a study on landslides and subsidence … of international academic research for multidisciplinary...

JOURNAL OF INTERNATIONAL ACADEMIC RESEARCH FOR MULTIDISCIPLINARY Impact Factor 1.625, ISSN: 2320-5083, Volume 3, Issue 6, July 2015

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A STUDY ON LANDSLIDES AND SUBSIDENCE ALONG NATIONAL HIGHWAY 31A, DARJEELING DISTRICT, WEST BENGAL

SANDIPAN GANGULY*

PALASH PATRA** AVIJIT GURAY***

*Guest Lecturer, Dept. of Geography, Suri Vidyasagar College, West Bengal, India **Assistant Teacher in Geography, Ghoga Junior High School, West Bengal, India ***Assistant Teacher in Geography, Muktarpur High School, West Bengal, India

ABSTRACT

The traffic flow on NH 31A, connecting Sevoke of West Bengal with Gangtok of

Sikkim through Kalimpong is frequently hampered due to occurrence of landslides and

subsidence, especially during the rainy season, triggered by faulty constructional activities.

Proper strategies should be adapted to prevent landslides and subsidence in this tectonically

vulnerable zone, a part of Darjeeling-Sikkim Himalaya. Keeping tune with this fact, the paper

attempts to study the landslide phenomena on a specific portion of NH 31A (Sevoke-

Kalimpong section), running parallel to the right bank of river Tista. The paper takes into

account the causes of landslide and description of two landslide and one subsidence zone in

the study area. The paper also deals with some suggestive techniques for the prevention of

landslides. The study was undertaken in the pre-monsoon period of year 2012 with the help

of primary survey.

KEYWORDS: Drainage Density, Landslide, Relative Relief, Slope Failure, Subsidence.

INTRODUCTION

For nearly last three decades, parts of Darjeeling Himalayan ranges have suffered

mass destruction due to initiation and aggravation of large scale landslide causing extensive

danger to communication lines, land and forest, human habitation etc. The problems have

been aggravated since early sixties when human activities progressed at a faster pace.

Consequent to development of a network of roads, new habitable areas were etched out,

cutting through mountain ranges, by resorting to deforestation Thus parts of the slopes after

road cutting and deforestation became unstable and susceptible to slope failure, where left

unprotected. The undercutting of slopes due to heavy monsoon precipitation, deforestation,

unscientific cultivation and construction activities, are known to combine with rather

complex and immature geological formation in the region to generate variety of landslide. A

number of small landslides (in their initial stage of development) have turned into major


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landslide mainly because of causal active factors and the mechanism could not be evaluated

scientifically for design and execution of appropriate control measures and whatever remedial

measures undertaken in the past, have proved to be either partly effective or a total failure.

Objectives:

The objective of the study was to study the characteristics and magnitude of landslide

in such an area with steep slope, high rainfall and precipitous material and to access the

impact of human activity on landslide initiation. The study also attempts to deal with some

effective suggestive measures for the prevention of the landslides.

Methodology:

The methodology of the study can be systematically divided into the following phases:

Pre-field: This includes mainly the library works regarding the information of geology,

physiography, climate, soil and hydrology of the study area and the study of relevant

literature.

Field: It includes the survey with the help of abney’s level, theodolite, clinometers etc to

determine the geomorphological characteristics of the study area.

Post-field: This includes mainly the assimilation and interpretation of the different evidences

which have been collected in the course of pre-field and field study.

The Study Area:

The study area is the Sevoke-Kalimpong section of NH 31A, which takes off from

Sevoke bridge and runs up to Gangtok which has been affected by number of major

landslides. The affected road runs on the right bank of Tista river. The entire hill slope has

been rendered unstable due to steeping of slope by toe erosion by the Tista river. During

monsoon season, the road is totally blocked by the downward movement of loose eroded

materials from the upper part of those slides which continues to be a major problem even

today.


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Fig. 1: Location Map of the Study Area

Results and Discussions:

Physical setting of the study area:

Geology: The Darjeeling Himalaya is separated from the for deep of the Ganga-Brahmaputra

plain by the active tectonic lines (Main Boundary Fault and Himalayan Font Tectonic line) in


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the zone of the subduction of the Indian block. It is built of a sequence of overthrusts pushed

southward and dipping to the north. Coming from the north, there are medium resistant

Darjeeling Gneisses, the Daling series of metamorphic rocks of varying resistance from

phylities to quartzities and the Damuda shales and sandstone interbedded with coal seams.

Table 1: Geological Succession of Darjeeling Himalaya

Age Series Lithological Character

Recent

(Holocene)

Pleistocene

Sub-areal formation (Soil

Alluvia, Colluvia), Raised

terraces

Younger flood-plain, deposits of the

rivers composed of sand, gravel, pebble

etc. and soil covering the rocks sandy,

clay, gravel, pebble, boulder etc.

representing older fluvial deposit.

Himalaya front tectonic line.

Pliocene to

Lower

Pleistocene

Siwalik Micaceous sandstones with salty bands

seams of geographic coal, silt and

minor bands of limestone.

Main boundary fault

Permian Damoda series (Lower

Gondwana)

Quartzitic sandstone with salty bands,

carboniaceous shales, seams of

graphitic coal, lampro-phyre sills and

minor bands of limestone

Thrust fault of nappe outlier.

Precambrian Daling series

Darjeeling Gneiss

Slates, Chlorite-sericite schists, mica

schists, coarse grained gneiss.

Source: Mallet, Gansser, Pawda and Saha


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Fig. 2: Geological Map of Darjeeling Hill

Physiography: The study area is divided into three broad physiographic divisions, these are

mountainous region, inter mountain low land and narrow river valley bounded by steep slope

cliffs are found. The terrain is very rugged all over the region.

.

Fig. 3: Topographical Map of the Study Area (Map No. 78 A/8)

The average height of the area ranges between 400 mts-800 mts. revealed by Survey of

India toposheet no. 78 A/8. Several morphometric analyses have been done to interpret the

topography and drainage of this region.


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Fig. 4: Relative Relief

Fig. 5: Average Slope (After Wentworth) According to relief analysis (After Smith) the relative height area is 800 metres. At both

the side of the river Tista relative relief is very high. Highest relief is observed in the western

and south-western part of the map.

On the basis of the average slope analysis it is found that the entire region is bounded by

very steep slope, ranging between 150-300, highest slope is observed in the south central part

of the region. All along the NH 31A the side of the mountain and the slope up to Tista river is

very steep to moderately steep. At places almost vertical scarp face drops to the Tista

following at 200-500 mts below.

Drainage: The main river of the entire hill section is Tista, all the streams are first order or

second order or third order tributaries to this main river. A number of small streams come


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down along the steep hill slope, which guide tremendous velocity and exclusive water and

erosion. During monsoon, most of the tributaries join with the main river at right angle and

thus forming parallel drainage pattern. In many parts of this region, drainage density is high.

Sharp water divides lye from north-eastern to south-eastern part of this map.

Fig.6: Drainage Density

Causes of Slope Failures:

Causative factors for landslide phenomena in the study are categorized into five types. A

complex action of more than of these factors creates conditions favourable or otherwise

landslide initiation and need not necessarily be the direct use or trigger mechanism.

Geological factors:

i. Physio-technical properties of the rock.

ii. Existence of tectonic elements such as fold, faults and thrust, adverse dip strike

orientation of the discontinuity planes etc. As a rule the stronger the resistance to

the shearing of rock mass, lesser is the susceptibility to landslide

a) Rock slides are rare in Tista valley north of Tung and in Lachung valley north of

Chungthang because the rock type of the area are hand resistant shearing although

geomorphological features i.e. high energy, relief are conductive to landslides.

b) The weak nature of the rock mass, adverse orientation of the discontinuity planes

Coupled with higher degree of relief explain the higher incidence of landslides.

Geomorphological factors:

The most important factor that is the condition favourable for initiation of landslide is high

degree of relief. The higher the degree of relief, the greater is the susceptibilities of the area


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to landslide incidences. The entire study exhibits high degree of relief. Adverse

geomorphological condition is influenced by favourable geographical conditions namely

existence of high resistant rock mass making high relief slopes. This explains stable nature of

slopes in certain areas of north Sikkim such as Phensong-Phodong, Tung-Chunthang and

those in Lachung and Tista valleys north of Chungthang though these areas possess high

degree of relief and could have been susceptible to landslides.

Tree tilted by Subsidence Sliding of the Road embankment Climatic factors:

i. Amount of precipitation

ii. Recurrence of cloud busters

iii. Depth in penetration of percolating water in soil.

These factors largely influence landslide susceptibility of an area. The higher rainfall and its

intensity are more favourable for the condition of landslide initiation. The Sikkim-Darjeeling

Himalaya, located in Tropics receives abnormally high precipitation exceeding 4000 mm

during monsoon months. The extensive rainfall reaches as high as 750 mm per hour that can

extend over as long period as 3 hours at a stretch.

Hydro-geographical factors:

i. Quantity of run-off

ii. Fraction of precipitation that percolates and becomes ground water

These are dependent upon the characteristics of the surface on which the precipitation falls.

Infiltration of precipitation plays a significant role in creating condition favourable for

landslide initiation. When the rate of precipitation is more than the infiltration capacity over

the land flows, there occurs initially deep erosion gullies. If the slope happens to be steep


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consisting of loose broken rocks or poorly consolidated soils, avalanches, slides and mud

flows occur following downpour.

Fig. 7: Cross Section Showing Slope Aspects Along the Study Area

Human activities:

i. Construction of new roads and establishment

ii. Agricultural practice

Those activities lead to mass scale deforestation, slope cutting and stagnation of water along

slopes. Deforested areas come under fast weathering and affect the equilibrium of the slope.

All these activities are detrimental to slope stabilities.

Fig. 8: Multiple and Complex Landslides


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Description of the Landslides:

The NH 31A, which takes off from Sevoke bridge and runs upto Gangtok, (along the Tista

river) has been affected by number of major landslides, especially during the rainy season

leading to the total disruption of vehicular traffic between the hills and the plains with the

consequent disastrous effect on the transport of goods and tourist operation. The undercutting

of slopes due to heavy monsoonal precipitation, deforestation, unscientific cultivation and

construction activities are known to combine with rather complex and immature geological

formation in the region to generate variety of landslides. The landslides in the Tista valley

along the NH 31A, have been dealt separately as their characteristics were compiled mostly

from GSI reports. The valley side of Tista and tributaries were undermined during the flood.

On these slopes were formed big rocks and earth slides, especially in the parts built by the

less resistant daling shales or by thick covers of waste materials. The niches are visible near

Tista bazaar. Most of the tongues were later carried away by the flood waters.

Description of the Transitional Slide: The transitional slide is observed 32.5 km away from

Siliguri on the left side of the road NH 31A. The road runs along the left side of the Tista

river. The base length of the slide is 17.5 mts. Boulders of different size from dust to small

boulders or angular fragments are the main eroded materials of the slide. Inclination of the

rock beds is towards the road. The rocks are mainly quartzite, very few phylite or slates

(slightly oxidized). The foliation plane is also dipping downwards producing a far weak slope

compared to rocks dipping upwards. Therefore the hill-slope itself is comparatively unstable.

Fig. 9: Transitional Slide

Description of the subsidence:

The subsidence area is observed 3 km away from Sevoke. The road runs along the left side of

Tista river. Total length of the portion of the road is 28 mts. The subsidence may have been

caused by many factors. Some geologically vulnerable areas are found. The vehicular

movement on the road is reciprocally higher than normal.


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Fig. 10: Subsidence along Sevoke Road

Description of the rotational slide:

This slide is located near Tista bazaar, 26.7 km away from Sevoke. Total length is 136.2 mts.

on the left with an average height of 10 mts, right side a bit larger than left side. Heavily

phylitic rocks with intrusion of quartz veins are found. Phylites are under erosion in high

rainfall areas producing erosional remnants which is found all along with slide. Rock

fragments of different size are present, which move along with the mobile mud. This is one of

the largest slides found along the road and extending downwards upto Tista river. South

western monsoon directly hits the hill slope, causing intense and heavy precipitation. Further

rainwater finds easy access and follows down the valley wards dipping discontinuity surface

such as foliation planes. Foliation and other joints cause fast weathering of the rock mass

rendering its soil. Thus a stick cover of the product of weathering is generated over the fresh

bed rock, downward percolation of rainwater increases the unit of surcharge soil mass and

tends to slide down together worth underline weathered rock over the dip slope under gravity.

In addition, the excessive saturation of the soil mass causes increase pore pressure which in

turn reduces the shearing resistance causing shear failure of the slope materials.

Fig. 11: Geographical Mapping of Rotational Slide


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Preparation of Landslide Zonation Map:

For a better understanding of the geographical distribution of landslide prone areas in

between Siliguri to Kalimpong, a map has been produced with the help of a ‘cheque list’(

Modified after Cooke and Doorncamp, 1974) from Topographical sheet 78 A/8 and direct

field observation.

Fig. 12: Landslide Zonation Map

Class-I: Very High Landslide Prone Areas:

These are the areas where landslides occur more frequently within a very short time period.

They are found along both the eastern and western part of the Tista river. It covers the area

which is situated by the side of the road NH 31A. For example, Tista Bazaar area, which is

located near the Tista bridge and west side of the NH 31A, is very high landslide prone area.

In the monsoonal season, landslide occurrence is very high in this area due to heavy rainfall.

Class-II: High Landslide Prone Areas:

It also includes the both side of the Tista river and Rilli Khola. In this area, slope is very

steep, drainage density and relative relief are very high, bed rock is fractured or joint. This

leads to instabilities on the valley side of the streams. Steep river gradient generally indicate a

phase of active and rapid incision, so the area is highly landslide prone.


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Class-III: Moderate Landslide Prone Areas: These areas are found near the Kalimpong town,

Naya Bazaar, Tirpai Bazaar, Chhoba, Durbindhara. In these areas, slope is moderate, relative

relief is medium.

Suggestive Measures to Prevent Landslide:

Natural process of land sculpturing such as weathering, erosion, slope failure etc. are active at

varied rates and to different degrees all over the mountainous terrain of Himalaya. The

process is comparatively faster in the Eastern Himalaya also known as Darjeeling Sikkim

Himalaya, mainly because the area receives intense and heavy precipitation owing to

unhindered access of south-western monsoon originating from Bay of Bengal. This results in

landslides and related slope stability problems.

While planning for construction of a road alignment or a railway line on a hilly terrain

especially in the Himalaya, a detailed geological investigation evaluating all essential

information or structural and tectonic behaviour of the slope material during and after the

slope cutting, stable and unstable zone which the proposed alignment may have to negotiate

is a pre-requisite and must. For example, adequate data have been collected on terrain

conditions such as geological set up, structural and tectonic behaviour of the slope materials.

Construction of building on the debris covered slopes should be restricted. In soil slopes,

building may be constructed but they should be follow proper engineering techniques.

Scientific terrace cultivation should be practised to prevent soil erosion.

Fig. 13: Different Systems to Prevent Landslide


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Conclusion:

From the analysis it can be said that the study area is highly susceptible to landsliding.

All the environmental factors here are favourable of landsliding. This has been aggravated by

human activities particularly road and building construction which disturb the natural slope.

This implies a practical problem to hill slope. The road remains unstable occasionally during

rainy season in almost each and every year. Thus the communication is greatly affected. So

proper steps should be rigidly implemented in case of construction activities, following the

underlying geological and the surrounding physiographical structures, to prevent the

landslides and ensure a safe and unaffected flow of traffic. Thus the overall equilibrium in

relation to the slope aspects of the study area (part of Darjeeling-Sikkim Himalaya) can be

properly maintained.

References:

1. Bhattacharya, A., Mishra, P., Ghoshal, T. B., Bahuguna, H. and Ghatak, T. 1998. A Geotechnical Appraisal of Landslides on 7th July 1998 along National Highway No. 55. G.S.I. (Editor) Progress Report.

2. Gautam, A. 2009. Geomorphology. Allahabad: Sharda Pustak Bhawan. pp 244-267. 3. Detwyler, T. R. 1971. Man’s Impact on Environment. New York: McGraw Hill. 4. Sarkar, A. 1977. Practical Geography- A Systematic Approach. Kolkata: Orient Longman. 5. Sharpe, C. F. S. 1938. Landslide and Related Phenomena. New York: Columbia University Press. 6. Singh, S. 2007. Geomorphology. Allahabad: Prayag Pustak Bhawan. pp 267-296. 7. Thornbury, W. D. 2004 Principles of Geomorphology. New Delhi: C B S Publishers and Distributors.

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