Friday, December 28, 2007

Landslide-prone Tungsung

Courtesy Mr Udhyan Rai Editor (19Sep2007)

Landslides and related phenomena are a common occurrence in and around Darjeeling. Records since 1849 show sharp acceleration in landslide rates. In 1899, the first major landslide occurred in Tungsung followed by others in 1934, 1950, 1968 and 1980.

The TungSung area of Darjeeling town is a densely populated commercial urban tract situated along the eastern spur of the Jalpahar • Lebong ridge and cover an area of 1.8 sq. Km.

The area is composed of rocks belonging to the Darjeeling Gneiss group. Such rocks are generally highly jointed and weathered. Fresh rock has been found along some very steep slopes.

Highly weathered rock and talus materials are found around St. Paul’s School, while the vertical rock face below the school joints dipping N 50 ˚W • 550 ˚E, N 75 ˚W • S 75˚ E, N 25˚ E • S 25˚ W and N 30˚ E • S 30˚ E. Foliation is also common and the prominent altitudes are N 30˚ E • S 30˚ W dip 6˚ and N 10˚ E • S 10˚ dip 25˚ .

Two major inclined faults have been recognized of which one is near the TungSung ZigZag and other near the Dengle Jhora. Granites also found around the St. Paul’s tennis court. Garnetiferous granite is found along the Tenzing Norgay road and a pegmatite vein has been identified near ZigZag.

A rocky slopes and hilltops show a thick mantle of brown sandy micaceous soil with rock fragments varying from large blocks to boulder and pebble sizes. The eastern slope of the Katakpahar • Jalpahar ridge along which TungSung is situated is one of the steepest parts of the town, where average slope is between 30˚ • 40˚ and as high as 60˚ • 65˚ in some places.

The slope show a predominance of convexity and rectilinear. However, minor concavity has developed due to slope failures e.g. Dengle Jhora and Manpari Busty.

The hill slopes are heterogeneous in profile, being commonly flatter towards the crest and steeper down hill. The overburden of course, textured soil has varying capacity to retain moisture and rest on rocks of variable dip, composition and decay and generally tends to slide down-slope as and when stress concedes resistance.

Landform and slope development is often controlled by the relative degree of physical and chemical weathering of rocks. Initially, the fragmentation is controlled in terms of size by the spacing of litho logical and structural discontinuities in the parent rocks followed by further splitting.

Bedrock is thus degraded into a colluvial mantle, which covers a large part of the TungSung area in the form of stress and debris.

Soil in the TungSung area is leached, gravelly and grey to brown in colour with soil • profile varying in the thickness from 0.1 m to 9.0m depending on the degree of slope,

Vegetation and nature of parent minerals. Very thick soil profiles ranging from 5 to 9 m have been found near Dengle Jhora, the youth hostel and St. Paul’s School main gate. Very shallow to skeletal soil • profiles have been located between Dengle Jhora and Manpari Busty. The two types of soil that have been identified are:
1. Micaceous sandy soil with poor cohesion, and
2. Brown, clayey soil with decomposed mica, sand and rock fragments with good cohesion.

While landslides generally occur during heavy rain but the action of the precipitated water is more like the trigger of a gun since it delivers the last blow when other factors have already brought the slope nearly to the point of failure.

The annual precipitation of this region ranges between 2650 to 2900mm. Rainfall is concentrated from June to September. High intensity rainfall associated with cloud burst has been found to occur quite frequently and perhaps contributes the trigger Mechanism that initiates the slide.

The ISI Seismic zone map of India 1971 places the TungSung area in zone V, and the related co-efficient should be taken into consideration in formulating the design and layout of buildings and other constructions.

The eastern spur of the Jalpahar • Lebong ridge is composed of soil, talus and scree materials, which are liable to slip unless suitable protective measures are adopted. The foliation dips of gneissic rocks are easterly at an angle varying from 15˚ to 25˚, while the dominant slope vary from 40˚ to 60˚ towards the east. Hence, if a suitable condition prevails, the top layers may either creep or slip downwards.

Although under normal circumstances such slopes may appear stable, sudden overloading of them through heavy precipitation or construction may reduce the shearing resistance of their component material causing them to ultimately yield under gravitational pull.

It has been observed that except in 1934, all major landslide events had occurred after heavy and continuous rain, which triggered the slides when other factors had already brought the slope near to the point of failure.

It is evident that the force causing slips is the combination of weight of the materials and gravity, with gravitational pull increasing when the slope has been steepened either due to under cutting by drainage elements or by human agencies.

In TungSung area, most buildings constructed after buildings, back cutting into the hill face (even in culverts or Jhoras) for developing new building sites. Widening of terraces by dumping of soil along their edge in Rock Ville area led slipping along the edge, which could not be prevented merely constructing retaining wall or protection wall.

Before taken up of any type of construction soil testing is very important but very few construction Cos. such as five elements construction Co stick to this rule. Deep soil in the area, which is derived from the underlying gneisses, shows progressive deterioration of shearing resistance by weathering following from formation of clay minerals from feldspar and biotitic.

Landslides are a pertinent natural phenomenon over the TungSung area. While they cannot be prevented entirely, they can be checked to a large extent. It is an obligation, therefore on the part of every individuals and concerned local elected and empowered agencies dealing with the welfare of people and beautiful mother town Darjeeling to implement remedial measures already suggested by professionals and their various studies from time tot time.

Slopes 0˚ to 15˚ are the most suitable for urban uses, 15˚ to 30˚ is suitable for certain specific uses, 30˚ to 45˚ is suitable for small residential construction, 45˚ to 60˚ can only be use under very specific categories. Slope above 60˚ is very dangerous to use.

Soil types of TungSung and their characteristics indicate that dominant soil is loamy sand to sandy loam. Perpetration resistance ranges between 4.5 kg /sq. cm to 6.6 kg /sq. cm, corresponding to the strength suitable for the construction of two to three storied buildings but unfortunately there are so many four to seven storied gigantic buildings standing in this area, which is very risky and dangerous in near future. Local administrative authorities and elected bodies must take strict necessary steps to check such dangerous constructions. Slope materials of this are is highly sensitive and no further construction should be allowed there.

Mr. Sangay Tashi Dugpa
Tenzin Norgay Road
Tenzing Norgay Building


Comment by praful rao

I met Mr Udhyan Rai, the webmaster and editor of (DT), just a few days back in Kalimpong. I was quite surprised that both savethehills (STH) and DT were working on the same issue 4 months ago when the mountains were cracking up all over the district.
I reproduce the above article and photo with his kind permission (italics are mine)

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