Engineering Guidelines on Landslide Mitigation Measures for Indian Roads

Scope of IRC:SP:106-2015 (Landslide Management)

The code covers comprehensive guidelines for landslide hazard assessment, monitoring, and mitigation relevant to Indian conditions.

Key Points from Scope:

• Landslide Features & Geometry: Defined in Clause 2.2 (see Table 2.1 for detailed descriptions).
• Hazard Mapping Scales (Table 4.1):

• Instrumentation Selection (Table 6.5): Instruments should be chosen based on:
  • Critical & complementary parameters
  • Ground & environmental conditions
  • Data acquisition needs
  • Instrument life, quality, and performance (range, resolution, accuracy, precision)

Instrument Performance Specification:

flowchart TD
    A[Scope of IRC:SP:106] --> B[Landslide Features & Geometry]
    A --> C[Hazard Mapping Scale Selection]
    A --> D[Instrumentation & Monitoring]
    C --> E[Regional Planning (1:50k-1:100k)]
    C --> F[Specific Policy (1:25k-1:50k)]
    C --> G[Project Planning (1:5k-1:25k)]
    C --> H[Site Planning (1:500-1:5k)]
    D --> I[Select Instruments by Parameters]
    D --> J[Consider Environmental & Ground Conditions]
    D --> K[Data Acquisition & Instrument Life]

IRC SP 106-2015: Terminology & Definitions for Landslides (Clause 2.2 & 2.3)

Key Landslide Features (Table 2.2)

Dimensional Features of Landslides (Table 2.3)

Visual Summary of Key Landslide Features

flowchart LR
    Crown --> MainScarp(Main Scarp)
    MainScarp --> Top(Top)
    Top --> Head(Head)
    Head --> MainBody(Main Body)
    MainBody --> Toe(Toe)
    Toe --> Tip(Tip)
    MainBody --> SurfaceRupture(Surface of Rupture)
    SurfaceRupture

Landslide Hazard Assessment - IRC SP 106 Key Points

1. Hazard Analysis Techniques (Table 4.3 Summary)

2. Landslide Inventory Mapping

• Purpose: Foundation for hazard mapping and risk assessment.
• Data: Past & active landslides, type, activity state, volume, geomorphic attributes (slope, lithology, soil, moisture).
• Mapping scale: Inventory maps at larger scale than hazard maps.
• Methods: Field surveys, aerial photo interpretation.

3. Hazard Assessment Approach

• Combine qualitative expert judgment with quantitative data.
• Determine susceptibility classes: High, Medium, Low.
• Use statistical models to estimate probability and magnitude.
• Stability models (safety factor) applied for detailed slope analysis.

Basic Formula for Safety Factor (Deterministic Analysis):

[ F_s = \frac{\text{Resisting Forces}}{\text{Driving Forces}} = \frac{c' + (\sigma - u) \tan \phi'}{\tau} ]

Where:

• ( c' ) = effective cohesion
• ( \sigma ) = normal stress
• ( u ) = pore water pressure
• ( \phi' ) = effective angle of internal friction
• ( \tau ) = shear stress

Conceptual Flow of Landslide Hazard Assessment

Key Formulas, Tables & Specifications for Field Investigations and Mapping (IRC SP 106 - Clause 5.1.2)

1. Objectives of Detailed Field Investigation

• Delineate type, size, mechanism of landslide
• Map aerial extent and direction of deformation
• Locate slide plane(s) and nature of landslide blocks
• Assess possibility of future movement
• Study groundwater distribution and soil types

2. Phases of Investigation (Fig. 5.1 summary)

• Preliminary investigation → Detailed field investigation → Subsurface exploration
• Use thematic maps, aerial photos, remote sensing & GIS
• Produce: Landslide Inventory, Susceptibility, Hazard Zonation, Risk Assessment maps

3. Subsurface Investigation (Table 5.1.2.5 summary)

4. Common In-situ Tests (Table 5.1)

5. Important Notes

• Use remote sensing and GIS for mapping and susceptibility analysis.
• Different approach for virgin areas (new roads)

Techniques for Monitoring Landslides (IRC SP 106 Highlights):

1. Drainage & Surface Protection (Clause 8.5)

• Surface Water Drains: Table drains with impermeable lining; must be regularly cleared.
• Surface Protection: Gabions and mattresses for erosion control and eco-compatible slope stabilization.
• Sub-soil Drains: Laid behind retaining walls; must have a fall of 1 vertical in 100 horizontal; use sand/gravel bed with graded stone/geotextile filter.
• Deep Underground Drains: Used in extreme cases to lower water table permanently; monitor flow changes unrelated to rainfall as warning signs.

2. Remote Sensing Methods (Clause 6.3.2.2)

• InSAR & DInSAR: Satellite or ground-based radar for detecting slope displacement.
• High-Resolution Optical/Infrared Imaging: IKONOS, QUICKBIRD, multi-temporal aerial photogrammetry.
• Ground-based Differential SAR Interferometry: For detailed slope motion.
• Aerial Photography: Visual interpretation for landslide inventory and evolution.

3. Key Specifications

4. Monitoring Instruments & References

• Use sensor-based early warning systems (SLEWS).
• Refer to ASTM D4750-87(2001) for subsurface liquid level measurement.
• Instrumentation includes tiltmeters, crack width gauges, and settlement devices (Bhandari et al.).

flowchart TD
    A[Landslide Monitoring] --> B[Surface Water Drains]
    A --> C[Surface Protection (Gabions)]
    A --> D[Sub-soil Drains]
    A --> E[Deep Underground Drains]
    A --> F[Remote Sensing]
    F --> G[InSAR / DInSAR]
    F --> H[Optical & Infrared Imaging]
    F --> I[Aerial Photography]

Summary:
Effective landslide monitoring combines drainage control, surface

Slope Stability Analysis - IRC SP 106 Key Points

1. Remedial Measures for Unstable Slopes (Table 8.1)

2. Drainage Specifications (Clause 8.5)

• Surface drains: Should have impermeable lining except in rock; regular clearing required.
• Sub-soil drains: Laid in sand/gravel beds with geotextile filter; slope minimum 1 vertical:100 horizontal.
• Deep underground drains: Used in extreme cases to lower water table permanently.

3. Basic Factor of Safety (FoS) Formula for Slope Stability

[ FoS = \frac{\text{Resisting Forces or Moments}}{\text{Driving Forces or Moments}} ]

• Typical methods: Limit equilibrium (e.g., Bishop, Janbu), Shear strength reduction.
• Shear strength parameters: cohesion (c), angle of internal friction (φ).

4. Decision Flowchart for Remedial Measures

flowchart TD
    A[Slope Instability Detected] --> B{Cause?}
    B -->|Geometry| C[Modify Slope Geometry]
    B -->|Water| D[Improve Drainage]
    B -->|Soil Strength| E[Internal Reinforcement]
    B -->|Load|

Planning for Existing and New Highways (IRC SP 106)

Key Points from Clauses 7.1.1 & 7.1.2:

• New Highways (7.1.1):

  • Alignment selection involves multidisciplinary inputs (transport, environmental, geological, geotechnical).
  • Identify and avoid hazardous zones or design protective measures if avoidance isn't possible.
  • Decisions impact construction, maintenance, safety, and costs long-term.
  • Follow detailed guidance in IRC:SP:20-2002 (Rural Roads Manual) and IRC:SP:48-1998 (Hill Road Manual).

• Existing Highways (7.1.2):

  • Focus on upgrading, widening, or local improvements.
  • Constraints: existing structures, utilities, topography, and traffic flow during construction.
  • Stability of slopes is a primary concern; protective works as per IRC:SP:48-1998 and IRC:SP:73-2007 (Hill Roads).

Essential Specifications & Formulas:

Typical Slope Stability Factor of Safety (FOS):

[ FOS = \frac{\text{Resisting Forces}}{\text{Driving Forces}} \geq 1.5 \quad \text{(for permanent slopes)} ]

Route Alignment Selection Flow:

flowchart TD
    A[Start: Define Project Objectives] --> B[Survey & Hazard Identification]
    B --> C{Hazardous Zone?}
    C -- No --> D[Select Alignment]
    C -- Yes --> E[Design Mitigation Measures]
    E --> D
    D --> F[Check Constraints (Structures, Utilities, Traffic)]
    F --> G[Finalize Alignment]
    G --> H

Landslide Risk Management & Mitigation (IRC SP 106)

Key Principles (Clause 4.6.2)

• Hazard Mapping: Identify and map landslide-prone zones.
• Avoidance: Prevent development in high-risk areas.
• Risk-Based Planning: For developed areas, assess and mitigate risks.
• Risk Communication: Inform stakeholders about landslide risks, especially in built-up zones.

Risk Management Framework (Clause 4.4, Fig 4.4)

1. Establish Scope/Goal
2. Hazard/Risk Identification
   • Characterize landslide (type, geometry, movement rate)
3. Risk Assessment
   • Elements at risk, probability, consequences
4. Risk Evaluation
   • Compare with tolerable risk
5. Treatment Options
   • Accept, avoid, reduce likelihood, reduce consequences
6. Implementation
7. Monitoring & Review

Common Formulas & Concepts

• Factor of Safety (FOS):
  [ FOS = \frac{\text{Resisting Forces}}{\text{Driving Forces}} \quad (FOS > 1 \text{ is safe}) ]

• Slope Stability Analysis:
  Use limit equilibrium methods (e.g., Bishop’s method) to compute FOS.

Typical Mitigation Measures

• Surface Drainage Control: Divert water to reduce pore pressure.
• Slope Reinforcement: Retaining walls, anchors, soil nails.
• Vegetation: Root systems stabilize soil.
• Grading: Reduce slope angle.
• Warning Systems: Monitoring movement and early warning.

flowchart TD
    A[Identify Hazard] --> B[Risk Assessment]
    B --> C{Risk Evaluation}
    C -->|Accept| D[Monitor]
    C -->|Avoid| E[Restrict Development]
    C -->|Reduce Likelihood| F[Engineering Measures]
    C -->|Reduce Consequences| G[Emergency Planning]
    F --> D
    E --> D
    G --> D

References: IRC SP 106:2015, Clause 4.6, Fig 4.4.

IRC SP 106: Remedial Measures & Slope Stabilization Techniques

Key Remedial Measures (Table 8.2 Summary)

Stabilization Methods (Table 8.1 Overview)

• Slope Geometry Modification: Remove/add material, reduce slope angle.
• Drainage: Surface drains, boreholes, tunnels, vacuum dewatering.
• Retaining Structures: Gravity walls, gabions, reinforced earth walls.
• Internal Reinforcement: Rock bolts, micro piles, soil nails, anchors, grouting.
• Bio-engineering: Vegetation for hydrological and mechanical effects.

Design Considerations

• Retaining walls must be founded below slip surface.
• Drainage effective if water table is above slip surface.
• Anchors and soil nails require specialist installation and monitoring.
• Bio-engineering suitable only for shallow or moderate slopes.

Decision Flow (Simplified)

flowchart TD
    A[Assess Slope Problem] --> B{Type of Failure?}
    B -->|Landslide| C[Stabilize: Reinforce or Remove]
    B -->|Surface Erosion| D[Retaining Wall & Drainage]
    C --> E{Is Drainage Needed?}
    E -->|Yes| F[Design Drainage System]
    E -->|No| G[Design Retaining Structures]
    D --> G
    F --> H[Surface Protection & Bio-engineering]
    G --> H

Key Formulas, Tables & Specifications for Retaining Structures (IRC SP 106)

1. Types of Retaining Structures (Table 8.1, Clause 8.1)

2. Drainage & Surface Protection (Clause 8.5)

• Surface drains: Should have impermeable lining except in rock; regular clearing mandatory.
• Sub-soil drains: Laid with sand/gravel bedding and geotextile filter; minimum slope 1:100 (V:H).
• Deep underground drains: Used in extreme landslide risk; permanently lower water table.
• Surface protection: Gabions, mattresses for flexible erosion control and eco-compatibility.

3. Design References

• IS:14458 (Parts 1-3) for retaining wall design & construction.
• IS:14458 (Part 10) for reinforced earth retaining walls.
• IRC:56-2011 for embankment slope erosion control.

4. Drainage Slope Specification

Minimum slope for sub-soil drains = 1 vertical : 100 horizontal (1%)

5. Stability Considerations (from Clause 7.3.2)

• Select retaining wall type based on slope geometry, soil conditions, and landslide risk.
• Incorporate drainage to reduce hydrostatic pressure.
• Use reinforced earth walls with polymer/metallic strips for flexible retaining.

Conceptual Diagram: Retaining Structure Components

graph TD
    A[Retaining Structure] --> B[Gravity Wall]
    A --> C[Crib-block Wall]
    A --> D[Gabion Wall]
    A --> E[Reinforced Earth Wall]
    A -->

IRC SP 106: Bioengineering & Greening Techniques (Clause 8.8)

Key Greening Techniques & Characteristics

Notes:

• Mulching mats require anchor pins and have limited root development space.
• Long-rooting grass offers non-invasive, quick cover but lacks long-term ecological balance.
• Fiber reinforcement promotes self-sustained vegetation with enhanced soil stability.

flowchart LR
    A[Greening Techniques] --> B[Mulching System]
    A --> C[Long-rooting Grass]
    A --> D[Fiber Reinforced Soil]
    B --> E[High adhesion, grass only]
    C --> F[Shotcrete holes, seasonal cover]
    D --> G[Diverse plants, soil strengthening]

For detailed application, refer to Table 8.8 and Clauses 8.3.7, 8.3.8.1 in IRC SP 106:2015.

Rockfall Protection Systems (IRC SP 106 - Clause 8.2.2 & Tables 8.3, 8.4)

1. Rockfall Drapery Systems (Table 8.3)

2. Rockfall Barriers (Table 8.4)

3. Flexible Barriers (IRC SP 106)

• Made of wire rings or high-strength mesh with energy absorption up to 8500 kJ.
• Supported by steel posts, anchor ropes, and deformable braking system.
• Fence fixed at bottom to hold rocks.
• Expensive; requires periodic cleaning; prone to damage under very high energy.

Key Design Considerations:

• Catchment Area: Essential for drapery and barrier systems, must be regularly cleaned.
• Rock Size: Draped nets typically limit to rocks ≤1.5 m diameter.
• Energy Absorption: Flexible barriers designed per European Technical Approval Guidelines.

Summary Diagram of Rockfall Protection Types

graph TD
  A[Rockfall Protection Systems]
  A --> B[Drapery Systems]
  A --> C[Barriers]
  B --> B1[Draped Mesh/Nets]
  B --> B2[Anchored Mesh/Nets]
  C --> C1[Earthen Barriers]
  C --> C2[Flexible Retaining Walls]
  C

IRC SP 106: Instrumentation and Monitoring Systems – Key Points

Instrument Selection Criteria (Table 6.5)

• Critical Parameters: Specify range, resolution, precision.
• Complementary Parameters: Monitor multiple parameters for complex behavior.
• Ground Conditions: E.g., diaphragm piezometers for low permeability soils.
• Environmental Conditions: Avoid hydraulic piezometers in freezing; prefer mechanical in tropics.
• Data Acquisition: Prefer instruments compatible with automatic data logging & alarms.
• Instrument Life: Choose durable instruments for long-term use.
• Instrument Quality & Performance: Balance cost vs accuracy, resolution, and precision.

Multi-Point Monitoring Systems (Table 6.4)

Common Instruments (Table 6.6)

Planning Steps (Clause 6.4)

1. Identify landslide & monitoring goals.
2. Select measurement types & instruments.
3. Define location, depth, and number of instruments.
4. Develop data acquisition method.
5. Manage & present data effectively.

Summary Diagram: Instrument Selection Factors

graph TD
    A[Instrumentation Selection]

Risk Assessment and Treatment (IRC SP 106)

Key Formulas

Quantitative risk estimation (Clause 4.6.10):

[ \mathbf{R = P \times P_s \times A \times P_v \times T \times S_e} ]

Where:

• R = Annual risk (probability of loss of property value/life)
• P = Annual probability of landslide occurrence
• P_s = Spatial probability landslide reaches property/person
• A = Area or asset value exposed
• P_v = Probability of vulnerability (damage given landslide reaches)
• T = Time exposure (duration of exposure)
• S_e = Severity or consequence factor

Risk Treatment Options (Clause 4.6.12)

• Accept: Only if regulator permits and risk is tolerable.
• Avoid: Relocate or redesign development to eliminate risk.
• Reduce Frequency: Slope stabilization (re-profiling, drainage, retaining walls, anchors).
• Reduce Consequences: Defensive structures (boulder fences), relocation, behavior amelioration.
• Manage: Monitoring & warning systems for early alerts.
• Transfer: Insurance or legal transfer of risk.
• Postpone: Further investigation if uncertainty is high; temporary risk acceptance.

Summary Table of Risk Treatment

flowchart TD
    A[Risk Assessment] --> B[Risk Estimation (R = P × Ps × A × Pv × T × Se)]
    B --> C[Risk Treatment Options]
    C --> D[Accept Risk]
    C --> E[Avoid Risk]
    C --> F[Reduce Frequency]
    C --> G[Reduce Consequences

IRC SP 106 - Case Studies & Applications: Key Points on Retaining Structures and Drainage

Retaining Structures (Clause 8.3.1)

• Retaining structures design references multiple codes and practices.
• Focus on stability, drainage, and eco-compatible surface protection.

Drainage & Surface Protection (Clause 8.5)

• Surface Water Drains: Table drains with impermeable lining preferred; must be regularly cleared.
• Surface Protection: Use gabions/mattresses for erosion control and natural aesthetics.
• Sub-soil Drains: Installed behind retaining walls; laid in sand/gravel bed with geotextile filter; minimum slope 1:100 (vertical:horizontal).
• Deep Underground Drains: For high-risk landslides; lower water table permanently; monitor flow changes for anomalies.

Instrumentation Selection (Table 6.5)

Typical Drain Slope Specification

Minimum slope for sub-soil drains = 1 vertical : 100 horizontal (1%)

Diagram: Drainage System behind Retaining Wall

flowchart LR
    A[Surface Water] --> B[Surface Drain (Impermeable lined)]
    B --> C[Sub-soil Drain]
    C --> D[Outlet]
    E[Retaining Wall] --- C
    F[Groundwater] --> C

References: ASTM A974/A975 (gabions), BS 8002 (earth retaining), and Australian Geoguide LR6 for retaining walls design details.

This concise summary captures key formulas, tables, and specifications for retaining structures and drainage from IRC SP 106, aiding practical application and monitoring.