EVALUATION OF SLOPE STABILITY ANALYSIS USING CONVENTIONAL METHODS: A CASE STUDY

Abstract

The irregularity and unforeseeable climate conditions are adding to the complexity to predict natural hazards. The hilly terrains are already having very complex geological and tectonic settings. Further, enhanced human encroachment and changing climatic conditions magnified the vulnerability of mountain slopes, such as the development of road connectivity in hilly regions, Dam, Tunnels and hydropower projects for electricity, etc. The passengers and residents in the hilly region are frequently affected by landslides. The landslides are increased either in monsoon or in the summer season due to the water percolating along with the weathered rock masses. Due to water percolation, rock masses get saturated, increases load, and reduces friction angle between the rock masses. All the drainages are flowing in the monsoon season and the capacity of drainage for block movement is very high at this time. Very big boulders were driven during runoff and few of them reaches on the road section resulting the damages of road bench, and sometimes causes causalities.

Full text

ISEG - Indian Naonal Group of IAEG Proceedings of Internaonal Conference EGCON - 2021
9th to 11th December 2021 (Virtual Mode)
452
EVALUATION OF SLOPE STABILITY ANALYSIS USING
CONVENTIONAL METHODS: A CASE STUDY
Rahul Kumar Verma, ; Singh, Rajesh; Sharma, Prateek; Mishra, Pranshu.
Rock Science and Rock Engineering Laboratory, Department of Geology, University of
Lucknow, Lucknow - 226 007, India
E-mail: getrahul9@gmail.com, georajeshsingh@lkouinv.ac.in,
prateeksharmarishi@gmail.com, mishrapranshu152@gmail.com
ABSTRACT
The irregularity and unforeseeable climate conditions are adding to the complexity to predict
natural hazards. The hilly terrains are already having very complex geological and tectonic
settings. Further, enhanced human encroachment and changing climatic conditions magnied
the vulnerability of mountain slopes, such as the development of road connectivity in hilly
regions, Dam, Tunnels and hydropower projects for electricity, etc. The passengers and residents
in the hilly region are frequently affected by landslides. The landslides are increased either
in monsoon or in the summer season due to the water percolating along with the weathered
rock masses. Due to water percolation, rock masses get saturated, increases load, and reduces
friction angle between the rock masses. All the drainages are owing in the monsoon season
and the capacity of drainage for block movement is very high at this time. Very big boulders
were driven during runoff and few of them reaches on the road section resulting the damages
of road bench, and sometimes causes causalities.
There is various method to analyse the slope instability such as Numerical, Conventional and
Chart method. Numerical methods have a broad application but these are very expensive and
high-end computation systems required. However, the conventional methods are cheap, but
the same requires geological knowledge about the site only. The present study is based on
the conventional methods along the Bhowali-Ramgarh-Mukteshwar road from Bhowali to
Mukteshwar in the district of Nainital, Uttarakhand, India. Slope stability analysis for this
road section is essential for the mitigation of landslides. Conventional methods are applying
for types of failure and classication of the slope examine using slope mass rating (SMR). In
addition, the chart method is used to calculate the factor of safety (FoS) for only wedge type
of failure.
Keywords: slope stability, conventional method, RMR, SMR and factor of safety (FoS).
Introduction
Landslides are a natural disaster that affects at least 15% of the land of India (Sharda, 2008).
Various kind of landslides frequently occur in Himalayan region of India due to natural and
anthropogenic engagements. Natural causes that trigger landslides such as seismic waves,
neotectonics activities and high intensity rainfall however it affects widely due to anthropogenic
involvements such as road widening, construction of bridges, dams, tunnels etc. In the month
of Oct. 2021, ash ood had been triggered landslides in Uttarakhand and casualties were
also happened mainly in Nainital district. Major problem in the hilly terrain, communication
and transportation with other places are entirely dependent on the road networks. Due to ash
ood and landslides activities, the Nainital city declared zero zone (cut off) for ve days from

ISEG - Indian Naonal Group of IAEG Proceedings of Internaonal Conference EGCON - 2021
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453
the other places. The present work is based on conventional methods to estimate the slope
stability conditions for three selected spots. The study area of this work is lying between N
29° 25’ 00’’, E 79° 34’ 00’’ and N 29° 27’ 00’’, E 79° 37’ 00’’ in district of Nainital along the
Bhowali- Ramgarh-Mukteshwar road and selected spots for study showing in the study map
(Fig.1). Surface of the failed wedges were easily recognisable in eld (gure 3 (b)). The eld
observation shows that the persistence of joints sets is very low. The spacing, aperture and
roughness of the discontinuities are very close to wide spacing, very tight to open aperture and
smooth to rough condition respectively. No lling observed in the discontinuities and no water
presented in discontinuities in pre-monsoon period. At the study area, the rock mass is slightly
weathered for the selected locations. The friction angle of the discontinuities was measured
using Tilt-meter in the laboratory
A
B
C
Fig.1: Showing study locations, Fig. 2: Estimation of factor A and B using Chart (A) and
stereographic projection (B &C).
Methodology
1. Kinematic Analysis (Markland Test, 1972)
Kinematic analysis is basically based on the geometrical relationship of the discontinuities
present in slope mass. The data required for the analysis can be measured and estimated easily
from eld excursion. This is basically a geometric relationship of three parameters such as (i)
Slope angle and direction (ii) dip amount and direction of discontinuities (iii) friction angle
along gentler discontinuity (Umrao et al. 2011; Umrao et al. 2015).
2. Limit Equilibrium Method (LEM)
The LEM is a traditional method of analysing slope stability that is used to estimate Factor
of Safety (FoS). LEM is based on method of slices. In this method, the material of rock mass
above the slip surface is divided into different vertical slices. The stability of the material above
the slip surface is analysed by considering the static equilibrium of the individual slices and
the entire equilibrium of the failing slope. The static equilibrium of slices can be achieved by
either removing or considering the interslice forces and the moment equilibrium of the slices.
Such methods determine the FoS along the critical slip surface as the ratio between the shear

ISEG - Indian Naonal Group of IAEG Proceedings of Internaonal Conference EGCON - 2021
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454
strength and the shear stress acting on the surface. The simplied Bishop’s method based on
method of slices has been used in present work for calculating the FoS at selected locations.
These methods work on failure is assumed to occur with the rotational mass of rock/soil on a
circular slip surface centred on a common point (Fig. 4). On the side of the slice assumption
is that the horizontal force acted i.e., there are no shear stresses between slides (Bishop 1955).
The total normal force is assumed to applied at centre of each slice base and derived by forces
summed in vertical direction. Bishop (1955) showed that the more accurate method is the
simplied Bishop method. Thus, LEM of the slopes in this work has been performed using
Bishop’s simplied method using Hoek and Brown failure criteria as implemented by Slide v.6
software (Slide 2010).
3. Chart Method
The chart method has been used for calculating the FoS of the wedge type failure. Hoek and
Bray (1977) has explained the chart method in detail and illustrated the ready to use charts.
Two basic key parameters affect the potential wedge movement are friction angle and plunge
amount of line of intersection of wedge forming joints.
The Hoek and Bray (1977) derived an equation to calculate the FoS and this equation is
simplied for dry slopes and discontinuity planes non-cohesive. The simplied equation is as
follows:
Friction angle (ф) measured by tilt meter instrument self-designed but similar to Alejano et al.
(2018).
4. RMR (Bieniawski, 1989) and SMR (Romana,1985)
The rock mass rating (RMR) and slope mass rating (SMR) are legible standard rating system in
which values can be used either to design the support system or comparing the slope characterizes.
The sum of rating of the rst ve parameters; intact rock strength (UCS or PLI), rock quality
designation (RQD), discontinuity spacing, conditions and ground water condition is termed as
Basic RMR and the sixth parameter discontinuity orientation is differently evaluated in each
case such as slope, dam and tunnel etc. (Bieniawski, 1989; Umrao et al. 2015).
SMR rating system was proposed by Romana (1985). The parameters F1, F2, F3 and F4 can
be evaluated by Eq. 2.
F1 depends on the parallelism of the strike of joints and slope, F2 incorporates inclination of
joints, F3 incorporates the daylight condition (Difference in dip amount of joint and slope)
and F4 refers the adjustment factor for the method of excavation and its value was xed for
different methods. These values are calculated based on table proposed by Romana (1985).
Result and Discussions
Rock mass was jointed at all locations and three set of joints were noticed with intersecting
at different angles during eld excursion. Figure: (3) showing eld photographs of all three
locations. Kinematic analysis examines mode of failure, whether it is wedge, planer or toppling
failure. Using stereographic projections, it was found that single wedge is formed within failure
criteria at location MK05 and double wedge in MK06 (g.2 (B&C)). Based on RMR and
Kinematic analysis, the SMR of each location was calculated. Calculated SMR values were
indicating two locations (MK05 and MK06) out of three were found in unstable condition due

ISEG - Indian Naonal Group of IAEG Proceedings of Internaonal Conference EGCON - 2021
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455
to planer failure and wedge failure (table 2). In addition to SMR, Chart method was used to
calculate the Factor of safety for wedge type of failure. Chart method for wedge of the slopes
MK05 and MK06 is showing critical unstable to unstable conditions (g. 2 (A) and table 3).
In addition to Kinematic and Chart methods, each slope was evaluated using traditional LEM
analysis (Slide v.6 software). The Factor of safety at all three slope locations were analysed
based on generalized Hoek and Brown criteria (Fig.4). The colour variation of the slip surface
indicates the FoS along the slip. It is showing in the gure 4, the minimum slip surface with
lowest FoS in the analysis. The FoS value at MK05 was 1.153, indicated that the slope is in
a critical state and it is unstable condition nearby. The FoS values at MK06 and MK07 were
1.758 and 1.662 respectively, indicating stable slope condition. In Slope MK06, Slope is higher
and steeper, which may be in critical condition due to higher load of rock mass.
J2
J3
J1
c
Fig. 3: Field photographs showing discontinuities present in slope mass (MK05-MK07)
Scale length 60cm.
Fig.4: Slopes signifying slip surfaces, safety factor contours and minimum safety factor
for cut slope models for all three locations (MK05-MK06).

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Table 1: Field measured parameters and calculated Rock Mass Rating (RMR) at
different Locations
Table 3: Factor of safety at various locations using chart method
Location Joints Wedge Under
Investigation
Stability
MK05 J1 35/15
J2 82/121
J3 67/174
Wedge (J2&J3)
FS = 0.432
Unstable Condition
MK06 J1 34/15
J2 73/310
J3 66/197
Wedge (J1&J2)
FS= 1.1458
Critical or Unstable
Wedge (J2&J3)
FS= 0.7652
Unstable Condition
Conclusions
Three methods were used to evaluate stability of rock slopes. The outcome of various methods
helps to estimate the stability condition more accurately. In the present study, Basic RMR
is varying from 62 to 72 where as SMR is varying 36 to 40 indicating the 60% percent of
probability of slope failure. The Probability was again rechecked with FoS calculated using
chart method and found that both Slope MK05 and MK06 have 100% probability of Slope
failure. But mode of failure will be wedge types i.e. slope will fail in form of Rock fall. The
silent outcome of this study that even LEM FoS of Slope MK06 showing stable condition, the
Chart method and kinematic method is indicating the rockfall problem at site. This outcome
was also veried from eld observation.

ISEG - Indian Naonal Group of IAEG Proceedings of Internaonal Conference EGCON - 2021
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Acknowledgements: The authors are thankful to Head, Department of Geology for valuable
suggestions and encouragements. This research is supported by DST NRDMS under the Grant
No. NRDMS/02/64/017.
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