A graphical approach for slope mass rating (SMR) PDF

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This paper has to be cited as: Tomás, R., Cuenca, A., Cano, M., García-Barba, J., A graphical approach for Slope Mass Rating (SMR). Engineering Geology, 124, 67-76, 2012. The final publication is available at Elsevier via: http://www.sciencedirect.com/science/article/pii/S0013795211002572 A graphica...

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This paper has to be cited as: Tomás, R., Cuenca, A., Cano, M., García-Barba, J., A graphical approach for Slope Mass Rating (SMR). Engineering Geology, 124, 67-76, 2012. The final publication is available at Elsevier via: http://www.sciencedirect.com/science/article/pii/S0013795211002572

A graphical approach for Slope Mass Rating (SMR)

R. Tomás, A. Cuenca, M. Cano, J. García-Barba

Departamento de Ingeniería de la Construcción, Obras Públicas e Infraestructuras Urbanas. Escuela Politécnica Superior, Universidad de Alicante, P.O. Box 99, E-03080 Alicante, Spain. [email protected]

Abstract Slope Mass Rating (SMR) is a commonly used geomechanical classification for the characterization of rock slopes. SMR is computed adding to basic Rock Mass Rating (RMR) index, calculated by characteristic values of the rock mass, several correction factors depending of the discontinuity-slope parallelism, the discontinuity dip, the relative dip between discontinuity and slope and the employed excavation method. In this work a graphical method based on the stereographic representation of the discontinuities and the slope to obtain correction parameters of the SMR (F1, F2 and F3) is presented. This method allows the SMR correction factors to be easily obtained for a simple slope or for several practical applications as linear infrastructures slopes, open pit mining or trench excavations.

Keywords: Geomechanical classification, SMR, basic RMR, stereographic projection

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1. Introduction Rock mass classification systems are a worldwide communication system for explorers, designers and constructors that facilitate characterization, classification and knowledge of rock mass properties. They provide quantitative data and guidelines for engineering purposes that can improve originally abstract descriptions of rock mass from inherent and structural parameters (Liu and Chen, 2007; Pantelidis, 2009) by a simple arithmetic algorithm (Romana, 1997). The main advantage of using a rock mass classification scheme is that it is a simple and effective way of representing rock mass quality and of encapsulating precedent practice (Harrison and Hudson, 2000). Nevertheless, rock mass classifications present some well-known limitations. Hack (2002) stated that generally rock mass classifications consider parameters related with slope geometry, intact rock strength, discontinuity spacing or block size and shear strength along discontinuities, some of which are difficult or impossible to measure (e.g. water pressure) or have a limited influence on slope stability (e.g. intact rock strength). Pantelidis (2008) referred to these parameters as “questionable”, including those that: (a) are unsuitable for use in slope stability problems, (b) are attributed into the systems in an erroneous manner, (c) although, in practice, they play significant role regarding stability of slopes, they exert a minor influence on the system, or, (d) present several major disadvantages related to their definition. All the previous mentioned causes can introduce some uncertainties during the rock mass characterization process that can affect the final computed indexes and the inferred geomechanical quality and parameters. As a consequence, rock mass classifications on their own should only be used for preliminary planning purposes or within the overall engineering design process (Bieniawski, 1997). Some of the existing geomechanical classifications for slopes are Rock Mass Rating (RMR, Bieniawski, 1976;1989), Rock Mass Strength (RMS, Selby, 1980), Slope Mass Rating (SMR, Romana, 1985), Slope Rock Mass Rating (SRMR, Robertson, 1988), Rock Mass Rating, Mining Rock Mass Rating (MRMR, Laubscher, 1990), Mining Rock Mass Rating modified (MRMR modified, Haines and Terbrugge, 1991), Chinese Slope Mass Rating (CSMR, Chen, 1995), Natural Slope Methodology (NSM, Shuk, 1994), Modified Rock Mass Rating (M-RMR, Ünal, 1996), Slope Stability Probability Classification (SSPC, Hack, 1998; Hack et al., 2003), modified Slope Stability Probability Classification (SSPC modified, Lindsay et al., 2001), Continuous

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Rock Mass Rating (Sen and Sadagah; 2003), Continuous Slope Mass Rating (Tomás et al., 2007) and an alternative rock mass classification system proposed by Pantelidis (2010). Among all geomechanical classifications listed above, SMR is universally used (Romana et al., 2001; 2003; 2005). It is derived from the basic RMR (Bieniawski, 1989), initially created for tunnelling applications, although its author also included proposals for slope correction factors in order to take into account the influence of the discontinuities orientation on slope stability. In practice, RMR is difficult to apply to slopes as there is no exhaustive definition for the selection of correction factors. The detailed quantitative definition of the correction factors (Itigaray et al., 2003) is one of the most important advantages of SMR classification. Both RMR and SMR are discrete classifications, computed by assigning a specific rating to each parameter included, depending on the value adopted by the variable that controls the parameter under consideration. The aim of this study is to propose a graphical method for the determination of Slope Mass Rating correction factors. The present work is devoted to define stereoplots that can be used in rock mass slopes studies in order to easily interpret and compute SMR correction factors.

2. Slope Mass Rating (SMR) classification The Slope Mass Rating (SMR) index, proposed by Romana (1985), is calculated by determining four correction factors to the basic RMR (Bieniawski, 1989). These factors depend on the existing relationship between discontinuities affecting the rock mass and the slope, and the slope excavation method. It is obtained using expression (1).

SMR  RMR b  (F1  F2  F3 )  F4

(1)

where: -

RMRb is the basic RMR index resulting from Bieniawski’s Rock Mass Classification without any correction. Therefore, it is calculated according to RMR classification parameters (Bieniawski, 1989).

-

F1 depends on the parallelism (A in Table 1) between discontinuity dip direction, j, (or the trend of the intersection line, i, in the case of wedge failure) and slope dip, s, (Table 1).

-

F2 depends on the discontinuity dip, j, in the case of planar failure and the plunge of the

intersection line, i, in wedge failure (B in Table 1). For toppling failure, this parameter

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adopts the value 1.0. This parameter is related to the probability of discontinuity shear strength (Romana, 1993). Table 1. Correction parameters for SMR (modified from Romana (1985) by Anbalagan et al. (1992)).

TYPE OF FAILURE P T W P/T/W P/W P/W T P

|j-s| ||j-s|A 180| |i-s| F1 B |j| ó |i| F2

j-s

W

C i-s

T P/T/W

F3

j+s

VERY FAVORAB LE

FAVORABLE

NORMAL

UNFAVORAB LE

VERY UNFAVORAB LE

>30º

30-20º

20-10º

10-5º

10º

10-0º

0º

0-(-10º)...