Geo-engineering classification with deterioration ...

Geo-engineering classiBcation with deteriorationassessment of basalt hill cut slopes along NH 66,near Ratnagiri, Maharashtra, India

ANURAG NIYOGI1, KRIPAMOY SARKAR

1,*, ASHOK KUMAR SINGH1

and T N SINGH2

1Department of Applied Geology, Indian Institute of Technology (Indian School of Mines), Dhanbad 826 004, India.2Department of Earth Sciences, Indian Institute of Technology Bombay, Mumbai 400 076, India.*Corresponding author. e-mail: [email protected]

MS received 1 October 2019; revised 25 January 2020; accepted 29 January 2020

Western Ghats (WG) in India is endowed with one of the most distinctive and picturesque mountainouslandscapes. The region encounters unprecedented rain and non-engineered excavations (especially alonghighway cut slopes), that turns out to be detrimental to the stability of rock and soil slopes. The presentresearch focuses on accounting the vulnerability condition of rock slopes of differential deteriorationintensities along National Highway corridor 66 (NH 66) at Ratnagiri–Sangameshwar (RS) stretch. Rockmass and associated slope stability condition were encapsulated, and studied locations were classiBedusing different geo-engineering classiBcation (GEC) systems such as RMR (Rock Mass Rating), SMR(Slope Mass Rating), CSMR (Continuous Slope Mass Rating), GSI (Geological Strength Index) and RDA(Rock slope Deterioration Assessment). The study introduces a new class in RDA for igneous rocks thatare susceptible to high deterioration. Some useful insights have been added and discussed in the light ofdeterioration-induced block failures which have been integrated with detailed kinematic analysis. Thepresent study recommends that RDA would prove more eDcient and informative in the preliminary stageof slope stability investigation for high deterioration susceptible igneous rock masses as compared to othergeo-engineering systems.

Keywords. Western Ghats; geo-engineering classiBcation; kinematic analysis; highway cut slopes.

1. Introduction

The importance of geo-engineering classiBcation(GEC) of rock-mass has already been well recog-nised. The advent of slope based rock mass classi-Bcation system has cut down the valuable time ofgeoscientist and geotechnical engineers for theproper decision-making process. Many of the pre-vious researchers have acknowledged the signifi-cance of discontinuity related failures in rockengineering projects (Selby 1980; Hoek and Bray

1981; Sarkar et al. 2016; Basahel and Mitri 2017;Kundu et al. 2017; Siddique et al. 2017; Pradhanet al. 2018) where rock, as well as discontinuityparameters primarily govern the stability condi-tion. Basalts are solidiBed lava Cow that formsa competent rock possessing high compressivestrength. A type of it known as blocky lava (Aalava) inherit fractures and joints. It depends upon aconjuncture of factors, both physical and miner-alogical, that interacts with the environment.Basalts are generally composed of minerals like

J. Earth Syst. Sci. (2020) 129:115 � Indian Academy of Scienceshttps://doi.org/10.1007/s12040-020-1378-0 (0123456789().,-volV)(0123456789().,-volV)

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feldspars, pyroxenes and olivine that are suscepti-ble to deterioration in an open environment as perBowen reaction series of minerals (Bowen 1922).Weathering of basalts leads to various physico-chemico-mechanical changes which invariablydegrade the strength of rock, unlike its fresh andunaltered form (Tugrul and G€urpinar 1997). TheBssured lava Cows, forming the extensive Deccantrap are most likely to incur structural disconti-nuities that are seen with discrete and randomfractures in the rock mass (Niyogi et al. 2016).The highway corridors along WG are experi-

encing severe threat of crumbling unstable geoma-terials with risks to the ecosystem and propertiessurrounding it. National Highway (NH) 66 is abusy highway which commercially connectsMumbai (Panvel) to Goa along with states ofKerala, Tamil Nadu and Karnataka stretchingparallel to the coastal region of western India.Malsej Ghat, Mahabaleshwar, Chiplun, and Puneare reported landslide-struck places in Maharash-tra’s WG region (Singh et al. 2016; Sharma et al.2017). Several researchers have attempted toanalyse the stability condition of slopes in theseareas using different techniques (Kainthola et al.2012; Singh et al. 2013; Sharma et al. 2016). Thephysical observations in the Beld conBrm the spa-tio-temporal deterioration of rock and soil slopesthat have been considered in the present study.Failure mechanism such as rockfall and rockslidewere reported in the region (Ansari et al. 2012;Ahmad et al. 2013; Niyogi et al. 2017). Themajority of slopes are previously failed or in criti-cally stable condition. In order to understand thissituation, rock structural analyses from Beld datawere carried out in the laboratory in order todemonstratively perceive it. Discontinuity-con-trolled slope failures are well followed by thekinematic study, which provides a detailed previewof the potential failure modes.Rock mass rating (RMR) introduced by Bieni-

awski (1973) is a well-established and extensivelyused classiBcation system worldwide for the pastfour decades that was mainly applied in theunderground excavation. It includes six parame-ters, one of which is a correction due to the orien-tation of discontinuities. Use of RMR classiBcationwas emphasized for tunnels with the addition ofBve rock mass parameters to get an RMRbasic

numerical value. Romana (1993) proposed somecorrection factors for slope failure if the disconti-nuities strike and dip are favourable or not. Inpractice, RMR is often applied to quickly assess the

rock mass condition of excavated rock slopes,particularly highway cut slopes.Slope mass rating (SMR), developed by Romana

(1985) is one of the most sought out and widelyaccepted method to characterise the health of theslope. SMR addresses all the shortcomings relatedto RMR, but retains its fundamental structure.Joint orientations are assigned a numerical valuefor rating. Continuous slope mass rating (CSMR)gives a continuous result that SMR, a discreteclassiBcation, is unable to address (Tom�as et al.2007). GSI (Geological Strength Index), introducedby Hoek (1994) proposed a qualitative classiBca-tion based on visual inspection that involves thediscontinuous conditions of the rock mass. It wasinitially developed for the estimation of rock androck mass properties. Associated with other con-ventional stability classiBcation, it does manage touphold the qualitative aspects.Rock deterioration assessment (RDA) has been

developed in the UK, and extensive work has beendone considering all types of rock slopes byNicholson (2000). The striking part of this classi-Bcation system includes the involvement of dete-rioration aspects with types of remedial measuresthat can be chosen accordingly.The present research involves 11 slopes that

were chosen depending upon its vulnerability con-dition along the road cut highway. These slopeswere structurally analysed using kinematic analy-sis that was used to design slope classiBcation.Selected rock slopes were classiBed by RMR, SMR,CSMR, GSI and RDA systems. Supportingempirical methods were provided for correlationbetween most inCuential systems. Deteriorationwas emphasised to demonstrate the weatheringrelated eAects on the slope stability.

2. Study area and geological setting

Ratnagiri is a coastal district of Maharashtrapopularly known as ‘Port city’ linking Mumbai toGoa. The study area lies under SangameshwarTaluka of Ratnagiri district along NH 66. Thishighway remains an epochal source of income forthe locals. Known for its tourist commuters, thesensitivity of road towards natural and man-madehazards is grave. Geologically, the region is a largevoluminous mass of Bssured lava Cow known as‘Deccan Trap’ covering approximately 500,000km2 over west-central India (Krishnan 1953). Hillshere are steep and Cat-topped with rugged

115 of 19 J. Earth Syst. Sci. (2020) 129:115

topography (Beane et al. 1986). Slopes of basalticrocks show prominent joint sets and randomlyoriented numerous fractures, varied from amyg-daloidal to vesicular to compacted with differentweathering intensities.With the prime focus on rock mass classiBcation

and deterioration assessment of vulnerable cutslope, 11 locations were chosen stretching around14 km from Ratnagiri to Sangameshwar (Bgure 1).The sites were selected according to the variedphysical conditions pertaining to the Beld scrutiny.The basalts here are essentially Aa/blocky lavaformation which belongs to the Deccan Volcanics(represented by basaltic Cows capped by laterites).Essentially Aa basalt Cow is of Punrandgarhformation that dates back to Upper Creta-ceous–Eocene–Oligocene (Alexander 1981; Sastri1981). Predominantly laterite capping covers mostof the coastal zones of Ratnagiri of quaternaryformation (Bgure 2).

3. Kinematic analysis of slopes

Slopes inheriting discontinuities can be struc-turally analysed. Discontinuity orientations areprimarily used over a stereonet to determine thepotential failure modes (Goodman and Bray 1976;Hoek and Bray 1981). It uses Beld observation

regarding discontinuities and slope face orientationtaking into account the slope geometry and angle offriction tested in the laboratory. It is a very handytool because of its ease to use and spurt resultgeneration. This technique is applied using Dips6.0 (Rocscience 2015). Predicted modes of failure,viz., wedge failure occurs when two discontinuitiesintersect each other in such a way that the line ofintersection has a plunge less than the slope anglebut higher than frictional angle. Toppling failureoccur when one or more discontinuities dip steeplyagainst the slope, and the rock mass should have itscentre of gravity outside the slope. Plane failure

Figure 1. Google image of the site locations along with NH 66.

Figure 2. Geological map with site location: ModiBed afterGeological Quadrangle map (47 G), 1963.

J. Earth Syst. Sci. (2020) 129:115 of 19 115

occurs when a potential discontinuity dips towardsthe slope face, and the dip angle of the disconti-nuity is lower than that of the slope but higherthan the friction angle on the discontinuity surface.All the 11 locations that were studied show slopesto be vertical to sub-vertical ranging from 82� to88�. The slope and joint orientations used forkinematic analysis are shown in table 1.Figure 3 describes joint-slope orientation rela-

tionship for various failures. RS1 is aAected byJ1–J2 intersection producing wedge failure withtoppling failure is also probable (a). RS2 showsoverhanging block where J1–J2 involves in wedgeformation and J1 initiates toppling failure type (b).RS3 and RS4 are an identical set of slope on eitherside of the road. These are vertical slopes wherefour sets of joints prevail. Three and one wedgesare observed for RS3 and RS4 with one and threetoppling failure, respectively (c and d). RS5 showroot wedging with two wedge and toppling failureswith J1–J2–J4, the most vulnerable sets (e). RS6experiences single wedge and toppling failure in thedirection of slope face (f). RS7 shows two wedges,but one of them plunging north is considered asinsignificant because failure intersection betweenJ2 and J3 is outside the critical zone of failure withJ2 most vulnerable towards planar type failure (g).RS8 shows wedging at 6� by J1–J2 and toppling byJ1 and J2 (h). RS9 having weak slope materialforming blocks due to J1–J2 intersection, topplingis seen to be oblique and direct type (i). RS10 andRS11 are quite identical in joint intersectionswhere J1–J2 intersects to produce wedge at 11� and7� with two toppling failures each with both J1 andJ2 taking part (j and k). Wedge type is the mostprominently occurring mode of failure, whereas

toppling type is also seen. Though the joint andslope relation shows the probability of planar fail-ure in the analysed slopes, it has been observedthat the basal plane dips out of the slope are lessthan the joint friction angle. Hence, the sliding willoccur as base-plane toppling instead of planar typefailures. The Beld evidence also suggests that theline of intersection dipping into the slope, whereasbasal plane dips out of the slope; that is why thefailure envelope is oppositely dipping against theslope and showing oblique type of toppling too.

4. Geo-engineering classiBcation (GEC)system

ClassiBcation techniques are an informative toolthat makes it easier to comprehend the stabilitycondition. This aid with speciBc guidelines owingto which numerical values are generated andcomprehensive information is employed fordesigning approach. The type of classiBcation sys-tem needed to be chosen for rock engineeringpractices depends upon the nature of rock mass andits association with open environment. There aretwo different outlooks to a system, viz., qualitativeand quantitative. Every classiBcation system hasits limitations and importance.

4.1 Rock Mass Rating unadjusted (RMRbasic)

Bieniawski was the pioneer to put up a rock mass’sparameter-based rating system that could beinformative about the engineering aspects of anexposed rock mass known as Rock MassRating (RMR). It was also considered as CSIR

Table 1. Mean slope and joint orientations with dip/dip direction of studied locations (in �).

Site

location

Slope

orientation (S0)

Joint orientation

J1 J2 J3 J4

RS1 86/315 84/320 85/274 9/303 –

RS2 88/315 86/301 84/358 7/303 –

RS3 88/285 88/255 84/328 2/0 89/213

RS4 88/110 86/117 81/173 1/65 82/234

RS5 82/333 83/323 85/262 22/298 85/6

RS6 87/0 83/15 80/63 17/23 –

RS7 82/275 82/318 80/271 12/313 –

RS8 85/9 90/63 87/329 4/8 82/22

RS9 84/334 86/48 85/355 16/357 79/283

RS10 86/348 86/41 84/346 27/354 –

RS11 82/9 81/30 85/314 7/14 –


geo-mechanical classiBcation as it was devised atthe South African Council of ScientiBc and Indus-trial Research, initially for characterisation of thetunnel. Bieniawski (1976, 1979) came up with

modiBcations which were expressed by six param-eters, viz., (i) uniaxial compressive strength(UCS), (ii) rock quality designation (RQD), (iii)spacing of discontinuities (DS), (iv) condition of

Figure 3. Kinematic analysis of studied locations showing various modes of failure and weathering conditions with Beldphotographs (a–k) represents location RS1–RS11.


Figure 3. (Continued.)


discontinuities (DC), (v) condition of groundwater(GW), and (vi) discontinuity orientation. Thediscontinuous condition has subdivisions, viz., (a)persistence of discontinuity, (b) aperture of dis-continuity, (c) surface roughness, (d) inBlling andweathering condition. Each parameter is given anumeric entity that signiBes ‘rating’. The sum ofthe Brst Bve parameters constitutes RMRbasic

(equation 1). Bieniawski (1989) extended theapplication of RMR in case of slopes, tunnels,foundations, etc.

RMRbasic ¼ UCSþ RQDþDSþDCþGW: ð1Þ

The use of volumetric joint count (Jv) wasconsidered for RQD calculation (Palmstrom 1982)as per the correlation equation (equation 2)

RQD ¼ 115�3:3� Jv: ð2Þ

RQD is a useful parameter that uses cores ofrock and its recovery of equal or more than 10 cm,but if not possible, then Jv can be used (Zhang2016).

Figure 3. (Continued.)


Table 2 shows the RMRbasic and its associatedclasses for each slope. Unadjusted ratings ofRMRbasic varies from 47 to 65. Most of the slopescome under a fair condition with RS2, RS3 andRS5 fall under good condition of the rock mass asdescribed by Bieniawski (1993). RS1, RS5 and RS6experiences wet dripping condition at lower partwith RS5 having root wedging.

4.2 Slope Mass Rating (SMR)

SMR came into existence when Romana (1985)recognised and perceived the significance of quan-tiBcation for relative orientation of slope withrespect to discontinuities present in the rock mass.It led to an accurate assessment of different failuretypes by the involvement of four adjustment fac-tors. In yesteryears, several sets of modiBcationswere introduced to quantify the problem faced(Romana 1991, 1993, 1995; Anbalagan et al. 1992;Romana et al. 2001, 2003). Use of RMR basic withadjustment of factors contributed to the formula-tion of SMR (equation 3).

SMR ¼ RMRbasic þ F1 � F2 � F3ð Þ þ F4; ð3Þ

where F1, F2, F3 are adjustment factors dependingupon the relative orientation of slope and discon-tinuity for a different mode of structurally con-trolled failures and F4 is related to excavationmethod. F1 depends upon parallelism between dipdirection of discontinuity and slope face which iscalculated using dip direction of slope and discon-tinuity causing planar and toppling mode of fail-ures, while for wedge mode, dip direction of theslope and the trend of the line formed by theintersection of two discontinuities forming wedge

failure are considered for calculation of F1. F2 refersto dip angle of discontinuity in planar and wedgemode of failure, while for toppling mode it remains1. F3 is related to the relationship between slopeinclination and dip of discontinuity, causing planarand toppling failures.The calculated values of SMR show that most of

the slopes fall under class III and IV, i.e., partiallystable to unstable condition for different modes offailure. Slopes of RS5–RS11 are unstable to par-tially stable because of prominent weatheringcondition. In most cases, the slopes are partiallystable against wedge failure, whereas slopes RS1,RS5, RS7 and RS9 are completely unstable. Con-sidering toppling type of failure, the majority ofslopes come under partially stable condition, butwith the future frame of reference, it cannot beneglected due to its prominence in the area. Planarfailures are not a dominant failure mode in Beldcondition, but RS7 has shown to be completelyunstable. RS5, RS6, RS9 appears to have a higherpossibility of rock topple (table 3).

4.3 Continuous Slope Mass Rating (CSMR)

Tom�as et al. (2007) noticed that the SMR lack incontinuity when real-time Beld condition was con-cerned. It is because of SMR show a discretebehaviour which cannot signify the transitionalchange in class intervals. To cope up with thislimitation, more precise continuous functions weregenerated. The adjustments were formed asaccordance with failure modes.

F1 ¼16

25� 3

500atan

1

10Aj j � 17ð Þ

� �; ð4Þ

Table 2. Ratings for parameters deBning RMRbasic of studied locations.

Site

location UCS RQD DS DC GW RMRbasic

Rock mass

classes

RS1 4 13 15 14 7 53 Fair

RS2 7 13 15 15 15 65 Good

RS3 7 8 15 16 15 61 Good

RS4 7 8 15 13 15 58 Fair

RS5 7 13 15 17 10 62 Good

RS6 4 13 15 9 10 51 Fair

RS7 4 13 10 10 10 47 Fair

RS8 4 13 10 13 15 55 Fair

RS9 2 17 10 8 15 52 Fair

RS10 4 17 15 14 7 57 Fair

RS11 4 13 10 10 15 52 Fair


F2 ¼9

16þ 1

195atan

17

100B � 5

� �: ð5Þ

For wedge failure and planar failure:

F3 ¼ �30þ 1

3atanC ; ð6Þ

and for toppling failure:

F3 ¼ �13� 1

7atan C � 120ð Þ; ð7Þ

where, for wedge failure, A is the angle formedbetween the intersection of the two discontinuities

(the plunge direction) and the slope dip direction.B corresponds to the discontinuity dip (bj) indegrees, for planar failure and toppling and to theplunge (bi) of the wedge failure intersection line. Itsvalid range is for values of B lower than 45�. Forhigher values, F2 is set to 1. C expresses dip rela-tionship and is equivalent to bj – bs for planarfailure, bi – bs for wedge failure and bj + bs fortoppling failure, where bj is the joint dip angle, andbs is the slope dip angle (equations 4–7).It shows a similar result to that of SMR. Most of

the slopes are of class III which falls under partiallystable condition. RS5–RS11 are most vulnerable as

Table 3. Ratings for parameters deBning SMR of studied locations.

Site

location RMRbasic

Failure

types F1 F2 F3 F4 SMR

SMR

class Stability condition

RS1 53 W 0.70 1 �50 0 18 V Completely unstable

T1 0.70 1 0 0 53 III Partially stable

T2 0.15 1 �25 0 49.25 III Partially stable

RS2 65 W 0.15 1 �50 0 57.5 III Partially stable

T1 0.85 1 0 0 65 II Stable

T2 0.70 1 �6 0 60.8 II Stable

T3 0.15 1 �25 0 61.25 II Stable

RS3 61 W1 0.15 1 �60 0 52 III Partially stable

W2 0.15 1 �60 0 52 III Partially stable

T 0.70 1 0 0 61 II Stable

RS4 58 W 0.15 1 �50 0 50.5 III Partially stable




RS5 62 W1 0.70 1 �50 �8 19 V Completely unstable

W2 0.15 0.15 �60 �8 52.65 III Partially stable

T1 1 1 �25 �8 29 IV Unstable

T2 0.15 1 �25 �8 50.25 III Partially stable

RS6 51 W 0.15 1 �50 �8 35.5 IV Unstable

T 0.15 1 �25 �8 35.25 IV Unstable

RS7 47 W1 0.85 1 �50 0 4.5 V Completely unstable

W2 0.15 0.15 �60 0 45.65 III Partially stable

P 0.85 1 �50 0 4.5 V Completely unstable

RS8 55 W1 0.15 1 �50 0 47.5 III Partially stable

W2 0.40 1 �50 0 35 IV Unstable



RS9 52 W1 0.15 1 �50 �8 36.5 III Partially stable

W2 0.70 1 �60 �8 2 V Completely unstable

T1 0.70 1 0 �8 44 III Partially stable

T2 0.15 1 �25 �8 40.25 IV Unstable

T3 0.15 1 �25 �8 40.25 IV Unstable

RS10 57 W 0.7 1 �50 0 22 IV Unstable

T 0.15 1 �25 0 53.25 III Partially stable

RS11 52 W 0.4 1 �50 0 32 IV Unstable




it shows unstable to partially stable condition.RS1, RS7, RS9 and RS10 are completely unsta-ble slopes for wedge failures. Toppling failuresobserved at RS5, RS6, and RS9 are unstable. RS7is completely unstable against planar failure(table 4).

4.4 Geological Strength Index (GSI)

Mid-1990’s saw an evolution of GSI, which wasdocumented by scholars depending upon thenature of physical parameters aAecting the

geo-engineering character of the rock. Hoek (1994)was the pioneer of introducing GSI to the geologi-cal community. GSI is a very convenient method toget a proper representation of the impression ofrocks’ mechanical behaviour by on-Beld observa-tions (Carter and Marinos 2014). Hard jointed rockto weak schistose and Cysches was accounted withMarinos et al. (2007) characterisation on hardrocks. More complex lithology like Cysches andmolasses to sheared and weak schistose werestudied by a number of researchers (Hoek et al.1998, 2005; Marinos and Hoek 2000, 2001; Marinoset al. 2006).

Table 4. Ratings for parameters deBning CSMR of studied locations.

Site

location RMRbasic

Failure

types F1 F2 F3 F4 CSMR

CSMR

class Stability condition

RS1 53 W 0.71 0.99 �53.86 0 15.14 V Completely unstable

T1 0.71 1 �0.47 0 52.66 III Partially stable


RS2 65 W 0.22 0.99 �56.23 0 52.75 III Partially stable

T1 0.87 1 �0.65 0 64.43 II Stable

T2 0.64 1 �1.76 0 63.87 II Stable

T3 0.16 1 �25.68 0 60.89 II Stable

RS3 61 W1 0.24 0.97 �59.24 0 47.21 III Partially stable

W2 0.18 0.96 �59.38 0 50.74 III Partially stable

T 0.64 1 �0.42 0 60.73 II Stable

RS4 58 W 0.16 0.99 �57.62 0 48.87 III Partially stable




RS5 62 W1 0.57 0.99 �45 �8 28.61 IV Unstable

W2 0.19 0.27 �59.69 �8 50.94 III Partially stable

T1 0.97 1 �25.53 �8 29.23 IV Unstable

T2 0.29 1 �25.58 �8 46.58 III Partially stable

RS6 51 W 0.18 0.99 �57.62 �8 32.73 IV Unstable

T 0.17 1 �25.66 �8 38.63 IV Unstable

RS7 47 W1 0.91 0.99 �53.86 0 �1.52 V Completely unstable

W2 0.15 0.18 �59.74 0 45.39 III Partially stable

P 0.94 0.99 �51.14 0 �0.59 V Completely unstable

RS8 55 W1 0.24 0.99 �57.62 0 41.31 III Partially stable

W2 0.34 0.99 �55.32 0 36.38 IV Unstable



RS9 52 W1 0.19 0.99 �56.85 �8 33.30 IV Unstable

W2 0.67 0.99 �58.10 �8 5.46 V Completely unstable

T1 0.74 1 �0.65 �8 43.51 III Partially stable

T2 0.16 1 �25.67 �8 39.89 IV Unstable

T3 0.19 1 �25.64 �8 39.12 IV Unstable

RS10 57 W 0.74 0.99 �53.86 0 17.54 IV Completely unstable

T 0.18 1 �25.67 0 52.37 III Partially stable

RS11 52 W 0.51 0.99 �55.32 0 24.07 IV Unstable




GSI is a charted representation that providesqualitative information on the strength of geoma-terials using different geological conditions thataAect them. These conditions are depicted on thehorizontal and vertical axes of the Beld observation

chart correlation of which deBnes the strengthscenario of the rock slope. With its association, arange of value is put up describing the actualstrength condition of geomaterials. QuantiBcationof GSI chart was attempted by many researchers in

Figure 4. GSI observations of 11 locations: (a) GSI1999 chart (after Sonmez and Ulusay 1999), (b) GSI1999 chart vs. GSI2017 plot,and (c) GSI2017 vs. RMRbasic plot.

Table 5. Site location details of GSI1999 chart, GSI2017 and RMRbasic.

Site

location SCR SR

GSI1999 chart

(approx. values) GSI2017 RMRbasic

RS1 12 40 45 46.5 55

RS2 13 40 47 49.0 65

RS3 15 31 47 51.5 61

RS4 13 30 44 45.7 58

RS5 15 35 49 52.9 62

RS6 9 47 40 40.9 51

RS7 8 35 36 35.4 47

RS8 11 40 43 44.0 55

RS9 6 50 34 35.0 52

RS10 11 50 46 47.5 57

RS11 8 47 38 38.4 52


the past. Sonmez and Ulusay (1999) came up withaxes variables like structural parameter usingvolumetric joint count (Jv) for rating known asstructural rating (SR) and geological discontinuityparameters such as roughness, inBlling materialsand weathering intensity rated as surface conditionrating (SCR). Other variables involve rock blockvolume (Vb) and joint condition factor (JC) (Caiet al. 2004), correlations with RMR (RMR89) andQ-system (Q) (Hoek and Brown 1997).Morelli (2017) brought a more elaborated and

convenient chart using Hoek et al. (2013)approach. It compiles Bve possible correlations atone place, viz., SCR with Jv#, SCR with RQD anddiscontinuity spacing rating (DS), discontinuitycondition (DC) with DS, SCR with spacing ofdiscontinuity (DS) and SCR with RQD/Jn.Equations developed can be used independentlywith ground reality in the Beld and suggested thatSCR remains the most vital part of Bnding the best

suited GSI value. All possibilities of getting a GSIwas performed, and the best correlation was opteddepending upon how exactly it matches withgeoengineering aspects observed in the Beld(equation 8):

GSI2017 ¼ 2:5� SCRð Þ�8:59� ln Jv#� �

; ð8Þ

where SCR = discontinuity roughness rat-ing + discontinuity inBlling rating + discontinuityweathering rating and Jv# = discontinuity volumecount when Jv is 100 or less.GSI values were observed using GSI1999

chart developed by Sonmez and Ulusay (1999).From Bgure 4(a), it is evident that every site fallsunder blocky/disturbed structure condition. Sitelocations RS9 and RS12 show poor surface condi-tion as the slopes are highly weathered. Site loca-tions RS6, RS7, RS8, RS10, RS11 fall under fairsurface condition where slopes are moderate tohighly weathered in Beld observation. RS1, RS2,

Figure 5. Flow chart representing the RDA stages followed for studied location (modiBed after Nicholson 2004).


and RS4 have slopes in good, whereas RS3 and RS5show very good surface condition (table 5).In order to Bnd the usefulness of the adopted

formulation for getting GSI, two correlations wereestablished. First between GSI2017 equation andvalues achieved by GSI1999 chart (Bgure 4b).Values thus obtained using the chart are theapproximate values. A linear curve Btting givesa near-ideal correlation to derive an equation,that is:

GSI1999 chart ¼ 0:79593�GSI2017 þ 7:41296; ð9Þ

where R2 = 0.97.A second equation was developed between

GSI2017 and RMRbasic that suggest a positive cor-relation via linear Btting from Bgure 4(c), that is:

GSI2017 ¼ 0:97682�RMRbasic�10:18115; ð10Þ

where R2 = 0.74.Equation (9) can enhance the credibility of the

GSI1999 chart as individual empirical values can be

put up for similar rock type and their geologicalstructure condition. Equation (10) can come inhandy in a similar situation.

4.5 Rock slope Deterioration Assessment(RDA)

Nicholson and Hencher (1997) coined the termRDA (rock slope deterioration assessment) thataddresses the deterioration of rock slope undervarying environmental condition. RDA qualiBesfor the present study as it is specifically designed toconsider the non-obvious fractures for classiBca-tion. Chowdhury and Flentje (2002) describedsusceptibility, which can be related to deteriora-tion. The RDA uses the smallest bit of geotechnicaldetails required for establishing GEC. It is theGEC which meticulously addresses the deteriora-tion or simply erosional phenomena aAecting theslopes. Structural orientation related failures arewell understood, but deterioration related dis-lodging was discussed in RDA. The significance of

Table 6. Rock slope susceptibility parameter ratings for RDA1U (modiBed after Nicholson and Hencher 1997).

Discontinuity

spacing

Rating

(max. 35)

Discontinuity

aperture

(mm)

Rating

(max. 15)

Intact rock

strength

(MPa)

Rating

(max. 35) Weathering intensity

Rating

(max. 15)

[2 m 2 Closed-0.1 1 [200 2 Fresh/unweathered (I) 1

600 mm–2 m 8 0.1–0.5 3 100–200 5 Slightly weathered (II) 5

200–600 mm 16 0.5–1.0 7 50–100 10 Moderately weathered

(III)

10

60–200 mm 28 1.0–5.0 13 12.5–50 18 Highly weathered (IV) 14

\60 mm 35 [5.0 15 5–12.5 35 Completely weathered,

Residual soil (V)

15

Table 7. RDA1U rating for studied locations.

Site location

Discontinuity

spacing

rating (DSR)

Discontinuity

aperture

rating (DAR)

Rock strength

rating (RSR)

Weathering

index

rating (WIR) RDA1U

RS1 8 15 18 1 42

RS2 8 15 10 1 34

RS3 8 13 10 1 32

RS4 8 13 10 1 32

RS5 8 15 10 10 43

RS6 8 13 18 10 49

RS7 16 15 18 14 63

RS8 16 13 18 10 57

RS9 16 13 18 14 61

RS10 8 13 18 10 49

RS11 16 15 18 10 59


Table

8.RDA1Awithad

justmen

tfactor

ratingforstudied

location

s.

Site

location

RDA

1U

Adjustmentfactors

(AF)*

RDA

1A=

(RDA

1U+AF)

Aspect,

exposure,

clim

ate

change

(A)

Aspect

(B)

Groundwater

andsurface

runoA

(C)

Static

stress

(D)

Dynamic

stress

(E)

Vegetationcover

(H)

Slope

geometry

(J)

Rock

mass

structure

(K)

Tim

esince

excavation

(L)

Direct

disturbance

(M)

Highly

weathered/

soillike

slope

Rock

cut

slope

RS1

42

11d

21a

21b

12

41b

–53b

�32b

�32b

�11c

11

51

RS2

34

11d

21a

01

12

41b

–53b

�32b

�32b

�11c

31

43

RS3

32

11d

21a

01a

02

41b

–33a

02b

�32b

�11c

11

39

RS4

32

11d

01b

01a

02

41b

–33a

02b

�32b

�11c

11

37

RS5

43

11d

21a

21b

22

41b

–53b

�21c

34

�11c

31

62

RS6

49

11d

41a

21b

22

41b

–23c

32a

�12a

�11c

52

70

RS7

63

11d

21a

21b

22

41b

�21c

–32a

�32b

�11c

31

74

RS8

57

11d

41a

01a

22

41b

–13b

02b

�32b

�11c

31

68

RS9

61

11d

21a

01a

02

41b

�61b

–32a

�32b

�11c

62

67

RS10

49

11d

41a

11a

12

41b

–13b

02b

02b

�11c

11

61

RS11

59

11d

41a

01a

22

41b

�61b

–32a

34

�11c

52

74

*PreBxes

usedforrespectiveadjustmentfactors

are

thesameasin

theabridged

adjustmentfactors

from

Nicholson(2004).


this classiBcation lies in the fact that it pondersaround the critical condition of rock slopes andgenerates a quality based sensitivity analysis. Theclass provides a pathway to a systematic approachtowards hazard mitigation and its maintenancemeasures.RDA comprises of three stages; stage one, i.e.,

RDA1 involves four parameters with their corre-sponding rating, viz., discontinuity spacing rating(DSR), discontinuity aperture rating (DAR), rockstrength rating (RSR) and weathering intensityrating (WIR). A tedious process based on theo-retical Bndings, expected ratings of a value rangedepending on Beld monitoring of slope deteriora-tion with its parameters deBnes the parameter andrating values. Stage two, i.e., RDA2, recognises therelationship between deterioration mechanismsinvolved for speciBc rock type represented bydeterioration characteristics. Stage three, i.e.,RDA3 helps design site-speciBc remedial measuresby clubbing Brst and second stage information(Bgure 5). The unadjusted form of RDA, RDA1U

rating is a summation of rock mass and rockmaterial properties providing deterioration sus-ceptibility measurement of the excavated rockmass (table 6).RDA is a classiBcation which makes it a much-

elaborated system of quantiBcation. The mainmotive of introducing it lies in the fact that dete-rioration plays a significant role in the overallframework of stability analysis in the study region.The eAectiveness of this system is pronounced as itinvolves much smaller but significant factors asadjustments in the collective deterioration assess-ment. Site locations were assessed thoroughly, andutmost care was taken to select the adjustment

factors. Locations RS1–RS11 see a variation ofunadjusted RDA1U from 32 to 63 (table 7). Theratings thus achieved follows the equation(equation 11).

RDA1U ¼ DSRþDARþ RSRþWIR: ð11Þ

The adjustment factors (AF) are based on altitude,exposure and climate conditions, northing oreasting aspects, groundwater and surface runoA,static and dynamic stress, method of excavation,stabilisation and protective measure, vegetationcover, slope geometry, rock mass structure, timesince excavation and direct disturbance.Adjustments were made on visual observation,and thus ratings were chosen appropriately.Certain AF was excluded, viz., method ofexcavation since the slopes were already existing,stabilisation and protective measure as it was notpresent and time since excavation is not known(table 8). It brings to an adjusted RDA, i.e.,RDA1A, which give a numerical value for thedeterioration aAecting the study area.The RDA achieved by Nicholson (2004) was

extensively employed to study different rock types.It was well documented for igneous rock slope asto how susceptibility classes 2 and 3 are looselyinterconnected with basic RMR showing no prac-tical relation. Previously, correlation for the rockslopes having higher susceptibility towards deteri-oration (class 4 and 5) between RMRbasic andRDA1A were not discussed. However, the presentstudy addresses this issue and signiBes the deteri-orating eAect on rock mass due to differentialweathering. It was noted that environmentaldeterioration has a significant impact on theoverall rating of RDA1. The site locationsRS1–RS2 and RS3–RS4 were classiBed underclasses 2 and 3, respectively, whereas RS5–RS11were classiBed under class 4. It leads to a goodcorrelation between RDA1A vs. RMRbasic (Bgure 6;equation 12).

RDA1A ¼ �0:91086� RMRbasic þ 116:92623:

ð12Þ

The RDA1A class with Beld investigation helpedin working out for RDA2. The primary rock massfound were majorly blocky both in regular andirregular shapes as deBned for class 2 and 3.RS5–RS11, which are class 4 rock slopes, also formregular and irregular blocks with the consequenceof weathering over it. Primarily, deteriorationwould be fall (block, rock, and stone) with

Figure 6. Comparative relation between RDA1A vs. RMRbasic

of selective locations.


Table 9. Results of RDA stages for studied locations (after Nicholson 2004).

Site

location RDA1A

RDA2 RDA3

RDA

1A Class

Deterioration

susceptibility

Primary

rock

mass

Mechanism of

deterioration

Characteristics of

deterioration

Predicted remedial

measures

RS1 51 Irregular

blocky

Rockfall, stone

ravelling

Deterioration incurred

by fracture represents

variable size falls

Fracture sealing,

dentition of cavities, use

of bolting or meshing,

warning signs

3 Moderate

RS2 43 Irregular

blocky

Rockfall, stone

ravelling

Deterioration incurred

by fracture represents

variable size falls

Fracture sealing,

dentition of cavities, use

of bolting or meshing,

warning signs

3 Moderate

RS3 39 Regular

blocky

Blockfall,

rockfall,

Rock mass properties

induced failure,

deterioration

represents frequent falls

Scaling of loose blocks,

ditch design, Cexible

barrier

2 Low

RS4 37 Regular

blocky

Blockfall,

rockfall,


induced failure,

deterioration

represents frequent falls

Scaling of loose blocks,

ditch design, Cexible

barrier

2 Low

RS5 62 Regular

blocky*

Blockfall,

rockfall, stone

ravelling*


induced failure,

deterioration

represents frequent

falls*

Fracture sealing,

underpinning of

overhangs, shotcrete,

warning signs�

4 High

RS6 70 Regular

blocky*

Blockfall,

rockfall, stone

ravelling*


induced failure,

deterioration

represents frequent

falls*

Fracture sealing,

underpinning of


warning signs�

4 High

RS7 74 Irregular

blocky*

Wash erosion,

blockfall,

rockfall, stone

ravelling*


induced failure,

deterioration

represents frequent

falls*

Fracture sealing,

underpinning of

overhangs, shotcrete

warning signs�

4 High

RS8 65 Regular

blocky*

Blockfall,

rockfall, stone

ravelling*


induced failure,

deterioration

represents frequent

falls*

Fracture sealing,

underpinning of


warning signs�

4 High

RS9 67 Irregular

blocky*

Wash erosion,

blockfall,

stone

ravelling*


induced failure,

deterioration

represents frequent

falls*

Fracture sealing,

underpinning of


warning signs�

4 High

RS10 61 Regular

blocky*

Blockfall,

rockfall, stone

ravelling*


induced failure,

deterioration

represents frequent

falls*

Fracture sealing,

underpinning of


warning signs�

4 High

RS11 74 Irregular

blocky*

Wash erosion,

blockfall,

stone

ravelling*


induced failure,

deterioration

represents frequent

falls*

Fracture sealing,

underpinning of


warning signs�

4 High

*Outcomes of RDA2 from present study described for high deterioration rock slopes.

�Outcomes of RDA3 using the information of RDA1 and RDA2.


ravelling. RDA3 describes the remedial measurethat can be applied eDcaciously in the Beld underconsideration. Studied locations can be stabilisedby underpinning, fracture sealing, construction ofCexible barriers, designing of ditches in RS1–RS4,scaling of overhanging blocks with shotcreting incase of the weathered face in case of RS5–RS11(table 9). Research investigation shows primarilyregular blocky rock mass with the mostappropriate mechanism of deterioration beingblockfall/rockfall for RS5, RS6, RS8 and RS10with possible wash erosion at RS7, RS9 and RS11.

5. Conclusions

The significance of this paper relies on the resultsgenerated by the classiBcation studies done onbasaltic rock slopes. The deterioration processleads to the progressive degradation of rock massstrength, which increases the differential instabil-ity condition. Kinematic analysis proves to becrucial in achieving failure modes and most sus-ceptible zones eDciently. It serves to be a crucialtool to access the probability of failure. It is notsuitable for random fracture in a rock mass. Thuscritical joint orientations are only evaluated. Thedetected block failures that are governed byunfavourable joint orientations match with thefailure type seen in the Beld condition. Charac-terisation performed shows that RMRbasic repre-sents fair to good class rock slopes that underminethe actual Beld scenario. This result is taken up forSMR and CSMR calculation. SMR, the discreteform, shows that RS1–RS4 is partially stable to-wards wedge type failure and minor chances oftoppling failure may occur. RS5–RS11 are unsta-ble to completely unstable for wedge failure andneeds immediate action. CSMR, the continuousform, gives a Brm agreement to the results ofSMR. These are summed up with GSI observa-tions using Morelli (2017) chart over Somnez andUllusay (1999) chart. A generalised equation isformulated, achieving a good correlation betweenGSI2017 and GSI1999 and serves to be an alterna-tive for RMRbasic values in the Beld. The presentresearch elaborates and suggests some new classesin RDA classiBcation for igneous rock slopes thatare susceptible to high deterioration. A good cor-relation is obtained between RDA and RMRbasic,which implies the impact of weathering as themost inCuential factor in slope instability. SlopeRS1–RS4 conBrms to be regular blocky as it falls

in class 2 and class 3, since these slopes are gov-erned by structure driven failure and observedmechanism may occur as rockfall or blockfall.Thus, bolting or drape meshing with fracturesealing can be considered as eAective remedialmeasure along this busy highway. RS5–RS11 arehigh deterioration slopes for which probable rockmass and deterioration mechanisms are introducedin this paper. The proposed RDA classes forigneous rock slopes were made based on essentialRDA parameters that were well veriBed in theBeld conditions. Primary rock masses have beenobserved from the study are blocky (regular andirregular). RS7, RS9 and RS11 are highly weath-ered rock slopes and thus are the most vulnerablefor which shotcreting and underpinning of over-hangs is necessary to stop weathering fromapproaching its interior parts.Based on the current study, it can be suggested

that RDA performs well for high deteriorationigneous rock masses as compared to other geo-engineering systems. In the preliminary stage ofslope stability analysis, it provides valuable infor-mation regarding weathering related impactincorporated with detailed kinematic analysis.

Acknowledgements

The authors express their sincere gratitude to IndianInstitute of Technology (Indian School of Mines)Dhanbad, for the grant and providing Bnancial sup-port. The authors are also thankful to Rock Scienceand Rock Engineering (RSRE) Laboratory, IndianInstitute of Technology Bombay, for allowing toperform experiments and geotechnical tests.

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Corresponding editor: N V CHALAPATHI RAO


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