Landslide Mitigation - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 252

population during the same periods are 28.83 and 253 %. Such unprecedented
growth of household and population as a result of migration from surrounding
depressed rural areas is recorded at the places of Tindharia, Giddapahar, Gayabari,
Paglajhora, South Shivkhola Tea Garden, Jogmaya Tea Garden, Selim Hill Tea
Garden etc. Everyday functioning of such huge population for livelihood collection
put tremendous pressure on the land resources damaging slope and vegetation cover
leads to soil loss and slope failure. Most of the houses are made up of concrete
floors which exert immense pressure on fragile slope materials. On the other hand
tin/asbestos roofed houses release maximum concentrated flow of water on land and
erode the slope materials easily. Except Paglajhora, most of the houses are one
storied and a considerable percentage is two storied which puts excess weight on
weak lithology. Nearly 50 % houses of all the villages do not have roof top and
waste water disposal system and this situation favours the water to enter into the
soil and thus destabilize the slope materials and ultimately threatens the entire
settlement area. To resist the problems of slope failure caused due to human
intervention on unstable slope in the Shivkhola Watershed, the landslide prone
areas should not be allowed for further concentration of settlement. The steep slopes
must be avoided for the construction of roads. The restriction on the plying of heavy
loaded vehicles and on the construction of two or three storied buildings and the
imposition of wooden structured houses, the restriction on the illegal deforestation
are to be made. Introduction of afforestation and plantation programme for the
unstable sections, and the provision of appropriate roof top water and waste water
disposal system to release the water through pipes and concrete drains are to be
regularized effectively. Finally, the people should be made aware about the

unscienti c interaction between man and land or soil and that can minimize the

harmful impact of population pressure on land and bring the stability to the slope
material.
The analysis of critical slope angle, rainfall and height suggests that the Shiv-
khola Watershed is a most unstable section of Darjiling Himalaya. In the existing
weak lithological composition threshold slope of the unstable sections range from
21

. The large parts of Tindharia, Paglajhora,
Gayabari, and Shiviter possess the slope angle of more than 24

° to 26 ° with an average value of 24 °

that indicates
absolute instability. Around 100 mm/day rain may become critical to the landslide
prone sections in the watershed. The slope height ranging from 5.89 to 7.80 m. is
very much critical and beyond this there is a great chance of slope failure at the
places of Tindharia and Paglajhora. The special care must be taken to reduce the
height of back wall along the main road as well as the marginal part of the slided
scar to that of 5.00 m of less with respect to the below threshold slope angle of 24

°

.
As the rain water is the main triggering factor for reducing cohesion, angle of
internal friction and increasing the pore-water pressure which altogether increase
the driving force to the slope materials, the most suitable way would be the
application of synthetic plastic for covering the extreme unstable section mainly
along the Hill Cart Road not to percolate the rain water through soil and slope in the
rainy season. The installations of rain gauge stations at all the major landslide
locations for the continuous monitoring of rainfall and the relevance of cumulative

°