Comparison of Slope Mass Ratings Classification Systems: A Review

. Engineering rock mass classifications are vital for empirical approach to evaluate and predict engineering behavior of a rock mass. Now well established empirical relations between behavior of the rock mass and the rock mass properties with regard to specific engineering applications have become an important tool for resolving many geo-engineering issues related to mega engineering projects. Engineering classifications of Rock Masses have been applied in tunneling and underground mining with great success for many years. Some rock mass classification systems developed originally for underground, excavations were also modified and adopted for many different applications including slope stability applications. The rocky slopes in general as well as along road and rail tracks are important locales for slope analysis and stabilization. In this study five classification systems are thoroughly studied for rock stability assessment and compared on the basis of reports of various research paper published so far. The methods are Slope Mass Rating and it’s off shoots, such as Continuous Slope Mass Rating, Chinese Slope Mass Rating, Graphical Slope Mass Rating and Landslide Hazard Evaluation Factor. We have tried to work which of these method can best predict slope failure as a normal process of mass wasting and mass movement as well as triggering mechanism such as pore water pressure increase, sudden down pour, earthquakes etc. So as to work out structurally controlled failure mechanism to find suitable ways for safe rock slope cuts for road networks in hilly and mountains terrain.


Introduction
The "Brown Field" areas comprising of rocky terrains are subjected to infrastructural development as in the case of "Green Field" areas. But categorizations of "Rocky Grounds" are difficult due to lot of natural variability as compared to "Soil Ground". The engineering classification of "Rocky Ground" has been attempted first by Karl Terzaghi in (1946) known as "Rock Load Theory" for designing support for underground tunnels. This was followed by Rock Mass Rating by Bieniwaski, 1973, Rock Mass Quality by Barton et al, 1974 and became the base of future engineering classification of rock masses. These classifications were mainly for designing tunnel support system. Many of these methods have been modified by different researchers to be used for other geotechnical engineering issues, such as characterizing slopes and identifying their failure vulnerabilities.
Methods proposed and classification developed so far for slope analysis can be broadly classified in four categories they are kinematic analysis approach (based on stereo net projection), empirical methods, limit equilibrium and numerical modeling. A number of classification systems are available for analysis of slope stability, namely: 1. Slope Mass rating, (SMR, Romana et al., 1985;2001; 2. Chinese Slope Mass Rating (CSMR, Chen, 1995), 3. Continuous Slope Mass Rating (Tomás et al.2007), 5. Landslide Hazard Evaluation Factor (LHEF, 1995)

Classification systems in slope stability analysis
Slope stability is an important aspect of infrastructure development especially road and rail networks and causes immense distress in our own country (NDMA, 2009) .Rock engineering has an important part to play in stabilization of the rocky slopes. During the plan, development and post design periods of rock slope stability, designers, engineers, and geologists need to give close consideration to the rock conditions of the rock slope, to anticipate and control failure of the slope, ensure masses safety living in these landscapes and maintain road and rail network for economic activities. With passage of time and advancement in engineering and technology there is a need of classification system with uniform validity, which attracts researcher to propose a new system which could describe and hold validity in different geological and engineering aspects. It is the rock mass classification system adopted by researcher, engineers in the real field situations throughout the world as a base, with an intention to provide quantitative guidelines for analysis and practice According to Hack (2002), classification systems consider various parameters like geometry, slope, shear strength etc.
But it is the properties pertaining to water seepage and pressures are still difficult to determine precisely, despite the fact that water is the biggest cause of slope failures. Pantelidis (2009) stated that these properties are controvertible and employed with errors. In all the above mentioned methods the designated properties and their indices may result in misinterpretation due to many varied parameters related to geomechanical properties of rock. Rock mass classification system is used for design In geotechnical engineering for preliminary assessment and due to its simplicity (Duran &Douglas 2000), and it is the primary resource for assessment of stability based on structural and inherent parameters (Taherniya et al. 2014). These systems are serving as a foundation in the empirical designs which correlates the past experience to the present prevailing situation and state at the present site (Bieniawski 1990). Main reason for the popularity of rock mass classification systems is that they are a basic and powerful method of delineation rock mass quality and setting the basic frameworks for implementation and practice in the field (Harrison, Hudson ,2000) In tunneling and underground rock engineering these systems are applied to find the rock mass quality, in other processes and pre-design excavation (Aksoy 2008). these systems of classification plays vital role in quantification of the rock properties on the basis of past findings and experiences and for evaluation of the behavior of rock mass under external loading conditions (Milne et al. 1998). SMR provides preliminary information in the initial phase of the investigation (Romana et al. 2015).The stereographic analysis approach is quite friendly and easy to use in case of jointed rock mass for the finding and assessment of potential failure types and direction (Goodman 1976;Hoek and Bray 1981;Matherson 1988) and these systems were initially developed and used for underground excavation purposes (Hoek ,2007) and must be used for preliminary investigation purposes (Bieniawski, 1997). Rock slope and soil mass are complex, even after major researches we are still lagging in understanding its geological characteristics, mechanical properties, strength, and deformation (

Slope Mass Rating (Romana, 1985)
Slope Mass rating is the extension of one of the early engineering classification of rock masses i.e. Rock Mass Rating (RMR) by Bieniawski in 1973. Romana (1985) [9] used this engineering classification to analyze rocky slopes by following formula: Where,RMRBasic stands for Rock Mass Rating given originally in 1973 and taking into consideration five parameters plus the sixth one given by Romana (1995) F 1 , F 2 , F 3 and F 4 are different functions related to slope (table 2.5), defined as:      In last three decades or more, SMR is being used very frequently to get: 1. Geomechanical classification system for rating of rocky slopes.

2.
Preliminary investigations to find out the vulnerability of slope failure.

3.
Served as base indicator for engineering solutions on failing slopes.
But, some natural issues of rock slopes could not be factored in SMR and resulted in to poor results. Many scientists tried to incorporate these parameters and have incorporated extra factors to get more realistic results.

Chinese Slope Mass Rating System (CSMR)
It was developed by Chen (1995) to adopt SMR system to rock slope conditions in China. It is used as a national standard for slope in design and construction of Dams and Hydroelectric power Stations. It adapts two additional factors in SMR: 1) Height of slope, ξ (if more than 80meter).
ξ and λ and modified slope mass rating (SMR) formula as follows.
Where, ξand λ represents the slope height factor and discontinuity factor respectively. ξ and λ are significant factors and included in the framework of SMR in light of the fact that there are a few slope failures for which SMR indicates stable slopes. Therefore, these two factors are included to improve the classification system, while other parameters remained the same. The factor ξ is applicable only for heights greater than 40 m. However, this is an accepted system of classification in China and for application other than China it requires a number of corrections and modifications before using at any other place.
Where, H is the height of slope in meters and, λ=0.7 for closed joints and tightly interlocked bedding planes. λ = 1 for faults, λ = 0.8 to 0.9 for long weak seams filled with clay and large scale joints.

Graphical Slope Mass Rating (Tomas et al 2012):
This classification system is based on graphical approach to find the correction factors in basic SMR by using stereo

Landslide Hazard Zonation(IS:14496 Part II)
The Bureau of Indian Standard has given this code IS 14496 Part II, 1985, [5] for landslide hazard mapping based on ten causative factors with each factor given landslide hazard evaluation factor (LHEF) as 1 or 2, totaling 10 maximum points. (table 5.1). The area to be mapped for landslide hazard zonation is to be divided into different smaller regions using maps of 1: 50,000 to 1: 25,000 for macro zonation.
It has direct similarity with SMR in terms of its parameter B which is equivalent to F1, F2 and F3 of SMR (table 5.2).

Table 5.1: Causative Factors and Landslide Hazard Evaluation Factor
The similarity with SMR in terms of its parameter B which is equivalent to F1, F2 and F3 of SMR but with different rating values (table 5.2) are as follows: a) The extent of parallelism of discontinuity plane or the line of intersection of two discontinuity planes w r t slope orientation (STRIKE) i.e. F1 of SMR.
b) The difference in the dip or inclination of discontinuity plane or the line of intersection of two discontinuity planes w r t slope inclination (DIP) i.e. F3 of SMR.
c) Steepness or dip of the discontinuity or plunge of line of intersection of two discontinuity planes i.e. F2 of SMR  Showing angular relationship relation between (a) -orientation of slope and joint, (b)

-between inclination of slope and dip of joint and (c) -dip of the joint.
Depending upon the estimated value of each region the entire area can be identified in to five hazard zones as per the In this paper five classification systems thoroughly examined particularly those systems which are established for the assessment of stability of rock slope. The worth mentioning inferences drawn from the comparative study of the five methods, Slope Mass Rating (Romana, 1985), Chinese Slope Mass Rating System (CSMR), Graphical Slope Mass Rating (GSMR), Continuous Slope Mass Rating (Cont. SMR) and Landslide Hazard Evaluation Factor are as follows: SMR methods are found to be most suited for the rock slope that undergoes failure mechanism which is structurally controlled .it includes the combine effect of dip direction and dip, which includes F 4 as the effect of method of excavation which is correlated with parameters (F 1, F2, F3).SMR is slightly conservative. The extreme values of F3 (-60 and -30) proposed by Romana (1985) are something difficult to cope with, SMR does not take into account the effect of height.
The major drawback for Chinese SMR is that it is not applicable for height of slope below 80 m hence it is not suitable for rock cuts however it includes height and discontinuity condition. In case of favorable conditions to discontinuity this methods gives higher ratings in comparison to the original SMR. It needs to be equipped with considerable modifications as per requirements and number of corrections before applying at any other place. It is concluded that all empirical methods compared in this study are applicable to controlled failure mechanism and not consider the triggering factors like water presence. None of the above methods considers convexity or concavity of the slope, which generally encountered in the rock cuts hence suitable for linear structure. These methods can further improved by incorporation slope factor for height less than 80 m, effect of pore water and adjustment factors can further be improved analytically. Shape factors must be included which could add the effect of shape and curvature of the slope .the above mentioned limitations need to be addressed in the future study It can be concluded on the basis of various case studies that need to employ parameters given by different authors in SMR. LHEF can be used for reconnaissance study though it is exhaustive but has lesser number of classes and wide range of values. Also it gives hydrology lesser importance with rating value of 1 only. The LHEF is silent on remedial measures to be provided if prone to instability. All above methods need proper technical expertise especially in measuring orientations, inclinations, and identification of most problematic joint. If LHEF and SMR with addition of parameters related to height, used as combined method can give better insight to slope.
Finally, all these methods are heavily relying on discontinuities (joint) and their orientation with respect to slope. But, do not factor the dip of the rocks. It is concluded that the horizontality, inclinity and verticality of rock mass, especially in layered rocks and their orientation with slope should also be taken into consideration. Also presence of "Shear Zones" which are very common in rock masses and are venues of severe mass wasting have not been considered where the discontinuity related parameters are overwhelmed.