Dept. of Civil Engineering
University of Toronto
35 St. George St.
Canada, M5S 1A4
Our research group's focus is liquefaction of strong ground, and in particular the liquefaction of mine tailings mixed with binder and called Cemented Paste Backfill (CPB). The very nature of the underground mining environment and the methods used to extract ore result in boundary conditions and imposed static and dynamic loads that are fundamentally different from the near-surface natural soil deposits which have been the focus of traditional liquefaction studies. This provides the impetus to go beyond the traditional areas of liquefaction research and explore exciting new areas including the effects of binder hydration, unsaturated water phases, frequency dependance under sustained high frequency loading, heterogeneous in situ conditions, and also how rheology modifiers influence post-liquefaction behaviour.
Education and Designations
|Course Code||Title & Description||Instructor||Session||Location(s)||Day(s)||Start Time||End|
|CIV1420H||Prof. Murray Grabinsky||Winter 2017||GB217||Tuesdays||14:00||17:00|
|CME321H1||Prof. Murray Grabinsky||Fall 2016||Scheduled by the Office of the Faculty Registrar.|
|MIN511H1||Prof. Murray Grabinsky||Fall 2016||Scheduled by the Office of the Faculty Registrar.|
Mines produce a significant amount of waste material in the form of finely crushed rock and process water, called mill sands or mine tailings, and the management of these tailings poses a signficant engineering challenge. Too often the conventional forms of tailings disposal and management, in particular slurry deposition into tailings ponds, have not performed well resulting in significant adverse impacts on the environment, surrounding communities, and the operation of the mine itself.
breach of a conventional tailings dam and flow of loose, wet tailings into surrounding areas
Our research examines the engineering properties and behaviour of tailings that have been dewatered to the extent that they are similar in consistency to toothpaste. Such materials are often referred to as thickened tailings or paste tailings. Paste tailings may be either pumped to surface and stacked, thereby eliminating the conventional ponds altogether, or they may be mixed with cement and pumped back underground as Cemented Paste Backfill in order to stabilize the large underground voids created by mining. We have researchers looking at the application of paste tailings for both surface disposal and underground backfill.
Field sites are an important part of our research strategy and we have worked with mines located in Canada, South America, Africa, Europe, and Asia. Closer to home, our research laboratories are located in the Galbraith Building on the downtown campus of the University of Toronto. In addition to standard geotechnical apparatus for stiffness, strength, and permeability testing, we have developed specialized facilities including the following:
- A Dispersion Technologies DT1200 Acoustic/Electroacoustic analyser for characterizing, amongst other things, particle size distribution and zeta potential, for suspensions up to 50% solids content by volume. This device is very useful when combined with rheometry equipment, for studying the properties and behaviour of flowing paste.
- An Environmental Chamber for simulating drying paste in arid conditions. The development of suctions and the movement of moisture can be assessed within the deposited layers.
- A high frequency (up to 70 Hz fundamental frequency) servo-hydraulic triaxial test device, with embedded ultrasonic p- and s-wave transducers for simultaneous monitoring of small and large strain stiffness. Blast time histories from in situ monitoring projects can be fed into this machine in order to subject cemented paste backfill samples to blasting loads under controlled laboratory conditions.
Previous in situ monitoring projects with Canadian mines have shown that the as-placed properties of cemented paste backfill can vary significantly from the properties of "similar" samples prepared under controlled laboratory conditions. This has important implications for underground mining operations, especially in terms of the procedures they use to fill the underground voids (i.e., fill rates and stages of filling), the design and construction of the barricades used to retain the fill, the time the mine must wait before they can resume production blasting in proximity to the fill, and the ultimate strength of the fill especially in critical design situations that involve mining through or under wide spans of fill. A multi-year, multi-university research program is therefore being initiated to better develop the science and engineering of cemented paste backfill systems. To find out more about this project, contact us.
Evaporation From Surface Deposited Thickened Gold Tailings
by P. Simms, M. Grabinsky, and G. Zhan
9th International Seminar on Paste and Thickened Tailings
Limerick, Ireland, 3-7 April 2006
The rate of evaporation from thickened tailings is an important parameter for the management of surface deposition. Promoting evaporation in a freshly-deposited layer is desirable up to a point, since evaporation causes densification and strength gain, but excess evaporation will desaturate the tailings and consequently increase the risk of acid generation. Therefore, the ability to better predict the rate of evaporation would be a substantial advantage in deposition planning. This study, part of a larger project researching the surface deposition of non-plastic, thickened tailings at the Bulyanhulu Gold Mine in Tanzania, investigates drying from thickened gold tailings in the laboratory and in the field. Material presented in this paper includes laboratory comparison of evaporation from a small column (0.3 m diameter by 0.2 m in height) to evaporation from larger-scale tests (2 m by 1 m in plan and 0.1 m in thickness) performed on tailings from Bulyanhulu. These laboratory results are compared with field measurements.
Field Properties of Cemented Paste Backfill at the Golden Giant Mine
by K. le Roux, W.F. Bawden, and M.W. Grabinsky
Institution of Mining and Metallurgy: Mining Technology (Section A), 114(2):65-80 (2005)
Cemented paste backfill (CPB) has been used for almost a decade in the mining industry and is gaining popularity worldwide. However, its design is largely based on material that is prepared, cured, and tested in the laboratory environment. Replicating the field mixing, placement and curing processes in a laboratory is difficult, and there are questions as to how representative the laboratory material is of the actual field material. Anecdotal evidence suggests that the CPB sometimes under performs, as evidenced by excessive sloughing of the exposed paste wall, and sometimes over performs, as suggested by the stable excavations in the pastefill. Only by understanding the field performance of cemented paste backfill can the design be optimised while ensuring safety. A field investigation of the Golden Giant Mine’s cemented paste backfill was undertaken to quantify the in situ properties and to provide the data needed for mix design optimization. The investigation was two pronged, comprising in situ testing using a self-boring pressuremeter, and testing of undisturbed samples of CPB. The investigation showed that the bulk properties of the in situ backfill are more variable than laboratory prepared samples and, on average, tend to have a higher void ratio and lower degree of saturation. Field strengths derived from both the self-boring pressuremeter (SBP) and triaxial testing are variable but consistently higher than the laboratory samples and this may be attributed to a higher cohesion developed in the field CPB. These results suggest that the current backfill design at the Golden Giant mine may be conservative. The self-boring pressuremeter also provides an indication of the in situ stresses at the individual test locations. Together the stress measurements provide an indication of the overall stress distribution in the test stopes. The results suggest that there is a complex interaction of self-weight, stress arching and post placement mining induced stress influencing the stress distribution in a backfilled stope. The measured stresses fall within the range predicted by simple self-weight calculations and numerical modelling that considered stress arching and mining induced stresses.
Microstructural and Chemical Investigations of Cemented Paste Backfills
by T. Ramlochan, M.W. Grabinsky, and D.H. Hooton
Tailings and Mine Waste '04
Fort Collins, Colorado, 10-13 October 2004
This paper reports findings of microstructural and chemical investigations of four cemented paste backfill mixtures. The mictures were prepared from tailings, process water, and binders obtained from three participating mines. Scanning electron microscopy of polished sections revealed that the microstructures of the paste balcfils were largely void space that was highly connected. The hydration products did not effectively fill the interstitial space separating the tailings particles to form a cement matrix. This was attributed to hte high water contents used in the mixtures. The relative volume of interstitial space occupied by hydration products was greater when a binder consisting of blast-furnace slag and a small amount of Portland cement was used than when one with equal parts of Portland cement and fly ash was used. As a result, higher strengths were attained with the former than the latter at comparable water-to-cementitious-material ratios.
Self-desiccation of Cemented Paste Backfill and Implications for Mine Design
by M.W. Grabinsky and P. Simms
9th International Seminar on Paste and Thickened Tailings
Limerick, Ireland, 3-7 April 2006
The hydraulic and mechanical properties of Cemented Paste Backfill (CPB) that are of principle interest in Geomechanical Mine Design include rheology (both “closed conduit” and “open channel” flow), suction (the Soil Water Characteristic Curve or Water Retention Curve), permeability, stiffness, and static and dynamic strength (including resistance to liquefaction). All of these properties change as the binder in the CPB hydrates. In some, and perhaps most mining applications the rate of hydration, and therefore the rate of hydraulic and mechanical property change, occurs on a time scale comparable to the rate of CPB delivery to and filling of the stope. This means that the CPB’s properties are evolving even as it is being deposited and overprinted. This fact can have serious implications for how we interpret total stress cell results, how arching develops both within the stope and across the fill barricades, how fill barricades are designed and constructed, and how we evaluate the CPB’s ability to carry its own self-weight during filling (i.e., resistance to static liquefaction during filling) and subsequent mining (i.e., resistance to dynamic liquefaction during blasting in proximity to recent fills). This paper begins by considering some initially unexpected results from an in situ investigation that illustrates the interaction between rate of binder hydration and rate of stope filling. The framework for conducting tests to evaluate CPB’s evolving hydraulic and mechanical properties is then considered. Some initial test results involving static monotonic and cyclic loading of CPB are then reviewed, and the mine design implications of these test results are considered. The conclusions arising from this work are not yet meant to be used for practical design, but rather point to the extensive research and development that is still required before we can rationally carry out optimized design of CPB fills and their barricades.
Liquefaction Analysis of Early Age Cemented Paste Backfill
by K. le Roux, M.W. Grabinsky, and W.F. Bawden
Liquefied cemented paste backfill (CPB) may breach its retaining barricade and flow into the adjacent mine workings resulting in production losses, large clean-up costs and potential injury. To address this concern backfill designers use cementitious binders to improve the strength and stability of the material. Static tests and anecdotal evidence suggests this approach effectively reduced the liquefaction potential for cured CPB. However, hydration of cement and the resulting strength gain is time dependant, therefore early age cemented paste backfill may be vulnerable to liquefaction. Static undrained triaxial tests show that the Golden Giant CPB is unlikely to liquefy under self-weight conditions but cyclic testing showed that if a stress ratio greater than 0.16 is applied to early age CPB (3 hour cure) liquefaction occurs. As the material cures and gains strength the minimum stress ratio needed to induce liquefaction increases. The results suggest that the frequently used guideline that states that an unconfined compressive strength (UCS) of 100 kPa ensures liquefaction resistance may be conservative for this material under these conditions. A methodology for assessing the risk of liquefaction in response to blasting is also presented.