STUDYING THE EFFECT OF MECHANICAL PROPERTIES OF DISCONTINUITIES ON THE SLOPE STABILITY IN PART OF ANGOURAN OPEN PIT MINE, IRAN

chart.  Barton and Bandis [5] present a method for estimating JRC from the Jr component of the Q rock mass classification system.

2-3-ESTIMATION OF Ф b The basic friction angle can be estimated from direct shear testing on smooth rock surfaces that have been prepared by means of a smooth, clean diamond saw cut [6]. Barton and Choubey [4] reported that the basic friction angle for most smooth unweathered rock surfaces lies between 25? and 35?. Stimpson [7] suggested the use of tilt testing of diamond core samples for the estimation of the basic friction angle. He observed that the core surfaces produced by typical core drilling procedures are precut and smooth and therefore not dissimilar to a saw cut rock surface. The suggested tilt tests involve attaching two pieces of core to a horizontal base, ensuring that the core samples are in contact with one another and are not free to slide. A third piece of core is then placed on top of the first two pieces and the base is rotated about a horizontal axis until sliding of the upper piece of core along the two line contacts with the lower pieces of core begins. The following equation can then be used to estimate the basic friction angle. )tan155.1arctan( sA   (5) Where Ф A is the basic friction angle for the upper piece of core and αs is the angle at which sliding commences.

DIRECT SHEAR TESTS A number of multistage direct shear tests have been undertaken on samples of Joint Set 1. Testing was performed at the Rock Mechanics Laboratory of Shahrood University of Technology. Results are summarized in Table 1. The frequency histograms from which the distribution types shown in Table 1 have been identified are presented in Figure 3. Table 1. Summary of direct shear test results for Joint Set 1 Parameter Statistic Amount(19 tests) Peak Cohesion (KPa) Mean 616 Standard Deviation 103.8 Minimum 475 Maximum 780 Distribution Type Normal Peak Friction Angle (deg) Mean 36.2 Standard Deviation 6.5 Minimum 25 Maximum 48 Distribution Type Logistic 6 0 1 2 3 4 5 6

551.25 627.5 703.75 More Peak Cohesion (KPa)F r e q u e n c y 0 1 2 3 4 5 6 7 8

30.75 36.5 42.25 More Friction Angle (deg)F r e q u e n c y Figure 3. Cohesion and friction histograms for Joint Set 1

USING BARTON’S CRITERION

1-DETERMINATION OF THE PARAMETERS JRC AND JCS A summary of the JCS and JRC estimates for the three main joint sets are presented in Table 2. The discontinuities in the study area generally exhibit no wall softening due to weathering, and the JCS is therefore assumed to be equal to the UCS of the intact rock. JRC values for the three discontinuity sets in the study area were recorded during scanline mapping using the profiles of Barton and Choubey [4]. Table 2. JCS and JRC estimates for the four main joint sets Parameter Statistic Set 1 Set 2 Set 3 JRC Mean 10.5 7.2 8.9 Standard Div. 5.1 2.8 7.2 Minimum 2.0 2.0 2.0 Maximum 18.0 12.0 14.0 JCS (MPa) Mean 96.15 90.34 93.18 Standard Div. 22.54 26.33 30.94 Minimum 41.5 41.5 41 Maximum 124.5 124.5 124.1

2-DETERMINATION OF BASIC FRICTION ANGLE There are three main methods for estimating basic friction angle [8]:  Direct shear testing along saw cut samples  Tilt tests  Suggested values for both dry and wet samples In this study, we have used tilt tests for estimating basic friction angle. These tests present some important advantages regarding the usual shear tests, when very shallow and particular works are concerned. Their physical meaning points out clearly for the execution of tilt tests for the evaluation of shear strength of rock slopes studies. A number of 100 tilt tests were performed on three joint sets. Summary of these estimates are presented in Table 3. Table 3. Summary of basic friction angle results by tilt testing Parameter Statistic Set 1 Set 2 Set 3 Mean 35.38 37.51 37.94Basic Friction Angle Ф b Standard Div. 3.02 3.26 3.52 Minimum 31.5 32.3 32 Maximum 40 42.6 43.4 7

COMPARISON BETWEEN DIRECT SHEAR TESTS RESULTS AND BARTON’S CRITERION The direct shear testing was undertaken using a four stage testing procedure on each of the samples. Any small-scale surface irregularities may have been sheared off by the initial stages of the tests, resulting in flatter curve and a lower basic friction angle [9]. There is significantly less potential for the shearing of asperities during tilt testing due to the relatively low normal stress levels involved. For an improved comparison between direct shear test and Barton’s criterion, plots are employed here. A plot which shows normal stress versus shear stress is presented in figure 4. 0 1000 2000 3000 4000 5000 6000

1000 2000 3000 4000 5000 Normal Stress (KPa)S h e a r S t r e s s ( K P a ) Barton's Criterion Direct Shear Test Best Fit Figure 4. Comparison between Direct Shear Test results and Barton’s Criterion

CONCLUSION Because of the existence of irregularities on joint planes, non-linear relationship normally exists between shear strength and normal stress at low normal stress levels. Generally, the gradient of the shear strength curve is increasing with decreasing normal stress. Hence, in particular at low normal stress levels, as relevant at most slope stability cases, it is of the greatest importance that the selection of shear strength parameters is adjusted to the normal stress level in question. The main purpose of this paper has been to put emphasis on the importance of reliable determination of shear strength parameters, not in detail to discuss testing-method for achieving reliable results. However, as previously underlined, results from small scale shear tests will often be uncertain due to scale effects. At important slopes, the selection of shear strength parameters should preferably be based on large scale shear tests (in-situ or lab). Back analyses based on previously failed rock slopes may also be extremely valuable for evaluating shear strength parameters. ACKNOWLEDGEMENTS This paper is based on part of experiments of a research thesis [10] will be submitted to the Department of Mining and Geophysical Engineering, Shahrood University of Technology. The authors wish to acknowledge the support of the university during the period that this work has been undertaken. 8 REFERENCES

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