The general objectives of the study is to analyze the landslide events and the risk in the temporal and the spatial scale.
a) To find out the socioeconomic and environmental impacts of the landslides.
b) To discuss the causes of landslides on the basis of the historical events.
c) To examine the triggering factor of the landslide events such as rainfall, geology and relief etc.
e) To predict the probability of the occurrence of the landslides in the future.
f) To give recommendations on the measures to be taken for the use area.
Due to the lack of the scientific equipments, details works in the other aspect of the slopes instability such as infiltration rate and underground water system etc were not possible. There is no any meteorological station in these regions. Therefore the data of the climatic aspects landslides processes are very heterogeneous in the nature and interrelated with different physical, social and economic factors. Therefore it is not easy to overcome fully in this short time period by the single person due to the complex nature of the rock in the study area. As Karmacharya (1989) stated that lacks of the information about higher Himalayan explain the fact that remote area are not reported appropriately. Thus a landslide of the remote area far from reporter and those not directly effected socioeconomic conditions has not been reported appropriately which is also the limitation of the study.
Presently the transverse section of the slopes at this stretch is concave more or less in a wide gully. The soil is comprised of the dark grey- yellowish, loose dense previous moist with the angular fragments of the rocks at the upper part. i.e near to the crown of the slide while at the middle part failure materials is of black colored loose silty and the clayey materials with the rock fragments. The steep slope prior to failure loose colluviums deposits of the slope allowed the rain water to infiltrate and percolate towards the slide. Besides this highly weathered carbonaceous slate and silty phyllite with intercalation of the carbonate rocks, whereas under the influences of the water these materials losses strength as increasing the driving force multiple shear zones. However, extensive study and the investigation should be performed to found out this.
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| Fig: 4 Bioengineering with jute netting |
Rocks are dipping counter slope major three sets of the join observed. Multiple sharing phenomenon made slope rock jointed and due to surface runoff water whole rock mass becomes chemically weathered and rock changes in situ soil as slope covered materials. Due to increase of the pore water pressure when entered even through joints and being weathered rock mass and slope became unstable and failed. Due to removal of the failed materials which was acted as retaining force to materials of the upper part tension cracks were developed and phenomenon was repeated and the failure mechanism became active and continuous being steep slope. Due to such situation under the influence of the even little quality of the water cause fast removal and sweeping of the more materials. As the result slide area becomes wider deep gully erosion causes deep cutting of the weathered rock mass. Due to improper function of the drainage construction at crown another slide zone towards west to prior slide formed. It is also become wider as previous slide at the crown due to the erosion of the surface materials and finally deep cutting of the weathered rock mass. Further the rain is percolating from the tension cracks and the holes formed by the activity of the mouse located in the up hill slope contributed in the increasing pore water pressure on the top. Weak colluviums deposits. Which resulted in the reduction of the available shear strength of the soil considerably leading to failure of the slopes. The water runoff is flowing through this gullies relatively in the higher velocity which as a consequence liquefying the failed loose unconsolidated soil resulting in the debris flow during the torrential rainfall. Mass wasting hazard is the outcome of the several comprehensive factors of the geological and geographical environment that interacted in the different magnitude. On the basis of the historical events and relevant literature review and the attempt is made to derive the causes of landslide in Nepal. Although landslides are the complex interactions among geological, Geo-morphological, hydrological and anthropogenic factors for understanding purpose causes can be dealt by separating natural (intrinsic and direct) and anthropogenic factors. Inclination slopes itself is one of the most important factors causing landslides cause slope acquires gravitational force. Most of the study depicted that maximum numbers of the landslides occurred in the natural slope ranging between 25-45 degree. In addition to the slope angel slope aspect also influence to the slope failure process. North facing slopes are found the most hazard prone areas by some studies. In the present study the relation of the landslides with lithology, altitude average annual rainfall seismicity, population density, livestock density and the land use pattern. Expect these parameters the landslides is the triggering by other several factors like rock structure, weathering geotechnical properties of the soil and topographical as well as the different anthropogenetic factors. However due to the lack of the information other important factors like rock structure (fault thrust) weathering properties of the rock and soil cover layer, slope aspect and angle GLOF and EROSION factors could not analyze.
For forces are acting on the slope; driving force which tends to move earth materials down a slope, and resisting force which tend to oppose such movements. Slope stability is determined by computing a safety factor defined as the ratio of the resisting force to the driving force. Vegetation also plays an important factor for the slope stability because of the infiltration and the root system which helps strengthen the bond of the soil particles. It increases the stability of slope. There are three main factors that play an important role in the fate of slopes. A) Role of Geology B) Role of Slopes C) Role of Water D)
Prevention Measures for Landslides
A) For artificial slope: Avoidance, Relocation, Bridging, Tunneling (or open cut and over tunnel)
B) Surface Drainage: Drainage Channel or ditches, Prevention of leakages, Cleaning natural ditches.
C) Sub-surface drainage: Drainage tunnels, Counter for trenches, Deep seated counter fort drains, Vertical drill holes, Horizontal bore holes, Slope seepage ditches, Drainage wells of ferro-concrete, Drainage wells of linear plates.
D) Supporters: Retaining wall, Anchor- retaining wall, Cribworks, Gabions pilling works
E) Excavations: Removal, Flatting and benching
F) River Structural Works: Checks dams, Revetment , Spur dikes
G) Other Methods: Vegetation, Blasting and Hardening
B) For Natural Slopes: a) River Structural works, Check dams, Revetment, Groin Dikes b) Benching and diversions, c) Re-vegetation, Grass Seeding, Forestation
Mitigation Practice Adopted: Different activities were carried out for the stability purpose like civil engineering practices, bioengineering including jute netting. Still now the cliff is hanging. Slightly towards the road indications future hazards. The adopted mitigation measures are not enough to further stability. Detail study should be carried out for the permanent stability.
Present Scenario: Now a days, Krishnabhir Landslides is more or less stabilized. Different control measures/mitigation measures are adopted. Bioengineering practices is successful. Different concrete structures like Cascade drainage, Stone machinery, Gabion wall was constructed to stabilize the slope, and jute netting is also successful in these areas. But landslides can again induce due to weak geology, anthropogenic activity by Trisuli river.
Environmental Effects: a) Soil loss, Socioeconomic losses of Landslides, Damage to infrastructure, Road blocked, Diseases epidemics
Kinematics Tests for the Failure: For the plane failure, the joints, bedding or the foliation planes dip parallel planes dip parallel to the hill slope with an angle equal or lower than the hill slope angle. For the wedge failure, the inclination of the line formed by the two planes is less than the hill slope that is two planes of weakness intersects to defined tetrahedral block. Toppling failure, in stereo net the slope of the discontinuity plane and the hill slope seems in opposite direction.
Conclusions: Due to my study, the main causes of the present problem of km 57-000 Krishnabhir landslides are
a) Steep Slope
b) Loose thin colluviums deposits at the steep slopes
c) Weathered bedrock
d) Tectonic activities at the evidences of the fine fault gauge sheared and crushed zone
e) Improper Drainage Management
f) Water percolation through the tension cracks and the holes made by mouse holes located at the uphill side from the road level where cultivate land is situated.
g) Intensive rainfall, which is the triggering factor of the slide due to the infiltration, percolation and making slope materials low strength as increasing driving force.
References
Manandhar
(2000)
Karmacharya
(1989), Ghimire (
1993) and Khanal
(1996)
Sharma
(1976)
Fleming and Mool
(1983)
Dhital, khanal and Thapa
(1993)
DPTC/T.U
(1994)
