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Landslide stability assessment along Panchkula–Morni road, Nahan salient,

NW Himalaya, India

Article  in  Journal of Earth System Science · August 2019

DOI: 10.1007/s12040-019-1181-y

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 J. Earth Syst. Sci. (2019)128:148 
c Indian Academy of Sciences
https://doi.org/10.1007/s12040-019-1181-y

Landslide stability assessment along Panchkula–Morni

road, Nahan salient, NW Himalaya, India

Jaspreet Singh1,2 and Mahesh Thakur1 , *

1
Centre of Advanced Study in Geology, Panjab University, Chandigarh 160 014, India.
2 Department of Earth Sciences, Indian Institute of Technology, Roorkee, Uttarakhand 247 667, India.
*Corresponding author. e-mail: mahesh09@gmail.com

MS received 19 September 2018; revised 21 February 2019; accepted 26 February 2019

We report the slope stability analysis of three vulnerable sites (S1, S2 and S3) within the lower Siwalik
along the Panchkula–Morni road section in the Nahan salient, north-western Himalaya. Kinematic
analysis of joint data was conducted to understand the different modes of failure. Rock mass classification
techniques like rock mass rating, slope mass rating (SMR) and continuous SMR were used for stability
classification and the factor of safety was calculated using stability charts. At site S1, the instability is
controlled by the orientation of the discontinuity joint J1 which is parallel to the bedding and at site
S2, the slope fails due to the wedge. The Umri landslide site S3 is the product of a damage zone by
the normal faults which intersect at joint J3; a wedge is formed which falls in the critical zone. The
damage zone in the Umri landslide greatly affects the porosity and permeability of the rockmass and
acts as a pathway for the percolation of water during rainfall which reduces effective stress. The slope
failures are tectonically controlled results due to the high slope angles, structural discontinuities like
joints and faults and structural damage zones associated with the faults.
Keywords. Umri landslide; joint; RMR; SMR; CSMR; kinematic analysis; factor of safety and tectonics.

1. Introduction building the highest mountain range in the world

and continuously transforming in terms of the geo-
Landslides are natural disasters that affect the hilly morphology and tectonics of the region. Tourists,
areas and are mainly triggered by earthquakes, pilgrims and local people are the most affected
rainfall and other structural failures under the by the landslides that pose a great risk to life
influence of gravity. Himalaya is very prone to nat- and property (Bhambri et al. 2017; Kumar et al.
ural disasters like earthquakes, extreme rainfall and 2017a, b; Siddique et al. 2017). The development
landslides (Paul et al. 2000; Bali et al. 2009; Ray work in the Himalaya like roads, rail network and
et al. 2009; Kothyari et al. 2012; Bhambri et al. infrastructure projects have enhanced the land-
2017; Kumar et al. 2017a, b; Sah et al. 2018). The slide in many ways by affecting the natural flow
ongoing convergence along the Indian–Eurasian of water, increasing the load, erosion, weathering
plate resulting in a 3–28 mm/yr movement along and changing the slope angle of natural slopes
the main thrust zones of the different segments of (Umrao et al. 2011). Moreover, the failure in strat-
the Himalayas (Larson et al. 1999; Avouac 2003; ified rocks is dominantly controlled by the dip and
Bettinelli et al. 2006; Jade et al. 2017) has been the mechanical properties of the bedding planes
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148 Page 2 of 15 J. Earth Syst. Sci. (2019)128:148

and results in planar, circular or non-circular road (figures 1 and 2). Detailed geological and
failures depending upon the conditions (Bhambri geotechnical investigation of the slopes was carried
et al. 2017; Kumar et al. 2017a, b; Singh et al. out for stability assessment. Different rock mass
2017). The presence of faults, folds, shear zones and slope mass characterisation techniques like
and other tectonic structures are commonly asso- rock mass rating (RMR), SMR and CSMR were
ciated with rock slope failures (Agliardi et al. 2001; implemented for stability classification along with
Jackson 2002; Ambrosi and Crosta 2006). Lee and the kinematic analysis. Stability along all three
Nguyen (2005) found that the distance from the slopes is structurally controlled due to the different
tectonic fracture largely controls the instability joint sets and, particularly, at site S3, the normal
of the slope. Folded or inclined bedding provides fault within the slope favours the kinematic release
the surface for failure to occur while faults facil- of the rock blocks.
itate the development of lateral or rear-release
surfaces (Brideau and Stead 2009). The intersec-
2. Geology and tectonics of the area
tion of faults results in the zone of low rock quality
designation (RQD) value due to the intense joint-
Morni is one of the famous tourist hill stations in
ing or other type of fractures (Brideau and Stead
Panchkula district, Haryana. The study area lies in
2009). The rock mass damage associated with the
the Panchkula district of Haryana, India (figure 1)
faults greatly affects the porosity and permeability
where the rocks belong to the lower Siwalik forma-
of the rock mass (Shipton and Cowie 2001, 2003;
tion of the Siwalik group in the Himalayan foreland
Bergbauer and Pollard 2004).
basin (Kumar and Tandon 1985; Thakur et al.
Stead and Wolter (2015) mentioned investigating
2010) (figure 2). The Siwaliks are formed by the
slope stability, to first consider the influence of
accumulation of molasses deposits in the Himalaya
major structures such as faults, shear zones and
foreland basin and are late deformed by the tec-
persistence of bedding planes and then incorpo-
tonic events (Lave and Avouac 2000; Kothyari et al.
rated the mechanics of joints. The slope instabil-
2010; Jayangondaperumal et al. 2018). The lower
ity is often governed by fault planes because it
Siwalik rocks are dominantly composed of sand-
acts as a sliding or release surface and is also
stone and are mainly medium to fine-grained and
associated with the steepening of the beds due
these sandstones are hard and compactly interbed-
to drag-folding over the fault plane (Stead and
ded with clay and mudstones (Krishnan 2009).
Wolter 2015). Furthermore, the increase in seismic
The exposed sandstones of the lower Siwalik along
stress due to earthquakes overcomes the strength
the Panchkula–Morni road cut slope contain multi-
of the underlying rocks or soil causing failure in
ple joint sets. The stratigraphy of the Siwalik group
slopes (Newmark 1965; Meunier et al. 2007). Slope
is shown in table 1. In the south of the study area,
mass rating (SMR) is a commonly used parame-
Nahan thrust (NT) marks the boundary between
ter to understand the slopes (Pradhan et al. 2011).
the lower Siwalik and upper Siwalik rocks and
Sarkar et al. (2016) used continuous SMR (CSMR)
on the west, an active strike-slip fault (Kumar
and kinematic analysis techniques for a detailed
and Tandon 1985; Arora and Malik 2017). Fur-
investigation of slopes along National Highway-22,
ther north, it is bounded by the Eocene age marine
Himachal Pradesh. The Himalayan rocks contain
shales of the Subathu formation. The Medlicott
several sets of discontinuity planes and the non-
Wadia thrust (MWT) marks the boundary between
scientific slope cuts the blocks of different sizes
Subathu and the lower Siwalik rocks (Kumar and
formed due to multiple sets of discontinuities which
Tandon 1985; Thakur et al. 2010) as shown in
are highly vulnerable to sliding or falling. The
figure 2.
study area is close to Nahan thrust, where the
rocks have suffered deformation and damage due to
repeated earthquake activity in its vicinity (Nakata 3. Neotectonic movements in the area
1989). The changing slope face direction along
the road naturally, largely influences the stabil- The thrust fault between the lower Tertiary and the
ity along the different joint sets, so it is of prime Siwaliks was designated the main boundary fault
importance to investigate vulnerable slopes. Dur- (MBF) (Medlicott 1864; Pilgrim and West 1928;
ing a field investigation, three unstable slopes Auden 1934) and Thakur et al. (2010) assigned a
at sites S1, S2 and S3 located in the lower new name to the MBF as the MWT. The MWT
Siwalik were found along the Panchkula–Morni is an important fault of regional dimension and is
J. Earth Syst. Sci. (2019)128:148 Page 3 of 15 148

Figure 1. Location map: image shows the location of Haryana and a Hillshade view of the study area in the Morni region.

Figure 2. Geological map of the study area. Three unstable slopes (S1, S2 and S3) are marked in white colour on the map.

active in some regions between the MBT and the which formed a part of the Nahan thrust system.
HFT along its strike length of ∼700 km (Thakur A strike slip fault was first reported by Kumar
et al. 2010). Nakata (1989) demonstrated the active and Tandon (1985) in the Khetpurali area, based
faulting along the Nahan thrust, showing a 250 on an offset of geological formation in this area.
m right-lateral displacement of the Koshallia river The active signatures of the Ketpurali–Taksal fault
channel. Malik and Nakata (2003) identified the were like a sag pond, an offset of streams and many
extension of this Nahan fault trending along the more features as reported by Arora and Malik
NNW–SSE and spanning a distance of about 20 (2017). Compressional movement along the Nahan
km. They named this active fault, the Taksal fault, thrust at the Kalawar gallery with a slip vector
148 Page 4 of 15 J. Earth Syst. Sci. (2019)128:148

Table 1. Stratigraphic succession of the Siwaliks (Krishnan 2009).

Units Lithology
Upper Siwaliks Course boulder conglomerates with red and orange clay.
Sandstone, clay and conglomerate
Middle Siwaliks Massive sandstone with minor conglomerate.
Medium-coarse grained sandstone, locally thick
conglomerates
Lower Siwaliks Fine-medium grained sandstone, calcareous cement and
maroon, chocolate clay.
Red claystone with intercalations of medium to fine-grained
sandstone

has a magnitude of 10 mm/yr and is orientated 25◦ to 34◦ and the slope dip towards the north. At
in a direction 132◦ E (Sinvhal et al. 1973). Evi- site S2, the south dipping slope has a high slope
dence of palaeoearthquakes is shown by Malik and angle of 44−73◦ along the Nahan thrust which is a
Mathew (2005) from trench investigations across contact between the Nahan formation sandstone
the active Pinjore Garden fault in Pinjore Dun. and the upper Siwalik (figure 3). A high slope
Two events of reactivation of the Nalagarh thrust angle in the south facing slope along the Nahan
in the Quaternary west of the study area were thrust is due to the comparatively resistant and
reported by Philip et al. (2014). The trenches indurated sandstones of the lower Siwalik rocks
excavated across HFT for palaeoseismological stud- overriding the upper Siwalik sequence of mudstone
ies between Chandigarh and Kala Amb, Himachal and clays. This is the prime reason for the high
Pradesh ruptured and generated two major earth- slope angle and is one of the main reasons of
quakes (M ∼ 7 or 8) during the last 2000 yr (Malik the resulting instability of the slope in the region.
et al. 2003; Kumar et al. 2006). The study area is The slope angle at site S3 (Umri landslide) lies
tectonically active, bounded by active faults, and between 25◦ and 50◦ and it dips towards the
a local normal fault is also found at site S3 (Umri north. The slope angle is low at site S3, but the
landslide) as shown in figure 2. slope is unstable pointing towards the fact that
this may result due to unfavourable structural
discontinuities.
4. GIS-based observations and
Aspect is another important parameter; it
interpretations
defines the dip direction of the slope. The south-
facing slopes are humid and lack vegetation,
4.1 Slope and other aspects of the study area
whereas the north-facing slopes experience more
Topography features such as elevation and slope orographic rainfall and are relatively cool and have
angle, etc. largely influence the geomorphology of high moisture content (Chauhana et al. 2010).
the region, which, in turn, limits the extent of land- The aspect map is prepared from the DEM data
slides both temporally and spatially (Chauhana and divided into nine aspect categories (Mathew
et al. 2010). The Cartosat digital elevation model et al. 2005) listed as (a) north, (b) north-east,
(DEM) data (30 m) is downloaded from the (c) east, (d) south-east, (e) south, (f) south-west,
Bhuwan website (www.bhuvan.nrsc.gov.in) and is (g) west, (h) north-west and (i) flat. At sites S1
used to analyse the slope and other aspects of the and S3, the slope is north-facing which may contain
study area using ArcGIS 10.3 software. more moisture content which further reduces the
A high slope angle may cause an instability effective stress due to the increase in pore pressure
in the rock mass as it affects the drainage of and may destabilise the slope (figure 4).
the area. In many areas due to steeper slopes,
the bed’s daylights can result in the failure of the 5. Geotechnical aspects for landslide
slope; therefore, it is important to understand the investigation
spatial distribution of the slope angle, which is
prepared from the DEM data. The slope angle in 5.1 Field investigation
the study area varies from 0◦ to 70◦ as shown in Traverse mapping was carried out to collect the
figure 3. At site S1, the slope angle varies from dip-strike data (100 data points) along the
J. Earth Syst. Sci. (2019)128:148 Page 5 of 15 148

Figure 3. Slope angle of the study area using Bhuvan, Carto DEM. The slope angle varies from 0 to more than 70◦ .

Figure 4. Aspect map of the study area which shows the dip direction of the slopes with respect to the north.

Panchkula–Morni road cut slope covering all three clay and mud rocks as shown in figure 5(B). The
unstable slope sites S1, S2 and S3. The slope at houses near the site S3 show cracks in the roof and
site S1 is unstable in the NE direction (figures 5A walls due to the movement of basement rocks near
and 9A), whereas the slope at site S2 is unstable in the Umri landslide. Four joint sets are identified
the SW direction opposite to S1. The lithology at in the rock mass at site S3. At the crown of the
sites S1 and S2 which are located on the opposite landslide, a tensional crack of thickness 60 cm is
side of the slope (water divide) mainly consists of observed due to the circular failure at site S3 as
the lower Siwalik sandstone unit. At site S3 (Umri shown in figure 5(C). At the Umri landslide, two
landslide), sandstone beds are interbedded with the normal faults (figure 5D and E) are identified with
148 Page 6 of 15 J. Earth Syst. Sci. (2019)128:148

Figure 5. Field photographs: (A) At site S1, unstable road cut slope showing planar failure dominantly composed of
sandstone (location figure 1) (B) S3 (Umri landslide) near Umri village, the section is exposed due to landslide show-
ing interbedded sandstone and clay (location figure 1). (C) Tension crack with a thickness of 60 cm due to failure at site S3.
(D) Two faults identified in the field on the road cut slope at site S3. (E) Normal faults with hanging wall moving down
relative to the footwall at site S3.

a fault plane dipping at an angle of 55−330◦ . The (54.7 mm in diameter) and should be drilled with a
main body of the Umri landslide slope is still unsta- double-tube core barrel. The RQD is calculated by
ble and damages the road as shown in figure 6(A the indirect method of volumetric joints count (Jv)
and B). Deranged forest and traverse ridges are developed by Palmstrom (1982). Jv is described as
formed due to the piling up of debris down the a measure for the number of joints within a unit
slope of the landslide (site S3) as shown in the volume of rock mass defined by
figure 6(B). The dip-strike data collected during
the field study plotted on Stereonet indicates that  1
the regional trend of the lower Siwalik sandstone Jv = , i = 1, . . . , j,
Si
is due N40◦ E as shown in table 2 and figure 7. A
systematic 3D block model for site S3 (Umri land-
slide) is constructed which shows different joint sets where Si is the average joint spacing in metres for
and the normal fault found in the rock mass and the ith joint set and j is the total number of joints.
Stereonet depicts the orientation of joint sets and For site S1, S2 and S3 using joint spacing, Jv and
the fault at site S3 (figure 8). RQD are calculated as listed in table 3.

5.2 Rock quality designation 5.3 Rock mass classification

RQD is the most commonly used method to Rock mass classification systems are used for
characterise the degree of jointing in borehole various engineering design and stability analyses.
cores. It was developed by Deere (1963) to provide The system is developed to understand the field
a quantitative estimate of rock mass quality from conditions and provide ratings to determine the
drill core logs. It is defined as ‘the percentage of rock mass, to pre-determine excavations and other
intact core pieces longer than 100 mm in the total processes required for engineering purposes (Aksoy
length of core’. The core should be at least NX size 2008). The objective of rock mass classification is to
J. Earth Syst. Sci. (2019)128:148 Page 7 of 15 148

Figure 7. Stereograph with four regional joint sets marked

as J1, J2, J3 and J4. J1 is parallel to the bedding plane and
its high frequency is represented by poles countoured in red
colour.

industrial research, and since its development, it

has undergone several improvements (Bieniawski
1974, 1975, 1976). RMR is widely used in engi-
Figure 6. Field photographs of the Umri landslide in neering purposes like tunnelling, slope stability and
Haryana, India. The crown area is still unstable, causing dams (Siddique et al. 2017). RMR is calculated on
serious damage to road. the basis of six parameters (uniaxial compressive
strength (UCS), RQD, joint or discontinuity spac-
ing (DS), joint condition, ground water condition
Table 2. The average regional trend data of all
joints calculated using Stereonet (figure 8) along
and joint orientation). The rating is given to all
the Panchkula–Morni road. parameters depending upon their values and all
ratings are added to get the RMR of the rock mass.
Dip Dip
On the basis of the RMR values for a given struc-
direction amount
ture, the rock mass is sorted into five classes: very
Joints (deg) (deg)
good (RMR = 100−81), good (80–61), fair (60–
J1 (bedding) 39 38 41), poor (40–21) and very poor (RMR < 20).
J2 306 74 The RMR values for the slope faces at sites S1, S2
J3 240 58 and S3 are shown in table 5.
J4 159 69
The UCS of the intact rock (sandstone) was
tested in the Geotechnical Laboratory, Depart-
ment of Geology, Panjab University, Chandigarh by
understand the intrinsic properties of the rock mass extracting NX-type cores (approximately 54 mm
and how other external factors affect its behaviour diameter) as per the specifications given by the
(Milne et al. 1998). International Society of Rock Mechanics (ISRM
1978). The UCS of the lower Siwalik sandstone
5.3.1 Rock mass rating tested in the laboratory and has an average value
of 37.6 MPa as shown in table 4 and the RMR of
The RMR system was first developed by Bieni- the sites S1, S2 and S3 are calculated as shown in
awski (1973) at the South Africa scientific and table 5 which varies from 31 to 55.
148 Page 8 of 15 J. Earth Syst. Sci. (2019)128:148

Figure 8. Conceptual three-dimensional-block model of the Umri landslide showing the presence of the discontinuity sets
and the tectonic structure that influence the stability of the slope. Stereonet depicts the orientation of the fault and different
joint sets at site S3 (not to scale).

Table 3. RQD of the slopes and volumetric joint count (Jv).

Site J1 (m) J2 (m) J3 (m) J4 (m) Jv RQD (%)

S1 0.36 0.33 0.57 0.55 9.38 84
S2 0.36 0.33 0.57 0.55 9.38 84
S3 0.70 0.70 0.45 0.50 7.07 91

Table 4. Uniaxial compressive strength of tool to understand the stability in rocky slopes
the intact rock samples collected from the (Siddique et al. 2017). SMR is calculated from
Umri landslide (site S3). Bieniawski’s RMR by subtracting adjustment fac-
Sample UCS (MPa) tors of the joint–slope relationship and adding a
factor depending on the method of excavation:
1 36
2 39
3 38 SMR = RMRbasics + (F 1 · F 2 · F 3) + F 4,
Mean 37.6

where RMRbasics is calculated according to Bieni-

awski (1993) by adding the rating of only five
5.3.2 Slope mass rating parameters (UCS, RQD, joint or DS, joint con-
dition, ground water condition) and (F 1, F 2, F 3)
Romana (1985, 1991, 1993, 1995) proposed a are adjustment factors related to joint orientation
classification by considering joint orientation with with respect to the slope and F 4 is the correc-
respect to slope orientation to understand rock tion factor for the method of excavation which
stability known as SMR. SMR is a widely used includes natural slope and cut slope by different
J. Earth Syst. Sci. (2019)128:148 Page 9 of 15 148

Table 5. RMR of sites S1, S2 and S3.

DC
UCS RQD Description
Location (MPa) % DS P S R I W GW RMRB AOD RMR and class
S1 4 17 5 2 1 3 6 3 15 56 −25 31 Poor (IV)
S2 4 17 5 2 1 3 6 3 15 56 −25 31 Poor (IV)
S3 4 20 10 2 1 3 2 3 15 60 −5 55 Fair (III)
UCS: uniaxial compressive strength; RQD: rock quality designation; DS: discontinuity spacing; DC: discontinuity condition;
P : persistence; S: separation; R: roughness; I: infilling; W : weathering; GW: ground water; AOD: adjustment factor for the
orientation of discontinuity; RMRB = rock mass rating basic.

Table 6. SMR for sites S1, S2 and S3 calculated using the CSMR method.
Slope
site RMRB F1 F2 F3 F4 SMR Class Grade Failure
S1 56 0.94 0.87 −59.04 0 7.37 V Very bad Planar
S2 56 0.91 0.92 −59.27 15 21.17 IV Bad Wedge
S3 60 0.17 0.83 −58.94 15 66.26 II Good Wedge

methods. By using the above parameters, SMR is C is bj − bs for planar failure, bi − bs for wedge
calculated for different types of failure (Siddique failure and bj − bs for toppling failure.
et al. 2017).The SMR values for the slope faces at Note: The arctan values should be in degrees
sites S1, S2 and S3 (Umri landslide) are shown in and as is the dip direction of slope, aj is the dip
table 6. direction of the joint, bs is the dip amount of the
slope, bj is the dip amount of the joint, ai is the
5.3.3 CSMR plunge direction of the line formed by the intersec-
tion of two joints and bi is the plunge of line formed
In 1985, Romana proposed the SMR and suggested by the intersection of two discontinuities.
the ratings for F 1, F 2, F 3 and F 4 which are dis-
crete and depend mostly on the judgement of the
investigator; this may result in the deviation of the 5.4 Kinematic analysis
results of SMR values. Later, Tomas and Seron
(2007) introduced a new continuous function to Kinematic analysis is a method used to anal-
get more accurate values and it helps in delineat- yse the potential for the various modes of rock
ing the boundary values in pre-existing intervals. slope failures (plane, wedge, toppling failures), that
Equations given by Tomas and Seron (2007) are as occur due to the presence of unfavourably ori-
follows: ented discontinuities and it is also termed as the
  ‘geometry of motion’. Various failures like pla-
16 3 1 nar, wedge and toppling can be judged using
F1 = − arctan |A| − 17 ,
25 500 10 the parameters such as orientation of discontinu-
  ities or joints and internal friction angle of the
9 1 17
F2 = + arctan B−5 , rock.
16 195 100
1
F 3 = −30 + arctan C,
3 5.4.1 Planar failure
1
F 3 (t) = arctan (C − 120) ,
7 The plane on which sliding occurs must strike
parallel or nearly parallel (within approximately
where A is |aj − as | for planar failure, |aj − as − 180| ± 20◦ ) to the slope face. The sliding plane must
for toppling failure and |ai − as | for wedge failure. daylight in the slope face. The dip of the sliding
B is bj for planar failure and bi for wedge failure plane must be greater than the angle of friction and
and F 2 remains 1 for the toppling mode of failure the upper end of the sliding surface either intersects
and F 3(t) is for toppling. or terminates into a tensional crack.
148 Page 10 of 15 J. Earth Syst. Sci. (2019)128:148

5.4.2 Wedge failure quality (permeability coefficient, K > 10−3 cm/s)

(Pandit et al. 2016). The flowing water gives rise
The plunge of the line of intersection of discontinu- to seepage forces and water pressure which in
ities must be flatter than the dip of the slope face turn reduce the frictional effect and decreases the
and steeper than the average friction angle of the instability of the soil:
two slide planes.
Uniformity coefficient (Cu)
5.4.3 Direct toppling
= D60 /D10 = 0.73/0.071 = 10.28,
Direct toppling occurs when in strong rock individ- Coefficient of gradation (Cc)
ual columns are formed by a set of discontinuities
2
dipping steeply into the face. A second set of widely = D30 /D60 ∗ D10
spaced joints defines the column height (Hudson = 0.14 ∗ 0.14/0.73 ∗ 0.071 = 0.38.
and Harrison 1997).
RMR can be used to estimate the cohesion and
internal friction angle of the rock mass for sites S1,
S2 and S3 (Bieniawski 1993). The friction angle of 6. Factor of safety (FS)
rock mass and dip of the slope face used in the
kinematic analysis is shown in table 7. FS is the most common method of slope design, and
it is widely applicable to many types of geological
5.5 Sieve analysis conditions, for both rock and soil. FS is the ratio of
resisting and driving forces acting on the slope. For
Sieve analysis is the technique to differentiate the FS > 1, the slope is stable; if FS < 1, the slope is
grains on the basis of their size. It was used to unstable. A rapid check of the stability of a wedge
separate silt, clay and sand in the soil sample of can be made if the slope is drained and there is
slope S3 (Umri landslide). First, by weight, 100 g zero cohesion on both slide planes A and B. If the
of the dry soil sample was taken for sieve analysis. sliding surface is clean and contains no infilling,
After sieving, the independent weight of the soil then the cohesion is likely to be zero. Under these
in each sieve was measured to compare the rela- conditions, the FS of the wedge failure (site S2) can
tive amount of silt, clay/silt and sand in the soil. be calculated using the equation given by (Hoek
The sieve analysis data were plotted with size vs. et al. 1973)
percent finer by weight to prepare the particle size
distribution curve (figure 12). The uniformity coef- F = A tan φ + B tan φ.
ficient (Cu) and coefficient of gradation (Cc) were
calculated. If Cc is between 1 and 3 the soil is well The dimensionless factors A and B depend upon
graded, but if Cc < 1 soil is poorly graded. Poorly the dip amount and dip directions of the two planes
graded is further classified into gap-graded and uni- which join to form a wedge. The values of these
formly graded depending upon grain sizes. Also, it factors can be estimated from the wedge stability
is clearly depicted from the particle size distribu- charts for friction only. φ is the friction angle.
tion curve that the soil at site S3 is gap-graded The value of A and B estimated from the
because some intermediate sizes are missing. The friction-only stability charts by Hudson and Har-
gap-graded soils generally have excellent drainage rison (1997) and using values given in table 8
is 1.1:

Table 7. Friction angle (Bieniawski 1993) and F S = 2 ∗ 1.1tan 30 = 1.2.

dip of slope face (DEM; http:// bhuvan.nrsc.gov.
in).
Table 8. Wedge stability analysis for friction only (site S2).
Angle of Dip of slope
Slope friction face Dip Dip direction Friction angle
site (deg) (deg) (deg) (deg) (deg)
S1 30 60 Plane A 70 300 30
S2 30 60 Plane B 70 160 30
S3 35 50 Differences 0 140
J. Earth Syst. Sci. (2019)128:148 Page 11 of 15 148

Therefore, the FS is 1.2 which reveals that the slope the shaded region fulfilling the condition of planar
at site S2 is critically stable. failure. The intersection of joint J1 with joints J2
and J3 forms two wedges with the trend of line
of intersection being at N15◦ E and N80◦ E, respec-
7. Results and discussion tively, as shown in figure 9(C). Blocks formed due
to these wedges will slide along the joint J1 in the
The kinematic analysis and geomechanical clas- NE direction. The line of intersection of joints J2
sifications of the rock mass are very important and J3 is dipping into the slope which forms the
tools for the investigation of vulnerable slopes. The edge of block and the joint J1 acts as a surface
friction angle of rock mass used in kinematic anal- for direct toppling in the NE direction as shown
ysis is estimated from the RMR classification of in figure 9(D). The RMR value at site S1 is 31
rock mass (Bieniawski 1993), depending upon their (table 5), representing the poor quality of the rock
RMR values. The RMR rating is calculated using mass. The SMR (planar) value is 7 at site S1
the method given by Bieniawski (1993) and the which lies in class V (table 6) and the slope face is
SMR rating is calculated by the method given by classified as completely unstable. Hence, using
Romana (1985) and Tomas and Seron (2007). kinematic analysis and SMR, the slope face at
The kinematic analysis of site S1 shows three site S1 is unstable. Therefore, the instability along
modes of failure: planar, wedge and direct toppling the road cut slope at site S1 is dangerous for road
(figure 9). In figure 9(B), a pole to joint J1 lies in transport.

Figure 9. Kinematic analysis of the slope at site S1; S = slope face; c = cone of friction; φ = angle of friction; P1 = pole
to joint J1; L1 = line of the intersection of joint J2 and J3; L2 = line of the intersection of joints J3 and J4.
148 Page 12 of 15 J. Earth Syst. Sci. (2019)128:148

Furthermore, the kinematic analysis of site S2 earthquake vibrations as this region is highly active
(figure 10) clearly shows the formation of a wedge and pore water pressure due to rainfall can eas-
due to the intersection of the two joints J2 and ily destabilise the slope. Due to the Nahan thrust,
J3 and the intersection lies in the shaded region. the zone is highly fractured and damaged, the rock
The RMR value is 31 for site S2 (table 5) which mass loses its stability because of the daylighting
represents the poor quality of the rock mass and of the structural features and rainfall. Therefore,
the SMR (wedge) value is 21 which lies in class IV failure at sites S1 and S2 is mainly controlled by
(table 6). Hence, the slope at site S2 is classified the joint sets and the slope angle.
as unstable. The FS calculated using friction-only The kinematic analysis of site S3 (figure 11)
charts is 1.2 which defines the slope as critically sta- shows the mode of wedge failure, the wedge due
ble. A little increase in the disturbing factors like to the intersection of the fault plane and joint J3
dipping out of the slope. The wedges formed due to
the intersection of the fault plane and J1 (parallel
to bedding) and the intersection of joints J1 and
J2 also lie near the critical zone. The RMR value
is 55 (table 5) for site S3 which represents the fair
quality of the rock mass and the SMR (wedge) of
joints is 66 for site S3 which lies in class II (table 6).
Hence, the slope is classified as a stable slope with
failure in some blocks. This points towards the fact
that the rock mass instability at site S3 is gov-
erned by other factors. First, the wedge formed by
the fault plane and the joint J1 as shown in kine-
matic analysis (figure 11). The normal faulting in
the slope decreases the quality of the rock mass
and the damage caused favours the instability. In
addition, the fault plane also acts as a pathway for
the flow of water because of high persistence than
joints. The analysis of the soil samples as shown in
figure 12 from site S3 (Umri landslide) shows that
the soil is dominantly composed of unconsolidated
sand. The cohesion of unconsolidated sand mate-
Figure 10. Kinematic analysis of the slope at site S2; rial is generally taken as zero, so the only resisting
φ = angle of friction; S = slope face. force left is friction. The percolation of the water

Figure 11. Kinematic analysis of the slope at site S3; φ = angle of friction; S = slope face.
J. Earth Syst. Sci. (2019)128:148 Page 13 of 15 148

Acknowledgements

The authors express their sincere acknowledge-

ments to the Chairperson, Centre of Advanced
Study in Geology, Panjab University, Chandigarh
for providing the necessary laboratory and other
departmental facilities. They also like to thank Mr
Neeraj Kumar, Mr Gurvinder Singh, Mr Shivam
Chawla who helped with obtaining field data, and
Dr Girish Chander Kothyari, Institute of Seis-
mological Research, Gandhinagar for the valuable
Figure 12. Grain size distribution curve for the soil sample suggestions for improving the manuscript.
collected from site S3 (Umri landslide).

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Corresponding editor: N V Chalapathi Rao

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