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 Questa Weathering Study p 1 of 22 March 27, 2009 DRA-45 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. Questa Weathering Study p 2 of 22 March 27, 2009 DRA-45 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 Questa Weathering Study p 3 of 22 March 27, 2009 DRA-45 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.