3.0 STUDY OF MASS MOVEMENT AND STUDY OF PREVENTIVE AND CORRECTIVE MEASURES
Mass movement is one of the most challenging slope process related to the potential energy developed due to the gravitational stress may or may not influenced by the pore water pressure. Slope failure, landslides and debris flows are the major mass movement’s phenomena. As the mechanism of the mass movement differ the necessary treatments and stabilization measures are also different from its complex type of mass movements are frequent in the area, which makes the treatment more challenging.
3.1 Classification of mass movements:
1. Landslide
2. Slope failure
3. Debris flow
3.1.1 Landslides
A landslide or landslip is a geological phenomenon which includes a wide range of ground movement, such as rock falls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments. Although the action of gravity is the primary driving force for a landslide to occur, there are other contributing factors affecting the original slope stability. Typically, pre-conditional factors build up specific sub-surface conditions that make the area/slope prone to failure, whereas the actual landslide often requires a trigger before being released.
Large dimensional, slow to sometimes rapid but continuous movement of large weathered rock or soils on a clear slide surface are also called as the landslides. Sliding surface usually contains clay and the activities are influenced by the ground water. Treatments of the landslides are expensive and site specific. Effective treatments of the landslides demand the geo-technical investigation of the unstable area. Stability measures in the landslides are big challenges for many of the civil engineering structures.
Causes of landslides
Landslides occur when the stability of a slope changes from a stable to an unstable condition. A change in the stability of a slope can be caused by a number of factors, acting together or alone. Natural causes of landslides include:
· Groundwater (porewater) pressure acting to destabilize the slope
· Loss or absence of vertical vegetative structure, soil nutrients, and soil structure (e.g. after a wildfire)
· Erosion of the toe of a slope by rivers or ocean waves
· Weakening of a slope through saturation by snowmelt, glaciers melting, or heavy rains
· Earthquakes adding loads to barely-stable slope
· Earthquake-caused liquefaction destabilizing slopes
· Volcanic eruptions
Landslides are aggravated by human activities, Human causes include:deforestation, cultivation and construction, which destabilize the already fragile slopes,vibrations from machinery or traffic blasting,earthwork which alters the shape of a slope, or which imposes new loads on an existing slope,in shallow soils, the removal of deep-rooted vegetation that binds colluvium to bedrock,Construction, agricultural or forestry activities (logging) which change the amount of water which infiltrates the soil.
3.1.3 Derbis Flow
When the shear strength of the big slope material considerably reduced the rapid movements of the solid earth materials including large volume of water can take place. If the material is complex debris such phenomena are called debris flow. If the flow is fine the phenomena is called odd flow. Sometimes the viscous materials could spread down slope. The flow but continuous movement of the slope containing the thicker soil, largely without any distinct slip surface is recognized as creeping. Stability measures against flow spread and creeping are complex and demands considerably high costs.
3.1.2 Slope failure
Relatively small dimensional movements of weathered rock or soil layer in the slopes are called as slope failures could lead to more stable configuration may redistribute the rock material in less steep slopes and it relief the stress by reducing the high concentration of stress usually present at the valley bottoms. At the same time slope failure weaken the rock mass providing the already sheared surfaces at the residual strength, the reactivation of instabilities as once failed rock mass has not always reached complete stabilization. The slope failure loosen the rock mass and open the stress relief joints or fractures.
Slope failure depends upon the mainly in the following Factors.
1. Angle at which it is sustaining
2. Height
3. Material
4. Strength
5. Water stress in pores etc
Cleaning of the debris is considered as the best solution in case of the slope failure along the road but it could sometimes lead to disaster landslides.
3.2 Description of different mass movement observed in field visit
1st day Date :-2067/11/15
Time:-9:12 A.M
Location:-17+00 km in chainage from kalanki along Tribhuwan Highway
Following were the mass movement observed during field visit:-
1. Landslide (Fig 1)
2. Debris Failure (Fig 2)
3. Slope Failure (Fig 3)
4. Creep Failure (Fig 4)
Preventive measures for different mass movement
· Concrete machinery wall
· Gabion wall
· Planting grasses
· Proper drainage system e.g. Cascade
· Making balanced natural system i.e. removing disturbance in the area e.g. vehicle load etc. as possible.
· Bio-engineering
· Rock Anchoring
· Sealing of tension cracks
· Crib Wall
