DRA-41

DRA 41–WET AND DRY SIEVING OF QUESTA ROCK PILE MATERIAL

S. Nunoo, A. Fakhimi, and V.T. McLemore, October 27, 2008, revised March 27, 2009 (reviewed by D. van Zyl, G. Robinson)

1. STATEMENT OF THE PROBLEM How does the percentage of fines (minus No. 200 sieve material) observed in wet sieving compare to that of dry sieving of Questa rock pile material? In general, higher percentage of fines reduces the angle of rock pile material and that can affect its gravitational stability.

2. PREVIOUS WORK One aspect of the research study on the Questa rock piles is the geotechnical characterization of the rock pile material. distribution within the rock piles forms an important component of the geotechnical characterization evaluation. Gutierrez (2006) performed both dry and wet particle size analysis on Goathill North rock pile material, one of the nine rock piles at the Questa mine site, in accordance to the ASTM standards (2002) and U.S. Army Corps of Engineers (1970) methodology. Gutierrez (2006) reported that there were differences in the percent fines i.e. the percentage passed sieve No. 200, when dry and wet sieving were performed on the same material. The percent fines for the sample tested changed from 2.5% in dry sieving to 17.8% when wet sieving was conducted. Norwest Corporation (2005) selected samples during drilling of the roadside rock piles (i.e. Sugar Shack South) that were tested using wet sieving. The percent fines for samples ranged from 6% to 21% with an average of 14.6%. The summary of the Norwest Corporation (2005) results is shown in Table 1-1 (Appendix 1). To further investigate this issue, wet and dry sieving were conducted on selected samples of materials collected from the Questa mine rock piles and analogs. The results of this specific study are reported in this DRA.

3. TECHNICAL APPROACH Five samples collected from five different locations from the Spring Gulch, Sugar Shack West rock piles and the analog sites (Debris flow and Questa Pit Alteration Scar) of the Questa mine were used for this study (Fig. 1). The locations were selected based on accessibility, safety, and near the locations where in situ direct shear tests were performed (refer to DRA 47). No scalping was performed on the samples in the field other than removing very large rock fragments that could not be placed in the 5 gallon buckets. The samples were transferred to the laboratory and air dried. From each sample two representative splits were taken by the method of cone and quartering (ASTM, 1987) for wet and dry sieving. The minimum mass of a sample used for particle size analysis was related to the maximum particle size present in the bucket. Table 1 shows different size particles and the corresponding minimum mass of sample necessary to perform the test (U.S. Army Corps of Engineers, 1970).

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FIGURE 1. Location of samples used for wet and dry sieving, Questa mine, New Mexico.

TABLE 1. The minimum specimen weight required for particle size analysis based on the size of the largest particle in the sample (U.S. Army Corps of Engineers, 1970). Nominal diameter of the largest Approximate minimum mass particle inches (mm) of the sample (g)

3 (76.2) 6000 2 (50.8) 4000 1 (25.4) 2000 ½ (12.7) 1000 0.18 (4.75) 200 0.079 (2) 100

For the dry sieving, the air-dried sample was weighed and then poured into the top sieve (3-inch sieve) of the stack of sieves that was placed on a mechanical shaker. The shaking time ranged from 45 to 60 minutes to assure that the retained material on each sieve remained unchanged. The first step in the sample preparation for wet sieving was weighing the air-dried sample and then soaking it in the water for more than 60 minutes. Three sieves, No. 6, No. 10, and No. 200 were put into a stack with a bucket placed underneath. The soaked sample was poured into the top sieve (No. 6) of the stack, and was washed gently by hand with running tap water to separate the fines from the coarse materials. The soaked material was put into the sieves in several steps and then washed to allow better separation of the particles. The fine material that passed through the No. 200 sieve was collected in the bucket that was placed underneath the sieve stack. The wet sieving process was conducted with caution to assure that this bucket did not overflow as this causes loss of the fine material. The slurry collected in the bucket was left for a while for the water to become clear. The clear water was gradually siphoned from the bucket leaving behind the slurry. The fine material was then placed in the oven and the dry weight of the fines was measured. The retained material on the sieves was oven dried as . Dry sieving was conducted on this coarse portion of the sample to result in the

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gradation curve. Figure 2 shows some of the steps involved in this study. (ASTM, 1971; Head, 1980).

Three sieves used sample saturated in water Washing the samples through sieves

Oven dried samples after washing Retained samples after washing Water runs clean after washing the sample FIGURE 2. Some of the steps followed for wet sieving of samples.

4. CONCEPTUAL MODEL distribution is an important physical characteristic of a soil. The gradation curve and the percentage of fines control the and compressibility of soil. The of granular can be related to D10. The fine particles in the rock pile samples are weakly cemented to the coarse particles or are cemented into clumps. The wet sieving method allows for the separation of the fines from the coarse particles. It can be argued that wet sieving results in the “true” particle size distribution of the material. However, in the rock piles the fines are cemented to the coarse particles or are present in clumps. The rock pile material therefore will behave differently from the “true” particle size distribution and dry sieving may result in a better representation of the in situ rock pile material behavior.

5. STATUS OF COMPONENT INVESTIGATION The wet and dry sieving test results are summarized in Table 2. These results suggest that the percent fines of 1.9, 2.0, 3.1, 2.7, and 1.4 due to dry sieving were increased to those of 12.6, 9.0, 18.1, 11.0, and 22.1 due to wet sieving (samples MIN-SAN-0001, Debris Flow; QPS-SAN-0001, Pit Alteration Scar; SSW-SAN-0005, Sugar Shack West Rock Pile; SPR-SAN-0001, Spring Gulch Rock Pile; and SSW-SAN-0001, Sugar Shack West Rock Pile), respectively. The gradation curves for dry and wet sieving for these five locations are reported in Appendix 2 (Figs. 2-1 to 2-5). Samples from the same locations were screened through 1-inch sieve and were sent to a commercial laboratory (i.e. Golder Associates-Burnaby Laboratory) for additional geotechnical testing, including wet sieve analysis. The percent fines of the minus 1-inch field samples reported by Golder was between 13.3% to 24.8%. The Golder laboratory results are summarized in Table 3 and the corresponding gradation curves are reported in Appendix 3. Note that the percents fines in wet sieving conducted by Golder are in general higher than those from NMT tests because larger particles were present in

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NMT samples. The ranges and means of percent , , and fines from Norwest Corporation (2005; Appendix 1), Golder Laboratory and NMT are compared in Table 4.

TABLE 2. Summary table of particle size results conducted at New Mexico Tech. Note that two separate samples were collected from Sugar Shack West rock pile. The particle size ranges for gravel, sand, and fines are defined in Appendix 2. SAMPLE ID DESCRIPTION PARTICLE SIZE, DRY SIEVING PARTICLE SIZE, WET SIEVING % % SAND % % GRAVEL % SAND % FINE GRAVEL FINE MIN-SAN- Debris Flow 52.4 45.7 1.9 53.2 34.3 12.6 0001 QPS-SAN- Alteration 64.9 33.1 2.0 62.0 29.1 9.0 0001 Scar SSW-SAN- Sugar Shack 56.7 40.2 3.1 49.8 32.2 18.1 0005 West SPR-SAN- Spring Gulch 71.4 25.9 2.7 66.4 22.6 11.0 0001 SSW-SAN- Sugar Shack 46.4 52.3 1.4 33.2 44.7 22.1 0001 West

TABLE 3. Summary table of particle size results conducted by Golder Associates. SAMPLE ID DESCRIPTION PARTICLE SIZE (GRADATION) PARTICLE SIZE (GRADATION) (-1-INCH) WET SIEVING (-No. 4) WET SIEVING % % SAND % FINE % % SAND % FINE GRAVEL GRAVEL MIN-SAN- Debris Flow 62.3 20.7 17.0 0.0 72.6 27.4 0002 QPS-SAN- Alteration 40.9 42.5 16.6 0.0 71.9 28.1 0002 Scar SSW-SAN- Sugar Shack 66.4 8.8 24.8 0.0 62.6 37.4 0006 West SPR-SAN- Spring Gulch 41.8 44.9 13.3 0.0 68.2 31.8 0002 SSW-SAN- Sugar Shack 39.2 43.5 17.3 0.0 71.6 28.4 0002 West

TABLE 4. Ranges and means of gravel, sand, and fines from wet sieving of Questa materials reported by different laboratories. Laboratory % Gravel %Sand %Fines Range Mean Range Mean Range Mean Norwest 27-58 42.3 24-68 43.0 6-21 14.6 Golder 39.2-66.4 50.1 8.8-44.9 32.1 13.3-24.8 17.8 NMT 33.2-66.4 52.9 22.6-44.7 32.6 9.0-22.1 14.6

6. RELIABILITY ANALYSIS It is unclear how much cementation of fines to coarse particles and “clumping” is present in the rock pile materials and their variability within the rock piles. These wet and dry sieving results provide better insight in these characteristics. The differences in the fines from the particle size distribution between wet and dry sieving relates to the behavior of the rock pile materials. However, the wet sieving

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results do not represent the behavior of the rock pile materials as there is cementation of fines to coarse particles and clumping of fines throughout the rock piles. Unless there is a major change to the geohydrological regime in the piles, e.g. large zones of saturated flow, the cementation and clumping will not be impacted.

7. CONCLUSION OF THE COMPONENT Wet and dry sieve analyses were conducted on the material collected from the rock piles and analogs at the Questa mine. It was observed that the wet sieving resulted in higher percentages of fines compared to dry sieving. These results are consistent with those from other laboratory tests. The increase in fines is a result of the presence of water in wet sieving that dissolves the cementation and between particles and disintegration of clumps. However, it is believed that the increased percentage of fines observed in the wet sieve analysis may not represent the true behavior of the unsaturated rock pile material.

8. REFERENCES CITED ASTM, 1987, Standard Preparation of Soil Samples for Particle Size Analysis and Determination of Soil Constants (D421): Annual Book of ASTM Standards. American Society for testing and Materials (ASTM), West Conshohocken, PA. ASTM, 2002, Standard Test Method for Particle-Size Analysis of Soils (D422): Annual Book of ASTM Standards. American Society for Testing and Materials (ASTM), West Conshohocken, PA. ASTM, 1971 (Reapproved), Standard Test Method for Amount of Material in Soils Finer than No. 200 (75μm) (D1140): Annual Book of ASTM Standards. American Society for Testing and Materials (ASTM), West Conshohocken, PA. Gutierrez, L.A.F., 2006, The influence of mineralogy, chemistry and physical engineering properties on shear strength parameters of the Goathill North rock pile material, Questa Molybdenum mine, New Mexico: M. S. thesis, New Mexico Institute of Mining and Technology, Socorro, 201 p., http://geoinfo.nmt.edu/staff/mclemore/Molycorppapers.htm, accessed December 06, 2007. Head, K.H., 1980, Manual of Soil Laboratory Testing: Volume 1, and Compaction Test Engineering Laboratory Equipment Limited, Pentech Press Limited, Estover , Plymouth, Devon. Norwest Corporation, 2005, Questa Roadside Rock Piles 2005 Operation Geotechnical Stability Evaluation: unpublished report to Molycorp Inc., 210 p., 3 vol. U.S. Army Corps of Engineers, 1970, Grain Size Analysis: Engineering Design: Laboratory Soil Testing: Department of the Army Headquarters, Washington, DC, p. V1-V28.

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APPENDIX 1. WET SIEVE ANALYSIS BY NORWEST CORPORATION (2005)

TABLE 1-1. Wet sieve analysis on samples collected from a bore hole in Sugar Shack South rock pile (Norwest Corporation, 2005).

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APPENDIX 2. NEW MEXICO TECH DRY AND WET SIEVING ANALYSIS RESULTS

Particle Size Distribution U.S. Standard Sieve Size

3 2 1.5 1 3/43/8 4 6 10 16 304050 60100 200 100 90 Sample A(dry) 80 70 Sample B(wet_dry) 60 50 40 30 20 10 Percent Passing by Weight 0 1000 100 10 1 0.1 0.01 0.001 Grain Size, mm

BOULDERS COBBLES GRAVEL SAND Coarse Fine Coarse Medium Fine

FIGURE 2-1. Gradation curve for sample MIN-SAN-0001 (Debris Flow).

Particle Size Distribution U.S. Standard Sieve Size 3 2 1.5 1 3/43/8 4 6 10 16 30 4050 60100 200 100 90 Sample A(dry) 80 Sample B(wet_dry) 70 60 50 40 30 20 10 Percent Passing by Weight 0 1000 100 10 1 0.1 0.01 0.001 Grain Size, mm

BOULDERS COBBLES GRAVEL SAND SILT CLAY Coarse Fine Coarse Medium Fine

FIGURE 2-2. Gradation curve for sample QPS-SAN-0001 (Alteration Scar).

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Particle Size Distribution U.S. Standard Sieve Size 3 2 1.5 1 3/43/8 4 6 10 1 3 4 5 60100 200 100 90 Sample A(dry) 80 Sample B(wet_dry) 70 60 50 40 30 20 10 Percent Passing by Weight 0 1000 100 10 1 0.1 0.01 0.001 Grain Size, mm SAND BOULDERS COBBLES GRAVEL SILT CLAY Coarse Fine Coarse Medium Fine

FIGURE 2-3. Gradation curve for sample SSW-SAN-0005 (Sugar Shack West).

Particle Size Distribution U.S. Standard Sieve Size 342 1.5 1 3/43/8 610 16 304050 60100 200 100 90 80 Sample A(dry)

70 Sample B(wet_dry) 60 50 40 30 20 10 Percent Passing by Weight 0 1000 100 10 1 0.1 0.01 0.001 Grain Size, mm

SAND BOULDERS COBBLES GRAVEL SILT CLAY Coarse Fine Coarse Medium Fine

FIGURE 2-4. Gradation curve for sample SPR-SAN-0001 (Spring Gulch).

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Particle Size Distribution U.S. Standard Sieve Size

3 2 1.5 1 3/43/8 4 6 10 16 304050 60100 200 100 90 Sample A(dry) 80 70 Sample B(wet_dry) 60 50 40 30 20 10 Percent Passing by Weight 0 1000 100 10 1 0.1 0.01 0.001 Grain Size, mm

SAND BOULDERS COBBLES GRAVEL SILT CLAY Coarse Fine Coarse Medium Fine

FIGURE 2-5. Gradation curve for sample SSW-SAN-0001 (Sugar Shack West).

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APPENDIX 3. GOLDER LABORATORY WET SIEVING ANALYSIS RESULTS

Particle Size Distribution

U.S. Standard Sieve Size

342 1.5 1 3/43/8 6 10 16 30 40 50 60100 200 100 90 80 70 60 50 40 30 20 10 Percent Passing by Weight 0 1000 100 10 1 0.1 0.01 0.001 Grain Size, mm

BOULDERS COBBLES GRAVEL SAND SILT CLAY Coarse Fine Coarse Medium Fine

(a)

Particle Size Distribution

U.S. Standard Sieve Size

3 2 1.5 1 3/43/8 4 6 10 16 30 40 50 60100 200 100 90 80 70 60 50 40 30 20 10 Percent Passing by Weight Percent 0 1000 100 10 1 0.1 0.01 0.001 Grain Size, mm

BOULDERS COBBLES GRAVEL SAND SILT CLAY Coarse Fine Coarse Medium Fine

(b) FIGURE 3-1. Wet sieving analysis results of the samples collected from Debris Flow (MIN-SAN-0002), a) -1-inch field material, b) minus No. 4 sieve material.

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Particle Size Distribution

U.S. Standard Sieve Size

3 2 1.5 1 3/43/8 4 6 10 16 30 40 50 60100 200 100 90 80 70 60 50 40 30 20 10 Percent Passing by Weight by Percent Passing 0 1000 100 10 1 0.1 0.01 0.001 Grain Size, mm

BOULDERS COBBLES GRAVEL SAND SILT CLAY Coarse Fine Coarse Medium Fine

(a)

Particle Size Distribution

U.S. Standard Sieve Size

3 2 1.5 1 3/43/8 4 6 10 16 30 40 50 60100 200 100 90 80 70 60 50 40 30 20 10 Percent Passing by Weight Percent 0 1000 100 10 1 0.1 0.01 0.001 Grain Size, mm

BOULDERS COBBLES GRAVEL SAND SILT CLAY Coarse Fine Coarse Medium Fine

(b) FIGURE 3-2. Wet sieving analysis results of the samples collected from Pit Alteration Scar (QPS-SAN-0002), a) -1-inch field material, b) minus No. 4 sieve material.

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Particle Size Distribution

U.S. Standard Sieve Size

3 2 1.5 1 3/43/8 4 6 10 16 30 40 50 60100 200 100 90 80 70 60 50 40 30 20 10 Percent Passing by Weight 0 1000 100 10 1 0.1 0.01 0.001 Grain Size, mm

BOULDERS COBBLES GRAVEL SAND SILT CLAY Coarse Fine Coarse Medium Fine

(a)

Particle Size Distribution

U.S. Standard Sieve Size

3 2 1.5 1 3/43/8 4 6 10 16 30 40 50 60100 200 100 90 80 70 60 50 40 30 20 10 Percentby Weight Passing 0 1000 100 10 1 0.1 0.01 0.001 Grain Size, mm

SAND BOULDERS COBBLES GRAVEL SILT CLAY Coarse Fine Coarse Medium Fine

(b) FIGURE 3-3. Wet sieving analysis results of the samples from Sugar Shack West rock pile (SSW-SAN-0006), a) -1-inch field material, b) minus No. 4 sieve material.

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Particle Size Distribution

U.S. Standard Sieve Size

3 2 1.5 1 3/43/8 4 6 10 16 30 40 50 60100 200 100 90 80 70 60 50 40 30 20 10 Percent Passing by Weight 0 1000 100 10 1 0.1 0.01 0.001 Grain Size, mm

BOULDERS COBBLES GRAVEL SAND SILT CLAY Coarse Fine Coarse Medium Fine

(a)

Particle Size Distribution

U.S. Standard Sieve Size

3 2 1.5 1 3/43/8 4 6 10 16 30 40 50 60100 200 100 90 80 70 60 50 40 30 20 10 Percent Passing by Weight 0 1000 100 10 1 0.1 0.01 0.001 Grain Size, mm

BOULDERS COBBLES GRAVEL SAND SILT CLAY Coarse Fine Coarse Medium Fine

(b) FIGURE 3-4. Wet sieving analysis results of the samples from Spring Gulch rock pile (SPR-SAN-0002), a) -1-inch field material, b) minus No. 4 sieve material.

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Particle Size Distribution

U.S. Standard Sieve Size

3 2 1.5 1 3/43/8 4 6 10 16 30 40 50 60100 200 100 90 80 70 60 50 40 30 20 10 Percent Passing by by Weight Passing Percent 0 1000 100 10 1 0.1 0.01 0.001 Grain Size, mm

BOULDERS COBBLES GRAVEL SAND SILT CLAY Coarse Fine Coarse Medium Fine

(a)

Particle Size Distribution

U.S. Standard Sieve Size

342 1.5 1 3/43/8 6 10 16 30 40 50 60100 200 100 90 80 70 60 50 40 30 20 10 Percent Passing by Weight 0 1000 100 10 1 0.1 0.01 0.001 Grain Size, mm

BOULDERS COBBLES GRAVEL SAND SILT CLAY Coarse Fine Coarse Medium Fine

(b) FIGURE 3-5. Wet sieving analysis results of the samples from Sugar Shack West rock pile (SSW-SAN-0002), a) -1-inch field material, b) minus No. 4 sieve material.

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