stope is oriente at an aziuth of 9 an a ip of 9 an is locate at a epth of 45 in a zone where there were no ore prouction sequences planne teporarely. Consequently, the initial stress fiel was relatively stable an the access to the stope was safe. The pastefill pore pressures were not onitore in this stuy because any rainage of free water at the rock pile-shotcrete barricae was observe (no buil-up of the total earth pressures). y Stope height H 29 z Upper wall High ceent "Plug" height (7 ) Stope length L z Barricae pressure cell 4.2 (a) Longituinal view 7 Paste ischarge Lower wall pressure cell 8 Lower wall Pressure cells ounte on a cubic frae 1.5 Rock pile barricae Draw point Stope height H 29 y H/ W F/ W z Stope with B 11 (b) Transverse view Figure 1. Stope 8-1 FW geoetry an iensions with the location of the earth pressure cells. The evelopent of pressures in the pastefill was easure in three iensions corresponing to the, y an z aes at two locations (on the floor of the stope an at the plug/resiual fill interface) as shown on Figure 1a. A single pressure cell was place along each ais at these locations, 3 pressure cells on the floor of the stope (cells 1, 2 an 3) an 3 pressure cells at the plug/resiual fill interface (cells 4, 5 an 6). A single pressure cell was place on the lower wall (cell 7) at the sae longituinal ais as cell 4 an another pressure cell was place at iheight of the raw point of the stope at 1.5 fro the barricae (cell 8) on a longituinal ais (Fig. 1a). 3.2 Eperiental evices for pressure easureent Earth pressure cells use The earth pressure cells were oel TPC by RocTest, which were consiere appropriate for this type of easureent as suggeste by Weller & Kulhawy (1982). The oel TPC consists of a seale istribution pa copose of two 23- iaeter circular plates wele together aroun their peripheries an fille with e-aire oil. These pas were connecte via lengths of steel tubes to vibrating wire pressure transucers an 3-eter-long cables. This oel has a built-in 3 KΩ theristor which allows teperature reaings fro -55 C to +85 C (see Fig. 2). The cell capacity was 75 kpa with an accuracy of ±.5% of the full scale (i.e ± 3.75 kpa). The cells were capable of operating at up to twice the rate capacity with reuce accuracy. The reaings of the total pressure were taken using a oel MB-6T portable rea-out unit. Pa with sei-rigi surfaces 23 Vibrating wire pressure transucer with a built-in theristor 3 kω Electrical cable Figure 2. The RocTest pressure cell oel TPC with a vibrating wire transucer an a built-in theristor.

3 3.2.2 Device for the pastefill ass Figure 3a shows the orientation of the three total pressures,, σ y, σ z, which were easure in Stope 8-1 FW. Two sei-stiff cubic etal boes (6 c) were anufacture. Three pressure cells were ounte on three faces of each bo (Fig. 3b). This type of arrangeent has been eploye in the past for siilar easureents (Hassani et al. 1998, Hassani 1999). To aintain alignent the boes were asseble on a etal sei-stiff frae of 7.6 high (Fig. 3c). The frae is then place in the stope by eans of an in-house anufacture trolley ounte on two wheels an a echanis of pulley an cors. The trolley supporting the frae is firstly thorough to 3 insie the stope an then the frae is raise by pulling at the sae tie on two cors, one fro the raw point an the other one fro the upper gallery. In the absence of angle inicators on the evice, bans of fluorescent painting on the top of the frae allow its visual upright positioning in the stope. After its installation, it was observe that the evice ha a slight eviation fro vertical estiate at 5 egrees. This eviation was neglecte in our interpretations Device for the foot wall The cell which was ounte on the lower wall of the stope was fie to a 2--thick woo boar base on the recoenations of Weller & Kulhawy (1982). The corresponing aspect ratio (ratio of the cell iaeter to the boar thickness) was 11.5 (Fig. 4a). The evice was easier to set up on the wall an prevente irect contact between the cell an the rock face. A siilar evice has been use by Yang et al. (1998) for the easureent of the pressures evelope on concrete bo culverts uner highway ebankents Device for the barricae The TPC cell near the barricae was installe vertically at the intersection of two 6--iaeter steel retaining cables (Fig. 4b). In such a configuration the cell coul not unergo rotation, but coul possibly unergo longituinal isplaceent of 2 to 4 c (Harvey 24). Such a isplaceent coul cause a slight unerestiate of the pressure on the barricae. Woo boar (Ø2 ) _wall TPC cell Rock ass (lower wall) (a) Device for the of lower wall pressure ( ) easureent 2.1 Cable y z Draw point Pressure cell Cable (b) Device for the barricae pressure ( ) easureent Figure 4. Eperiental evices to instruent (a) the stope lower wall an (b) the raw point in front of the barricae. σ y σ z (a) Earth pressure referential syste showing vertical (σ z ), longituinal ( ) an transverse (σ y ) total pressures 6 c Earth pressure cell (RockTest oel TPC) 23 c 6 c 6 c (b) Sei-stiff cubic etal unit (bo) equipe with three TPCs TPCs unit 2,6 σ y2 TPCs unit 1 7 (c) Sei-stiff etal frae supporting the two pressure cell units 2 38,6 Figure 3. Eperiental evice for the instruentation of the paste backfill ass. 4 BACKFILLING OF THE STOPE 4.1 Paste backfill i esign The tailings fro the concentrator at the Doyon Gol Mine were use for the pastefill. At the outlet of the thickener the pulp is 6% solis by ass. It is then strippe of cyanie an route to isc-type filters where it is ewatere to 8% solis. The tailings pulp is then ie with the biner agents an water to create ceente paste. The i use at Doyon Gol Mine is 7% by ass of Portlan ceent for the "plug" which is use to fill the lower portion of the stope (7 in this case), an 3% by ass of biner (3% Portlan ceent an 7% slag) for the "resiual fill" use to fill the reainer of the stope (22 in this case). Because of the clayey nature of the Doyon Gol Mine tailings (ore than 4% clay fraction) its Specific Gravity is of 2.73 an the average solis concentration of the resulting pastefill is 7% by ass with an average slup of 21 (oisture content of 42.9%). This low solis concentration is ue to pastefill low value of bulk ensity (1.8 t -3 ) an to the fact that Doyon Gol Mine tailings contain about 5% fines (grains iaeter < 2 µ) while 15% of fines woul have been optial for the nees of the pastefill transport through pipes via gravity (Lanriault et al. 1997). Also the clayey nature of this ceente fill ehibits its strong water retention capacity.

4 The bulk unit weight (γ) of the Doyon pastefill is of 18 kn/ 3 (egree of saturation S r 1%) an its ry unit weight (γ ) is of 12.6 kn/ 3. The initial voi ratio (e ) an corresponing initial porosity (n ) of the pastefill is 1.18 an.54 respectively for the plug (7% Portlan ceent) an 1.17 an.54 for the resiual fill (3% biner). Table 1 presents the variation of the oisture content an the solis concentration of the Doyon Gol Mine pastefill after 7-, 14- an 28-ay curing tie. Table 1. Variation of the Doyon Gol Mine pastefill oisture content, w(%) an solis concentration by ass, C w (%) in the course of curing tie. Plug Resiual fill Curing Tie w (%) C w (%) w (%) C w (%) -ay ay ay ay The observe echanical strenght are rather weak an the average 7-ay copressive strength is about 17 kpa for the plug (7% Portlan ceent) an of 13 kpa for the resiual fill (3% biner agent). 4.2 Stope backfilling with pastefill Backfilling of Stope 8-1 FW with pastefill began four weeks following the last ining sequence (wall convergence was assue to be coplete by this tie) an was carrie out in three sequences (Fig. 5). The first sequence (345 tons of pastefill) consiste of the pouring of the plug (h 7.3 ) an laste 44 hours ( 2 ays) followe by a curing perio of 94 hours ( 4 ays). The secon sequence (1,339 tons of pastefill) consiste of the placeent of 18 of resiual fill an laste 19 hours ( 8 ays) followe by a curing perio of 37 hours. The thir sequence (882 tons of CPB) consiste of the copletion of the resiual fill by an aitional 2, the reaining 2 of the stope were left epty. The total uration of filling incluing the curing perios was 356 hours ( 15 ays) an a total of 14,266 tons of pastefill were place (Harvey 24). Elapse tie (hrs) t hrs (h 18 ) t 1 94 hrs (h 7.3 ) t 3 11 hrs (h 2 ) Upper wall TPCs unit 2 Plug (sequence 1) 3,45 tons Fill (sequence 3) 882 tons Fill (sequence 2) 1,339 tons TPCs unit 1 Lower wall σ y σ z lower wall TPC Barricae TPC Rock pile barricae Figure 5.The filling of Stope 8-1 FW with ceente paste backfill in three sequences. 5 RESULT OF PRESSURE MEASUREMENTS The pressure reaouts were taken fro the start of backfilling until 32 ays after the en of backfilling. Due to the geoetry of the stope (see Fig. 5), the fille heights (h) were calculate fro the pastefill quantities. The uration of filling an the stope volue were obtaine fro a CMS (Caving Monitoring Syste) scanning. Figure 6 shows the variation of fille height (h) with respect to the tie elapse since the start of filling. Fille height h () rst sequence Curing perio 2 n sequence 3 r sequence En of backfilling Elapse tie since the beginning of the filling (ay) Figure 6. Tie-history of the placeent of ceente paste backfill. 5.1 Internal pressure in pastefill uring placeent Pressure evelope at the floor of the stope Figure 7 shows the evolution of the vertical ( ), longituinal ( ) an transverse ( ) pressures at the floor of the stope uring the filling which laste 15 ays incluing the curing perios. After 4 ays the fille height, h, was 7.3 (actual plug height) an at the 15 th ay the fille height was 27. The point of easureent of the longituinal an transverse pressures was locate at elevation, z, of.3 an for the vertical one at elevation of.6. It can be observe fro Figure 7, that uring filling the longituinal pressure ( ) was the highest on the floor of the stope (Fig. 7). This pressure reache its aiu value (_a ) of about 15 kpa uring the secon sequence of filling (after the 1 th ay). This aiu value is alost twice that of the vertical ( ) an transverse ( ) pressures which were siilar in agnitue ( > ). It shoul be note that the 882 tons of pastefill ae uring the 3 r sequence of filling, fro the 13 th ay, ha no influence on the internal pressures at the floor, which actually began ecreasing. This reuction continue until ay 91.

5 Develope internal pressure (kpa) curing perio en of backfilling Elapse tie since the beginning filling (ay) Figure 7. Evolution of the internal pressures of the CPB at the floor of Stope 8-1 FW as a function of elapse tie since the beginning of the filling Pressure evelope at the plug/fill interface The TPC cells which were locate at elevation 7.6 for σ z2 an at elevation 7.3 for 2 an σ y2 began recoring pressures only after the pastefill rose to that level, soe 3 hours after the start of the placeent of the resiual fill (124 hours fro the start of filling). Figure 8 shows the evolution of the vertical (σ z2 ), longituinal (2 ) an transverse (σ y2 ) pressures at the plug/resiual fill interface fro the 5 th ay of filling. Again, it can observe that the longituinal pressure is the highest of the three easure pressures (2 > σ y2 > σ z2 ). The aiu value of the longituinal pressure (2 ) is about 53 kpa (σ y2 38 kpa an σ z2 25 kpa). Develope internal pressure (kpa) n sequence 3 r sequence 2 σ y2 σ z Elapse tie since the beginning of filling (ay) Figure 8. Evolution of the internal pressures of the pastefill at the elevation 7.6 (plug/resiual fill interface) uring the course of the filling of Stope 8-1 FW. 5.2 Lateral pressure on the lower wall The TPC cell locate on the lower wall also began registering pressures 3 hours after the start of placeent of the resiual fill. Figure 9 shows the evolution of the longituinal pressure (_wall ) eerte on the lower wall of Stope 8-1 FW. The reaings began the 5 th ay an the aiu value reache was about 45 kpa. Lateral pressure on wall _wall (kpa) 5 45 _wall n sequence 3 r sequence Elapse tie since the beginning of filling (ay) Figure 9. Evolution of the lateral pressure on the lower wall at elevation 7.6 uring the filling of Stope 8-1 FW. 5.3 Lateral pressure on the barricae Figure 1 shows the evolution of the longituinal pressure (_b ) eerte on the barricae at iheight (z' 2.1 ). It can be observe that the aiu pressure eerte on the barricae was about 54 kpa an was reache on the 1 th ay of filling. However, this pressure begins to ecrease just after having reache this aiu. Pressure on barricae _b (kpa) Curing perio 1 st sequence 2 n sequence 3 r sequence _barricae Elapse tie since the beginning of filling (ay) Figure 1. Evolution of the lateral pressure on the barricae at i-height of the lower gallery (z' 2.1 ) uring the course of the filling of Stope 8-1 FW. 6 DISCUSSION 6.1 Long-ter behaviour of pastefill The tie history of pressure easureents perits observation of both the variation of the evelope internal pressures an the effect of hyration of the

6 biner reagents. The pressures on the floor of the stope will becoe critical when they approach the copressive strength of the pastefill. For eaple, an increase in the transverse pressure (σ y ) woul probably inicate wall convergence. Fro the point of view of the ine prouction an safety, it is also iportant to know when the pressure being eerte on the barricae will be issipate. Finally, the longter evolution of the pressures in the pastefill allows estiation of its consoliation characteristics, an the stress reistribution ue to the ining at the vicinity of fille stope. Figure 11 shows the evolution of the internal stresses of the pastefill at elevation.6 as a function of the elapse tie since the beginning of the filling. It can be observe that after reaching their peak values, all the pressures ecrease after the en of the filling (15 th ay) until approiately the 11 th ay. Beyon 122 ays a continual increase in the longituinal ( ) an vertical ( ) pressures until 361 ays is observe. On the other han the transverse pressure ( ) increases an then began ecreasing. The sae tenencies were observe for the pressures (2, σ y2, σ z2 ) easure at elevation 7.6. The increase in the pressures beyon 122 ays is probably ue to the circulation of heavy achines on the top of the fill as well as new ining sequences at the vicinity of the Stope 8-1 FW. Pore pressure of pastefill is necessary for the calculation of the effective earth pressure, but was not easure in this stuy. Even if the rock pile barricae of the trial stope allows the rainage of free water, any rainage was quantifie. Due to its strong water retention capacity the Doyon Gol Mine pastefill reains saturate (oisture contents of about 38%) a long tie until 36 ays an the rainage perio, if any, oes not ecee 5 ays. Inee, in situ easureents of pore pressure of pastefill on barricae showe that pore pressure is negligible as shown on Figure 12 (Briges, 23). Develope internal pressure (kpa) Elapse tie since the beginning of the filling (ay) Figure 11. Long-ter evolution of the internal pressures of the pastefill at the floor of the stope. Figure 12. Pressures on barricae as a function of the fill height (After Briges, 23). 6.2 Effect of fille height on the evelope pressure Vertical pressure Figure 13 presents the evolution of the vertical pressure (σ z ) at elevation.6 (botto of the plug) an at elevation 7.6 (plug/resiual fill interface) copare to the theoretical overburen stress o the pastefill (γh). This coparison allows verification of the eistence of an arching effect. An arching effect woul reuce the agnitue of the vertical pressure at the floor of the stope (σ z < γh) which will be copensate by an increase in the longituinal pressure ( ) on the walls of the stope (Aubertin et al. 23; Li et al. 23, 24). The curves thus show that there was an arching effect in the fille stope. Vertical pressure σ z (kpa) γh Curing perio γh σ z σ z Fille height h () Figure 13. Variation of the internal vertical pressure σ z of the pastefill as a function of the fille height Longituinal pressure Figure 14 presents the evolution of the longituinal pressure at elevations.6 (plug) an 7.6 (plug/resiual fill interface an lower wall). The longituinal pressure at the botto of the stope ( )

7 is ore than twice that easure at the plug/resiual fill interface (2 ) as well as at the lower wall (_wall ). It is also note that the pressure eerte on the lower wall (_wall ) is slightly lower than that easure on the sae ais but at a istance of 3 (2 ). The aiu longituinal pressures an 2 were obtaine at a fille height of 22 while the aiu longituinal pressure on the lower wall (_wall ) was obtaine at a fille height of 18. Longituinal pressure (kpa) _wall Fille height h () Figure 14. Variation of the internal longituinal pressure of the pastefill as a function of the fille height Transverse pressure Figure 15 presents the evolution of the transverse pressure at elevations.6 (plug) an 7.6 (plug/resiual fill). This figure shows that the transverse pressure at the botto of the stope ( ) was twice that easure at the elevation 7.6 (2 ). The aiu lateral pressure was obtaine at a fille height of 19 while 2 reache a aiu value at a fille height of 14. Transverse pressure σ y (kpa) σ y σ y Fille height h () Figure 15. Variation of the internal transversal pressure σ y of the CPB as a function of the fille height Pressure on the barricae Figure 16 presents the evolution of the lateral pressure on the barricae (_b ) as a function of the fille height. This pressure increase continuously an reache its aiu at a fille height of 22. Beyon this fille height the placeent of aitional pastefill i not result in an increase in pressure on the barricae, on the contrary the pressure ecrease. Pressure on barricae _b (kpa) σ X_b Fille height h () Figure 16. Variation of the lateral pressure _b on the barricae as a function of the fille height. 6.3 Moeling pressures evelopent in the CPB uring stope filling The results of the pressure easureents presente in this paper clearly show that the pressures ecrease slightly, by about 8 kpa, (ecept for the pressure on the barricae, see Fig. 1) uring the curing perio (between the 2 n an the 5 th ay). This inicates that the evelopent of the internal pressures ay be inepenent of the hyration of the biner reagent (see Fig. 7). Consequently, the oinant factor appears to be the fille height. Moreover, uring pastefill placeent it is helpful to know the evolution of the pressures evelope within the pastefill base on the fille height. Accoringly, we propose siple 3D oels to allow the preiction of both the three-iensional pressures (, σ y, σ z ) evelope in the pastefill an the pressure eerte on the barricae (_b ) uring filling Fille height-epenent 3D oel to preict the internal stresses of pastefill Accoring to the results presente, the internal stresses of the CPB increase graually as a function of the fille height to soe aiu values. These stresses then reaine relatively constant at the en of the filling. With regar to the botto of the stope, the longituinal pressure ( ) was alost twice that evelope vertically ( ) or transversely ( ). This type of variation suggests that the pressure at the

8 floor of the stope epens on the unit weight of the pastefill, an ore iportantly on the iensions of the stope, apparently as a result of the arching effect. Thus, the variation of the longituinal pressure ( ) epens on its aiu value ( ) a an on the fille height (h). This variation of can be escribe by an eponential relationship (as propose by the Marston theory; see McCarthy 1988 an Aubertin et al. 23), which can be forulate as follows: ( h z) ( σ ) 1 ep σ ( h) (1) a a where a is a constant of proportionality; h is the fille height (); z is the elevation (); an h z. The aiu longituinal pressure, ( ) a, epens on the overburen stress of the CPB (γh) an can be estiate by the following relationship: ( σ ) γ( H z) H (2) a 3 ( B + L) where γ is the bulk unit weight of the CPB (kn/ 3 ); H is the total height of the fille stope (); z is the elevation (): z at the floor of the stope, z H at the top of the fille stope; B is the stope with; an L is the stope length. Substituting Equation 2 into Equation 1 an assuing that the constant a is half of the stope with B (a B/2) leas to the following 3D oel: ( H z) 2( h z) ( ) 1 ep B + L B γh σ ( h) 3 (3) where z h H. Longituinal pressure (kpa) easure easure Fille height h () Figure 17. Coparison between eperiental ata an curves of the longituinal pressure at two elevation points (z1.6 an z2 7.6 ) using Equation 3: γ 18 kn/ 3, H 29, B 12, L Figure 17 shows the easure longituinal pressures at the elevations.6 ( ) floor of the stope an 7.6 (2 ) plug/fill interface copare to the values using Equation 3. Note that this 3D oel escribes the longituinal pressure at the floor of the stope ( ) reasonably well, but at the plug/resiual fill interface (2 ) is not as accurate. Fro the results of pressure easureents presente in this paper (e.g. Fig. 7) one can reasonably assue that the vertical pressure (σ z ) evelope in a stope backfille with pastefill is approiately equal to the evelope transverse pressure (σ y ). Fro this figure one can also consier that the longituinal pressure ( ) at the floor of the stope is about 1.8 ties the transverse pressure [ 1.8 (σ y σ z )]. This observation is not true for the pressures easure at the plug/fill interface (see Fig. 8). Consequently, the transverse an vertical pressures can be evaluate using the following relationship: ( H z).185 γh 2( h z) σ y z ( h) 1 ep B + L (4), B Figure 18 shows the easure transverse pressures at the elevations.6 ( ) an 7.6 (σ y2 ) copare to the values using Equation 4. It is note that the oel reasonably well preicts the transverse pressure at the floor of the stope ( ), but less accurately at the plug/resiual fill interface (σ y2 ). Figure 19 shows the easure vertical pressures at the elevations.6 ( ) an 7.6 (σ z2 ) copare to the values using Equation 4. Note again that the oel preicts the vertical pressure at the floor of the stope ( ) rather well, but preicts that at the plug/resiual fill interface (σ z2 ) less well. Other approaches evelope to oel the stresses in backfille stopes have been presente in recent copanion papers (Aubertin et al. 23; Li et al. 23, 24). Transversal pressure σ y (kpa) σ y2 easure easure σ y Fille height h (ay) Figure 18. Coparison between eperiental ata an curves of the transversal pressure σ y at two elevation 1 2

9 points (z1.6 an z2 7.6 ) using Equation 4: γ 18 kn/ 3, H 29, B 12, L 21. Vertical pressure σ z (kpa) σ z2 easure easure σ z σ z Fille height H () Figure 19. Coparison between eperiental ata an curves of the vertical pressure σ z at two elevation points (z1.6 an z2 7.6 ) using Equation 4: γ 18 kn/ 3, H 29, B 12, L Fille height an tie-epenant 3D oel to preict the internal pressures of the pastefill The forulation of Equations 3 an 4 oes not take into account the elapse tie uring the stope filling with pastefill. In these relationships the only paraeter which can vary with tie is the bulk unit weight (γ) of the pastefill. However, this paraeter is constant in the initial forulation of Equations 3 an 4. To take the tie factor into account in these equations we propose a relationship escribing the evolution of the bulk unit weight of the pastefill with tie (γ*) as follows: γ γ * (5) γ γ t 1+ γ ta where γ is initial bulk unit weight of the pastefill (kn/ 3 ); γ is the ry unit weight of the pastefill (kn/ 3 ); t is the tie elapse since the beginning of pastefill placeent in the stope (ay); t a is the aiu elapse tie (ay) at which γ γ (t a is estiate to be approiately 2 years or 758 ays). Substituting Equation 5 into Equations 3 an 4 leas to the oels to preict the evolution of the internal pressures of the pastefill as a function of elapse tie since the beginning of the filling as follows: γh ( H z) 2( h z) (6) σ ( t) 3 ( B + L) γ γ 1+ γ t t a 1 ep 1 B 2 σ y, z ( t).185 γh γ γ 1+ γ ( B + L) ( H z) t t a 2( h z) 1 ep B (7) Figure 2 shows the longituinal pressure easure at the elevation.6 ( ) copare to the values using Equation 6. It can be observe that the oel preicts the longituinal pressure at the floor of the fille stope ( ) as a function of the fille height an elapse tie rather well, thus inicating that Equation 5 is well forulate. Longituinal pressure s (kpa) (easure) () Elapse tie since the beginning of the filling (ay) Figure 2. Coparison between eperiental ata an curve of the longituinal pressure at the elevation point z1.6 using Equation 6: γ 18 kn/ 3, γ 12.6 kn/ 3, H 29, B 12, L 21, t a 758 ays D oel to preict pressure on barricaes Because of the copleity of the paste backfill the aaptation of the Rankine theory of earth pressures is not conucive to the preiction of the lateral (or longituinal) pressure eerte on the barricae. Even though the Rankine passive an active earth pressures equations are siple to use, they nevertheless nee the intrinsic paraeters of the pastefill (c an φ) which can be obtaine fro laboratory tests. Fro the analysis of the eperiental results presente in this paper we propose a siple 3D eponential oel to preict the lateral pressure eerte by the pastefill on the barricae (σ b ) as a function of fille height which is given by the following relationship: γ( h z') 4 ( B + L)( h z') σb ( h) ep (8) 2 9 L B where γ is the bulk unit weight of the pastefill (kn/ 3 ); h is the fille height (); z' is the elevation of the point of easureent () in the raw point; B is the stope with (); an L is the stope length (). Substituting Equation 5 into Equation 8 leas to a oel to preict the lateral pressure on the barricae

10 as a function of elapse tie since the beginning of the filling: γ( h z' ) σ b ( t) γ γ t 2 1+ γ t a 4 ( B + L)( h z' ) ep 9 L B (9) Figure 21 shows the easure lateral pressure on barricaes of two stopes of ifferent size (large an sall) copare to the values using Equation 8. Barricae 1 (large stope) is that of the stope stuie in this paper while the barricae 2 is that of another instruente fille stope (sall stope) which is not presente herein. It can be note that Equation 8 preicts the lateral pressure on the barricae of the large stope (barricae 1) rather well, but preicts the lateral pressure on the barricae of the sall stope (barricae 2) less accurately. Pressure on barricae σ b (kpa) Barricae 1 (B12, L21, H29) -Barricae 2 (B3.5, L23, H22) (barricae 1) easure (barricae 2) easure (barricae 1) (barricae 2) Fille height h () Figure 21. Coparison between eperiental ata an curves of the longituinal pressure σ b on the barricae (z'1 2.1 an z'2 1.8 ) using Equation 8: γ 18 kn/ 3. 7 CONCLUSION The objective of this stuy was to follow the evolution of internal stresses in ceente fill uring an after stope filling. To reach this objective, a stope at the Doyon Gol Mine (Cabior Inc., Canaa) was instruente at various points using earth pressure cells (oel TPC). The stresse inuce in the pastefill (σ longituinal, σ transversal σ y an σ vertical σ z ) as well as on the lower wall an on the barricae (lateral or longituinal pressure) were recore uring an after placeent of pastefill. The resulting ata inicate that the internal longituinal pressure of the pastefill is higher than the transversal an vertical pressures. This tens to confir the eistence of an arching effect which evelops in the stope uring filling. In orer to have tools for stability analysis of the fille stopes with pastefill, four 3D oels were propose to preict the internal pressures an the pressure on barricaes an both as a function of the fille height (Eqs. 3 & 8) an as a function of elapse tie since the beginning of backfilling (Eqs. 5 & 9). The propose oels responses are in goo agreeent with the eperiental ata. This result is very encouraging to the on-going stuy of groun control using pastefill in unergroun ines. The results presente herein are base on the easureents ae in a specific stope fille with a specific type of pastefill an ay not be applicable elsewhere. ACKNOWLEDGMENTS This research was supporte by the Fon e l'université u Quebec en Abitibi-Téiscaingue (FU- QAT), IRSST an parts of NSERC an NATEQ. The authors gratefully acknowlege their support. The authors woul also like to thank our ining partner, Cabior Inc. (Mine Doyon) for their collaboration in the copletion of this work. REFERENCES Aubertin, M., Li, L., Arnoli, S., Bele, T., Bussière, B., Benzaazoua, M., Sion, R. 23. Interaction between backfill an rock ass in narrow stopes. In P.J. Culligan, H.H. Einstein, A.J. Whittle (es), Soil an Rock Aerica 23, vol. 1, pp Essen: Verlag Glückauf Essen (VGE). Been K., Brown E.T., Hepworth N. 22. Liquefaction potential of pastefill at Neves Corvo Mine, Portugal. Trans. Institution Mining an Metallurgy, April 22, A47-A58. Bele T., Benzaazoua, M., Bussière B., Dagenais A.-M. 22. Effects of settleent an rainage on strength evelopent within ine paste backfill. Proceeings of Tailings an Mine Waste'2, 27-3 January 22, Fort Collins, Colorao, Balkea : Rottera, pp Benzaazoua M., Bele T. 2. Optiization of sulfie-rich paste backfill itures for increasing long-ter strength an stability. Proceeings of 5 th Conference on Clean Technology for Mining Inustry, Santiago, M.A. Sánchez, F. Vergara & S.H. Castro es., University of concepción, Vol. I, pp Benzaazoua M., Bele T., Bussière B. 22. Cheical aspect of sulfurous paste backfill itures. Ceent an Concrete Research, Vol. 32 (7), pp Bernier, R.L., Li, M.G., an Moeran, A Effects of tailings an biner geocheistry on the physical strength of paste backfill. Suburry'99, Mining an the environent II. Eite by N. Golsack, P. Belzile, Yearwoo an G. Hall. 3: Briges M. C. 23. A New Era Of Fill-Retaining Barricaes. AMC's newsletter Digging Deeper on current events an oern ining ethoology. e Corson D.R Fiel evaluation of hyraulic backfill copaction at the Lucky Friay Mine, Mullan, Iaho, U.S. Bureau of Mines, RI 7546.

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