The role of oversize particles on the shear strength and deformational behavior of rock pile material

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1 ARMA 8-4 The role of oversize particles on the shear strength and deformational behavior of rock pile material Fakhimi, A. Associate Professor, Department of Mineral Engineering, New Mexico Tech, Socorro, NM, USA & Department of Civil Engineering, University of Tarbiat Modarres, Tehran, Iran Hosseinpour, H. Graduate Student, Department of Mineral Engineering, New Mexico Tech, Socorro, NM, USA Copyright 8, ARMA, American Rock Mechanics Association This paper was prepared for presentation at San Francisco 8, the 4 nd US Rock Mechanics Symposium and nd U.S.-Canada Rock Mechanics Symposium, held in San Francisco, June 9- July, 8. This paper was selected for presentation by an ARMA Technical Program Committee following review of information contained in an abstract submitted earlier by the author(s). Contents of the paper, as presented, have not been reviewed by ARMA and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of ARMA, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of ARMA is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 3 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgement of where and by whom the paper was presented. ABSTRACT: Large rock fragments were encountered during the in situ shear testing on the Questa mine rock pile materials, New Mexico, USA. The in situ shear boxes used in the field were 3 cm 3 cm and 6 cm 6 cm in size. In order to evaluate the effect of these oversize particles on the friction angle and cohesion of the rock pile material, several laboratory shear tests with a box 6 cm 6 cm in size were conducted where the oversize particle was simulated using stainless steel spheres that were placed along the shear plane. The experimental findings suggest that the presence of the oversize particle causes an increase in friction angle while the cohesion can either decrease or increase depending on the size of the oversize particle with respect to the size of shear box and the location of the oversize particle along the shear plane. 1. INTRODUCTION The effect of oversize particles on shear strength of soil has been studied by some researchers with emphasis on the role of scalping on the mechanical behavior of rock fill dam material. Holtz and Gibbs [1] conducted several triaxial tests on river sand and gravel and quarry material to investigate the effect of the density, maximum particle size, particle shape, and amount of gravel on the shear strength of the material. All specimens were saturated before testing and the loading rate was slow enough to allow free drainage of the material without generating any excess pore water pressure. To study the effect of specimen size on the shear strength, very small (34.9 mm 76. mm), small (8.5 mm 6.4 mm), medium (15.4 mm 381 mm), and large (8.6 mm mm) specimens were tested. Test results on sand specimens (passing No. 4 sieve size) compacted at 7% relative density showed similar friction coefficients irrespective of the specimen size except for the very small specimen that showed marked increase in shear strength. The authors attributed this increase in shear strength to the appreciable coarse sand of mm (3/16-inch) maximum size that resulted in a maximum particle size to cell diameter ratio of.14. Hennes [] investigated the effect of particle size, shape, and grading on the friction angle of dry, rounded gravel and crushed rock. The shear test apparatus was 15.4 mm 34.8 mm (6-inch 1-inch) in size. He concluded that as the maximum particle size increases, the friction angle increases. The same conclusion was made as some complementary triaxial tests were conducted by this author. Note that in this research work, the specimens were compacted following identical vibration and tapping of the materials without verifying the resulting porosity of the specimens. Vallerga et al. [3] did not observe any change in friction angle of granular materials using traixial tests for the situation that the specimen diameter to maximum particle size ratio was larger than about 15. In the series of tests conduced, the compaction of specimens were controlled to achieve a fixed void ratio irrespective of the maximum particle size. Rathee [4] in his study of granular soils, using a 3 cm 3 cm shear box and maximum applied normal stress of about 3 kpa, concluded that the ratio of specimen width to maximum particle size that will not affect the shear strength is in the order of 1 to 1 for sand-gravel mixture. Based on this author, the friction angle of sandgravel mixture increases with the increase in the maximum particle size while for almost uniform size

2 gravel, the friction angle was almost constant with increasing the maximum particle size. Su [5] performed triaxial tests and suggested that the static strength of a soil with oversize particles in the floating state is mainly determined by the soil matrix in the far field; tests should be performed on the soil matrix alone at the dry density of far field matrix to obtain the shear strength of the soil with the floating oversize particles. Cerato and Lutenegger [6] studied the scale effect of direct shear tests on sands. Their study showed that the friction angle measured by direct shear testing can depend on specimen size, and that the influence of specimen size is also a function of sand type and relative density. They concluded that for dense sand, the friction angle measured by direct shear testing reduces as the box size increases and that a box width to maximum particle size ratio of 5 or beyond should be used in order to minimize the size effect on the friction angle of sand. Furthermore, these authors claim that the thickness of shear zone in a direct shear test is not only a function of the mean particle size (D5) but also it depends on the height and width of the box. While the above research works illustrate no change or increase in friction angle with the increase in the maximum particle size or the ratio of maximum particle size to the box width, there have been reports that suggest reduction in friction angle with increase in particle size. For example, Becker et al. [7] used a biaxial apparatus to measure the friction angle of earth dam material in a plane strain condition. They concluded that in general the friction angle decreases as the maximum particle size is increased but the difference in friction angle between the small size material and the large size material reduces as the confining pressure increases. Nieble et al. [8] conducted direct shear tests on uniform crushed basalt and showed that as the maximum particle size increases, the friction angle decreases. His study showed that a maximum particle size less than 5% of the shear box width should be used to avoid size effect in measuring friction angle. Kirkpatrick [9] studies on Leighton Buzzard sand with uniform particle sizes using triaxial tests showed reduction of friction angle as the mean particle size was increased while the porosity was kept a fixed value. Large triaxial tests on gravelly soils with parallel straight-line grading curves and with maximum particle sizes of minus sieve No. 1, 6.35 mm, 1.7 mm, 5.4 mm and 5.8 mm were conducted by Leslie [1]. All gradings had a uniformity coefficient of 3.3. Based on these series of tests, he concluded that the intermediate particle sizes rather than the large particles had a greater influence on the shear strength and that the influence of oversize particles may not be as important as was suspected. To eliminate or reduce the effect of oversize particles on the shear strength, ASTM standard D38-98 [11] recommends that the maximum particle size in a shear test must be no larger than one tenth of the shear box width and one sixth of the specimen height. The above literature survey indicates that there is no general agreement on the effect of oversize particles on the shear strength of soil. Note also that almost all the research work in this field has been concentrated on cohesionless material. Furthermore, the authors of this paper are not aware of any research work that studied the effect of one oversize particle on the shear strength of rock pile material. All the above studies seem to be related to earth rock fill dam or sandy-gravelly materials and the effect of scalping on their shear strength. In order to measure the shear strength of the near surface material in some of the rock piles at the Questa mine, New Mexico (USA), several in situ shear tests using boxes 3 cm 3 cm and 6 cm 6 cm were conducted. The details of the shear box apparatus and its accessories have been reported elsewhere [1]. In majority of the in situ tests, one or two oversize particles were encountered in the rock pile block along the shear plane that can modify the shear strength and deformational behavior of the material. To address this issue, laboratory studies were performed on Questa rock pile material with oversize particles whose results are reported in this paper.. SAMPLE PREPARATION Experimental studies were conducted on the material collected from Spring Gulch rock pile that passed through sieve No. 6 using a 6 cm 6 cm shear box. The original and scalped gradation curves are shown in Figure 1. To reduce the errors associated with material variations, the rock pile material retained on sieve numbers 1, 16, 3, 4, 5, 7, 1, and passed sieve No. 1 were collected in separate containers and mixed together according to the gradation curve of Figure 1 for direct shear testing. The air dried material was mixed thoroughly with water to result in 6% water content. This water content is about the average value of the unsaturated rock pile materials that was measured in the field. Each sample was compacted in three layers in the shear box by tapping it carefully to result in a dry density of 167 kg/m 3. Note that in all shear tests, the dry density of the soil matrix was kept equal to 167 kg/m 3 irrespective of the size of the oversize particle. The normal stresses used for shear testing were.5 kpa, 4.3 kpa, and 68. kpa that are similar to those used for in situ shear testing. The oversize particles were simulated by using stainless steel spheres or balls having diameters of.66 cm, 1.8 cm, and 1.9 cm. The reason for using spherical particles was to reduce the number of

3 parameters involved in this study; non- spherical particles can have different degrees of sphericity and angularity that add to the complexity of the problem. The oversize particles were accommodated along the shear plane at the center and the left and right sides of the shear box to investigate whether the location of oversize particle modifies the shear strength. Figure shows a photo of the shear box in which the left and right sides of the box with respect to the location of applied shear force have been depicted by letters L and R, respectively. Percent Passing by Weight U.S. Standard Sieve Size 3/ / Grain Size (mm) In-situ Fig. 1. Gradation curves (in situ and lab) of the material used for experimental study. L Fig.. A photo of the shear box showing tilting of the top plate, direction of applied shear force, and right (R) and left (L) sides of the box. Lab 3. DIRECT SHEAR TEST RESULTS In total, 68 direct shear tests were conducted. To assure the consistency and repeatability of the shear tests results, normally more than one test was performed for a fixed ball size, ball location, and applied normal stress. Table 1 shows the results of all the shear tests. All the shear tests were conducted at a displacement rate of approximately.5 mm/min. Figure 3 shows the resulting Mohr-Coulomb failure envelopes. Each envelop is a regression line passing through all the points of shear R Direction of applied force test results with the same ball size and ball location. The average shear stress-shear displacement curves for the shear tests at the normal stress of 68. kpa are shown in Figure 4. Figures 3 and 4 suggest that the presence of the oversize particle changes the shear strength of the material, and that this effect is more pronounced when the largest oversize particle (1.9 cm in diameter) is used. Table 1. The results of laboratory direct shear tests. Ball Diameter (cm) Ball Location N/A Normal Stress (kpa) Test Number 1 Number of Tests Peak Shear Stress (kpa)

4 To investigate the dilatational behavior of the material, the average normal displacements at the center of top plate versus the shear displacement for the normal stress of 68. kpa are shown in Figure 5. Except for the largest ball, the dilatational behavior of the material remained almost unchanged. As expected, the test with largest ball shows more dilatation, indicating that more energy is needed to shear the material that is consistent with the higher peak shear stress in Figure 4. Note that in Figure 5 the points corresponding to the peak shear stresses are shown with larger symbols, suggesting that a greater shear displacement corresponding to the peak shear stress is observed for the shear tests with the two largest balls. Peak shear stress (kpa) Ball diameter = 1.8 cm Normal stress (kpa) Shear stress (kpa) 1 Normal stress = 68. kpa Ball diameter = 1.8 cm Peak shear stress (kpa) Ball diameter = 1.8 cm Normal stress (kpa) Shear stress (kpa) 1 Normal stress = 68. kpa Ball diameter = 1.8 cm Peak shear stress (kpa) Ball diameter = 1.8 cm Normal stress (kpa) (c) Fig. 3. Mohr-Coulomb failure envelopes for the ball located at the a) left side, b) right side, and c) middle of the shear box. Shear stress (kpa) Normal stress = 68. kpa 4 Ball diameter = 1.8 cm (c) Fig. 4. Shear stress vs. shear displacement for ball located at the a) left side, b) right side, and c) middle of the shear box.

5 Normal displacement (mm) Normal stress = 68. kpa No ball Ball diameter = 1.8 cm Table. The measured friction angles, dilation angles, and cohesions in the direct shear tests. Ball diameter (cm) Location of the ball (Degree) Dilation angle (Degree) (Normal stress 68. kpa) c (kpa) Normal displacement (mm) Normal stress = 68. kpa No ball Ball diameter = 1.8 cm D =.66 D = 1.8 D = N/A Normal displacement (mm) Normal stress = 68. kpa (c) No ball Ball diameter = 1.8 cm Fig. 5. Normal displacement vs. shear displacement for the ball located at the a) left side, b) right side, and c) middle of the shear box. 4. DISCUSSION OF THE RESULTS The friction angles (G), dilation angles (H) and cohesion intercepts (c) from direct shear tests are reported in Table. The dilation angles corresponding to the peak shear stresses and a normal stress of 68. kpa were obtained from Figure 5 using the following equation [13]: dyy dv tan( ) = = (1) d xy du where J yy and K xy are normal and shear strains, and v and u are vertical (normal) and shear displacements, respectively. Note that this equation is valid only if the shear plane is assumed to be a zero extension line. As expected, the data in Table show that in general with increase in friction angle, the dilation angle increases. The friction angles, dilation angles, and cohesions of the material with and without the oversize ball shown in Table indicate how the shear strength and deformational characteristics of the material are affected with the ball size. In Figure 6a, the variation of friction angle as a function of normalized ball diameter with respect to box width is shown. It is clear that with increase in the ball diameter, the friction angle increases. The maximum increase in the friction angle is for the test with the largest ball located in the left side of the box that shows a friction angle of 48. compared to the no ball situation whose friction angle is 41.. The friction angle of 48. is higher than those of 46.5 and 45.1 for the situations where the largest ball is accommodated in the middle and right side of the box, respectively. Preliminary observations suggest that depending on the location of the oversize ball in the shear box, different kinematical constraints in the box deformation are realized that affect the shear strength of the material. For example, even though the final vertical

6 displacement of the center of top platen was almost the same in all the three tests shown in Fig. 5, the tilting of top platen (Fig. ) was slightly different depending on the location of the oversize ball. Discrete element numerical modeling of the shear box is currently underway to better clarify the reason for the change of friction angle with the location of oversize ball. Note also that tilting of the top platen causes a non-uniform distribution of soil density and normal stress within the deformed specimen and that can be another reason for sensitivity of the shear tests results with the largest ball to the ball location. It is important to realize that the increase in friction angle due to presence of oversize ball is only a couple of degrees even when the normalized ball diameter is equal to %. This indicates that the recommendation regarding the maximum allowable particle size in a direct shear tests by ASTM (maximum particle size / box width <.1) can be somewhat relaxed, at least for the rock pile material in this study and the given range of low normal stresses. Figure 6b illustrates the cohesion intercept versus the normalized ball diameter. Except for the largest ball located at the left side of the box, the cohesion reduced due to presence of oversize particle. Depending on the location of the ball and its size, the decrease in cohesion intercept could be 1 to 4% with an average of about 15%. Nevertheless, cohesion in general is more sensitive to soil remoulding compared to friction angle, and its measurement in the lab might be questionable. A hybrid discrete-finite element analysis is underway by the authors to investigate the effect of oversize particles in more detail. This numerical investigation should help to elucidate the shear band formation and the deformation pattern around the oversize particle in order to better interpret the experimental findings reported in this paper. 5. CONCLUSION There is no general agreement in the literature on the effect of oversize particles on the shear strength of soil. To study this problem, several direct shear tests with specimens 6 cm 6 cm in size were conducted on the Questa mine rock pile materials. This study should help to interpret the in situ shear tests conducted in the mine area where oversize particles along the shear plane were encountered. To simulate the oversize particles, spherical balls made of stainless steel with different diameters were placed along the shear plane. The results of direct shear tests suggest that in general both the friction angle and dilation angle increase due to the presence of the oversize particle, even though the effect of an oversize particle is much more pronounced on the friction angle compared to that on the dilation angle. The recommendation made by ASTM regarding the allowable maximum particle size seems to be too conservative for the case studied, i.e., for the low normal stress used and the rock pile material investigated. Instead of a limit of.1 for the ratio of maximum particle size to box width, a ratio of. can be applied with only minimal increase in friction angle (about degrees). The results of this study show also that cohesion is normally underestimated if oversize particles are present in the shear box. Friction angle (degree) Cohesion (kpa) ASTM recommended limit 38 Ball at the left of box Ball in the middle of box 36 Ball at the right of box ASTM recommended limit Ball diameter/box width 4 Ball at the left of box Ball in the middle of box Ball at the right of box Ball diameter/box width Fig. 6. Friction angle and cohesion vs. normalized ball diameter. The allowable maximum particle size / box width recommended by ASTM is shown with the dashed line.

7 AKNOWLEDGMENTS This project was funded by Chevron Mining Inc. (formerly Molycorp Inc.), and the New Mexico Bureau of Geology and Mineral Resources (NMBGMR), a division of New Mexico Institute of Mining and Technology (New Mexico Tech). David Jacobs, and Dirk Van Zyl reviewed an earlier version of this manuscript and their comments are appreciated. REFERENCES 1. Holtz, W.G. and H.J. Gibbs Triaxial shear tests on pervious gravelly soils. Journal of the Soil Mechanics and Foundation Division; In Proceedings of the American Society of Civil Engineers. Vol. 8, pp Hennes, R.G The strength of gravel in direct shear. ASTM special technical publication. Vol. STP 131, pp Vallerga, B.A., H.B. Seed, C.L. Monismith, and R.S. Cooper Effect of shape, size, and surface roughness of aggregate particles on the strength of granular materials. nd Pacific Area National Meeting of the American Society for Testing Materials. Los Angles, California, USA. 4. Rathee R.K Shear strength of granular soils and its prediction by modeling techniques. Journal of the Institution of Engineers (India). Part C1, Civil Engineering Division, Vol. 6, Special Issue on Hydrology and Hydraulic Engineering, pp Su, W Static strength evaluation of cohesionless soils with oversize particles. PhD dissertation. Washington State University, Pullman, WA. 6. Cerato, A.B., A.J. Lutenegger. 6. Specimen size and scale effects of direct shear box tests of sands, Geotechnical Testing Journal, Vol. 9, No. 6, pp Becker, E., C.K. Chan, and H.B. Seed Strength and deformation characteristics of rockfill materials in plane strain and triaxial compression tests, Report No. TE 7-3 to State of California Department of Water Resources, Department of Civil Engineering, University of California Berkeley, California, USA. 8. Nieble, C.M., Silveira, and N.F. Midea Some experiences on the determination of the shear strength of rock fill materials. In Proceedings of the Second International Congress of the International Association of Engineering Geology. Sao Paulo, Brazil, pp. IV- 34.1, IV Kirkpatrick, W. M Effect of grain size and grading on the shearing behavior of granular materials, Proceedings of the sixth International Conference on Soil Mechanics and Foundation Engineering, Montreal, Canada. 1. Leslie, D.D Large-scale triaxial tests on gravelly soils. Pan-American Conference on Soil Mechanics and Foundation Engineering. Vol. 1, pp ASTM standard D38-98, 3, Annual book of ASTM standards, Section four, Construction, Volume Fakhimi, A., K. Boakye, D. Sperling, and V. McLemore. 8. Development of a modified in situ direct shear test technique to determine shear strength of mine rock piles. Geotechnical Testing Journal. Vol. 31, No. 3, paper on line Simoni A., and G.T. Houlsby. 6. The direct shear strength and dilatancy of sand-gravel mixtures. Geotechnical and Geological Engineering, 4, pp