Backfill Practices for Sublevel Stoping System

Transcription

1 Backfill Practices for Sublevel Stoping System Muhammad Zaka Emad 1, Isaac Vennes 1, Hani Mitri 1, and Cecile Kelly 2 1 McGill University, Montreal, Canada 2 Vale Manitoba Operations, Thompson, Canada Abstract. Sublevel stoping mining methods are very popular in Canadian metal mines since they enable maximum ore recovery due to pillar-less mining. Sublevel stoping mining methods are generally employed to extract steeply dipping tabular ore deposits characterized by relatively long strikes and having various thicknesses. Ore is recovered from each sublevel in a planned sequence of primary and secondary stopes with delayed backfill in a manner to create a stress shadow to avoid high stress buildup in ore pillars. Backfill material is engineered to withstand the exposed height of the primary stope it occupies when adjacent secondary stope is being mined, thus avoiding dilution of precious ore which may result from backfill failure into the secondary stope. Thus, backfilling is one of the most critical tasks for sublevel stoping mining systems with delayed backfill. The selection of the type of backfill is a substantial component of backfill design at preliminary stage of mining production. This paper presents a review of backfill practices with emphasis on backfill operations in North America. Operations employing cemented rockfill and paste fill are presented. Backfill design recipes, properties and placement methods are discussed. The merits, demerits and major difficulties associated with two backfill operations are discussed in this paper. Backfill operation at a case study mine is presented and preliminary recommendations are made based on a review of best practices. Keywords: underground mining, sublevel stoping, mine backfill, cemented rockfill. 1 Introduction Many Canadian metal mines have adopted sublevel stoping method or one of its variations, such as blasthole stoping, vertical crater retreat (VCR), or vertical block mining (VBM) for the extraction of steeply dipping ore bodies [1]. In these methods, the ore body is divided into blocks or stopes, which are mined out while following a pyramidal mining sequence in transverse-retreat directions. In VBM, stope production is carried out in three or four blasts or lifts. Each lift is blasted and mucked before blasting the next one. Backfill is required to fill up the stopes once they are mined out. Backfill can be defined as, the filling of an excavation with waste material. It is being used as a filling material for stopes in cut and fill and open stoping mining methods. Backfilling is a widespread practice in Canadian metal mines. It has proven to be a good passive support in hard rock C. Drebenstedt and R. Singhal (eds.), Mine Planning and Equipment Selection, 391 DOI: / _38, Springer International Publishing Switzerland 2014

2 392 M.Z. Emad et al. metal mines. It decreases ore dilution from hanging-wall and footwall slough, and it maximizes ore recovery [2-6]. Backfill dilution is a prime concern while mining adjoining stopes for pillar-less mining. The stability of exposed backfill face is governed by many factors, some of them are stope dimensions, stiffness of backfill, cohesion and blast induced vibrations from adjacent stope being mined [4, 7]. Backfill can be classified into the following three main types; 1.1 Hydraulic Fill Hydraulic backfill also known as slurry backfill is composed of a classified, permeable, low density blend of mill tailings, sand, rock and water having and average pulp density of around 60% to 70% solids by weight [7, 8]. Fill material is transported through a network of pipelines at high velocity while maintaining turbulence to achieve good suspension of solid particles. Extensive transport water is required for conveying fill through the mine backfill transportation system. Consequently, there is considerable water seepage from stopes after placement, which must be pumped back to surface [8-10]. The fine particles are not accommodated in this system and are rejected to surface in a tailings pond. Hydraulic backfill may or may not incorporate a binder, depending on the purpose of the backfill. The mines practicing hydraulic fill also require a strong barricade to contain fill mass. Additional drainage is required to drain out seeped water from fill [9, 10]. This method requires huge binder quantities as most of the binder seeps out of the stope with water. Cured hydraulic backfill is generally very weak as compared to other backfill types. That is why it is practiced for overhand cut and fill operations and is not exercised for mines practicing open stoping methods with greater exposures [9-11]. 1.2 Paste Fill Paste backfill is composed of thickened mill tailings generated during mineral processing which are mixed with additives such as Portland cement, lime, pulverized fly ash, and smelter slag. Paste fill utilized full range of tailings and tailings to be disposed are lesser in quantity compared to hydraulic fill systems [9-11]. It includes sand/mine tailings, water and cement is added to enhance strength properties of fill. Its bulk density is 75 80% solids by weight. Paste fill is relatively consistent backfill having high pulp density with about 15% minus 45 microns (325 mesh) fines content [9]. The consistency of paste fill is similar to that of tooth paste. Paste fill is the most popular backfilling method for massive ore bodies employing sublevel mining systems and it has higher initial costs and moderate operational costs. It is generally used for large operations having high mining rates and long mine life. A smaller operation does not justify the costs involved with installation, maintenance of a paste fill plant, and paste fill distribution system. A detailed review of paste fill operation is presented in Section 2.

3 Backfill Practices for Sublevel Stoping System Cemented Rockfill [1]: Cemented rockfill (CRF) is also known as consolidated rockfill. It is composed of sized or un-sized aggregates coated with Portland cement slurry having an overall cement content of 4 6%. Binder contents below 4% is inappropriate for binder coating on aggregate and binder contents greater than 6% are less economical. Developmental waste can also be used in place of aggregates or can be blended with aggregates in CRF [8, 9]. CRF is capable of bearing active pressures, providing not only ground support but also improves wall rock stability. No additional drainage is required for CRF systems and high strengths can be achieved by proper placement. Quality control and segregation can be difficult to control. Stope access, backfill raise orientation, binder contents, particle size distribution, water to binder ratio etc. are critical design parameters [8, 10, 12]. Sometimes, Portland cement is blended with fine slag or fly ash for cost effectiveness, but inclusion of flyash increases curing time of CRF. CRF has higher strength than paste fill and hydraulic fill, when properly practiced [8, 10]. CRF is generally employed for smaller operations having low mining rate or narrow vein ore bodies being mined with open stoping mining method with delayed backfill. A detailed review of CRF practices is presented in Section 3. 2 Review of Paste Fill Practice [1] Paste fill is the most popular backfilling method used for open stoping mining method with delayed backfill. It requires mill tailings to be used as a raw material. A backfill plant is also required for paste thickening and temporary storage. Backfill plant accepts mill tailings from the mine mill or tailings pond, reduces their water contents to % using paste fill thickener technology [9]. It then mixes binder and additives for achieving desired properties of paste fill. A network of pipes with valves and pumps are required for delivering paste fill underground. Gravity driving for paste is generally preferred [8], which requires a controlled range of bulk density values of paste. The main paste fill line should be drilled and cased separately from other mine services. This is done to avoid production delays in case there is a blowout of main fill line. Backfill plant monitoring and pipeline monitoring system is also required to avoid system failure. Design of backfill plant and paste fill line is based on backfilling rate and pipeline wear. Mine backfill team must devise a plan to deal with any blockage, plugging or blowout of pipelines [13]. Pressure losses are higher in paste fill systems are around 10MPa/Km. Pipeline wear is a major issue for paste fill operation, it can be reduced by increasing percentage solids [8]. Variable pipeline diameters for different sections should be eliminated to decrease pipe wear [13, 14]. A typical layout of paste fill system is shown in Fig. 1 [15].

4 394 M.Z. Emad et al. Fig. 1 Paste fill processing inside a backfill plant [15] Increasing solids in fill increases the strength of backfill [14, 16], which means that paste fill is stronger than hydraulic fill, the relationship is presented in Fig. 2. High solid contents in paste fill above 70% require a positive displacement pump rather than centrifugal pump for horizontal transport of fill. The relationship is presented in Fig. 2 [13]. Pumps are not generally employed for paste fill operations and gravity flow is the choice by decreasing horizontal pipe lengths. A comparison of specifications for small and large tailing disposal system is presented in Tab. 1 [17]. The literature review has unveiled the following advantages [9, 18] of using paste fill: 1. It reduces mining cycle due to an early development of compressive strength and nonstop filling 2. It reduces binder utilization due to its homogeneous fill nature 3. It consumes the whole range of mill tailings 4. No critical velocity is required to keep backfill in suspension [10, 11] 5. Its operation is cleaner

5 Backfill Practices for Sublevel Stoping System 395 Paste Cake Slurry Centrifugal Pump Positive Displacement Pump % Solids Fig. 2 Effect of % solids on pumping requirements [13] Table 1 Comparison of specifications for a small and large mine backfill systems [17] Small system Large system Slurry Nickel tailings Kimberlite slimes Tonnage 150 t/h 1570 t/hr Volumetric flow rate 190 m 3 /hr 3640 m 3 /h Pipe diameter 170 mm 450 mm Pipeline length 5800 m 6500 m Discharge pressure 2700 kpa 2600 kpa Number of Pumps 6 duty and 6 standby 12 duty and 6 standby Absorbed power 330 kw 7200 kw 6. It provides flexibility to mine adjacent, underneath or through the fill [8] 7. It reduces long term environmental impacts [19] 8. Better mixing can be achieved [20] 9. Lower cost for constructing a barricade and there is no drainage from stope [19]

6 396 M.Z. Emad et al. The literature review presents following disadvantages [9, 21] of paste fill: 1. Extensive technical supervision and monitoring is essential for successful operation [21] 2. There is a need for better dewatering technology from stopes for better performance [9] 3. High pipeline pressures at greater depths leading to pipeline blow-out and pipe wear [9, 14] 4. Higher exposures of paste fill may lead to backfill dilution 5. Pipelines can get plugged due to slow sedimentation of fill 6. Sticky tailings can pose problems in handling and pumping [8] 7. Fill may leak through jointed rock when poured in an uncased hole[20] 8. Dynamic loading from adjacent production stope may incur dilution of precious ore [20] 9. High dynamic loads on pipeline may lead to failure [10] 10. Blended tailings may segregate when placed in stope [10] 11. Tailings exposed to weather conditions may change properties [16, 21] 3 Review of Cemented Rockfill Practices Cemented rock fill is employed for narrow vein deposits being mined with open stoping mining method with delayed backfill. It is a stiff backfill and it can be used for higher exposures for achieving greater recoveries. CRF system comprise of rock aggregate source, aggregate processing system, binder slurry storages, mechanical mixer and a placement mechanism. Backfilling with CRF may require slurry mixer for preparing binder slurry in a desired ratio which is to be preprogrammed in the mixer. The cement slurry is pumped to desired stope to be filled by a network of pipes and pump. Rock aggregates are dumped in a surface raise which divides into finger raises terminating at different levels. The rock is mixed with cement slurry and is dumped in the stope by load-haul-dump (LHD), truck or by a backfill raise. A small berm is sometimes constructed by dumping waste rock to contain fill inside the stope [9, 22]. Fig. 3 shows a typical schematic of CRF system. Rock aggregate for CRF should be graded for increased particle interlocking which leads to better CRF performance. This is due to the fact that particle size distribution has the largest impact on CRF strength and density [12, 23, 24]. The aggregate can be graded to maximize CRF density for increased strength based the Talbot distribution curve [23]. For longhole stopes, the Talbot distribution curve is adapted to obtain excess fines, increasing aggregate flowability [23]. However, it is found that excessive fines in CRF will take away most of the binder. On the other hand, deficiency of fines in CRF will lead to higher void ratio [12, 23-25]. Particle size distribution also affects the efficiency of the cement [26]. For example, a poorly graded aggregate with a high void ratio will have cement pool in the voids rather than coat particles [27].

7 Backfill Practices for Sublevel Stoping System 397 Fig. 3 Schematic of cemented rockfill preparation system Rock aggregate shape and rock fabric may influence CRF strength as cleavage planes can contribute to failure. Flat or elongated particles can slide through the screens and can create an uncontrolled particle size distribution [22, 24, 28]. Physical durability of aggregates is also very important if the aggregate is to be rehandled especially when dropped from a backfill raise [12, 23]. Attrition of the aggregate can cause excess fines, reducing CRF strength. Aggregate physical durability can be checked with the Los Angeles Abrasion test and aggregates with an LAA value below 30 are good for CRF. Water to cement ratio should be for open stoping mining methods. The range 0.8 to 1.2 depends on weather conditions 0.8 for wet aggregate and 1.2 for very dry aggregates [22, 23, 29]. The water to cement ratio also depends on required cement slurry flowability [23]. Aggregate moisture contents may vary with weather and a variation in moisture contents due to wet weather will categorically influence on water to cement ratio [23, 24]. Snow, rainfall and dry periods can have an influence on strength due to a change in water to cement ratio in CRF. It has been observed that an increase in water contents may flush away the fines and binder, whereas a decrease in water content will lead to lack of flow-ability in CRF [22, 23, 29]. In such a case preprogrammed fill recipes should be changed accordingly. Aggregate clay contents also need to be monitored as high clay content may behave in a similar fashion to excessive fine particles that is to take away the cement contents. Thin clay layer

8 398 M.Z. Emad et al. on aggregates may reduce cohesion of cured CRF [23-25]. Cemented rockfill columns are generally designed on the basis of limit equilibrium methods available in the published literature [22, 30-32]. These methods provide static strength requirements for backfill columns. Some mines design their CRF stopes based on experience and previous practices by other operations in near vicinity [22]. The required UCS is achieved by varying CRF practice with respect to binder, particle size distribution, placement method, water contents etc. Placement method is particularly important in controlling segregation and acquiring suitable CRF strength across the entire stope; Segregation of the aggregate can lead to a wide variation of CRF strength across a stope [28, 33]. Placement method can be adjusted by for example changing the orientation or the position of the fill raise. When the fill raise orientation and position is designed properly, one will obtain well cemented, high strength zones near future exposed faces and obtain unavoidable, poorly cemented, low strength zones in the center of the stope or near faces that will not be exposed [28]. The extent of the aggregate segregation also depends on stope geometry and stope size, with large stopes requiring multiple fill points to limit segregation [28]. A quality control program is essential for any product, and especially CRF which needs to be tested and monitored in the laboratory and underground [22, 23, 25, 28]. Tab. 3 shows different fill recipes and strength of CRF for different operations in North American mines. Table 2 Typical mix specifications for North American mines [23, 34] Mine Aggregate Aggregate % Binder UCS (Psi) Top Size (in) Coarse/Fine Deep Post / Carlin East / Deep Star / Rodeo / Meikle / Bullfrog / Turuoise Ridge / Kidd Creek / Bousquet Birchtree / The literature review presents following advantages [22-24, 34] of using CRF: 1. Raw material for CRF is readily available 2. It is a stiff backfill and required stiffness can be achieved with lesser binder 3. High stiffness enables greater exposures 4. Capital cost is low

9 Backfill Practices for Sublevel Stoping System Its operation is simple 6. Better practices can decrease dilution levels 7. Higher exposures can be achieved by better practice The literature review presents following disadvantages [22-24, 34] of using CRF: 1. Low mining flexibility 2. Moderate operational costs 3. Difficulty to tight fill 4. Segregation 5. Improper aggregate size distribution leads to low strength 6. Water to cement ratio is hard to control 7. Failure in adjacent stopes being mined 8. Blast induced vibrations may destabilize CRF 4 Backfill Practices at the Case Study Mine The case study mine is located in northern Manitoba. The mine is practicing sublevel stoping mining method with delayed backfill. Cemented rockfill is being used as a backfill material, which is prepared by mixing rock aggregate with binder slurry (blend of type 10 Portland cement and type C fly ash in 30:70). The rock aggregate used is graded and generally comes from developmental work and a nearby open pit mine. The type of rock is biotite-schist (with minerals biotite 16 24%, quartz 28 42% and feldspar 35 55%) which has a low porosity with an average UCS of 99 MPa and Young s modulus of 56 GPa. The rock is graded and passed through sieves to achieve ¼ size range, no extra fines are added. Rock aggregate is then conveyed to the mine using trucks. The rock is then dumped in rockfill raise, which divides into many finger raises ending at different levels. Fill raise is filled to the surface in summer but in winter the level of rock is dropped to 300 feet from surface to avoid freezing of aggregates. The level is maintained within 500 feet from surface to avoid impact damage. An estimated capacity of rockfill raise is nearly 9 tons/ft. A 20 ton binder silo underground is connected with an 8 binder line via main shaft. Binder is conveyed pneumatically through binder line and compressors are used to supply pressure to keep the binder suspended. The water to binder ratio is generally kept at 0.54 (observed to be 0.7 during underground visit) using the flash mixer. The water used for binder preparation comes from nearby river. A flash mixer is used for mixing binder slurry and a Moyno pump is used for pumping slurry to stope. Slurry line is flushed after every shift to avoid plugging of slurry line. The ore body is mined transversely, by developing sill drifts on top and bottom of the planned stopes. The stopes are then drilled and blasted from the top or bottom drifts, the dimensions of stope are 60 x 40 x 100 (l x w x h) and there are generally three stopes across the thickness of the orebody. The ore is mucked from bottom sill and the stope is then backfilled from top sill drift using LHD s. The binder slurry is

10 400 M.Z. Emad et al. prepared in colloidal mixers at different levels with a mixture of Portland cement, fly ash and water. The binder slurry is then pumped to each sub-level. There are two mixing methods being practiced at the case study mine. First method is termed as sump method in which a LHD operator loads aggregate from finger raise, it then dumps aggregates in a sump and showers binder slurry over it as practiced by Junction mine in Australia [8]. The LHD loads CRF from sump and then dumps it in an open stope. The backfilled stope is cured for at least 28 days before mining adjacent secondary stopes. Second method used for mixing is termed as bucket method in which a bucket load of aggregate is showered by binder slurry before placement in the stope. The mine has two 12 hour shifts per day and the current average placement rate of backfill is 500 tons per shift. The stope which will not be exposed in the future are filled with unconsolidated rockfill. 5 Conclusion This review of backfill practices has highlighted some of the issues associated with different types of mine backfill. Rationalized solutions to some of the inherent problems related to different backfilling methods have been presented. The review of cemented rockfill practices at the case study mine has enabled a thorough review of backfill operation and shed light on industrial practices. Acknowledgements. This work is jointly funded by a grant from the Natural Sciences and Engineering Research Council of Canada (NSERC) and Vale Canada under the Collaborative Research and Development (CRD) Program. The authors are grateful for their support. References 1. Zhang, Y., Mitri, H.S.: Elastoplastic stability analysis of mine haulage drift in the vicinity of mined stopes. International Journal of Rock Mechanics & Mining Sciences 45, (2008) 2. Aithchison, G.D., Kurzeme, M., Willoughby, D.R.: Geomechanic considerations in optimizing the use of fill. In: Mine Filling Proceedings of the Jubilee Symposium, Australian Institute of Mining and Metallurgy, Queensland, Australia, pp (1973) 3. Dickhout, M.H.: The Role and Behaviour of Fill in Mining Jubilee Symposium on Mine Filling. Aus IMM (August 1973) (2,8,9,10) 4. Archibald, J.F., et al.: Underground Mine Backfill 1 & 2. Internet course publication of InfoMine Inc. (2003) 5. Jung, W.J., et al.: Effects of rock pressure on crack generation during tunnel blasting. IJ. Japan Explosives Soc. 62(3), (2001) 6. Kump, D.: Backfill - Whatever it takes. Mining Engineering 53(1), (2001)

11 Backfill Practices for Sublevel Stoping System Hassani, F., Archibald, J.: Mine Backfill. Canadian Institute of Mining, Metallurgy, and Petroleum (1998) 8. Grice, A.G.: Recent Minefill Developments in Australia. In: Minefill 2001: Proceedings of the 7th International Symposium on Mining with Backfill Society of Mining, Metallurgy & Exploration, Seatle (2001) 9. Hassani, F., Archibald, J.: Mine Backfill. Canadian Institute of Mining, Metallurgy, and Petroleum, Montreal (1998) 10. Henderson, A., Jardine, G., Woodall, C.: The implementation of paste fill at the Henty gold mine. In: Bloss, M. (ed.) Minefill 1998: The Sixth International Symposium on Mining with Backfill, pp Australasian Institute of Mining and Metallurgy, Brisbane (1998) 11. Currie, R.: The preparation of pastefill and its use at some Canadian mines. In: Bloss, M. (ed.) Minefill 1998: The Sixth International Symposium on Mining with Backfill, pp Australasian Institute of Mining and Metallurgy, Brisbane (1998) 12. Chen, D., Messurier, M.L., Mitchell, B.: Application of cemented aggregate fill at Barrick s Darlot Gold Mine. In: Aimin, Z. (ed.) Minefill 2004: Proceedings of the 8th International Symposium on Mining with Backfill, pp The Nonferrous Metal Society of China, Beijing (2004) 13. MacKenzie, A.T., Rantala, P.A.: Pastefill transportation techniques and predictive rheology. In: Stone, D. (ed.) Minefill 2001: The 7th International Symposium on Mining with Backfill, pp Society for Mining, Metallurgy & Exploration, Seatle (2001) 14. Jewel, R.J., Fourie, A.B.: Paste and thickened tailings - a guide, 2nd edn. Australian Centre for Geomechanics (2005) 15. Slottee, J.S.: Update on the Application of Paste Thickeners for Tailings Disposal - Mine Paste Backfill. In: International Seminar on Paste and Thickened Tailings Paste and Thickened Tailings Paste 2004 (2004) 16. McGuiness, M.: Pipeline wear and the hydraulic performance of pastefill distribution systems: the Kidd Mine experience. Masters Seminar-1 Presentation (2012) 17. McGuinness, M., Cooke, R.: Pipeline wear and the hydraulic performance of pastefill distribution systems: the Kidd Mine experience. In: Minefill 2011, 10th International Symposium on Mining with Backfill, pp The Southern African Institute of Mining and Metallurgy (2011) 18. van Gool, B.S., Karunasena, W., Sivakugan, N.: Effects of blasting on the stability of paste fill stopes at Cannington Mine. James Cook University, Australia (2007) 19. Skeeles, B.E.J.: Design of paste backfill plant and distribution system for the Cannington project. In: Bloss, M. (ed.) Minefill 1998: The Sixth International Symposium on Mining with Backfill, pp Australasian Institute of Mining and Metallurgy, Brisbane (1998) 20. Bloss, M.L., Mathew, R.B.: Mining with paste fill at BHP Cannington. In: Stone, D. (ed.) Minefill 2001: The 7th International Symposium on Mining with Backfill, pp SME, Seatle (2001) 21. Archibald, J.F., Souza, E.M.D., Beauchamp, L.: Safe Canadian practices in mine backfill operations. In: Minefill 2011, 10th International, Symposium on Mining with Backfill, pp The Southern African Institute of Mining and Metallurgy (2011)

12 402 M.Z. Emad et al. 22. Stone, D.M.R.: The optimization of mix designs for cemented rockfill. In: Glenn, H.W. (ed.) Minefill 1993: The 5th International Symposium on Mining with Backfill, pp The South African Institute of Mining and Metallurgy, Johannesburg (1993) 23. Stone, D.: Factors that affect cemented rockfill quality in Nevada mines. In: Hassani, F., et al. (eds.) Minefill 2007: The 9th International Symposium on Mining with Backfill, pp Canadian Institute of Mining and Metallurgy, Montreal (2007) 24. McKay, D.L., Duke, J.D.: Mining with backfill at Kidd Creek no. 2 mine. In: Hassani, F.P., Scoble, M.J., Yu, T.R. (eds.) Minefill 1989: Proceedings of the 4th International Symposium on Mining with Backfill, pp A.A.Balkema / Rotterdam / Brookheld, Montreal (1989) 25. Bloss, M.L., Greenwood, A.G.: Cemented rockfill research at Mount Isa Mines Limited In: Bloss, M. (ed.) Minefill 1998: The Sixth International Symposium on Mining with Backfill, pp Australasian Institute of Mining and Metallurgy, Brisbane (1998) 26. Peterson, S., Szymanski, J., Planeta, S.: A Statisitcal Model for Strength Estimation of Cemented Rockfill in Vertical Block Mining. In: Minefill The Australasian Institute of Mining and Metallurgy, Brisbane (1998) 27. Annor, A.B.: A Study of the Characteristics and Behaviour of Composite Backfill Material. Department of Mining and Metallugical Engineering. McGill University, Montreal (1999) 28. Farsangi, P.N.: Improving cemented rockfill design in open stoping. Department of Mining and Metallurgical Engineering. McGill University, Montreal (1996) 29. Reschke, A.E.: The use of cemented rockfill at Namew Lake mine, Manitoba, Canada. In: Glenn, H.W. (ed.) Minefill 1993: The 5th International Symposium on Mining with Backfill, pp The South African Institute of Mining and Metallurgy, Johannesburg (1993) 30. Mitchell, R.J.: Earth structures engineering. Allen & Unwin, Boston (1983) 31. Mathews, K.E., Kaesehagen, F.E.: The development and design of a cemented rock filling system at the Mount Isa Mine Australia. In: Jubilee Symposium on Mine Filling, Mount Isa, 1973, pp The Australian Asian Institute of Mining and Metallurgy, North West Queensland Branch Mount Isa (1973) 32. Yu, T.R.: Mechanisms of fill failure and fill strength requirements. In: Kaiser, P.K., McCreath, D.R. (eds.) 16th Canadian Rock Mechanics Symposium, Sudbury ON Canada, pp (1992) 33. Yu, T.R.: Some factors relating to the stability of consolidated rockfill at Kidd Creek. In: Hassani, et al. (eds.) Innovations in Mining Backfill Technology, pp (1989) 34. Yu, T.R., Counter, D.B.: Backfill practice and technology at Kidd Creek Mines. The Canadian Mining and Metallurgical Bulletin 76(856), (1983)