ABSTRACT 2. TAILINGS TYPES AND TESTS 1. INTRODUCTION. the Ball Penetration Test (BPT) and the Field Vane Test (FVT).

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1 CORRELATIONS OF SHEAR STRENGTHS OF SOFT OIL SANDS TAILINGS ASSESSED BY DIFFERENT IN SITU METHODS Srboljub Masala, Barr Engineering, Calgary, Alberta Navdip Dhadli, Shell Canada Energy, Calgary, Alberta ABSTRACT This paper presents correlations of shear strengths measured by three different field testing methods in tailings deposits at the Shell Canada Tailings Testing Facility at the Muskeg River Mine site. The testing was conducted using the Cone Penetration Test, the Ball Penetration Test and the Field Vane Test. The materials tested consisted of: (a) two Non-Segregating Tailings deposits with different gradations, and (b) two Thickened Tailings deposit with different gradations. Characterization of the deposits was performed while they were at different stages of consolidation, but generally soft, with shear strengths lower than 20 kpa. The strength conversion factors N kt and N ball were calculated for Cone and Ball Penetration Test results, respectively, for each tested deposit, with the Field Vane Test adopted as the reference measurement technique. The customized strength conversion factors were, on average, close to the commonly applied values N kt = 15 and N ball = 10.5 for the Non-Segregating Tailings, but deviated from typical values for the Thickened Tailings. For all tailings types evaluated, the scatter of data was significant with the coefficients of variance of up to 50% for the strengths calculated. It was concluded that more work is needed to collect additional experience with the field methods for strength measurements in soft oil sands tailings. Caution should be exercised when making generalizations, which should be limited to specific tailings types. 1. INTRODUCTION This paper presents correlations of shear strengths measured by three different field testing methods in tailings deposits at Shell Canada Ltd. (Shell) Testing Tailings Facility (TTF) at the Muskeg River Mine (MRM) site. The testing was conducted using the Cone Penetration Test (CPT), the Ball Penetration Test (BPT) and the Field Vane Test (FVT). 2. TAILINGS TYPES AND TESTS Table 1 presents summary data on Shell s tailings deposited at TTF [1]. The materials tested consisted of: - two Non-Segregating Tailings (NST) deposits with different gradations (Cells 5 and 6); - one Thickened Tailings (TT) deposit (Cell 4); and. - one Treated Thickened Tailings (TTT) deposit, of a paste consistency (Cell 1). Both TT and TTT were produced in a paste thickener, the former with the paste thickener operating in a high-rate thickener regime. The NST was made by mixing the paste thickener underflow with dewatered coarse tailings. The sand-to-fines ratios were originally determined on the 74-micron basis, from the fines contents FC 74 that were obtained by wet sieving. The fines content FC 44 and the sand-to-fines ratios SFR 44 on the 44- micron basis were calculated from a materialspecific correlation with the SFR 74. Characterization of Shell TTF deposits was performed at the following approximate ages: - 10 months for NST in Cell 6 and TTT in Cell 1, - 3 months for NST in Cell 5 and - 1, 8 and 10 months for TT in Cell 4. Strength testing was performed at three locations in each cell: upstream (A), middle (B) and downstream (C), approximately at the quarters of a cell length. The strengths were investigated using three field methods: FVT, CPT and BPT. The FVT testing was performed by Geoforte, Edmonton, using custom equipment with vane sizes D:H (diameter to height) of 4 x 8 cm and 6 x 12 cm. Geoforte also performed sampling of the deposits for determination of geotechnical index

2 properties, which was conducted by Shell s Calgary Research Centre (CRC). The penetration tests were performed by Conetec Investigations Ltd., Vancouver. The CPT sounding was performed in general accordance with the ASTM standard D5778, using a cone equipment with a maximum tip capacity of 200 bars, tip area of 15 cm 2 and friction sleeve area of 225 cm 2. The BPT sounding was performed using a standard cone pushing frame with a maximum BPT tip capacity of about 80 bars, and the standard ball of 11.3 cm in diameter, with projected area of 60 cm 2. The BPT penetration rate was a commonly used value of 2 cm/s. 3. CHARACTERIZATION RESULTS The calculated strength data for all three test methods are presented in Figures 1-3 for Cells 4, 5 and 6. All three locations in a cell are shown in the same diagram, for comparison. Strength data for TTT in TTF Cell 1 are not presented. The FVT testing was not performed at the mid-cell location B, and it had to be repeated on location C. The vane results in Cell 1 were assessed as invalid they were very noisy, the curves wavering, and the peak strengths generally low - below 2 kpa, so it was decided not to include them in this analysis. No BPT peak testing was performed at location A in Cell 6. Forty five NST and fifteen TT strength data points were used in this analysis. Only peak strengths were considered. 3.1 Calculation of strengths The strengths presented in Figures 1-3 were calculated using the formulae: s BPT u q N net ball q b v0 N u 1 a ball A A where: q b is the ball penetration resistance, v0 the total vertical overburden stress, u the pore pressure measured just behind the joint between the ball and push rods, A s the cross sectional area s p of the cone shaft, and A p the projected area of the ball; s CPT u q N net kt qt N kt v0 where: q t is the measured tip resistance corrected for unequal end area pore pressure effects on the cone tip, and N kt is the undrained shear strength conversion factor; and s FVT u 6T 7D max 3 where: T max is the maximum value of measured torque (corrected for the apparatus and rod friction), and D is the vane diameter. The CPT and BPT strengths were calculated from the net tip penetration resistances using the common values of the strength conversion factors N kt = 15 and N ball = 10.5, respectively. No correction for the rotation rate was applied to the vane data. It should be noted that the vane test were performed with non-uniform rotation rates, so that a correction for shear rate sensitivity should be applied to normalize the data set. The vane rotation rates were, in general, higher than the recommended standard rates; occasionally, they were up to an order of magnitude higher. Therefore, the normalization with respect to shear rate would reduce the strength data used. However, the reduction would be small, about 10%, and would not substantially affect the correlations. 4. DATA SELECTION 4.1. Preliminary Data Screening Initial screening of the TTF data included the following: Indurate surface crust: None of the data points fall within a strong, partially dried surface crust, extending to about 0.4 m of depth. Disturbed materials zones during testing: Some of the strength tests were located near the elevations of CPT pore pressure dissipation tests, which were found to affect the response of soil in continued penetration. Similar is valid for the

3 cycling tests in BPT for determination of the remoulded strengths. It was difficult to estimate to what extent the tailings response was affected by the proximity to these, potentially disturbed zones, so no points were discarded. Test performance: Similar comments are valid when pushing rods were added or removed during CPT and BPT, with characteristic spikes in the penetration resistance curves at regular 1-metre intervals. Although some vane tests were located in the vicinity of these depths, none of the CPT/BPT points were discarded from further analysis. Pore pressure response as indicator of mode of deformation (undrained / drained): Dynamic pore pressure curves were inspected for visible deviations from the hydrostatic pore pressure lines, but it was concluded that this may not be decisive, as explained in Section 4.2. Only one Shell TTF strength test point was actually discarded: the lowest point at the upstream location (A) in Cell 6, which was suspected to be a mixture of NST tailings with the cell bottom material (lean oil sand or overburden soil) Estimation of Mode of Stress-Strain Behaviour No specific screening for the stress-deformation mode of behaviour (drained, undrained or partially drained) was performed on the vane data. The penetration results were screened for the mode of stress-deformation behavior. The majority of CPT data shows generation of positive pore pressures during penetration, which should correspond to the expected volumetric deformation behavior of the contractant structure of soft NST tailings. Sometimes, this response is not apparent, but the data cannot be discarded because of a possibility that pore pressure generation can be suppressed by dilation of coarse granular structure (coarser material is typically found in the neighbourhood of a discharge point). Therefore, individual data points were not examined for pore pressure response, but two more general, global approaches were chosen. It is generally accepted by the geotechnical community that, at a standard rate of CPT penetration of 2 cm/s, undrained response will occur if the permeability of the soil is less than about 10-5 cm/s [2]. Using the pore pressure dissipation tests during CPT in the actual deposits [3] and applying four different methods for estimation of the permeability from CPT data [3, 4 and 5], it was found that the TTF NST permeability should be within the range of 10-8 to 10-6 cm/s, which is an order of magnitude below the undrained behaviour limit value. An alternative approach to an estimate of the NST mode of behaviour was taken from reference [6]. For shallow circular foundations in calcareous silts and sands, it was found that the limits for undrained and drained behaviour corresponded to the non-dimensional velocities V of about 0.01 and 30, respectively. It was found later that these limits may be used as a first estimate of the degree of drainage in the CPT field tests [7]. The following formula is used for the CPT V factor, and a similar one for the BPT V factor: v d V c v where: v is the penetration rate, d is the cone diameter and c v the coefficient of consolidation. The values for c v in Shell NST tailings were estimated from CPT data and varied within the range cm 2 /min. The normalized velocity range for CPT was calculated as V CPT = , which is much above the undrained behaviour criterion. The corresponding V range for the BPT test was determined as V BPT = , again higher that the undrained behaviour criterion. 4.3 Presence of Air Bubbles (Gassy Soils) The longitudinal velocities v p in Shell s NST tailings were consistently low ( m/s or about 15-25% of the velocity in water), which was explained by partial saturation of tailings stream due to air bubbles entrained during pumping from the TTF plant and discharge into the cell. These bubbles were visible in the cores taken from the tailings, but the amount of gas could not be measured during sampling. The corresponding air volume fraction was numerically estimated to be within the range % and the degree of saturation greater than 99%. However, these results were taken as an illustration of low sensitivity of the

4 applied model rather than as an indicator of the undrained behaviour. It should be noted that the stress-deformation of gassy soils can exhibit phenomena like subdued excess pore pressure response, which will imply different resistance to penetration and other field strength tests. That effect could not be investigated during NST tailings characterization. However, the volume of air bubbles could be more precisely estimated in TT in Cell 4 based on stereological image processing techniques. The air volume was found between 2% and 6% and the corresponding degree of saturation between 92% and 97%. Based on certain literature data on gassy sands, for example [8], the specimen with the initial degree of saturation greater than 90% responded in a strain-softening manner similar to that of fully saturated specimens. Therefore, the measured strengths were likely undrained, with a possibility that some results were obtained in a partially drained mode of behaviour. 5. STRENGTH CORRELATIONS The vane results were assumed as the primary data set, i.e. the FVT was adopted as the reference method. The basis for this decision is that the vane method measures strength directly, while the penetration methods estimate strength through correlations. The following approach was adopted and consistently implemented in data interpretation and comparison. For each testing location A, B, C in a cell, the CPT and BPT penetration resistances, corresponding to the elevations at which the vane tests were performed, were calculated as the averages over a 20-cm interval centered at the vane test elevation. The correlations between FVT and CPT / BPT data were established separately for the two NST cells. The correlations were carried out using inverted forms of the equations for undrained strength: q Nball s q N kt s BPT net FVT u CPT net FVT u Figures 4-7 present the plots of correlation data: undrained vane strengths and the net tip penetration resistances. The strength conversion factors N kt and N ball were obtained as the slopes of the linear best-fit lines for these data sets. They are presented in Table 2. Calculated N ball factors for two Shell NST materials are very close to the commonly used value of 10.5 and, on average, identical to it. Therefore, using the standard value of N ball = 10.5 for BPT data interpretation results in strengths consistent with the FVT measurements. The N kt factors for Shell NST are a little below the commonly used value of 15, which means that the standard CPT strength prediction with N kt = 15 will, on average, slightly underestimate the shear strengths obtained by the FVT. 6. ANALYSIS AND CONCLUSIONS 6.1. Comparison of Strength Correlations for Shell NST and Natural Sandy Soils The strength conversion factors N ball and N kt for Shell s NST materials are closer to the commonly applied values, and their variation is smaller than in natural sandy soils [5]. This can be expected in the pilot deposits, where a tighter control over tailings production and deposition can be exercised. It is also consistent with a relatively high solids content of Shell NST and a smaller variation of its SFR compared with natural soils, where soft and widely graded deposits show higher variation in properties and behaviour than compact and uniform deposits. A caveat for the use of the pilot data for the larger scale operations is that the larger variation of properties at a larger scale of operations may diminish the predictive capability of a model developed at a smaller scale Uncertainty in Strength Conversion Factors The range of variation of N ball and N kt factors for Shell NST material is too wide for a single value to be chosen as representative of the whole tailings type. Take for an example the BPT penetration data. Although the N ball = 10.5, consistent with the commonly used value in practice, could be recommended, the variation of the N ball values in the range of 6 to 15 implies that the coefficient of variation for the calculated strengths would be about 50%. This inherent variation in the

5 parameters obtained from statistical considerations (correlations) may present risks without cautious application. Therefore, the field penetration tests may be safely applied for strength assessment of a tailings deposit when: - the deposit is well understood, including: zonation of different tailings, possible stratification, geotechnical index properties, strength development processes (sedimentation, consolidation, desiccation, etc.) and similar; - a strong correlation is established between a penetration test results and actual, operational strengths obtained or confirmed by other, different methods; - there is sufficient understanding of influences on the tailings strength and the adopted strength correlation, such as: tailings type; particle size distribution; plasticity of fine fraction, rate sensitivity, etc.; - a value of N ball or N kt is selected such that it ensures that the strength equal to or greater than the calculated value is present in the majority of the deposit, within a margin of error sufficient for the purpose of strength assessment or another specific application; - updates of the above data are performed on a regular basis Representativeness of Field Vane as Reference Method for Strength Measurement The FVT method is probably the most direct means available to measure strength in soft tailings deposits. The method involves uncertainties in performance and data interpretation, including: rotation rate sensitivity of certain soils/tailings, soil anisotropy, progressive failure, empirical corrections for actual in situ strength, vane size, operator competence, etc. Back calculations from case histories - actual failures of geotechnical structures - showed that in many cases vane strengths overpredicted the strength values at failure for natural soils. This led to the concept of correction factors that should be applied to raw measured FVT data to adjust calculated FVT strengths and obtain reliable strength values for design stability analyses; for example see [9]. However, practical implementation of this concept faced numerous difficulties, so that application of correction factors has not become an element of the FVT standardization. Although vane is extensively used for strength measurement in the oil sands industry, the authors are not aware of any assessment of the need for correction factors for FVT data when applied to oil sands tailings problems. Insufficient information exists on application of FVT in soft oil sands tailings. The magnitudes of the mentioned influential factors in soft tailings have not been quantified. Additional factors affecting FVT results in oil sands tailings should be considered, such as the percentage of bitumen and chemical treatment of tailings. It is our opinion that more work is needed to collect additional experience with FVT application in soft oil sands tailings. Generalization should be postponed, and FVT should be used as the only strength testing method only after careful consideration of all available information for an individual deposit. It is recommended to regularly use FVT in conjunction with the penetration methods. Particular attention should be paid to the performance of the vane test as seemingly the adopted reference method in oil sands industry. It is suspected that occasional large differences between FVT strengths and the CPT / BPT estimated strengths can be attributed to the variability in the execution of the FVT Strength Correlation for Shell s TT (Cell 4) The conversion factor N ball = 13.5 for peak strength of TT in Cell 4 is significantly higher than the commonly used N ball = This variation may be attributed to finer gradation, and the presence of air and bitumen. BIBLIOGRAPHY [1] Matthews, J.; and S. Masala Tailings Research at Shell s Muskeg River Mine Tailings Testing Facility. Tailings and Mine Waste 09, Banff, Alberta, November 1-4, [2] Schnaid, F.; Bedin, J.; and L.M. Costa Filho Piezocone in Silty Tailings Materials. Studia Geotechnica et Mechanica, Vol. XXIX, No. 1-2, pp [3] Robertson, P.K.; and R.G. Campanella Interpretation of Cone Penetration Tests Part II: Clay. Canadian Geotech. Journal, Vol. 20, No. 3.

6 [4] Teh, C. I.; and G.T. Houlsby An Analytical Study of the Cone Penetration Test in Clay. Geotechnique, 41, No. 1, pp [5] Mayne, P.W Stress-strain-strength-flow parameters from enhanced in-situ tests. Proceedings, Int. Conf. on In-Situ Measurement of Soil Properties & Case Histories [In-Situ 2001], Bali, Indonesia, May 21-24, 2001, pp [6] Finnie, I.M.S.; and M.F. Randolph Punch-through and liquefaction induced failure of shallow foundations on calcareous sediments. Proceedings, International Conference on Behaviour of Offshore Structures BOSS 94, Boston, 1, pp [7] House, A.R.; Oliveira, J.R.M.S.; and M.F. Randolph Evaluating the Coefficient of Consolidation Using Penetrometer Tests. International Journal of Physical Modelling in Geotechnics, 3, pp [8] Grozic, J., P.K. Robertson and N.R. Morgenstern The behaviour of loose gassy sand. Canadian Geotechnical Journal, 36, [9] Bjerrum, L Problems of Soil Mechanics and Construction on Soft Clays and Structurally Unstable Soils (Collapsible, Expansive and Others). 8th International Conference on Soil Mechanics and Foundation Engineering, Moscow, USSR, Vol. 3, pp Table 1: TTF Tailings Deposits, Deposition and Characterization Data Tailings TTF End of Solids Date of Testing Cell Deposition content (%) SFR 44 Shell TTT 1 Dec. 04, 2007 Oct , Shell TT 4 Nov. 09, 2009 Dec. 7-11, 2009; Jul , 2010; Sept , Shell NST Nominal SFR 74 =2.8 (measured) Nominal SFR 44 =4.8 (estimated) Shell NST Nominal SFR 74 =2.4 (measured) Nominal SFR 44 =4.1 (estimated) 5 Jun. 25, 2008 Sept. 20 Oct. 20, Nov. 21, 2007 Sept , Table 2: Strength Conversion Factors for TTF NST and TT Tailings Shell NST SFR 44 =4.8 (estimated) Shell NST SFR 44 =4.1 (estimated) TTF Cell CPT strength conversion factor N kt BPT strength conversion factor N ball Average Range Average Range Shell TT 4 n/a n/a

7 Figure 1: Shear Strengths in TTF Cell 4 (TT) Figure 2: Shear Strengths in TTF Cell 5 (NST, SFR 44 = 4.8)

8 Figure 3: Shear Strengths in TTF Cell 6 (NST, SFR 44 = 4.1)

9 CPT qt net (kpa) Ball Peak qb net (kpa) Ball Peak qb net (kpa) CPT qt net (kpa) TTF Cell 5 - NST SFR 44 =4.8 BPT - FVT Strength Conversion Factor TTF Cell 5 - NST SFR 44 =4.8 CPT - FVT Strength Conversion Factor 300 Nball average = 10.1 Nball range = 6-15 R² = Nkt average = 14.5 Nkt range = R² = Vane Peak Shear Strength (kpa) Loc A Loc B Loc C Nball = Vane Peak Shear Strength (kpa) Loc A Loc B Loc C Nkt = 14.5 Figure 4: Cell 5 - Determination of N ball factor Figure 6: Cell 5 - Determination of N kt factor TTF Cell 6 - NST SFR 44 =4.1 CPT - FVT Strength Conversion Factor TTF Cell 6 - NST SFR 44 =4.1 BPT - FVT Strength Conversion Factor 350 Nkt average = 13.4 Nkt range = 8-19 R² = Nball average = 10.8 Nball range = R² = Vane Peak Shear Strength (kpa) Loc A Loc B Loc C Nkt = Vane Peak Shear Strength (kpa) Loc A Loc B Loc C Nball = 10.8 Figure 5: Cell 6 - Determination of N ball factor Figure 7: Cell 6 - Determination of N kt factor

10 Ball Peak qb net (kpa) TTF Cell 4 - TT SFR 44 = BPT - FVT Strength Conversion Factor 100 Nball average = 13.5 Nball range = R² = Vane Peak Shear Strength (kpa) Loc A Loc B Loc C Nball = 13.5 Figure 8: Cell 4 - Determination of N ball factor