This document provides an overview of landslides, including: - Landslides involve the downslope movement of soil, rock, and organic materials under gravity. They can be triggered by rain or earthquakes. - The driving forces of landslides are gravity and the mass/weight of material. The resisting forces are friction and cohesion. When driving forces exceed resisting forces, landslides occur. - Common types of landslides include rockfalls, rockslides, rock slumps, earth flows, soil creep, and complex slides. - Landslides frequently impact the Himalayan region and western ghats of India. Mitigation techniques include non-engineering methods like proper land use planning and tree planting,

1. Natural Disaster Mitigation
and Management (NDMM)
Course Instructor:
Aditya Kapoor
Assistant Professor
Civil Engineering Department
SGT University, Gurgaon

2. Lecture-6
Hazard #3:Landslides

3. Introduction to Landslides
• Landslide is a general term used to describe the downslope movement of
soil, rock, and organic materials under the effects of gravity and also the
landform that results from such movement (refer next slide for graphical
representation)
• May be triggered by rain or earthquake
• Also commonly known as mass wasting

4. Pictorial Representation
of
land Slides

5. Introduction to Landslides
• Landslide is a general term used to describe the downslope movement of
soil, rock, and organic materials under the effects of gravity and also the
landform that results from such movement (refer next slide for graphical
representation)
• May be triggered by rain or earthquake
• Also commonly known as mass wasting

6. Mechanism behind Landslides
• There are two type of forces acting on a sloped mass:
• Driving Force: Gravity
• Resisting force: Friction and Cohesion
• When driving forces overcome the resisting force, landslide takes place
• Factors driving the land slide:
• Slope steepness
• Moisture content
• Material weight

7. Mechanism behind Landslides: Driving Force
• The mass of the sloping material drives it down slope
• Greater the steepness or mass, greater the driving force (refer figure
below)
Driving Force: F=m*g*Sin(α)
Where, m=mass of the object, g= acceleration due to gravity
Driving force is proportional to α (steepness) and mass
Hence chances of landslide are more in the case on right (α2 > α1)

8. Mechanism behind Landslides: Driving Force
Effect of Rainfall
• As discussed earlier, soil is made of solid particles with pores between
them
• Upon rainfall, these pores will get filled with water and the weight of the
soil mass may increase
• This increased mass increases the driving force, which inturn increases
the chance of landslide (as discussed in the last slid)

9. Mechanism behind Landslides: Resisting Forces
•The two resisting forces are:
1. Frictional Force
2. Cohesion

10. Mechanism behind Landslides: Resisting Forces
• Resisting Force#1: Frictional Force
• Frictional force (f) acting on a body is calculated by using the expression:
f=µ×m×g×Cos(𝛼), Where, µ=coefficient of friction, m= mass, 𝛼= angle of
slope, g=acceleration due to gravity

11. Mechanism behind Landslides: Resisting Forces
• Resisting Force#2: Cohesion
• Cohesion
• Results from a static charge attraction between clay particles.
• Acts like a glue that acts between clayey masses

12. Condition for Landslide: Driving Forces>Resisting Forces
Mathematical Expression Governing Landslide:
F > f+c
Driving
Force
Resisting
Force

13. Factor of Safety (FoS): Driving Forces>Resisting Forces
• Mathematically, Factor of Safety is the ration of resisting force to the
driving force (refer equation below).
FoS =
Resisting Force
Drving Force
=
f+c
F
FoS> 1 is a favourable condition as it implies that resisting forces are
greater than driving forces
FoS< 1 is an unfavourable condition as it implies that driving forces are
greater than resisting forces, and landslide will happen

14. Effect of vegetation on landslides
• Vegetation is a significant factor in slope stability for three reasons:
1) Vegetation provides a protective cover that cushions the impact of
falling rain. This cushion allows the water more time to infiltrate the
slope rather than running downhill, retarding surface erosion.
2) Plant roots add strength and cohesion to slope materials. They act like
steel rebar reinforcements in concrete, anchoring soil and
unconsolidated material, and increasing the resistance of a slope to
landsliding

15. Modes of Landslides
1) Rockfall
2) Rock slide
3) Rock Slump
4) Earth Flow
5) Soil creep
6) Complex slide

16. Modes of Landslides
1) Rockfall
Rockfall, in which blocks of bedrock
fall through air and accumulate at the
base of the cliff as a talus slope
(Talus: a slope formed especially by
an accumulation of rock debris)

17. Modes of Landslides
2) Rockslide
Blocks of bedrock slide down a
bedding plane in sedimentary rock

18. Modes of Landslides
3) Rock slump
Slump, in which cohesive blocks of
soft earth material slide down a curved
slip surface

19. Modes of Landslides
4) Earth Flow
mud, sand, rock, and other material are mixed with water and become partially
liquified

20. Modes of Landslides
5) Soil Creep
Soil creep, causing telephone poles
to tilt and trees trunks to become
curved

21. Modes of Landslides
6) Complex Slide
Complex landslide, consisting of upper
slump and lower flow

22. Land Slides in India
• Landslides mainly affect the Himalayan region and the western ghats of India.
Landslides are also common in the Nilgiri range. It is estimated that 30 percent
of the world’s landslides occur in the Himalayas.
• Due to tectonic activities, the Himalayas have a history of landslides that has no
comparison with any other mountain range in the world. Landslides are also
common in western ghat.
• (refer landslide vulnerability map on the next page)

23. Landslides in India
Landslide vulnerability
map of India

24. Mitigation of Landslide Hazard
• Non-engineering Solutions
1. Proper land use
2. Tree planting on slopes
• Engineering Solutions
1. Providing surface drainage
2. Providing Sub-surface drainage
3. Removal of unstable slope materials
4. Construction of pile foundation and retaining walls
5. Installation of rock screens

25. Mitigation of Landslide Hazard: Non Engineering Solutions
1. Proper land use
• Avoiding populating the landslide prone area
2. Tree planting on slopes
• Roots of trees bind their root zone and provide stability

26. Mitigation of Landslide Hazard: Engineering Solutions
1. Surface drainage
• By making the surface of the hill slope impermeable to limit the infiltration
of water
• Performed by using a technique called shotcreting in which concrete is applied at very
high velocity. Steel mesh may be used for reinforcement

27. Mitigation of Landslide Hazard: Engineering Solutions
2. Sub Surface drainage
• Subsurface drains are installed to drain away the excess water in the soil

28. Mitigation of Landslide Hazard: Engineering Solutions
3. Removal of unstable slope materials
• Removal of slope mass from very steep slope lowers the angle of slope, and
thus reduces the driving forces which in-turn reduces the probability of
occurrence of land slide
• In other words it increases the FoS (Factor of Safety)
• This process is also known as Grading

29. Mitigation of Landslide Hazard: Engineering Solutions
4. Construction of pile foundation and retaining walls
• Deep piles anchor into the unstable soil mass and provide stability
• Retaining wall holds the unstable soil mass back
• Drains are also installed to help drain out the subsurface water contained
in the soil mass behind the retaining wall
(refer the figure on the next slide)

30. Mitigation of Landslide Hazard: Engineering Solutions
4. Construction of pile foundation and retaining walls

31. Mitigation of Landslide Hazard: Engineering Solutions
5. Installation of rock screens
• Also known as rock curtains
• Consists of heavy steel cable mesh that
provides stability to the underlying soil
mass

32. Mitigation of Landslide Hazard: Engineering Solutions
6. Drill and Bolt
• Consists of 12 to 15 m long bolts installed across the discontinuities or the
potential failure surfaces
• Bolts are anchored to the stable soil mass
• At the surface, these bolts are attached to concrete frames (or elements)
(Refer to the illustrations on the next slide)

33. Mitigation of Landslide Hazard: Engineering Solutions

34. Next Lecture: Types of Landslides and their
Mitigation

35. Thank You!