Nilgiris

V B MajiDept. of Civil Engineering

IIT Madras, Chennai, India

Rain Induced Landslides in the NilgirisDistrict of Tamil Nadu, India

S S Chandrasekaran and V SenthilkumarVIT Vellore, India

• Dept. of Science of Technology, Govt. of India,

NRDMS

• District officials, Nilgiris

Acknowledgements

2

Landslide risk zones - India

The NilgirisModerately high hazard

NDMA, GOI

Year Location No. of people

killed

1948 Assam 500

1954 Himalayas 30

1956 Himalayas 27

1958 Himalayas 52

1968 Bihar, W.Bengal 1000

1978 Northeastern 64

1979 Lahaul,Pangi 230

1979 U.P 42

1980 U.P 150

1986 U.P,H.P 88

1988 U.P, Haryana, H.P,

Punjab

255

1989 Himalayas 41

1989 North India 45

1990 Sikkim 30

1990 Nilgiris, Tamil Nadu 36

1992 Aizawal 60

1993 Bombay 20

1993 Nilgiris,Tamil Nadu 40

1995 Mizoram 40

Year Location No. of people

killed

1996 Kulla,H.P 400

1997 Darjeeling Hills 23

1997 Gangtok 28

1998 H.P 26

1998 Assam 48

1998 Malpa Village,U.P 239

1998 Masuna Village, U.P 37

2000 Moradabdad 43

2000 Ghatkopar 58

2001 Rudraprayag, Uttranchal 27

2001 Chamba District, H.P 16

2001 Amboori Village, near

Trivandrum

38

2004 Joshimath-Badrinath 17

2004 Tehri Dam 9

2005 Assam 12

2009 Nilgiris 45

2013 Uttarakhand, Himachal,

(Flood & Landslide)

5750

2014 Malin, Pune,

Maharashtra

200

Some Major Landslides in India

Scene of the landslides in Nilgiris (Nov. 9-11, 2009) district of Tamil Nadu. killed 43 people and left nearly 100 others wounded. (Xinhua/STR) , Chandrasekaran et al. (2013)

• Nilgiris is a part of Western Ghats and one of the oldest mountain rangeslocated at the tri-junction of Tamilnadu, Kerala, and Karnataka states ofIndia

• The District is basically a hilly region, lying at an elevation of 1000 to 2600meters above Mean Sea Level (MSL)

• It’s latitudinal and longitudinal location is (Lat 11̊ 12’N to 11̊ 37’N) & (76̊30’E to 76 ̊ 55’E)

• The District has an area of 2,552 km2

• Nilgiris is an important tourist center in southern India

• Nilgiris Mountain Railway (NMR) line declared as World Heritage site byUNESCO

Nilgiris

6

7

Location - Nilgiris

8

Nilgiris Mountain Railway (NMR)

Nilgiris Mountain Railway (NMR) line declared as World Heritage site by UNESCO

World Heritage Nilgiris Mountain Railway gets suspended due to frequent landslides specially during monsoon

• Landforms of Nilgiris region have been classified into two types namelyDodabetta landform and Ootacamund landform

• Dodabetta landform has many high peaks with steep slope and rockescarpments with or without soil cover, whereas the Ootacamund landformhas gentle topography with a thick soil formation.

• The rock formation of study area is mainly of charnockite andgranetiferous quartzo-felspathic gneisses belonging to Archaeanmetamorphic rocks

• The rock formation is overlain by lateritic soil. The soil is yellowish toreddish brown in colour formed due to intense physical and chemicalweathering.

• The overburden thickness of soil varies from less than a meter to 32meters

Geology and Geomorphology

10

11

Geological map of Nilgiris district

• Since the Nilgiris district is located in the tropical zone, it receives

rainfall during both southwest and northeast monsoons

Southwest - (June to September)

Northeast - (October to December)

• The average annual rainfall of this district is about 1700 mm

• Generally, entire Coonoor, Kothagiri taluks and part of

Udagamandalam taluk receive rainfall from northeast monsoon and

other parts of the district receive rainfall from southwest monsoon

Niligiris: Rainfall

12

13

S.No Location Event date Landslide

type

Cause Remarks

1 Marappalam 11-11-1993 Debris

flow

Heavy

rainfall

Eleven people were killed. Three zigs of road network

(NH-67) and a rail network damaged for about 300 m.

Two busses with passengers have been buried in the

debris.

2 Metuppalam-

Coonoor road

network (NH-67)

11-11-1998 Rock fall Heavy

rainfall

A massive rock weighing about 20m tonnes fell on the

road network and traffic has been closed for two days.

The rock has been blasted and removed from the road

(Ganapathy et al. 2010).

3 Pudukadu Dec 2001 Debris

flow

Heavy

rainfall

Deries flow damaged two bridgegs at pudukadu along

Mettupalayam – Coonoor road network and resulted in

Complete clousere of traffic (Ganapathy et al. 2010).

4 Silver bridge 13-11-2006 Debris

flow

Heavy

rainfall

A bridge located along Mattupalayam- Coonoor stretch

has been completely washed away. Transport has been

cut off for about two weeks.

5 Burliyar 13-11-2006 Debris

flow

Heavy

rainfall

Debries flow damaged the road network and disturbed

the traffic for a week and damaged one check post

located at Burliyar

6 Kallar 13-11-2006 Debris

flow

Heavy

rainfall

A hundred year old fruit farm located at kallar has been

completely collapsed.

7 Burliyar 13-11-2006 Rock fall Heavy

rainfall

Under heavy rainfall a massive rock fell on road at

burliar near hairpin bend number two. The same

phenomena has been observed in many places along

Mettupalayam - Coonoor rail route and closed the traffic

for a week

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8 Hillgrove 10-11-2009 Debris flow Heavy rainfall Railway track near Hillgrove station dislocated and

washed away. Rail service has been stopped for about

two weeks

9 Kurumbadi 10-11-2009 Debris flow Heavy rainfall Landslide damaged the resort located at kurumbadi and

killed one security guard of that resort.

10 Lovedale 10-11-2009 Earth slide

(Rotational)

Heavy rainfall

and loading at

slope crest

Earth slide removed the mass of the soil surrounded the

foundation of a local shop. About half portion of shop

left hanging at the slope crest.

11 Manjur 10-11-2009 Earth flow Heavy rainfall

and improper

cutting of slope

for tea plantation

Improper planning and cutting of slope for tea plantation

led to landslide under heavy rainfall. Houses were buried

into the debris and the residents of those houses were

killed due to landslide.

12 Kothagiri 10-11-2009 Earth slide

(Translational)

Heavy rainfall

and loading at

slope crest

The building foundation exposed due to slope failure

under heavy rainfall and vertical cut at the toe and

loading at the slope crest.

13 Marappalam 10-11-2009 Debris flow Heavy rainfall A portion of road and rail network at marappalam has

been damaged and completely washed away. Transport

has been cut off for about a month.

14 Achanakkal 10-11-2009 Debris

avalanche

Heavy rainfall

and Blocking of

drainage above

slope crest

Due to blocking of drainage above the slope crest and

accumulation of water at slope crest led landslide. Seven

people were killed due to collapse of a house during the

landslide.

15 Madithorai 10-11-2009 Earth slide Heavy rainfall Slide occurred at Madithorai along Kothagiri - Ooty road

network. Half of the road has been slided and transport

has been cut off for about two weeks.

15

Marappalam - debris flow Silver bridge - debris flow Burliar - debris flow

Lovedale- earth slide Manjur – earth flow Burliyar – Rock fall

Marappalam- Debris flow Achanakkal - debris avalanche Madithorai - Earth slide

Factors Causing Landslide

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GROUND CONDITIONS

GEOMORPHOLOGICAL PROCESSES

PHYSICAL PROCESSES

MAN-MADE CAUSES

Landslide causing factor - Rainfall

Water and slope failures

Water influences the stability of slopes in many ways-decreasing

suction, positive pore water pressure and seepage forces reduce

the shear strength of soil.

It is often impossible to isolate one effect of water and identify it

as a single cause of failure (Duncan and Wright, 2005).

Sowers (1979) stated: "In most cases, several "causes" exist

simultaneously; therefore, attempting to decide which one finally

produced failure is not only difficult but also technically

incorrect”.

Sowers (1979) “Often the final factor is nothing more than a

trigger that sets a body of earth in motion that was already on the

verge of failure. Calling the final factor the cause is like calling

the match that lit the fuse that detonated the dynamite that

destroyed the building the cause of the disaster”.

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Excavation of slope at toe

Unplanned vertical cut

Blocking of drainage systemsLoading the slope at crest

Deforestation

The Marappalm location has experienced landslides repeatedly in the

past.

Marappalam 1993 landslide

Debris flow damaged three zigs of road network (NH-67) and a rail

network for about 300 m.

Two busses with passengers have been buried in the debris.

The debris flow also damages several houses and a mosque located

along the run out area (Gupta et al. 2003)

Marappalam 2009 landslide

Damaged the Coonoor - Mettupalayam National Highway (NH-67)

a life line of Nilgiris and NMR rail road for about 100 m

The road and rail traffic was suspended for more than a month

About Marappalam landslides (1993 & 2009)

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20Google earth view of Marappalam 1993 & 2009 landslides

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View of Damaged road, rail network and retaining wall at 2009

Marappalam landslide location

Marappalam 2009 landslide

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Contour map of Marappalam 2009 landslide

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Longitudinal

topographic

profile of

Marappalam

slope

Slope angle - 25 ᵒ to 30ᵒ

Debris volume - 172125 m3

SPT N value and Vs (MASW)

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Descriptions BH location I BH location II BH location III BH location IV

Clay % 6 7 18.3 7

Silt % 37.2 43.6 60.5 39.2

Sand % 54.8 46.6 11.3 50.1

LL % 48 37 40.6 42

Ip% 15.1 12.4 12.4 13.8

USCS soil classification SM (Silty Sand) MI (Sandy silt) MI (Sandy silt) SM (Silty Sand)

Specific gravity 2.65 2.69 2.67 2.64

Moisture content (%) 17.1 18.6 19.4 16.6

Hydraulic conductivity m/s 3.85 x 10-6 4.66 x 10-7 5.00 x 10-7 4.70 x 10-6

Properties of soil

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Repeated direct shear test apparatus

Sample consolidation

0

5

10

15

20

25

30

35

40

45

50

0 5 10 15

Sh

ear

stre

ss (

kP

a)

Shear displacement (mm)

Cycle 1

Cycle 5

Typical shear stress vs shear displacement curve for

50 kPa normal stress

Repeated direct shear test

Properties Marappalam location

Peak friction angle (degree) 27

Residual friction angle (degree) 22

Peak cohesion (kN/m2) 13

Residual cohesion (kN/m2) 6

Steady state shear resistance or

residual shear strength under

50 kPa normal stress (kN/m2)

35

The following tests were conducted on collected rock

samples

Uniaxial compressive strength test (UCS)

Triaxial compression test

Point load index test (irregular samples)

Tensile strength test (Brazilian tension test)

Test on rock core samples

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Properties BH-2 BH-3 BH-4

Density (kg/m3) 2670 2910 3050

UCS (MPa) 105 133 106

Tensile strength (MPa) 5.4 11.1 9.50

Young's modulus E (MPa) 3102 9739 3775

Poisson’s ratio 0.25 0.25 0.25

Shear modulus G (MPa) 1241 3896 1511

Shear wave velocity (m/s) 681 1156 703

Friction angle φ (degree) 55

Cohesion c (MPa) 5

Properties of rock

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Borehole

Number

Depth Vs from lab

test (m/s)

Vs from field

MASW test

(m/s)

BH II 2.50 m – 6.00 m 681 624

BH III 16.00 m – 18.00 m 1156 1064

BH IV 18.00 m – 17.50 m 703 641

Comparison of Vs from Laboratory

and Field test results

29

• Based on the SPT-N value and shear wave velocity value (Vs)

of subsurface material, identified the subsurface details,

density of the subsurface material increases with depth.

• The results of laboratory investigations revealed that the soil is

Sandy silt (MI) and Silty sand (SM) type which consist low

permeability and intermediate plasticity with moisture content

ranges from 16% to 20%

• The field and laboratory investigations revealed that, with low

permeability and intermediate plasticity was the main source

of subsurface materials involved in debris flow

Observations – field investigation

30

Weathering and its influence• The six-grade weathering classification of Little 1969 has been used to

create weathering profile for Marappalam slope

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weathering

classification (Little

1969)

Six grade weathering profile of

Marappalam slope

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0.00 – 3.00 m 3.00 – 6.00 m

6.00 – 9.00 m 9.00 – 15.00 m

Microscopic image of subsoil at different depth

Highly porous structure with

Kaolinite clay matrix

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Energy dispersive X-ray spectroscopy (EDX) spectrum

X-ray diffraction pattern

Kaolinite clay mineral

[Al2Si2O5(OH)4]

Landslides, 2017 , 14: 1803-1814.

• The weathering characteristic of Marappalam slope consist all six grades(Grade VI to I) starting from a completely weathered residual soil slope toslightly weathered rock followed by fresh rock

• The SEM images and Energy dispersive X-ray spectroscopy (EDX)spectrum and X-ray diffraction pattern revealed that the upper layer ofMarappalam slope from 0.00 to 3.00 m depth are covered with Kaoliniteclay mineral [Al2Si2O5(OH)4]

• The iron concentration (Fe) present in the soil formation is the main reasonfor red in soil colour. The presence of iron concentration makes the soilmaterial harder during dry periods due to cementing action between ironand aluminium oxide that increase the matric suction thus improve stabilityof slopes during dry periods

• Whereas, during the monsoon period the clay minerals formed byweathering can lead to reduction in matric suction and effective shearstrength of the soil. Under the influence of pore water which could be thedominant reason for debris-flow type instabilities in the residual soils

Weathering and its influence

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• Vibrating wire piezometer (EPP-30V) has been installed in boreholes which are drilled at

Marappalam 2009 landslide location to measure the pore water pressure in soil

• The capacity range of the vibrating wire piezometer is 0.5 MPa. Five piezometers are

installed at three different boreholes

• Two piezometers installed at 7.50m and 12.00m depth in borehole I and 8.00m and 11.00m

depth in borehole III. One piezometer has been installed at 9.00m depth in borehole IV

• The readout unit (Edi-51 V) is connected with piezometer through 4 core cable and the pore

pressure is being monitored using the readout unit

Monitoring of pore water pressure

35Installation of piezometers

36

S.No Event Date

Average

Daily Rainfall (on the

event date)

Average 5-days

antecedent rainfall

1 15-11-92 116 409

2 16-11-92 25 510

3 25-11-92 0 305

4 11-11-93 369 215

5 11-11-94 36 303

6 17-12-96 68 300

7 16-10-99 157 37

8 23-11-99 115 63

9 24-11-00 205 47

10 16-11-01 112 84

11 17-11-01 67 196

12 27-12-01 156 184

13 14-11-06 60 82

14 11-11-09 285 477

15 20-10-14 46 170

Rainfall events and average daily rainfall and average 5AD

• The rainfall threshold is represented by an empirical equation

The threshold reveals that, either very high magnitude of daily rainfall or very

high amount of five-day antecedent rainfall or combination of both is required

to trigger landslides.

• When the daily rainfall crosses 225mm there is a possibility of landslide occurrence

even when there is no antecedent rainfall.

• When R5ad exceeds 110 mm, even a continuous normal rainfall is capable of

triggering a landslide

Rainfall threshold

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RT = 225 – 2.04 R5AD

ASCE, International Journal of. Geomechanics, 2018, 18(9): 05018006

Effective stress contours (a) End of first rainfall event (b) End of second rainfall event

FLAC analysis

Failure surface

Maximum shear strain rate with plasticity indicator (a) at the end of first rainfallevent (b) at the end of second rainfall event

International Journal of Geomechanics, ASCE (2018) 8:9.

Landslide simulation analysis

• LS-RAPID, an Integrated model simulating the initiation and motion ofearthquake & rain induced rapid landslides

• This simulation model has been developed to asses the initiation and motion oflandslide triggered by earthquakes, rainfalls or the combined effects ofrainfalls and earthquake

• All limit equilibrium stability analysis methods (Fellenius, Bishop, Janbu,Spenser) assume that the whole sliding surface will fail at once

• However, in large scale landslides, weak zones or areas subjected by higherpore pressure will firstly fail, and the failure area will expand around the initialfailure zone, then finally a whole landslide mass will start to move (Sassa et al.2010)

• This simulation model can reproduce this “Progressive failure phenomenon”

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Landslide simulation analysis

• The fundamental theory behind the LS-RAPID programme is, at the initial

stage soil mass remains stable under the peak friction coefficient (tan φp)

• Failure will occur due to rainfall that develops excess pore water pressure

within the soil mass

• Development of excess pore pressure will lead to shear strength reduction

from peak to residual state (or) steady state in progress with shear

displacement

• When the shear strength reaches to residual or steady state, there is no

further strength reduction but shear displacement will proceed under a

constant shear resistance (Sassa et al. 2010; Igwe et al. 2014)

41

• The topographic input data were created using Digital Elevation Model

(DEM) with 2.50 m spatial resolution from Cartosat-I satellite data (NRSC,

ISRO, Hyderabad)

Topography input data

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Delineation of unstable mass

43

44Landslides, 2017 , 14: 1803-1814.

• The initiation of landslide occurred till 3.00 sec and the motion of the landslide starts afterinitiation at 3.00 s with velocity of 4.5 m/s at which strength reduction started occurring inprogress with shear displacement (DU=6mm)

• At 10.40 s (v = 21.3 m/s), just a small amount of the earth material from the source wasmoved towards the valley due to progressive failure at which initial failure zone extendsfurther due to reduction in shear resistance

• The slide attained peak velocity of 68.3 m/s at 20.50s and remaining materials movingtowards the valley. At 31.9 s (velocity = 34.00 m/s) almost all the materials at the scarp(Source area) had reached near the valley which shows that there is no further strengthreduction and only the deformation takes place with constant shear resistance.

• At 42.2 s (velocity = 13.8 m/s) virtually every unstable earth material formed at the headscarp had reached the valley. Finally, at 66.7 s (velocity = 0 m/s) the movement graduallycame to a halt

• The pore pressure generation ratio ru = 0.3 with 95% saturation in the source area cause rapidlandslide and considered as a critical value to trigger landslide in Marappalam

• The travel distance, distribution area and debris volume of landslide observed from thesimulation analysis are found to be similar as observed from the topographical survey

Numerical simulation discussion

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• The field, laboratory and numerical landslide simulation analysis elaboratesthe cause and failure mechanism of Marappalam 2009 landslide in detail

• Since the landslide occurred at the beginning of monsoon season, it can benoted that the short duration but intense rainfall caused landslide atMarappalam

• Generally in residual soil slope at tropical region, matric suction is theprinciple force keeping the slope stable during dry period (Rahardjo et al.2001; Tasi and Chen 2010; Tasi and Wang 2011)

• When the rainfall infiltration saturates the slope thus decreases the matricsuction and leads to development of positive pore water pressure which inturn reduce the shear resistance of the soil resulted in a progressive failureas observed in the Marappalam 2009 landslide.

• Study gives a clear idea on subsoil profile of Marappalam slope which consistof information on nature and sequence of various subsoil material (soil androck) that might be useful to perform slope stability analysis

• The rainfall threshold identified from this study can be used in landslide earlywarning systems precisely for Marappalam location.

Summary

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