CORRELATIONS OF SHEAR STRENGTHS OF SOFT OIL SANDS TAILINGS ASSESSED BY DIFFERENT IN SITU METHODS

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1 CORRELATIONS OF SHEAR STRENGTHS OF SOFT OIL SANDS TAILINGS ASSESSED BY DIFFERENT IN SITU METHODS Srboljub Masala, Barr Engineering and Environmental Science Canada Navdip Dhadli, Shell Canada Energy 1

novel oil sands tailings treatment options - tailings thickening, and non-segregating

2 Shell Tailings Testing Facility (TTF) 1 Tailings Testing Facility (TTF) at the Muskeg River Mine (MRM) site since June 2007 Used to demonstrate the viability of various novel oil sands tailings treatment options - tailings thickening, and non-segregating tailings (NST) processes - at pilot scale Large deposition cells, 20 m x 50 m at base, 5 m deep 2

3 Shell Tailings Testing Facility (TTF) 2 Tailings deposited: Non-Segregating Tailings (NST) two deposits with different gradations: Cell 5 with SFR 44 = 4.8 Cell 6 with SFR 44 = 4.1 Thickened Tailings (TT) - two deposits with different thickening processes and gradations: Cell 1 with SFR 44 = Cell 4 with SFR 44 = Methods for in situ strength measurement: Direct measurement: Field Vane Test (FVT) Indirect measurement: Cone Penetration Test (CPT) Ball Penetration Test (BPT) 3

4 TTF Strength Measurement Results - Example NST with SFR 44 = 4.1 Undrained Shear Strength S u (kpa) Location A Undrained Shear Strength S u (kpa) Location B Undrained Shear Strength S u (kpa) Location C Typical conversion factors for penetration tests used: 15 for CPT and for BPT. 4

5 TTF Strength Correlations - BPT-FVT Determine a custom value of conversions factor N ball adopting field vane as reference Cell 5 with NST SFR 44 = 4.8 Cell 6 with NST SFR 44 = N = ball q s 44 BPT net FVT u TTF Cell 5 - NST SFR 44 =4.8 BPT - FVT Strength Conversion Factor TTF Cell 6 - NST SFR 44 =4.1 BPT - FVT Strength Conversion Factor 300 Nball average = 10.1 Nball range = 6-15 R² = Nball average = 10.8 Nball range = R² = Ball Peak qb net (kpa) Average Nball = 10.1 Range Ball Peak qb net (kpa) Average Nball = 10.8 Range Vane Peak Shear Strength (kpa) Vane Peak Shear Strength (kpa) Loc A Loc B Loc C Nball = 10.1 Loc A Loc B Loc C Nball =

6 TTF Strength Correlations - CPT-FVT Determine a custom value of conversions factor N kt adopting field vane as reference Cell 5 with NST SFR 44 = 4.8 Cell 6 with NST SFR 44 = q N kt = s CPT net FVT u 44 TTF Cell 5 - NST SFR 44 =4.8 CPT - FVT Strength Conversion Factor TTF Cell 6 - NST SFR 44 =4.1 CPT - FVT Strength Conversion Factor 350 Nkt average = 14.5 Nkt range = R² = Nkt average = 13.4 Nkt range = 8-19 R² = CPT qt net (kpa) Average Nkt = 14.5 Range CPT qt net (kpa) Average Nkt = 13.4 Range Vane Peak Shear Strength (kpa) Vane Peak Shear Strength (kpa) Loc A Loc B Loc C Nkt = 14.5 Loc A Loc B Loc C Nkt =

7 TTF Strength Correlations - BPT-FVT Determine a custom value of conversions factor N ball adopting field vane as reference Cell 4 with TT SFR 44 = TTF Cell 4 - TT SFR 44 = BPT - FVT Strength Conversion Factor Nball average = 13.5 Nball range = R² = 0.97 N = ball q s BPT net FVT u VFT data in Cell 1 (a TT type) were assessed of low quality and the correlations with CPT and BPT were not developed. 80 Ball Peak qb net (kpa) Average Nkt = 13.5 Range Custom conversion factors Nball produce lower values of undrained strength than common conversion factors. Vane Peak Shear Strength (kpa) Loc A Loc B Loc C Nball =

8 Issues with Undrained Strength and Field Measurements 1 Field methods are influenced by a number of in situ factors and details of testing procedures - data screening for effects of: surface crust, disturbed zones during testing, test performance (e.g. rod replacement in CPT/BPT) No proof is usually provided that the measured strengths are really undrained - Estimation of mode of stress-strain behaviour during strength testing is required The only controllable factor in field conditions is deformation rate - penetration rate for CPT/BPT and rotation rate for FVT 8

9 Issues with Undrained Strength and Field Measurements 2 Three screening criteria for undrained behaviour with penetration methods: Permeability lower than a certain limiting value (can be estimated from penetration test data themselves) (Alternative) Penetration rate faster than a certain limiting value, obtained using a normalized velocity criterion Degree of saturation higher than a certain limiting value (gassy soil air bubbles in soft fine tailings are common) 9

10 Issues with FVT as Reference Method FVT involves uncertainties in performance and data interpretation; it is sensitive to: rotation rate, soil anisotropy, progressive failure, vane size, operator competence, etc. Case histories - actual failures of geotechnical structures - showed that in many cases vane strengths overpredicted the strength values at failure for natural soils correction factors were introduced Specific for oil sands tailings: No correction factors for oil sand (no failures, known and published) Percentage of bitumen Chemical treatment, esp. flocculation 10

11 Conclusions Custom BPT and CPT strength conversion factors are close to commonly applied values for NST, but BPT factors are higher in TT (no data for CPT in TT) Uncertainty of conversion factors is high; the coefficient of variation about 50% FVT involves uncertainties in performance and data interpretation, especially corrections for actual in situ strength (from case histories) Insufficient information exists on application of FVT in soft oil sands tailings factors: percentage of bitumen and chemical treatment of tailings Recommended to use FVT (punctual data) in conjunction with the penetration methods (continuous records) Another TTF field investigations campaign is in progress expect updates 11

12 FVT in flocculated tailings - video Courtesy of Mr. Carl Sorensen, Barr Engineering 12

Geotechnical Performance of ATA -Treated Fluid Fine Tailings

Geotechnical Performance of ATA -Treated Fluid Fine Tailings T. Moore, Thurber Engineering Ltd., Canada C. Zhang, Thurber Engineering Ltd., Canada M. Pinheiro, Thurber Engineering Ltd., Canada Outline