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Questa Weathering Study p 1 of 22 March 27, 2009

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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Questa Weathering Study p 2 of 22 March 27, 2009

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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Questa Weathering Study p 3 of 22 March 27, 2009

FIGURE 1. Location of samples collected, Questa mine, New Mexico. Table 1. Summary of Triaxial Test Conditions by Golder

SAMPLE ID

INITIAL DIMENSIONS

HO (mm)

DO (mm)

AO (mm2)

GS (ASSUMED) w (%)

γdry (kN/m3)

SAT. (%) eo

MIN-SAN-0002

204.9 101.5 8099 2.76 9.3 18 48 0.54 205.4 102.0 8164 2.76 9.3 17 46 0.56 205.8 101.3 8067 2.76 9.3 18 48 0.54 204.7 101.6 8107 2.76 9.3 18 47 0.54

QPS-SAN-0002

204.5 101.3 8059 2.76 11.6 17.62 60 0.53 205.4 101.3 8067 2.76 11.6 17.50 59 0.55 204.7 101.3 8059 2.76 11.7 17.57 60 0.54 204.7 101.6 8107 2.76 9.5 17.51 48 0.54

SSW-SAN-0006

204.9 101.5 8083 2.76 9.9 17.5 50 0.55 204.8 101.4 8075 2.76 9.2 17.6 47 0.54 204.7 101.4 8083 2.76 9.0 17.6 46 0.54 204.2 101.1 8026 2.76 11.0 17.5 56 0.54

SPR-SAN-0002

204.9 101.4 8075 2.76 9.3 18 47 0.54 204.5 101.0 8014 2.76 9.2 18 48 0.52 204.6 101.3 8063 2.76 9.0 18 47 0.53 204.5 101.1 8026 2.76 10.3 18 53 0.54

SSW-SAN-0002

204.5 101.4 8079 2.76 9.4 18 48 0.53 204.5 101.6 8107 2.76 9.2 18 47 0.54 204.6 101.3 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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Questa Weathering Study p 4 of 22 March 27, 2009

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.

SAMPLE ID DESCRIPTION SWI

FAILURE AT MAXIMUM STRESS RATIO Q/P'

Q/P' MAX

εf (%)

Total Cell

Pressure (kPa)

∆U (kPa)

σ1' (kPa)

σ3' (kPa)

φ' (DEGREES)

c (kPa)

SSW-3

Brown silty, gravelly sand with some clay. Gray

layer with paste pH of 5.7

2 0.77 8.24 69 60 75 9

37.1 40.0 0.65 20.08 690 564 593 126 0.68 9.4 1380 1116 1347 264 0.63 20.25 2760 2186 2425 574

SSM-6

Brown silty, sandy, gravel, trace to some clay. It is

yellow in color with paste pH of 2.9. It

is similar to SSM-3.

4

0.77 22.08 69 48 170 21

35.6 63.8 0.67 21.35 345 230 615 115 0.67 21.65 690 443 1133 247 0.63 21.82 1380 898 1956 482 0.63 20.05 2760 1759 3923 1001

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Questa Weathering Study p 5 of 22 March 27, 2009

TABLE 3. Golder Triaxial Testing Results Note that Q = (σ1 – σ3)/2 and P’ = (σ1’ + σ3’)/2.

SAMPLE ID DESCRIPTION SWI

FAILURE AT MAXIMUM STRESS RATIO Q/P'

Q/P' MAX

εf (%)

Total Cell

Pressure (kPa)

∆U (kPa)

σ1' (kPa)

σ3' (kPa)

φ' (DEGREES)

c (kPa)

MIN-SAN-0002

Yellowish brown, silty sand and

gravel, trace clay, angular to

subangular particles

3

1.04 19.31 45.8 46.8 45.8 -1

41.0 10 0.69 15.27 131.9 109.1 123.6 22.8 0.57 15.67 276.5 199 241.5 77.5 0.68 17.48 651.0 530.3 627 120.7

QPS-SAN-0002

Yellowish brown sand and gravel,

trace silt little clay, angular to

subangular particles

4

0.78 17.71 41.5 32.4 73.3 9.1

40.0 10 0.75 13.55 135.9 117.4 132.3 18.5 0.67 15.91 392.5 326 333.8 66.5 0.66 14.74 658.7 511.5 728.7 147.2

SSW-SAN-0006

Yellowish brown gravelly silty sand trace clay, angular

to subangular particles

3

0.78 11.76 40.4 33.6 54.6 6.8

40.5 4 0.91 18.54 135.5 130.5 102.4 5 0.66 16.25 367.2 305.7 296.3 61.5 0.66 8.39 676.5 542.6 654.7 133.9

SPR-SAN-0002

Yellowish brown sand and gravel,

trace silt and clay, angular to

subangular particles

2

1.0 18.56 38.9 38.8 35.9 0.1

43.0 5 0.73 16.88 139.1 122.1 110.8 17 0.7 17.88 368.7 312.4 322.6 56.3 0.7 15.5 678.4 561.8 653.3 116.6

SSW-SAN-0002

Yellowish brown, sand and gravel,

trace silt and clay, angular to

subangular particles

4

1.12 18.75 45.2 47.5 40.2 -2.3

41.0 15 0.81 18.28 128.1 117.2 107.3 10.9 0.56 15.5 216.7 166.1 180.8 50.6 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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Questa Weathering Study p 6 of 22 March 27, 2009

0

10

20

30

40

50

MIN-SAN-0002

QPS-SAN-0002

SSW-SAN-0006

SPR-SAN-0002

SSW-SAN-0002

Rock Piles

Coh

esio

n (k

Pa)

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

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

piles

30

33

36

39

42

45

MIN-SAN-0002

QPS-SAN-0002

SSW-SAN-0006

SPR-SAN-0002

SSW-SAN-0002

Rock Piles

Fric

tion

Ang

le (d

egre

es)

Friction Angle (degrees) 12"Friction Angle (degrees) 2.4"Friction Angle (degrees) Triaxial (4")

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

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Questa Weathering Study p 7 of 22 March 27, 2009

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

MIN-SAN-0002 12-inch Shear Box 17

4-inch triaxial samples 19.9 2.4-inch Shear Box 32.2

QPS-SAN-0002 12-inch Shear Box 16.6

4-inch triaxial samples 20 2.4-inch Shear Box 32.2

SSW-SAN-0006 12-inch Shear Box 24.8

4-inch triaxial samples 28.2 2.4-inch Shear Box 42.3

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Questa Weathering Study p 8 of 22 March 27, 2009

Sample ID Box type Percent Fines

SPR-SAN-0002 12-inch Shear Box 13.3

4-inch triaxial samples 18.4 2.4-inch Shear Box 37.3

SSW-SAN-0002 12-inch Shear Box 17.3

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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Questa Weathering Study p 9 of 22 March 27, 2009

APPENDIX 1. THURBER LABORATORY TRIAXIAL TEST RESULTS

SSW-3

0

400

800

1200

1600

2000

2400

2800

0 4 8 12 16 20

Axial Strain (%)

Dev

iato

r str

ess

(kPa

)

10psi100psi200psi400psi

SSW-3

1

2

3

4

5

6

7

8

0 4 8 12 16 20

Axial Strain (%)

Stre

ss ra

tio

10psi100psi200psi400psi

SSW-3

0

500

1000

1500

2000

2500

0 4 8 12 16 20

Axial Strain (%)

Pore

Pre

ssur

e C

hang

e, k

Pa 10psi100psi200psi400psi

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Questa Weathering Study p 10 of 22 March 27, 2009

SSM-6

0

400

800

1200

1600

2000

2400

2800

0 4 8 12 16 20 24

Axial Strain (%)

Dev

iato

r str

ess

(kPa

)

10psi50psi100psi200psi400psi

SSM-6

1

2

3

4

5

6

7

8

0 4 8 12 16 20 24

Axial Strain (%)

Stre

ss ra

tio

10psi50psi100psi200psi400psi

SSM-6

0

400

800

1200

1600

2000

2400

2800

0 4 8 12 16 20 24

Axial Strain (%)

Pore

Pre

ssur

e ch

ange

(kPa

)

10psi50psi100psi200psi400psi

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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

0

50

100

150

200

250

300

0 2 4 6 8 10 12 14 16 18 20

Axial Strain (%)

Dev

iato

r Str

ess

(kPa

) CIU 1CIU 2CIU 3CIU 4

MIN-SAN-0002

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 2 4 6 8 10 12 14 16 18 20

Axial Strain (%)

stre

ss ra

tio

CIU 1CIU 2CIU 3CIU 4

MIN-SAN-0002

0

100

200

300

400

500

600

0 2 4 6 8 10 12 14 16 18 20

Axial Strain (%)

Exce

ss P

ore

Pres

sure

(kPa

)

CIU 1CIU 2CIU 3CIU 4

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Questa Weathering Study p 12 of 22 March 27, 2009

QPS-SAN-0002

0

50

100

150

200

250

300

350

0 2 4 6 8 10 12 14 16 18 20

Axial Strain (%)

Dev

iato

r Str

ess

(kPa

) CUI 1CIU 2CIU 3CIU 4

QPS-SAN-0002

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 2 4 6 8 10 12 14 16 18 20

Axial Strain (%)

Stre

ss ra

tio CUI 1

CIU 2

CIU 3

CIU 4

QPS-SAN-0002

0

100

200

300

400

500

600

0 2 4 6 8 10 12 14 16 18 20

Axial Strain (%)

Exce

ss P

ore

Pres

sure

(kPa

)

CIU 1CIU 2CIU 3CIU 4

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Questa Weathering Study p 13 of 22 March 27, 2009

SSW-SAN-0006

0

50

100

150

200

250

300

0 2 4 6 8 10 12 14 16 18 20

Axial Strain (%)

Dev

iato

r Str

ess

(kPa

)

CIU 1CIU 2CIU 3CIU 4

SSW-SAN-0006

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 2 4 6 8 10 12 14 16 18 20

Axial Strain (%)

Stre

ss ra

tio

CIU 1CIU 2CIU 3CIU 4

SSW-SAN-0006

0

100

200

300

400

500

600

0 2 4 6 8 10 12 14 16 18 20

Axial Strain (%)

Exec

ess

Pore

Pre

ssur

e (k

Pa)

CIU 1CIU 2CIU 3CIU 4

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Questa Weathering Study p 14 of 22 March 27, 2009

SPR-SAN-0002

0

50

100

150

200

250

300

0 2 4 6 8 10 12 14 16 18 20

Axial Strain (%)

Dev

iato

r Str

ess

(kPa

) CIU 1CIU 2CIU 3CIU 4

SPR-SAN-0002

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 2 4 6 8 10 12 14 16 18 20

Axial Strain (%)

Stre

ss ra

tio

CIU 1CIU 2CIU 3CIU 4

SPR-SAN-0002

0

100

200

300

400

500

600

0 2 4 6 8 10 12 14 16 18 20

Axial Strain (%)

Exce

ss P

ore

Pres

sure

(kPa

) CUI 1CUI 2CIU 3CIU 4

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Questa Weathering Study p 15 of 22 March 27, 2009

SSW-SAN-0002

0

50

100

150

200

250

300

0 2 4 6 8 10 12 14 16 18 20

Axial Strain (%)

Dev

iato

r Str

ess

(kPa

)

CIU 1CIU 2CIU 3CIU 4

SSW-SAN-0002

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 2 4 6 8 10 12 14 16 18 20

Axial Strain (%)

Stre

ss ra

tio

CIU 1CIU 2CIU 3CIU 4

SSW-SAN-0002

0

100

200

300

400

500

600

0 5 10 15 20 25

Axial Strain (%)

Exce

ss P

ore

Pres

sure

(kPa

) CIU 1CIU2CIU 3CIU 4

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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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Questa Weathering Study p 22 of 22 March 27, 2009

Mohr Circle Diagram for SSW-SAN-0002