Causes and Mechanism of Landslide: A Case of Krishnabhir Landslide along Prithvi Highway


Fig:1 Supportive mechanism stone machinery wall 
Dr. Kabiraj Poudel (Lecturer, Geologist TU), Subash Duwadi 

Abstract: Nepal is the country of the young mountains. Most of the area of Nepal falls under the mountainous terrain (i.e 83 percent). The land area is fragile and rugged topography. So the entire area of Nepal is seismically hazardous. The rainfall pattern of Nepal is quite irregular and most of the rainfall occurs on the monsoon season, so the mass movement and the related phenomenon are common. Every year due to landslide and related hazards huge amount of the property and many numbers of lives are lost. The Geo-environmental study of Krishnabhir Landslide was concentrated mainly in the cause and impacts. Slope instability is the universal phenomenon. The problem becomes more in tectonically active mountainous areas. Being predominantly a mountainous country, Nepal has many example of the mass movement hazards. Out of the total area of Nepal more than 80 percent of the land is elevated above 3000m from the mean sea level with the considerable relief feature and the mountain drainage. Hence young and the unstable geology, steep slope, rugged mountain topography, energetic river and the streams with their steep gradient and the swift flow, intense rainfall and frequent earthquakes contribute significantly to the process of the mass wasting.In this way, on the one hand the physical characteristics of the Nepalese hill slope make the young and the fragile mountain one of the most prone areas in the world. On the other hand, the high rate of the population growth causing additional demands and pressure on the environment distributing the environmental balance. Marginal lands prone to the natural hazard have been encroaching for the cultivation and human settlements. As a result, the frequency of the occurrence of all type of the hazards, its magnitude and the loss of the lives and property has been increasing in the process and the most damaging to the work of the man because they are an ever-present and frequent danger for the people and their property in the area. The rapidly increasing constructions of the infrastructures such as roads, irrigation canals and dams without due considerations given to the natural hazards, is contributing triggering landslides.

Objectives of the Study
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.
d) To study the losses of the landslides during the same periods.
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.
g) Hazard Mapping of the area.

Limitations of the Study
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.

a) Desk top Work
b) Field Work
c) Lab Work
d) Nature and Source of Data  

Introduction of Krishnabhir Landslides
Fig:3 Sudden fall of huge mass of soil and rock debris 
Krishanabhir-Kurintar sector of Prithvi Highway of Nepal, which is suffered from frequent slope failure problems every year, has been considered for this study. This sector is a part of 200 Km long Prithvi Highway of Nepal, which connects Kathmandu, the capital with the tourist city, Pokhara, landslide and other mass movements in every rainy season, which block the highway sometimes for several days (Bhattarai et al, 2001) are the chronic problems of this highway. Due to the lack of information on the potential failure sites, road clearance equipments are neither properly placed nor the pre-monsoon failure prevention strategies are well considered. The study of the geological map shows the rocks of Midlands group (Upper Precambrain to late Paeozoic) of Lakharpata sub groups (Lakharpata formation, Syanja formation, Sangram formation and Galyang formation) underlies the study area. Limestone, Shale, Slate, Phyllites and Dolomites are the main dominant rocks along the alignment. Several major fault/thrust lines pass over the area. The average drainage density of the study area is about 3.26Km/Kmz. Many large small streams along both side of the highway dissect the area. There are numerous slope failures along the catchments of these streams. Those slopes failures are responsible for the huge volume of the mass movements down streams.

Causes and Mechanism
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.

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.

Manandhar (2000) 
Karmacharya (1989), Ghimire (1993) and Khanal (1996)
Sharma (1976)
Fleming and Mool (1983)
Dhital, khanal and Thapa (1993)
DPTC/T.U (1994)

Subash Duwadi
Nepal (,

Note: This research was presented in the College of Applied Sciences (CAS-N) and DP-net program of Disaster Risk Reduction in 2010.

Team Member:        Dr. Kabiraj Poudel (Lecturer at TU, Geologist)
                                      Subash Duwadi (Environment Services Professional)
                                      Hiramani Sapkota
                                      Dil Bahadur Karki
                                      Biraj Nepal


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