Tailings and Mine Waste Conference

If it creeps, does it matter? Znidarčić, Dobroslav 2015-10

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Proceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015 If it creeps, does it matter? Dobroslav Znidarcic University of Colorado, Boulder, USA ABSTRACT Creep effects are among the last phenomena in soft tailings behavior still needing comprehensive study. Evidence of creep effects have been reported, but rational evaluation of its significance on tailings disposal facility design is still lacking. The paper presents a parametric study in which the nonlinear finite strain consolidation theory is applied to evaluate potential creep effects on the storage capacity and settlement rates of a hypothetical tailings disposal facility. The analyses compare consolidation results for highly compressible tailings having low hydraulic conductivity with and without creep effects. The increase in storage capacity and reduction in settlement rates are quantified in the paper. Key Words: creep, consolidation, tailings, disposal, numerical modeling 1 INTRODUCTION Over the past three decades numerical models used for predicting field behavior of fine mine tailings have reached the point of development where they are applied in routine engineering practice with confidence. Appropriate laboratory testing procedures are also routinely used to generate accurate constitutive relationships for nonlinear finite strain consolidation models in the form of void ratio – effective stress and void ratio – hydraulic conductivity relationships. In most cases such modeling efforts lead to reasonable predictions of field performance of tailings disposal facilities and allow for rational design of such facilities during mine operation as well as during and after mine closure. In some instances however, it is expected that tailings will continue to “creep” and reduce in volume, causing additional settlements over extended periods of time. Though the creep behavior has been a subject of numerous studies since 1930s, it is fair to say that even today we do not have a proper testing procedure for quantifying its significance or to include the effects into comprehensive numerical models for predicting the field performance. One of the reasons why the problem has not been solved, might be the fact that the early efforts were based on the comparison of observed settlement rates to the rates predicted by the conventional Terzaghi’s consolidation theory. The Terzaghi theory is based on a number of simplifying assumptions that make it inadequate for predicting the field behavior of soft soils such as fine tailings generated in mining operations. The theory neglects the self-weight effects of materials and assumes constant values of consolidation properties. These assumptions are not at all justified when modeling field behavior of fine tailings since the consolidation driving force is primarily the self-weight of the material and the consolidation properties change by several orders of magnitudes during the consolidation process. This is particularly true for hydraulic conductivity whose accurate determination still poses significant challenges at low effective stresses. Some more recent efforts in evaluating the creep behavior of fine tailings have recognized the shortcomings of the conventional consolidation theory, but more work is needed before both convenient testing methods and reliable models are fully developed and prepared for routine application in the industry (Beuth et al, 2014). Further research into creep effects will hopefully resolve some of the fundamental scientific issues, but what might be more interesting from an engineering point of view is the consequence of the creep effects for the design of tailing storage facilities. This paper addresses the second issue through a parametric study in which hypothetical creep effects are evaluated by comparing consolidation behavior of several soil columns exhibiting some creep effects to the columns undergoing only primary consolidation due to the self-weight of tailings. Several assumptions are made regarding the creep effects on the compressibility and hydraulic conductivity relationships Proceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015 for a Mature Fine Tailings sample from an Oil Sands mining operation. Based on published data on other materials the assumptions are believed to be reasonable and realistic.  2 CREEP CONCEPTS Slurries undergo significant volume changes upon their deposition due to sedimentation, consolidation and possibly creep. In all three processes the volume changes are directly related to the expulsion of pore water contained within the slurry. The sedimentation and consolidation processes are well understood and rational theoretical numerical models are available to predict these processes for any combination of deposition scenarios with the appropriate boundary conditions (Gibson et al, 1967, Pane, 1985). Testing procedures to determine relevant material characteristics are also well developed and routinely used in engineering practice. However, the creep behavior prediction has not reach that level of development and there is no consensus on either a theoretical framework or testing procedures that provide rational and reliable analysis of field performance of slurries undergoing creep deformations. Thus, it is not possible to ascertain how significant is the creep effect in any disposal scenario and how much does it contribute to the volume changes in comparison to the sedimentation and consolidation processes. It is also not possible to establish the effect that creep has on volume change rate. Does it increase or decrease the settlement rate when compared with the consolidation rate of a material that would not exhibit creep behavior. Answering these questions rationally is of interest to mining industry as the fine tailings undergo field settlements that last decades if not centuries. The creep related deformations could then be significant even if the creep rate is relatively low. It is noted that during the creep process the pore water must also be expelled from the pores as in saturated materials any volume change can only be accomplished by removing water from the material. The water expulsion is controlled by Darcy’s law and any rational creep theory must account for this effect. Unfortunately, this aspect of creep behavior has not received adequate attention in studies to date. The basic premise of any creep theory is that the relationship between void ratio and effective stresses is not unique but it depends on either rate of void ratio change or on time that the material is subjected to a certain effective stress level. This is the main, and possibly the only, difference between the classical nonlinear consolidation theory and a rational creep theory. In the consolidation theory the uniqueness of the void ratio-effective stress relationship is assumed a priori, but in all other aspects of slurry behavior (mass conservation, equilibrium and flow relations) the theory is rational and complete without any other restrictive assumptions. A rational creep theory should retain the essential features of the nonlinear consolidation theory and include a void ratio-effective stress relationship that is modified to account for any rate dependency of this relationship. Szavit-Nossan (1988) has developed such theoretical framework and this work provides a solid base for the implementation of a rational treatment of the creep effects in slurries such as fine tailings. Unfortunately the theoretical developments have not been verified by any experimental results and cannot be considered as a definite answer to creep questions that can be readily implemented in practice.  Bartholomeeusen (2003) studied experimentally the sedimentation, consolidation and creep processes for a number of slurries and presented convincing evidence that creep could play a significant role in a slurry settlement behavior, especially at very low effective stress levels. His experimental data analysis included a procedure similar to the work of Szavits-Nossan (1988) consistent with the nonlinear finite strain consolidation theory.  The work by Szavits-Nossan (1988) and Bartholomeeusen (2003), based on rational mechanics, address fundamental issues in creep behavior of slurries. They provide a solid theoretical base for studying creep behavior and provide experimental evidence that in soft soils creep at high void ratio (low effective stress) affects the settlement process significantly. It is not surprising that creep effects would be more significant at low effective stresses as the overall volume change in that range is much higher and the magnitude of the creep effects should be higher as well. At higher effective stresses the overall compressibility of soils is much smaller and any creep effect Proceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015 would have diminishing significance as well. Though the creep effects might be observed at all stress level their practical implications will most likely be less significant when the material becomes stiff enough.  The creep effects in the early stages of the consolidation process is of a particular interest to mining as they influence the early water release and affect the near surface behavior of disposed tailings. The top surface characteristics affect any atmospheric drying and therefore shear strength development and trafficability of the surface. The optimal timing of the next layer deposition will also greatly depend on the performance of top surface upon deposition. Clearly, a rational approach in analyzing the consolidation processes in the early stages upon deposition and in the near surface layer of the disposed fine tailings is needed and will enhance our ability to design and predict the behavior of containment facilities and tailings deposits. 3 CONSOLIDATION AND CREEP CHARACTERISTICS Three sets of consolidation characteristics are selected for the parametric study. The first set of compressibility and permeability characteristics are obtained directly form the Seepage Induced Consolidation Test on a MFT sample as reported by Znidarcic et al (2011). The second set is created by modifying the compressibility characteristics of the MFT sample to account for creep effects. It was assumed that the creep effect will be most prominent in lower effective stress range and that the effect will gradually diminish as the effective stress increases, but will still be noticeable. The hydraulic conductivity function was not changed from the relationships for the MFT sample as it can be argued that the hydraulic conductivity depends only on the void ratio irrespectively how this void ratio is reached: due to the effective stress increase or due to creep. This compressibility relationship is identified as Creep I in the paper. The third set is created by again modifying the compressibility characteristics of the MFT sample, but this time the creep effects were assumed to be roughly the same for all stress levels (same void ratio change for any stress level) creating a “parallel” curve in the semi-logarithmic plot. This sample is identified as Creep II in the paper. The hydraulic conductivity relationship is again considered to be unchanged due to creep. Figure 1 shows the compressibility characteristics for the three materials and Figure 2 shows the unique hydraulic conductivity relationship for all three materials. Table 1 lists the material parameters characterizing the compressibility and hydraulic conductivity relationship used in the analyses. Parameters A, B and Z define compressibility characteristics in Equation 1, and parameters C and D define the hydraulic conductivity in equation 2.    e=A(’+Z)B         (1)   k=CeD          (2)  in which e is void ratio, ’ is vertical effective stress and k is the hydraulic conductivity. Table 1. Model parameters used in the analyses ______________________________________________________________________ Sample  Specific  Compressibility    Hydraulic conductivity gravity    A B Z  C  D       kPa  m/day _____________________________________________________________________________ MFT  2.6  3.27 -0.195 0.074  2.16e-6  3.71 Creep I  2.6  2.54 -0.165 0.01  2.16e-6  3.71 Creep II  2.6  2.80 -0.222 0.05  2.16e-6  3.71 _____________________________________________________________________________  Proceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015  Figure 1. Compressibility characteristics for MFT, Creep I and Creep II materials  Figure 2. Hydraulic conductivity relationship used in the analyses 4 MODELING SCENARIOS  In order to evaluate the creep effects on the disposal of fine mine tailings three hypothetical scenarios were considered: thin lift, thick lift and deep deposit. The thin lift had an initial height of 1 m, thick lift 10 m and deep deposit has an initial height of 50 m. While the selected values do not represent any particular disposal facility it is believed that they span the values that could be found in many mining operations. The goal here was to investigate if the creep effects might be more pronounced in one disposal scenario than in others. Figures 3, 4 and 5 present the analyses results for column heights of 1m, 10m and 50 m, respectively. Each figure has three height vs time curves, one for each material, MFT, Creep I and Creep II.   0.000.501.001.502.002.503.003.504.004.505.000.1 1 10 100 1000Void ratioEffective stress (kPa)MFTCreep ICreep II0.000.501.001.502.002.503.003.504.004.505.001.00E-07 1.00E-06 1.00E-05 1.00E-04 1.00E-03Void ratioHydraulic conductivity (m/day)Proceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015  Figure 3. Height vs time plots for 1 m tall column and three materials  Figure 4. Height vs time plots for 10 m tall column and three materials 5 10 15 20 25 30Height (m)Time (years)MFTCreep ICreep II0246810120 100 200 300 400 500 600 700Height (m)Time (years)MFTCreep ICreep IIProceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015  Figure 5. Height vs time plots for 50 m tall column and three materials Several observations are noted from these results. No dramatic changes in predicted settlement rates are observed in these plots irrespectively of the column heights analyzed. The materials exhibiting creep effects will produce, as expected, lower final heights. This difference will be noticeable only after extended period of time, well beyond the time frame of engineering interest. It was assumed in these analyses that the creep effects keep up with the consolidation process, though in reality they may lag behind. In that case it is expected that the creep settlements will be even more prolonged. It is of particular interest to note that the initial settlement rates are not at all affected by creep. This is not surprising as these rates are directly related to the hydraulic conductivity at high initial void ratio which is expected not to depend on any creep effects. The Creep I material exhibits the largest settlement for 1 m column, while the Creep II material has largest settlement for the 50 m column. This is the direct consequence of the compressibility characteristics presented in Fig. 1. The maximum effective stresses are 2.5 kPa, 25, kPa and 125 kPa for column heights of 1 m, 10 m and 50 m, respectively. Thus, the compressibility characteristics only up to these stress levels are of interest in each analysis. This again highlights that the settlements in soft fine grained soils are completely controlled by hydraulic conductivity and compressibility characteristics of the material. They determine how much water will be expelled from the slurry (compressibility) and how easy this water will flow out (hydraulic conductivity). It doesn’t matter what causes the volume change: the increase in the effective stress or creep. The results are the same. A more rigorous analysis might result in somewhat slower settlement in case of materials exhibiting creep effects. 5 CONCLUSIONS These simplified analyses do not definitely answer the question of the importance of creep effects on the field settlement rates and magnitudes for fine grained slurries in mining operations. However, from an engineering point of view it appears that the presence of creep in a material might not have as significant effect as one might initially imagine. The results indicate that at best the creep effects will create larger ultimate settlements, but at least the initial settlement rates are completely controlled by the hydraulic conductivity of the material. Again, correct determination 01020304050600 1000 2000 3000 4000 5000Height (m)Time (years)MFTCreep ICreep IIProceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015 of consolidation properties (compressibility and hydraulic conductivity relationships) are of paramount importance for any reliable prediction of field performance of soft slurries. An accurate determination of hydraulic conductivity at high void ratios is particularly critical.  The evaluation of creep effects requires excessively long laboratory testing, which is the main reason why the question of creep has not been settled yet. Proper theoretical framework and reliable laboratory testing procedures have been developed and are available for further investigation of this topic. However, significant additional efforts are needed before these procedures can be routinely used in engineering practice. In the meantime the analyses similar to the ones presented in this paper could be used to at least approximately evaluate the potential creep effects in tailings disposal facilities.  6 REFERENCES Bartholomeeusen, G., 2003, Compound shock waves and creep behaviour in sediment beds, PhD Dissertation, University of Oxford, Oxford, England Beuth, L., Dykstra, C., and Jacobs, W., 2014, Numerical Modeling of hydraulic sand capping on creeping ultra-soft tailings, 4th International Oil Sands Tailings Conference Proceedings, December 7-10, Lake Louise, AB Canada Gibson, R.E., England, G.L., and Hussey, M.H.L., 1967, The Theory of One-Dimensional Consolidation of Saturated Clays, I. Finite Nonlinear Consolidation of Thin Homogeneous Layers, Geotechnique, Vol.17(3), pp. 261-273. Pane, V., 1985, Sedimentation and Consolidation of Clays, PhD Dissertation, University of Colorado, Boulder  Szavits-Nossan, V., 1988, Intrinsic time behavior of cohesive soils during consolidation, PhD Dissertation, University of Colorado, Boulder  Znidarcic, D., Miller, R., van Zyl, D., Fredlund, M., and Wells, S., 2011, Consolidation Testing of Oil Sand Fine Tailings, Tailings and Mine Waste ‘11 conference proceedings, November 6-9, Vancouver, BC, Canada, ISBN: 978-0-88865-815-9, pp.251-257. 


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