Dra 45. Triaxial Tests S

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DRA-45

DRA 45. TRIAXIAL TESTS S. Nunoo, D. van Zyl, V.T. McLemore and G. Ayakwah, November 17, 2008, revised March 27, 2009 (reviewed by A. Fakhimi, Erich Rauber)

1. STATEMENT OF PROBLEM How do the triaxial test results compare to the direct shear test results obtained for saturated samples? The shear strength results from the triaxial tests provide a comparison to the direct shear tests performed on Questa materials.

2. PREVIOUS WORK The triaxial compression test is used to measure the shear strength of a soil under controlled drainage conditions. In the conventional triaxial test, a cylindrical specimen of soil encased in a rubber membrane is placed in a triaxial compression chamber, subjected to a confining fluid pressure, and then loaded axially to failure. Connections at the ends of the specimen permit controlled drainage of pore water from the specimen. Triaxial testing of soil and rockfill samples is a well established approach to obtain shear strength parameters under well-controlled stress and drainage conditions (Bishop and Henkel, 1962, Leps, 1970, Marsal, 1973). Recent results for large scale tests (1 m diameter by 2 m tall triaxial specimens) on mine rock pile materials have been reported by Linero, et al (2007) and Valenzuela, et al (2008). The major advantage of testing large scale specimens is that larger particles can be included in the testing that better simulate field conditions. Uhle (1986) performed statistical evaluations on a large range of laboratory test results obtained for rockfill dam materials and concluded that: • Axial and volumetric strains at failure and particle breakage, tend to increase with: (1) increasing uniformity of a rockfill sample, (2) increasing grain size of uniform rockfill, (3) increasing angularity of particles, (4) increasing void ratio, (5) decreasing strength of rockfill material, (6) increasing confining pressure, (7) increasing normal stress at a given confining pressure, (8) decreasing compressive strength of intact rock from which rockfill is obtained, and (9) increasing the rockfill saturation; and • Angle of internal friction φ, tends to increase with : (1) increasing compressive strength of intact rock from which rockfill is obtained, (2) increasing coefficient of uniformity of the rockfill, (3) increasing or decreasing maximum particle size (no universal conclusion) , (4) increasing particle angularity, (5) decreasing void ratio, (6) decreasing confining pressure, (7) decreasing rockfill saturation, (8) increasing particle surface roughness.

Dawson, et al (1998) evaluated the liquefaction behavior of three kinds of carbonaceous waste rock materials under saturated conditions where static liquefaction had been identified as a failure mode resulting in runouts over long distances. The results indicated a typical strain-softening behavior under the undrained isotropic conditions indicating that despite differences in waste material the steady-state friction angles were basically the same and close to the field gradation waste dump angle of repose value of 37-38o. In evaluating the liquefaction of sands and other finer grained materials Jefferies

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and Been (2006) emphasize the importance of loose contractive behavior of such materials as a pre-requisite to liquefaction. Experience shows that materials of higher saturated hydraulic conductivity such as rock pile and heap leach materials must be close to 100 percent saturated before it will liquefy (Been, 2008).

3. TECHNICAL APPROACH URS Corporation (2003) reported the results of isotropically consolidated undrained triaxial tests performed by Thurber Engineering Ltd using a 6 inch diameter triaxial apparatus at a range of confining pressures, from low to high. These results also were reported by Norwest (2005). Two types of material were used, one with about 19% fines (SSW-3) and the other with about 29% fines (SSM-6). Minus 1-inch rock pile material was used and compacted to a target density of 1,922.3 kg/m3 at 5 and 9% water contents. The samples were collected near test pits SSW-1/SSW-2 (SSW-3) on Sugar Shack West and test pits SSM-3/SSM-5 (SSM-6) on Middle Rock Pile (Fig. 1). Effective confining pressures of 68.9, 344.7, 689.5, 1,379, and 2,757.9 kPa were used for the tests. After saturating the specimen consolidation occurred in stages, varying from 1 stage for the 10psi test to 4 stages for the 400psi test. Shearing was carried out at constant strain rate of 0.08 %/min or 0.009 in/min and pore water pressure and deviator stress recorded electronically during shearing. Shearing continued to a strain of 8% to 20%, but generally close to 20%. In 2007 five samples, called mega samples, taken from the Spring Gulch and Sugar Shack West rock piles, and the Debris flow and Pit Alteration Scar (analog sites) of the Questa mine were collected (Fig. 1). The sample locations were selected near some of the places where in situ direct shear tests were performed. Note that two samples from the Sugar Shack West rock pile were collected at two different locations with different visual weathering intensity. From each location, the minus 1 inch material collected in the field was placed into three 30 gallon plastic drums and shipped to the Golder Associates- Burnaby Laboratory for triaxial and direct shear testing. A total of fifteen 30 gallon plastic drums of material were shipped to the Golder Associates-Burnaby Laboratory. DRA 49 reports on the direct shear testing. Minus 0.5-inch material was used and compacted to a target dry density of 1800 kg/m3 at water contents of 9 to 12 percent. After back pressure saturating the specimens, consolidated undrained triaxial tests with pore pressure measurement were conducted. The specimen dimensions were 4 inch diameter and 8 inch height. Each test series included four individual shear tests using different cell pressures ranging from 397 to 1099 kPa. It was assumed that the specific gravity of the material was 2.76. Table 1 is a summary of test conditions.

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FIGURE 1. Location of samples collected, Questa mine, New Mexico.

Table 1. Summary of Triaxial Test Conditions by Golder

INITIAL DIMENSIONS SAMPLE ID HO DO AO GS γdry SAT. 2 3 (mm) (mm) (mm ) (ASSUMED) w (%) (kN/m ) (%) eo 204.9 101.5 8099 2.76 9.3 18 48 0.54 MIN-SAN- 205.4 102.0 8164 2.76 9.3 17 46 0.56 0002 205.8 101.3 8067 2.76 9.3 18 48 0.54 204.7 101.6 8107 2.76 9.3 18 47 0.54 204.5 101.3 8059 2.76 11.6 17.62 60 0.53 QPS-SAN- 205.4 101.3 8067 2.76 11.6 17.50 59 0.55 0002 204.7 101.3 8059 2.76 11.7 17.57 60 0.54 204.7 101.6 8107 2.76 9.5 17.51 48 0.54 204.9 101.5 8083 2.76 9.9 17.5 50 0.55 SSW- 204.8 101.4 8075 2.76 9.2 17.6 47 0.54 SAN-0006 204.7 101.4 8083 2.76 9.0 17.6 46 0.54 204.2 101.1 8026 2.76 11.0 17.5 56 0.54 204.9 101.4 8075 2.76 9.3 18 47 0.54 SPR-SAN- 204.5 101.0 8014 2.76 9.2 18 48 0.52 0002 204.6 101.3 8063 2.76 9.0 18 47 0.53 204.5 101.1 8026 2.76 10.3 18 53 0.54 204.5 101.4 8079 2.76 9.4 18 48 0.53 SSW- 204.5 101.6 8107 2.76 9.2 18 47 0.54 SAN-0002 204.6 101.3 8059 2.76 9.0 18 47 0.53 204.7 101.6 8107 2.76 10.3 17 51 0.55

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4. CONCEPTUAL MODEL The Mohr-Coulomb criterion is the most common shear strength model used in geotechnical engineering for shear test interpretation. Many geotechnical analysis methods and programs require use of this strength model. The Mohr-Coulomb criterion describes a linear relationship between shear strength and normal stress.

The formulation of the model is given by Eqn.1:

(1)

where c' is the cohesion intercept, φ' is the effective friction angle and τ′ and σ′n are shear stress and effective normal stress, respectively.

5. STATUS OF COMPONENT INVESTIGATION The deviator stress (σ1 – σ3) and pore water measurement versus axial strain results from Thurber Engineering Limited reported in Norwest (2005) are shown in Appendix 1 and the results from Golder are reported in Appendix 2. Notice that the samples show no or little softening behavior under the applied confining pressures .Tables 2 and 3 present results reported in Norwest (2005) and for the Golder triaxial test results, respectively. The corresponding effective stresses (i.e. σ´1 and σ´3) for maximum stress ratio were used to obtain the effective cohesion intercept and friction angle. The effective stresses were used to draw Mohr circle diagrams and then the best fit straight line (failure envelope) tangential to the Mohr circles for all the stress circles was selected as shown in Appendix 3. Note that the 3rd circle for both MIN-SAN-0002 and SSW-SAN-0002 do not get close to the envelope. This is because of the best fit straight line used since the failure envelope is non-linear.

TABLE 2. Triaxial Test Results from Thurber as reported by URS Corporation (2003) and Norwest (2005). Note that Q = (σ1 – σ3)/2 and P’ = (σ1’ + σ3’)/2. FAILURE AT MAXIMUM STRESS RATIO Q/P' SAMPLE DESCRIPTION SWI Total ID Q/P' εf Cell ∆U σ1' σ3' φ' c MAX (%) Pressure (kPa) (kPa) (kPa) (DEGREES) (kPa) (kPa) Brown silty, gravelly sand with 0.77 8.24 69 60 75 9 SSW-3 some clay. Gray 2 0.65 20.08 690 564 593 126 37.1 40.0 layer with paste pH 0.68 9.4 1380 1116 1347 264 of 5.7 0.63 20.25 2760 2186 2425 574 Brown silty, sandy, 0.77 22.08 69 48 170 21 gravel, trace to some clay. It is 0.67 21.35 345 230 615 115 SSM-6 4 35.6 63.8 yellow in color with 0.67 21.65 690 443 1133 247 paste pH of 2.9. It 0.63 21.82 1380 898 1956 482 is similar to SSM-3. 0.63 20.05 2760 1759 3923 1001

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TABLE 3. Golder Triaxial Testing Results Note that Q = (σ1 – σ3)/2 and P’ = (σ1’ + σ3’)/2. FAILURE AT MAXIMUM STRESS RATIO Q/P' SAMPLE DESCRIPTION SWI Total ID Q/P' εf Cell ∆U σ1' σ3' φ' c MAX (%) Pressure (kPa) (kPa) (kPa) (DEGREES) (kPa) (kPa) Yellowish brown, 1.04 19.31 45.8 46.8 45.8 -1 silty sand and MIN-SAN- gravel, trace clay, 3 0.69 15.27 131.9 109.1 123.6 22.8 41.0 10 0002 angular to 0.57 15.67 276.5 199 241.5 77.5 subangular particles 0.68 17.48 651.0 530.3 627 120.7 Yellowish brown 0.78 17.71 41.5 32.4 73.3 9.1 sand and gravel, QPS-SAN- 0.75 13.55 135.9 117.4 132.3 18.5 trace silt little clay, 4 40.0 10 0002 angular to 0.67 15.91 392.5 326 333.8 66.5 subangular particles 0.66 14.74 658.7 511.5 728.7 147.2 Yellowish brown 0.78 11.76 40.4 33.6 54.6 6.8 gravelly silty sand SSW-SAN- trace clay, angular 3 0.91 18.54 135.5 130.5 102.4 5 40.5 4 0006 to subangular 0.66 16.25 367.2 305.7 296.3 61.5 particles 0.66 8.39 676.5 542.6 654.7 133.9 Yellowish brown 1.0 18.56 38.9 38.8 35.9 0.1 sand and gravel, SPR-SAN- trace silt and clay, 2 0.73 16.88 139.1 122.1 110.8 17 43.0 5 0002 angular to 0.7 17.88 368.7 312.4 322.6 56.3 subangular particles 0.7 15.5 678.4 561.8 653.3 116.6 Yellowish brown, 1.12 18.75 45.2 47.5 40.2 -2.3 sand and gravel, SSW-SAN- 0.81 18.28 128.1 117.2 107.3 10.9 trace silt and clay, 4 41.0 15 0002 angular to 0.56 15.5 216.7 166.1 180.8 50.6 subangular particles 0.69 17.6 669.5 544.1 677.6 125.4

Figures 2 and 3 shows friction angle and cohesion intercept values from the triaxial and direct shear tests under saturated conditions. The direct shear tests were conducted using 2.4 inch and 12 inch samples by Golder laboratory. Figure 3 suggests that the friction angles from 12-inch shear box and the triaxial cell are consistent (except for tests on sample SSW-SAN-0006) while the small shear box results in lower friction angles.

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30 12inch Cohesion(kPa) 2.4inch Cohesion (kPa) 4inch trixial Cohesion (kPa) 20 Cohesion (kPa) Cohesion

0 MIN-SAN- QPS-SAN- SSW-SAN- SPR-SAN- SSW-SAN- 0002 0002 0006 0002 0002 Rock Piles

FIGURE 2. Cohesion intercept from direct shear and triaxial testing for the various rock piles

39 Friction Angle (degrees) 12" Friction Angle (degrees) 2.4" Friction Angle (degrees) Triaxial (4") 36 Friction Angle (degrees) Angle Friction 33

30 MIN-SAN- QPS-SAN- SSW-SAN- SPR-SAN- SSW-SAN- 0002 0002 0006 0002 0002 Rock Piles

FIGURE 3. Friction angle from direct shear and triaxial testing for the various rock piles

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6. RELIABILITY ANALYSIS The triaxial testing reported here was performed by experienced laboratories and the data are reliable. The interpretation of the results followed a standard approach and the results can also be considered as reliable. Only one approach of data interpretation (maximum stress ratio and associated Mohr circles) was applied and it is possible that slightly different shear strength results would be obtained if the maximum deviator stress values at various strains are selected for the interpretation of the shear strength parameters. The interpretation also did not consider a curved envelope as was used in DRA 49.

7. CONCLUSIONS OF THE COMPONENT The shear strength parameters obtained from the triaxial testing performed by Thurber Engineering and Golder Associates resulted in the following results (Nunoo, 2009): • The friction angles obtained from the triaxial tests on the Questa rock-pile materials are high (φ>36 degrees) and similar to friction angles obtained by direct shear testing (φave>40 degrees, DRA-42, Table 3-3). • The friction angles for the samples tested by Thurber Engineering were 36 and 37 degrees, while the samples tested by Golder Associates had friction angles of 40 to 43 degrees. • The cohesion intercepts for the samples tested by Thurber Engineering were 40 and 64 kPa, while the samples tested by Golder Associates had cohesion intercept values of 4 to 15 kPa. • The friction angle values obtained for the triaxial test results and the 12 inch shear box tests performed by Golder Associates are similar. • The cohesion intercept values obtained for the triaxial test results are 50 percent or more lower that those obtained from the 12 inch shear box tests performed by Golder Associates.

In conclusion, the results obtained for the large samples under saturated conditions consistently show higher peak friction angles than the smaller sample sizes. In Table 4, the percent fines for different samples are compared indicating that smaller samples contain higher percentage of fines. This explains the lower friction angles obtained for 2.4-inch samples and emphasizes the importance of size effect in shear testing of Questa mine material.

TABLE 4. Percent Fines of Samples of Questa Mine Material Obtained from Golder Laboratory Results and Thurber Engineering Limited Sample ID Box type Percent Fines 12-inch Shear Box 17 MIN-SAN-0002 4-inch triaxial samples 19.9 2.4-inch Shear Box 32.2 12-inch Shear Box 16.6 QPS-SAN-0002 4-inch triaxial samples 20 2.4-inch Shear Box 32.2 12-inch Shear Box 24.8 SSW-SAN-0006 4-inch triaxial samples 28.2 2.4-inch Shear Box 42.3

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Sample ID Box type Percent Fines 12-inch Shear Box 13.3 SPR-SAN-0002 4-inch triaxial samples 18.4 2.4-inch Shear Box 37.3 12-inch Shear Box 17.3 SSW-SAN-0002 4-inch triaxial samples 19.6 2.4-inch Shear Box 33.4 * SSW-3 6-inch triaxial samples 9.5 * SSM-6 6-inch triaxial samples 14.6 Note:*- average value was used.

8. REFERENCES CITED Bishop, A.W. and Henkel, D.J., 1962, The measurement of soil properties in the triaxial test, Second Edition, Arnold, London. Dawson, R.F., Morgenstern, N.R. and Stokes, A.W., 1998, Liquefaction flowslides in Rocky Mountain coal mine waste dumps. Canadian Geotechnical Journal, 35: 328-343. Jefferies, M.G. and Been, K., 2006, Soil liquefaction, a critical state approach, Taylor and Francis, London, UK. Leps, T.M., 1970, Review of shearing strength of rockfill, Journal of Soil Mechanics and Foundation Division, ASCE, Vol. 96, No. SM4, July, p. 1159-1170. Linero, S., Palma, C., and Apablaza, R., 2007, Geotechnical characterization of waste material in very high dumps with large scale triaxial testing, Proceedings of International Symposium on Rock Slope Stability in Open Pit Mining and Civil Engineering, 12-14 September 2007, Perth, Australia, p. 59-75. Marsal, R.J., 1973, Mechanical properties of rockfill, In: Embankment-Dam Engineering (Casagrande Volume) Hirschfeld R.C. and Poulos, S.J. (Eds.), John Wiley and Sons, pp 109-200. Norwest Corporation, 2005, Questa Roadside Rock piles 2005 Operation Geotechnical Stability Evaluation: unpublished report to Molycorp Inc., 210 p., 3 vol. Uhle, R.J., 1986, A statistical analysis of rockfill data – shear strength and deformation parameters with respect to particle size, M.S. Thesis, Department of Civil Engineering, Colorado State University, 212 p. URS Corporation, 2003, Mine rock pile erosion and stability evaluations, Questa mine: unpublished Report to Molycorp, Inc., 4 volumes. Valenzuela, L., Bard, E., Campana, J. and Anabalon, M.E., 2008, High waste dumps – challenges and developments, In: Rock Dumps 2008, Fourie, A. (Ed.), Australian Centre for Geomechanics, Perth, Australia, pp 65-78.

9. TECHNICAL APPENDICES

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APPENDIX 1. THURBER LABORATORY TRIAXIAL TEST RESULTS

SSW-3 2800 10psi 100psi 2400 200psi 400psi 2000

1600

1200

800 Deviator stress (kPa)

400

0 048121620 Axial Strain (%)

SSW-3 8 10psi 100psi 7 200psi 400psi 6

4 Stress ratio

1 0 4 8 12 16 20 Axial Strain (%)

SSW-3 2500

2000 10psi 100psi 200psi 1500 400psi

1000

PorePressure Change, kPa 500

0 0 4 8 12 16 20 Axial Strain (%)

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SSM-6

2800 10psi 2400 50psi 100psi 2000 200psi 400psi 1600

1200

Deviator stress (kPa) 800

400

0 0 4 8 12162024 Axial Strain (%)

SSM-6 8

Stress ratio 10psi 3 50psi 100psi 2 200psi 400psi 1 0 4 8 12 16 20 24 Axial Strain (%)

SSM-6 2800 10psi 50psi 2400 100psi 200psi 2000 400psi

1600

1200

800 Pore Pressure change (kPa) 400

0 0 4 8 12162024 Axial Strain (%)

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APPENDIX 2. GOLDER ASSOCIATES TRIAXIAL TEST RESULTS CIU1, CIU2, CIU3, CIU4, are Isotropic Consolidated Undrained with corresponding confining pressures of 45.8, 131.9, 276.5, 651 kPa respectively.

MIN-SAN-0002 300

250 CIU 1 200 CIU 2 CIU 3 150 CIU 4

100 Deviator Stress (kPa)

0 0 2 4 6 8 101214161820

Axial Strain (%)

1.2 MIN-SAN-0002

1.0

0.8

0.6 stress ratio 0.4 CIU 1 CIU 2 CIU 3 0.2 CIU 4

0.0 02468101214161820

Axial Strain (%)

MIN-SAN-0002 600

500 CIU 1 400 CIU 2 CIU 3 300 CIU 4

200

Excess Pore Pressure (kPa) Pore Excess 100

0 02468101214161820 Axial Strain (%)

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QPS-SAN-0002 350

300

CUI 1 250 CIU 2 CIU 3 200 CIU 4

150

Deviator Stress (kPa) Stress Deviator 100

0 02468101214161820 Axial Strain (%)

QPS-SAN-0002 0.9

0.8

0.7

0.6

0.5 CUI 1 CIU 2 0.4 CIU 3 Stress ratio Stress 0.3 CIU 4 0.2

0.1

0.0 0 2 4 6 8 101214161820 Axial Strain (%)

QPS-SAN-0002 600

500

400

300 CIU 1 CIU 2 200 CIU 3 CIU 4

Excess Pore Pressure (kPa) 100

0 0 2 4 6 8 10 12 14 16 18 20

Axial Strain (%)

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SSW-SAN-0006 300 CIU 1 CIU 2 250 CIU 3 CIU 4 200

150

100 Deviator Stress (kPa)

0 02468101214161820

Axial Strain (%)

SSW-SAN-0006 1.0 0.9 0.8 0.7 0.6 CIU 1 0.5 CIU 2 0.4 CIU 3 Stress ratio CIU 4 0.3 0.2 0.1 0.0 0 2 4 6 8 10 12 14 16 18 20 Axial Strain (%)

SSW-SAN-0006 600 CIU 1 CIU 2 500 CIU 3 CIU 4 400

300

200

Execess Pore Pressure (kPa) 100

0 02468101214161820

Axial Strain (%)

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SPR-SAN-0002 300

250 CIU 1 CIU 2 200 CIU 3 CIU 4 150

100 Deviator Stress (kPa)

0 0 2 4 6 8 10 12 14 16 18 20

Axial Strain (%)

SPR-SAN-0002 1.2

1.0

0.8

0.6

CIU 1 Stress ratio Stress 0.4 CIU 2 CIU 3 CIU 4 0.2

0.0 0 2 4 6 8 101214161820

Axial Strain (%)

SPR-SAN-0002 600

500 CUI 1 CUI 2 CIU 3 400 CIU 4

300

200 Excess Pore Pressure (kPa) 100

0 0 2 4 6 8 10 12 14 16 18 20 Axial Strain (%)

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SSW-SAN-0002 300

250 CIU 1 CIU 2 200 CIU 3 CIU 4 150

100 Deviator(kPa) Stress

0 0 2 4 6 8 101214161820

Axial Strain (%) SSW-SAN-0002 1.2

1.0

0.8

0.6

Stress ratio CIU 1 0.4 CIU 2 CIU 3 0.2 CIU 4

0.0 02468101214161820

Axial Strain (%) SSW-SAN-0002 600

500 CIU 1 CIU2 400 CIU 3 CIU 4

300

200 Excess Pore Pressure (kPa) 100

0 0 5 10 15 20 25 Axial Strain (%)

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APPENDIX 3: MOHR CIRCLE PLOTS

Mohr Circle Diagram for SSM-6

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Mohr Circle Diagram for SSW-3

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. Mohr Circle Diagram for MIN-SAN-0002

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Mohr Circle Diagram for QPS-SAN-0002

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Mohr Circle Diagram for SSW-SAN-0006

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Mohr Circle Diagram for SPR-SAN-0002

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Mohr Circle Diagram for SSW-SAN-0002

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