Transportation Geotechnics¶

Session Summary

Session 10 of GeoVadis (GAIC 2025) investigates the demanding geotechnical challenges of highway and railway infrastructure. Core themes encompass back-to-back Mechanically Stabilized Earth (MSE) walls under cyclic train loading, recycled rubber grids from waste conveyor belts for ballast degradation control, 3D dynamic train-track-subgrade modeling, geocell-stabilized expansive pavement subgrades, EPS geofoam lightweight highway embankments, and pavement structural responses to electric vehicle (EV) axle loading.

1. Back-to-Back MSE Walls Under Heavy Rail Loading (S. Attara, B. Umashankar)¶

Constrained Approach Embankments¶

In narrow transportation corridors and bridge approach ramps, retaining walls are constructed back-to-back with overlapping or connected geogrid reinforcement layers. Attara and Umashankar analyze the dynamic performance of back-to-back MSE walls subjected to cyclic train loading (\(P_{axle} = 250\,\text{kN}\)): - Overlapping vs. Connected Reinforcement: When the distance between wall faces (\(W\)) is narrow (\(W/H < 1.4\)), connecting opposing geogrid layers directly prevents outward wall face bulging and reduces maximum lateral wall displacement by over \(40\%\) compared to unconnected overlapping sheets. - Dynamic Lateral Earth Pressure: High-speed wheel passage generates transient dynamic earth pressure spikes that attenuate exponentially with depth. Maximum geogrid tensile strains concentrate within the upper \(0.3H\) zone immediately below the track slab.

       Back-to-Back MSE Wall Geometry Under Cyclic Train Surcharge
       ┌────────────────────────────────────────────────────────┐
       │   Rail Tracks & High-Speed Train Axle Loads (250 kN)   │
       │   ══════════════════════════════════════════════════   │
       │   ┌────────────────────────────────────────────────┐   │
       ├───┴────────────────────────────────────────────────┴───┤
       │◄── Facing 1      Compacted Granular Backfill   Facing 2──►│
       ││ ════════════════════════════════════════════════════ ││
       ││ Connected Continuous Geogrid Reinforcement Layers   ││
       ││ ════════════════════════════════════════════════════ ││
       ││ (Zero Lateral Overlap Slippage; Bounded Deflection)  ││
       ││ ════════════════════════════════════════════════════ ││
       ├───┬────────────────────────────────────────────────┬───┤
       │   │ In-Situ Subsoil Foundation Bed                 │   │
       └───┴────────────────────────────────────────────────┴───┘

2. Waste Conveyor Belt Rubber Grids in Ballasted Tracks (S. Hettiyahandi, B. Indraratna, T. Ngo, Y. Qi, et al.)¶

Circular Economy in High-Speed Rail Track Maintenance¶

Ballast breakage and settlement account for over \(50\%\) of track maintenance budgets worldwide. Professor Indraratna’s research group develops perforated rubber grids upcycled from end-of-life industrial conveyor belts: - Energy Dissipation: When placed beneath the ballast bed, the high damping ratio and resilience of the rubber grid absorb impact shock waves generated by wheel-rail irregularities. - Ballast Degradation Reduction: Large-scale cyclic triaxial tests (\(500,000\) cycles at \(f = 15\,\text{Hz}\)) demonstrate that rubber grids reduce the Ballast Breakage Index (\(BBI\)) by \(45\% - 50\%\) compared to unreinforced ballast. - Track Resilient Modulus (\(M_R\)): The composite layer dampens cyclic track acceleration while preserving track elasticity, significantly reducing ballast tamping frequencies.

3. Geocell Stabilization of Expansive Pavement Subgrades (S. Srivastava, B. Umashankar, P.K. Mahopatra, et al.)¶

Controlling Differential Swelling-Shrinkage Cracking¶

Pavements constructed over high-plasticity expansive black cotton soils suffer from severe seasonal differential heave, longitudinal cracking, and alligator fatigue. Srivastava et al. investigate 3D geocell confinement mattresses installed at the subgrade-base interface: - All-Around Cellular Confinement: Geocell pockets confine granular aggregate, generating high localized hoop stresses that convert vertical wheel loads into wide horizontal stress distributions. - Traffic Benefit Ratio (\(TBR\)): Cyclic plate load testing and finite element modeling show that geocell reinforcement increases the Traffic Benefit Ratio (\(TBR\)) by \(2.5 - 3.8\), permitting a \(30\%\) reduction in asphalt layer thickness while completely suppressing surface rutting over wet-dry seasons.

4. Lightweight EPS Geofoam Embankments on Ultra-Soft Clays (V.V. Butle, G.S. Parvathi, A. Mandal, V. Srinivasan)¶

Zero-Consolidation Settlement Over Coastal Mud¶

Constructing conventional fill embankments (\(18 - 20\,\text{kN/m}^3\)) over deep coastal clays induces colossal consolidation settlement and global slope instability. Butle et al. evaluate ultra-lightweight Expanded Polystyrene (EPS) geofoam blocks (\(\gamma \approx 0.15 - 0.25\,\text{kN/m}^3\)): - Gravity Load Compensation: Replacing \(3\,\text{m}\) of earth fill with EPS geofoam reduces total overburden pressure by over \(90\%\), eliminating primary and secondary consolidation settlement. - Dynamic Rail/Highway Performance: FE simulations confirm that reinforced concrete load-distribution slabs atop EPS blocks bound cyclic compressive strains well within elastic limits (\(\epsilon < 1\%\)), ensuring long-term fatigue durability.

5. Pavement Structural Demands Under Electric Vehicles (P. Chaudhary, S. Saride)¶

Impact of Heavy Battery Packs on Pavement Longevity¶

Electric vehicles (EVs) feature heavy battery packs that increase vehicle curb weight by \(20\% - 35\%\) compared to equivalent internal combustion engine (ICE) vehicles: - Axle Load Spectra Shifts: Chaudhary and Saride analyze pavement damage ratios using heavy vehicle simulator data and Kenlayer viscoelastic modeling. - Fatigue & Rutting Damage: The heavier EV fleet increases bottom-up asphalt tensile strains (\(\epsilon_t\)) and subgrade compressive strains (\(\epsilon_v\)), accelerating subgrade rutting rates by \(28\% - 42\%\). - Design Recommendations: Standard design codes must adopt revised Equivalent Single Axle Load (ESAL) truck factors and specify polymer-modified binders or geosynthetic interlayers to preserve pavement service life.

6. Transportation Geotechnics Instrumentation Checklist¶

Transportation Asset	Primary Structural Sensor	Dynamic / Traffic Sensor	Key Monitored Performance Metric
Back-to-Back MSE Wall	High-Precision Inclinometer Casings	Dynamic Earth Pressure Cells	Wall lateral deflection, dynamic horizontal thrust
Ballasted Railway Track	Under-Sleeper Load Cells	Multi-Axis Triaxial Accelerometers	Dynamic sleeper contact stress, track vibration (\(a_{rms}\))
Geocell Highway Subgrade	Subgrade Soil Pressure Cells	Wheel Path Displacement Transducers (LVDT)	Subgrade vertical stress distribution, surface rut depth
EPS Geofoam Embankment	Multi-Point Magnetic Extensometers	Sub-Slab Earth Pressure Cells	Foundation clay settlement, geofoam elastic strain
Heavy EV Pavement Section	Asphalt Strain Gauges (Transverse/Longitudinal)	Pavement Multi-Depth Deflectometers (MDD)	Bottom-up tensile strain \(\epsilon_t\), resilient deflection