Landslides Causes, Prevention, and Rehabilitation | Geology Optional Notes for UPSC PDF Download

Understanding Landslides

Definition of Landslide

• Nearly all exposed rocks and soils undergo weathering and erosion due to natural processes known as mass wasting.
• Mass movement refers to the constant slow downhill movement of disintegrated rocks, rock debris, and resulting soil under the influence of gravity.
• If there is a sudden transport of a large amount of material after a period of slow movement, it is termed as a landslide.
• Landslides are a natural process and are essential for soil formation, which serves as the foundation for vegetation, particularly in agriculture.
• However, they also pose a significant natural hazard, potentially leading to the loss of natural resources, infrastructure, property, and even lives in areas prone to landslide hazards.

Causes of Landslides

Natural Causes

• Gravity is the primary force driving landslide movements, followed by the infiltration of water into the earth materials.
• Natural causes such as heavy rainfall, earthquakes, volcanic eruptions, and slope angle steepness can trigger landslides.
• Vegetation removal, which can destabilize slopes, is another natural factor contributing to landslides.

Anthropogenic Causes

• Human activities like deforestation, construction, mining, and irrigation can significantly increase the susceptibility of an area to landslides.
• Improper land-use planning and inadequate drainage systems can exacerbate the risks of landslides in certain areas.

Causes of Landslides

• Natural Causes

  • All slopes degrade due to weathering and material movement downhill.
  • When slope angle exceeds the angle of repose, landslides can occur.
  • Landslide types range from earth failures to rock failures.
  • Potential slip surfaces like weak planes along bedding and joints can trigger landslides.
  • Presence of salts, clay, and altered rocks can create weak planes for slope failures.
  • Pore water pressure and joint water weaken rocks, making them susceptible to sliding.
  • Erosion at the toe or from factors like wind can destabilize slopes.
  • External factors like river cutting, freezing/thawing, earthquakes, and volcanism can also induce landslides.

• Anthropogenic Causes

  • Human activities like improper construction and excavation in hilly areas lead to slope failures.
  • Construction of buildings, roads, and infrastructure without proper consideration can trigger landslides.

Classification and Terminologies of Landslides

• Definition of Landslide: Landslide is a broad term encompassing various types of mass movements that can be categorized based on the materials involved and the type of movement.
• Types of Landslides:
  • Topple: Involves the forward rotation of material around a pivot point.
  • Fall: Material falls freely through the air.
  • Slide: Movement occurs along a well-defined surface.
  • Spread: Material spreads out in various directions.
  • Flow: Material moves downslope as a viscous fluid.
  • Complex: Combination of different types of movements.
• Classification based on Material Involved and Type of Movement:
  • Rock:
    • Rock Topple
    • Rock Fall
    • Rock Slide
    • Planar / Wedge
    • Combination of any of the above
  • Soil:
    • Slump
    • Earth Slide
    • Translational / Rotational
    • Earth Spread
    • Earth Flow
    • Saturated
    • Combination of any of the above
  • Mixed (Debris):
    • Debris Topple
    • Debris Slump
    • Debris Slide
    • Translational / Rotational
    • Debris Spread
    • Debris Flow
    • Saturated
    • Combination of any of the above
  • Other Materials:
    • Block Topple
    • Ice Block Fall
    • Ice Block Slide
    • Snow Spread
    • Avalanche / Sollifluction
    • Mud Flow
    • Volcanic Ash Flow
    • Saturated
    • Combination of any of the above
• Slump (Rotational Failure):
  • Common in soil with rotational failure surfaces.
  • Often found in areas with intensely weathered rocks or on soil-rich slopes.
  • Example: A slump occurs when a section of a hillside saturated with water slumps down due to gravity, creating a visible scar on the landscape.

Understanding Landslides

Main Types of Landslides:

• Foot
• Toe
• Transverse ridges (Terraces)

Geological Terms:

• Crown
• Left flank
• Right flank
• Slip plane

Rock Fall and Topple:

Rock fall refers to the free falling action of rocks from vertical slopes or cliffs due to undercuts by natural elements like glaciers, rivers, or sea waves. When rocks dislodge along very high-angle slopes, it's termed as Apling failure.

Rock Slide:

Rocks slide along weak surfaces or joints dipping towards the slope. This commonly occurs in rocky areas, especially when joints are filled with clay acting as a lubricant when saturated with water. Rock slides can be planar or wedge-shaped.

Debris Slide:

Weathered rock material mixed with soil slides along rotational failure surfaces, often resulting in complex landslides. These slides are also known as debris slides.

Fig. 2: Schematic diagram of a landslide in homogenous unconsolidated medium with a curvy-planar failure surface.

Illustration depicting a landslide scenario in a uniform, unconsolidated medium with a curvy-planar failure surface.

Fig. 3:

• Rock Fall (a): Rocks free-falling without ground contact on vertical slopes.
• Apling Failure (b): Rocks dislodging along very high-angle slopes.
• Planar Rock Slide (c): Rocks sliding along planar weak surfaces or joints.
• Wedge Rock Slide (d): Rocks sliding along weak surfaces or joints.

These examples illustrate different types of landslides based on their characteristics and causes.

Paper:

Study Material: Hydrogeology and Engineering Geology

Module:

Topic: Mass Movements, Causes of Landslides, and their Remedial Measures

Mass Movements and Landslides

Flows

• Flows involve the rapid and consistent movement of very fine-grained material saturated or oversaturated with water.
• Earthflows and mudflows are terminologies used based on the involvement of non-cohesive and cohesive soil, respectively.
• Ash flows refer to fast-moving volcanic ash saturated with water. If hot and incandescent, they are known as nuee ardente or lahars.

Avalanches

• Snow avalanches are fast-moving snow occurrences along steep slopes, commonly found in areas with permanent or heavy snowfall.
• Triggers for snow avalanches include earthquakes, rain, sudden vibrations from blasting, flying aircraft, and human interventions.

Creep

• Creep involves extremely slow movement that is not easily observable but can be indirectly identified.
• Signs of creep include trees with curved stems, tilted monuments, distorted fences, and bends in telephone or electric poles on creeping slopes.

Types of Landslides

• Rock Fall: Occurs on vertical slopes with closely spaced joints.
• Topple Failure: Happens on high-angle slopes with prominent vertical joints.
• Planar Slide: Involves rock sliding along joint/bedding planes with a low angle of dip.
• Wedge Failure: Common on slopes with conjugate joint sets.

Fig. 3 illustrates the different types of landslides on rocky slopes, depicting various scenarios such as rock fall, toppling failure, planar rock slides, and wedge failure.

Paper: Hydrogeology and Engineering Geology

Module: Mass Movements, Causes of Landslides, and their Remedial Measures

Landslide Hazard Zonation

• Solifluction: Occurs in periglacial regions where freeze-thaw cycles are common. Saturated soil or rock debris in the active layer may fail at slopes less than 5° inclinations.
• Caving: Common in areas with limestone. Seeping waters dissolve the rock, creating caverns. As the caves expand, the roof can collapse, leading to subsidence of structures.
• Landslide Hazard Zonation (LHZ) Map: A crucial tool to assess slope vulnerability. It considers various ground characteristics and factors influencing slope stability to mitigate landslides:
• 
  
  • Assists planners and engineers in selecting suitable sites for infrastructure development.
  • Guides in taking preventive measures against construction issues in hilly terrains.
  • Identifies vulnerable areas for proper mitigation strategies to prevent further deterioration.
  • Locates critical zones near dams and reservoirs to prevent siltation and water overflow.
  • Helps in rerouting transportation and water pressure tunnels away from landslide-prone areas.

By understanding and utilizing Landslide Hazard Zonation maps, communities can proactively address the risks associated with slope failures and minimize potential damages.

Landslide Hazard Zonation (LHZ) Mapping

• Overview:

  LHZ maps are created by integrating various factors involved in mass wasting and movement. These maps, produced using GIS techniques, help in identifying areas with varying levels of vulnerability. The scale of these maps can range from 1:50,000 for regional assessments to 1:2,000 for micro-scale zonation, depending on the project size.

• Factors to Include:
  • Lithology
  • Topography and Slope Morphometric Analysis
  • Structural Geology
  • Surface Hydrology
  • Forest Cover and Vegetation
  • Anthropogenic Land Use
  • Groundwater Condition
• Methods for Assessment:

  Various methods can be employed to assess slope susceptibility to failure. Two indigenous methods, one developed by CBRI, Roorkee, and another by BIS in collaboration with GSI (IS: 14496 Part I and II-2004), are briefly discussed. Additionally, the Slope Mass Rating method by Romana (1995) is also explored.

• Landslide Susceptibility Score:

  The Landslide Susceptibility Score (LSS) developed by Roorkee involves assigning ranks and weightage to different factors. The LSS is calculated using a specific formula. A higher LSS indicates a higher susceptibility to landslides. For instance, an LSS greater than 300 signifies high susceptibility, 300 to 200 indicates moderate susceptibility, and less than 200 represents low susceptibility.

Factors and Categories for Calculating Landslide Susceptibility Scores

• Geology:
  • Hydrology
  • Slope
  • Overburden Thickness
  • Slope and Discontinuity Relation
  • Joint and Fractures
  • Weathering
  • Rock mass
  • Lithology
• Vegetation:
  • Flowing
  • Wet
  • Dry
  • Flat
• Slope:
  • 0 - <15°
  • 15-30°
  • 30-45°
  • > 45°
• Thickness:
  • <1m
  • 1 - 2m
  • 2 - 3m
  • > 3m
• Slope Type:
  • Dip Slope
  • Oblique Slope
  • Opposite Slope
  • Flat Slope
• Soil and Boulder:
  • Thinly Bedded
  • Thick Bedded
  • Massive
  • Schist / Shale*
  • Sandstone* / Limestone*
  • Gateway to All ses
  • Gneiss / Quartzite*
  • Basalt*
  • Rhyolite*
  • Granite* / Granulite*
  • Barren
  • Sparse
  • Moderate
• Agriculture:
  • Thick
  • Added by the author to make it broad based and realistic

Paper: Hydrogeology and Engineering Geology

Module: Mass Movements, Causes of Landslides and their Remedial Measures

Overview of Landslide Hazard Mapping

• Bureau of Indian Standards (BIS) has devised a code for landslide hazard mapping involving ten causative factors, each assigned a Landslide Hazard Evaluation Factor (LHEF) of 1 or 2, summing up to a total of 14.
• For landslide hazard zonation, the designated area is segmented into smaller regions using maps ranging from 1:50,000 to 1:25,000 for macro zonation and 1:10,000 to 1:5,000 for micro zonation.

Landslide Hazard Zonation/Evaluation

Table 3: Causative factors and their corresponding LHEF according to IS: 14496 Part II

Table 4:

• S.No.
• 1. GEOLOGY
• 2. Lithology
• 3. Structure
• 4. Slope Morphology
• 5. Relative Relief
• 6. Land Use
• 7. Land Cover
• 8. Hydrological Conditions
• 9. Rainfall
• 10. Landslide Incidence

Based on the assessed value of each region, the entire area can be classified into five hazard zones as depicted in Table 4.

Zones for total estimated hazard as per IS: 14496 Part II

• <4.90 - Very Low Hazard
• 4.91 - 7.00 - Low Hazard
• 7.10 - 8.40 - Moderate Hazard
• 8.41 - 10.50 - High Hazard
• > 10.50 - Very High Hazard

Table 4 also includes the categorization of LHEF for different hazard zones.

Zone Description

• Zone I: Very Low Hazard
• Zone II: Low Hazard
• Zone III: Moderate Hazard
• Zone IV: High Hazard
• Zone V: Very High Hazard

The classification is vital for understanding the level of risk associated with different areas prone to landslides.

Paper and Module Information

• Paper: Hydrogeology and Engineering Geology
• Module: Mass Movements, Causes of Landslides, and their Remedial Measures

Preparedness, Mitigation, and Civil Engineering Interventions

• Types of Regions

  • Plain areas: characterized by low angle slopes to flat tracts.
  • Mountainous areas: high angle slopes, mostly in valleys or valley flanks.
  • Juncture of mountains and plains: ground with different slope angles.

• Landslide Occurrences

  Regions prone to landslides: mountainous and juncture areas.

  Challenge: Inhabitants building on unsafe slopes due to limited habitable areas.

  Role of Civil Engineers: Providing safe infrastructure and guiding safe construction.

• Landslide Countermeasures

  Identification of prone areas through hazard zonation maps.

  Factors influencing development decisions: slope angle, rock types, past occurrences.

• Rock Fall Solutions

  Methods: rock trap ditches, rock catching nets, shelters, wire meshing, fencing.

  Considerations: Material size, probability of fall events, weathering state of rock mass.

Monitoring Landslide-Prone Areas

• Monitoring landslide-prone areas at different spatial scales is crucial for prevention and preparedness.
• Ground surveying is essential, involving measurements at fixed reference points.
• Methods like monitoring rock displacement and crack widening using instruments like glued studs and inclinometers in boreholes help track any movement.

Prevention Strategies

• Utilize landslide zoning maps to consider various options based on the risk levels identified.
• Implement slope modification techniques such as excavation, filling, and adjusting pore water pressure.
• Reduce the weight of rock masses through dewatering and enhance drainage systems.
• Enhance slope stability through vegetation like turfing and afforestation, which strengthen the soil.

Structural Support

• Construct gabion walls, concrete buttresses, retaining walls, dental masonry, and concrete spray for mass support.
• These structures provide essential reinforcement to slopes susceptible to landslides.

Additional Measures

• Ground anchors like grouted dowels, rock bolts, bored piles, and rock anchors enhance tensile strength and counteract landslide forces.

By implementing these strategies and measures, we can effectively safeguard against landslides, protecting infrastructure and communities.

Geology Ground Support Methods

• Toe
• Ditch
• R
• Shear
• Key
• Anchored Wall
• Earth Trap
• Bank Ditch
• C Pattern
• Bolting
• Chain Link Fencing
• Weak Rock

Fig. 5 Different kinds of ground support to stabilize the ground.

Undercutting or lateral scouring by river or sea waves, known as toe, can lead to landslides. This can be controlled by erecting gabion walls, verabili, tripods, and other specific options, especially to stay safe from rockfall.

• Rock Bolts
• Bored Piles
• Strong Rock
• Net in Catenary
• Grady to lay
• Layered Rocks
• Suspended Netting

Fig. 6 Some other methods for mitigating the problem of rockfall to protect a road/rail track.

These methods may include rip rap or keeping road/rail tracks safe from rockfall.