Slope Management in Hill Roads and Landslides

yogendra borse

Hill slopes across highways 

 
            A) Based on height

            B) Base on slope inclination & presence of overhangs

            C) Based on Geological classifications

Definition of Hill slopes across Highway

Convex slope segments commonly occur in the upper parts of soil man taled slopes, as near the drainage divide

Straight slope segments are dominated by mass movement processes.

Talus slopes are a type in which debris piles up to a characteristic angle of repose.

When new debris is added to the slope, thereby locally increasing the angle, the slope adjusts by movement of the debris to re-establish the angle.

Concave slopes are especially common where overland-flow runoff transports sediment derived 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

                    Colluviums

                    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 macrofeatures 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) :- This method utilizes the refraction of seismic waves on geologic layers -rock/soil units in order to characterize the subsurface conditions and geologic structure. Shots are deployed at surface and recordings are made using geophones.
            

Some natural phenomena that can trigger a landslide are
        Earthquakes, volcanic eruptions, and seismic shaking
        Intense rainfall, rapid snowmelt, glaciers melting, and changes in water level
        Stream/coastal erosion and natural dam failure
        Rising of groundwater or increase of pore water pressure
        The undercutting of cliffs and banks by water erosion due to 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 :- Zonation of highway hill slopes based on their vulnerability classes (methods and applications)

Identification/estimation/calculation of risk due to vulnerable highway hill slopes

        Terrain/Slope inventory
        Slopes
            Man made slopes

            Natural slopes
            Rock slopes

 
            Debris slopes/Block Slopes

            Soil slopes

 
            Composite slopes

            Bedding controlled slopes

        Geology
            Discontinuities

 
            Shear zones and Faults/thrust Contact surfaces

             Slickensides surface
            Weak materials e.g. clay seams
            Colluvium- material characteristics
            Alluvium – Characteristics
            Ancient landslide
            Slope morphology
            Drainage morphology
            Vegetation type

            Slide type
            Micro features on the slope
            Cracks/fracturs/subsidence
            Sinking/cavities
            Bulging
            Deformed ground
            Break in slope
            Hummocky ground
            Drainage system

 
        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
            N. 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 at Toe Slope,
            Rock or Gravel Fill at Toe of Slope
            By Replacing Existing Soil with Low density material, such as Geofoam

            Application of Geosynthetics for hill slopes stabilizations


    Selection of Structures for Protection-
            Debris Arrestors,
            Retaining Wall - PCC wall section Calculator

                Rock and Earth Fill Buttress at Toe of Slope,
                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 of increase of Shear Strength of Soil Cementation,

             Freezing, Electro-Osmosis,
             Compaction, Rock Bolting, Blasting at Toe

Note: - Maintenance of slope protections structures are equally important

Drainage System, <-- click on link to read 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 --> Express way, National Highway, State Highway, 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 the area driving the landslide (with possible substitution by lightweight fill)
✓ Adding material to the area maintaining stability (counterweight berm or fill)
✓ Reducing general slope angle

DRAINAGE
✓ Surface drains to divert water from flowing onto the slide area (collecting ditches and pipes)
✓ Shallow/deep trench drains filled with free draining geomaterials (coarse granular fills and geosynthetics)
✓ Buttress counter forts of coarse-grained materials (hydrological effect)
✓ Vertical (small diameter) boreholes with pumping or self-draining
✓ Vertical (large diameter) wells with gravity draining
✓ Sub-horizontal or sub-vertical boreholes
✓ Drainage tunnels, galleries or adits
✓ Vacuum dewatering
✓ Drainage by siphoning
✓ Electro-osmotic dewatering
✓ Vegetation planting (hydrological effect)

RETAINING STRUCTURES
✓ Gravity retaining walls
✓ Crib-block walls
✓ Gabion walls
✓ Passive piles, piers and caissons
✓ Cast-in situ reinforced concrete walls
✓ Reinforced earth retaining structures with strip/ sheet - polymer/metallic reinforcement element
✓ Buttress counterforts of coarse-grained material (mechanical effect)
✓ Retention nets for rock slope faces
✓ Rockfall attenuation or stopping systems (rocktrap ditches, benches,fences and 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)