Slope Management in Hill Roads and Landslides

Hill Slopes across Highways – Classification

• A) Based on height
• B) Based on slope inclination & presence of overhangs
• C) Based on Geological classifications

Definition of Hill Slopes across Highways

• Convex slope segments – commonly occur in upper parts of soil mantled slopes, near drainage divide
• Straight slope segments – dominated by mass movement processes
• Talus slopes – debris piles up to characteristic angle of repose; adjusts by movement when new debris is added
• Concave slopes – common where overland-flow runoff transports sediment from upper slopes
• Virgin slopes
• Highway cut slopes
• Manmade slopes

Components of Hill Slope across Highways

Geological, Geomorphological, Geohydrological, Geotechnical, Ecological/Environmental Interactions and Interrelationships between Slope Components

Investigations of Hill Slopes & Processes

Geological Investigations – Type of material, structures

Identification of material:

A) Soil Strata

• Sandy soil
• Clayey soil
• Residual soil
• Talus
• Colluvium
• Moraines, glacial
• Alluvium, Loess

B) Rocky Strata

• Type of Rock
• Weathering extent
• Joint pattern, Joints, Faults, Folds, Fractures

C) Mixed Strata

• Mix of soil + boulders
• Properties and classification

Geomorphological Investigations

Type of slope, steepness and various features on the slope

Micro and macro features of morphology:

• Erosion of slope
• Presence of streams
• Presence of waterfalls
• Presence of vegetation
• Extent of Erosion
• Piping phenomenon

Geohydrological Investigations

• Drainage network, springs, Rivers etc.
• Surface and Sub-surface Drainage Investigations
• Watershed Management

Geotechnical Investigations

Strength properties of the material

Seismic Refraction Tomography (SRT): Utilizes refraction of seismic waves on geologic layers to characterize subsurface conditions and geologic structure. Shots at surface, recordings via geophones.

Natural Phenomena that can Trigger Landslides

• Earthquakes, volcanic eruptions, seismic shaking
• Intense rainfall, rapid snowmelt, glacier melting, changes in water level
• Stream/coastal erosion and natural dam failure
• Rising groundwater or increase of pore water pressure
• Undercutting of cliffs and banks by water erosion (tidal waves)
• Anthropogenic action

Highway Hill Slope Vulnerability and Risk Assessment

• Highway hill slope rating system
• Rating criteria for identification and classification of highway slopes
• Suitability/vulnerability and potential for hill slope failure
• Zonation of slopes – based on vulnerability classes (methods & applications)

Identification / Estimation / Calculation of Risk due to Vulnerable Highway Hill Slopes

Terrain / Slope Inventory

• Man-made slopes
• Natural slopes
  • Rock slopes
  • Debris slopes / Block Slopes
  • Soil slopes
  • Composite slopes
  • Bedding controlled slopes

Important Factors / Features

• Geology – Discontinuities, Shear zones, Faults, Slickensides, Weak materials, Colluvium, Alluvium, Ancient landslide
• Slope morphology, Drainage morphology, Vegetation type, Slide type
• Micro features: Cracks/fractures/subsidence, Sinking/cavities, Bulging, Deformed ground, Break in slope, Hummocky ground
• Hydrology: Slope hydrology, Catchment, Ephemeral drainage, Topography vs Drainage flow, Scouring/Erosion
• Geohydrology: Perched groundwater flow, Water table, Streams/springs
• Landslides: Active/dormant, Debris flow , Rockfall, Slumps, Slides, Subsidence
• Infrastructure: National Highway, SH-MDR-VR, Railways, Irrigation Canals, Dams etc.

Highway Hill Slope Protection Structures (Construction & Maintenance)

Stabilization of Slopes

• Excavation at Top of Slope
• General Flattening of Slope
• Benching of Slope
• Complete Removal of Unstable Mass
• Earth Fill / Rock or Gravel Fill at Toe of Slope
• Replacing Existing Soil with Low density material e.g. Geofoam
• Application of Geosynthetics

Selection of Structures for Protection

• Debris Arrestors
• Retaining Wall (PCC wall section Calculator)
• Rock and Earth Fill Buttress at Toe
• Cribs or Gravity Retaining Wall
• Breast Wall, Toe Wall
• Pile Walls
• Benching, Filter beds, Easing of Slopes
• Bitumen/Asphalt mulching, Chutes and Sloping Aprons, Turfing
• Caisson Toe of Slope
• Barriers at Toe anchored by Tie-Back Method
• Increase of Shear Strength: Soil Cementation, Freezing, Electro-Osmosis
• Compaction, Rock Bolting, Blasting at Toe

Note: Maintenance of slope protection structures is equally important.

→ Drainage System (detailed article)

Attributes / Factors for Slope Management Inventory

• A – Location
• B – Age of cutting
• C – Type of slope
• D – Mode of failure
• E – Type of lithology
• F – Thickness of soil/debris on affected slopes
• G – Thickness of overburden
• H – Vegetation on affected slope
• I – Vegetation above / beyond affected slope
• J – Height of cutting
• K – Slope of cutting
• L – Cut slope angle vs. Uphill natural slope
• M – Water conditions
• N – Deformation characteristics (cracks/subsidence/failure)
• O – Joint sets
• P – RMR (rock slope)
• Q – Direction of bedding plane / foliation
• R – Natural Drainage Preservation
• S – Type of Road (Expressway, NH, SH, MDR, ODR, VR, NP etc.)
• T – Number of Road
• U – Chainage of Landslide location

Brief List of Remedial Measures for Unstable Slope

Modification of Slope Geometry

• Removing material from driving area (with possible lightweight fill substitution)
• Adding material to maintaining area (counterweight berm/fill)
• Reducing general slope angle

Drainage

• Surface drains to divert water
• Shallow/deep trench drains with free-draining geomaterials
• Buttress counterforts (hydrological effect)
• Vertical boreholes (pumping / self-draining)
• Sub-horizontal / sub-vertical boreholes
• Drainage tunnels, galleries, adits
• Vacuum dewatering, Siphoning, Electro-osmotic dewatering
• Vegetation planting (hydrological effect)

Retaining Structures

• Gravity retaining walls, Crib-block walls, Gabion walls
• Passive piles, piers, caissons
• Cast-in-situ reinforced concrete walls
• Reinforced earth structures (strip/sheet – polymer/metallic)
• Buttress counterforts (mechanical effect)
• Retention nets, Rockfall attenuation systems (ditches, benches, fences, walls)
• Protective rock/concrete blocks against erosion

Internal Slope Reinforcement

• Rock bolts, Micro piles, Soil nailing, Anchors
• Grouting, Stone/lime cement columns
• Heat treatment, Freezing, Electro-osmotic anchors
• Vegetation planting (root strength mechanical effect)