Rock mass disturbance effects on slope assessments using limit equilibrium method

Transcription

1 Southern Cross University 23rd Australasian Conference on the Mechanics of Structures and Materials 2014 Rock mass disturbance effects on slope assessments using limit equilibrium method S Sheng Z Qian Winston Brackley A J. Li Publication details Sheng, S, Qian, Z, Brackley, W, Li, AJ 2014, 'Rock mass disturbance effects on slope assessments using limit equilibrium method', in ST Smith (ed.), 23rd Australasian Conference on the Mechanics of Structures and Materials (ACMSM23), vol. II, Byron Bay, NSW, 9-12 December, Southern Cross University, Lismore, NSW, pp ISBN: epublications@scu is an electronic repository administered by Southern Cross University Library. Its goal is to capture and preserve the intellectual output of Southern Cross University authors and researchers, and to increase visibility and impact through open access to researchers around the world. For further information please contact epubs@scu.edu.au.

2 23rd Australasian Conference on the Mechanics of Structures and Materials (ACMSM23) Byron Bay, Australia, 9-12 December 2014, S.T. Smith (Ed.) ROCK MASS DISTURBANCE EFFECTS ON SLOPE ASSESSMENTS USING LIMIT EQUILIBRIUM METHOD S. Sheng School of Engineering,, Geelong, Australia. Z. Qian* School of Engineering,, Geelong, Australia. (Corresponding Author) Winston Brackley School of Engineering,, Geelong, Australia. A.J. Li School of Engineering,, Geelong, Australia. ABSTRACT From the current literature and some related practice, the rock mass disturbance was found having significant influence on evaluating rock slope stability, especially for the rock slope composed by poor quality rock masses. However the effect of this disturbance on the overall rock slope stability has not been thoroughly investigated. This paper adopts the limit equilibrium method (LEM) and the latest version of the Hoek-Brown failure criterion to investigate the rock mass disturbance effects on the rock slope stability evaluations. In addition, this paper considers a range of recommended damage zone thickness. Some stability charts will be proposed and potential influence zones are discussed. KEYWORDS Safety factor, stability number, chart solution, disturbance factor. INTRODUCTION Currently limit equilibrium method (LEM) is the most popular method to evaluate rock slope stability. Not only the concept of the LEM is simple, but also the results can be obtained quickly. However, most commercial geotechnical software based on the LEM applied to rock slopes may only accept a linear criterion and require the conventional Mohr Coulomb soil parameters (Sofianos et al. 2006). In fact, using equivalent Mohr-Coulomb linear model to estimate the safety factor will ignore non-linear nature of the rock mass failure envelope and thus predicted failure surface shapes would be different. This classical problem has attracted wide attention in the literature where it has been shown that the Mohr-Coulomb criterion is not adequate to describe rock mass strength and thus it would produce poor estimates of safety factors (Li et al. 2011). Fortunately, for better estimating the stability of rock slopes and overcome the shortcomings of a linear criterion, the Hoek-Brown failure criterion was proposed (Hoek et al. 1997; Hoek 1983). The Hoek-Brown failure criterion is probably the most widely accepted fit to represent the non-linear characteristics and the strength properties of heavily jointed rock mass. Based on the latest Hoek-Brown failure criterion (Hoek et al. 2002), recent investigations (Li et al. 2011; Li et al. 2012) showed that the rock mass disturbance would play an important role to the rock This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this license, visit 739

3 slope stability evaluation. Hence, the aim of this research is to investigate the influence of the rock mass disturbance on rock slope stability assessments. In particular, the rock mass disturbance which can vary in a slope is taken into account. It means the rock mass is inhomogeneous. This paper focuses on using the LEM and Hoek-Brown failure criterion to build stability charts for convenient purposes. More specifically, the commercial LEM software, SLIDE, with Bishop s simplified method (Bishop 1955) is used to investigate the rock slope stability based on the Hoek-Brown failure criterion. In addition, the results obtained will be presented as chart solutions. METHODOLOGY Hoek-Brown Failure Criterion The Hoek-Brown failure criterion is used usually to estimate rock mass strength by practicing engineering instead of the Mohr-Coulomb criterion. The generalized Hoek-Brown failure criterion is non-linear criterion and defined by Hoek et al. (2002), shown as the following equations: where ' ' 1 3 ci m b a ' 3 s ci GSI 100 mb mi exp 28 14D GSI 100 s exp (3) 9 3D 1 1 GSI a e e (4) 2 6 ci is uniaxial compressive strength. Equations above showed that m b, s and a are all depending on the Geological Strength Index (GSI) (Marinos et al. 2007). The ranges of GSI investigated in this paper are from 10 to 90. Based on the Hoek Brown failure criterion (Hoek et al. 2002), D which is disturbance factor used to represent the rock mass disturbance. In this paper, D = 0, D = 0.7 and D = 1.0 are considered for the undisturbed rock masses, good blasting and fully disturbed rock mass respectively. In addition, N is a dimensionless parameter used to present the rock slope stability, which is defined in Equation (5). This form was firstly proposed by Li et al. (2008), however only homogeneous rock slopes were investigated. N ci HF where γ is the unit weight, H is the slope height and F is the safety factor. (1) (2) (5) Based on the Equation (5), F 1 indicates that the rock slope is unsafe. As pointed out by Marinos et al. (2006), it is appropriate to simulate a distribution of disturbance factor that decreases as the distance from the slope surface increases. In this study, the value of D is assumed to linearly decrease with the increase of distance from the slope surface, as shown in Figure 1. The initial disturbance factor (D 0 ) at slope surface is assumed as either 1.0 or 0.7 which decreases linearly in Region 1. The magnitude of D 0 adopted in this paper is based on the suggestion of Hoek et al. (2002). The rock masses in Region 2 are undisturbed, and thus D is a constant (D = 0). In Figure 1, T represents the thickness of the damage zone that suggested by Hoek et al. (2000). Table 1 shows the ranges of all parameters considered in this study. RESULTS AND DISCUSSIONS The solutions of rock slope stability number for β = 15 are shown in Figure 2 where thickness of the damaged zone is 0.5H. It can be observed that the stability number increases with a reduction of the GSI or m i. This finding agrees with the presented results of Li et al. (2008). In addition, a comparison between Figures 2(a) and (b) indicates that the change of the stability number is more significant between various GSI values when D 0 = 1.0. It means that the more serious rock mass disturbance can make the slope stability evaluation more sensitive. ACMSM

4 Figure 1. Effects of disturbance factor on rock slopes (Modification) Table 1. Parameters investigated in this study GSI 10, 20, 30, 40, 50, 60, 70, 80, 90 m i 5, 10, 15, 20, 25, 30, 35 D 0 0.7, 1.0 β 15, 30 T 0.5H, 1.0H (a) D 0 = 0.7 (b) D 0 = 1.0 Figure 2. Limit equilibrium analysis solutions of stability numbers for rock slopes T = 0.5H Some comparisons have been made and shown in Table 2 using selected data. It can be found in Table 2 that N increases with increasing slope angle and the thickness of damaged zone. The later trend indicates that the rock mass is more critical when the disturbance zone is larger, as expected. Table 2 also shows N for undisturbed rock slope (D = 0 and T = 0.0H) presented by Li et al. (2008) previously. The difference in N can be seen obviously. It should be noted that this discrepancy will influence slope stability estimations. Table 2. Some slope details picked comparisons D 0 β m i GSI T N H H H D = H 0.99 The graphs from SLIDE displayed in Figure 3 present the disturbed zone and obtained slip surfaces for β = 15, D 0 = 0.7 and T = 0.5H. It was found in Figure 3 that the potential slip surface is shallower when GSI is lower. The depth of the failure surface increases with increasing GSI. Due to the fact that ACMSM

5 N decreases with increasing GSI, the depth of the failure surface increases with N decreasing. In addition, a comparison between Figures 3 (a) and (c) showed that the failure surface is almost unchanged when m i is different. When GSI 40, the slip surface is deeper than the suggested damage zone. It implies that the damage zone should be fully considered in a slope stability assessment. (a) GSI = 10, m i = 5 (b) GSI = 30, m i = 5 (c) GSI = 10, m i = 35 (d) GSI = 40, m i = 35 (e) GSI = 20, m i = 5 (f) GSI = 90, m i = 5 Figure 3. Potential slip surface for rock slope for β = 15, D 0 = 0.7 and T = 0.5H ACMSM

6 For D 0 = 1.0, smaller potential failure surfaces can be found in Figure 4 by comparing to Figure 3. It means that, if the size of damage zone is unchanged, the slip surface would have higher potential to be controlled in the damage zone (Region 1) for lower GSI. However, this phenomenon is insignificant when GSI = 90. (a) GSI = 10, m i = 5 (b) GSI = 30, m i = 5 (c) GSI = 10, m i = 35 (d) GSI = 40, m i = 35 (e) GSI = 20, m i = 5 (f) GSI = 90, m i = 5 Figure 4. Potential slip surface for rock slope for β = 15, D 0 = 1.0 and T = 0.5H ACMSM

7 CONCLUSIONS This research adopted the LEM to estimate the rock slope stability and presented as chart solutions by considering the effects of the rock mass disturbance based on the Hoek-Brown failure criterion. The simulations are done by varying the thickness of the damage zone and initial disturbance factor which are caused by blasting. The obtained stability numbers are significantly different when comparing the undisturbed rock slopes to disturbed rock slopes. The results shows that the rock mass disturbance will influence rock slope stability evaluations significantly. More parametric studies should be done in the future works in order to provide more comprehensive stability charts. REFERENCES Bishop, A. W. (1955) "The use of the slip circle in the stability analysis of slopes", Geotechnique, vol. 5, No. 1, pp Hoek, E. (1983) "Strength of jointed rock masses", Geotechnique, vol. 33, No. 3, pp Hoek, E. and Brown E. (1997) "Practical estimates of rock mass strength", International Journal of Rock Mechanics and Mining Sciences, vol. 34, No. 8, pp Hoek, E. and Karzulovic A. (2000) "Rock mass properties for surface mines", Slope Stability in Surface Mining, WA Hustrulid, MK McCarter and DJA van Zyl, Eds, Society for Mining, Metallurgical and Exploration (SME), Littleton, CO, pp Hoek, E., Carranza-Torres C. and Corkum B. (2002) "Hoek-Brown failure criterion-2002 edition", Proceedings of NARMS-Tac, pp Li, A., Merifield R. and Lyamin A. (2008) "Stability charts for rock slopes based on the Hoek Brown failure criterion", International Journal of Rock Mechanics and Mining Sciences, vol. 45, No. 5, pp Li, A., Merifield R. and Lyamin A. (2011) "Effect of rock mass disturbance on the stability of rock slopes using the Hoek Brown failure criterion", Computers and Geotechnics, vol. 38, No. 4, pp Li, A., Cassidy M., Wang Y., Merifield R. and Lyamin A. (2012) "Parametric Monte Carlo studies of rock slopes based on the Hoek Brown failure criterion", Computers and Geotechnics, vol. 45, pp Marinos, P., Hoek E. and Marinos V. (2006) "Variability of the engineering properties of rock masses quantified by the geological strength index: the case of ophiolites with special emphasis on tunnelling", Bulletin of Engineering Geology and the Environment, vol. 65, No. 2, pp Marinos, P., Marinos V. and Hoek E. (2007) "Geological Strength Index (GSI). A characterization tool for assessing engineering properties for rock masses", Underground works under special conditions. Taylor and Francis, Lisbon, pp Sofianos, A. and Nomikos P. (2006) "Equivalent Mohr Coulomb and generalized Hoek Brown strength parameters for supported axisymmetric tunnels in plastic or brittle rock", International journal of rock mechanics and mining sciences, vol. 43, No. 5, pp ACMSM