TRANSPORTATION ENGINEERING –I (CE-404) - Unit 4 Short Notes

UNIT 4: RAILWAY, BRIDGE, AND TUNNEL ENGINEERING

I. RAILWAY ENGINEERING FUNDAMENTALS & TRACK COMPONENTS

Hauling Capacity & Tractive Effort

• Hauling Capacity: Maximum load (tonnes) a locomotive can pull on a level track at a specified speed. It is a function of the locomotive's power.

• Tractive Effort (TE): The pulling force (in kN) exerted by the locomotive at the drawbar to move the train. It varies with speed.

• Key Difference: Hauling capacity is a mass (tonnes), while tractive effort is a force (kN). Hauling capacity is derived from tractive effort considering gradients and resistances.

Derivation of Tractive Effort (TE) Formula:

The total tractive effort required to move a train is the sum of forces to overcome all resistances:

$$ TE = R_a + R_g + R_c + R_t $$

Where:

• $$\displaystyle R_a $$ = Resistance due to acceleration (if any, often neglected for steady speed)

• $$\displaystyle R_g $$ = Gradient resistance = $W \times \sin \theta \approx W \times g\%$ (where $W$ is total weight, $g\%$ is gradient in percent)

• $$\displaystyle R_c $$ = Curve resistance (for each degree of curve or radius $R$)

• $$\displaystyle R_t $$ = Basic train resistance (rolling friction, air resistance, etc., often given by Davis formula: $$\displaystyle R_t = A + BV + CV^2 $$)

For a locomotive on a level track with no curve, the maximum available TE at the drawbar is:

$$ \boxed{TE_{available} = \mu \times W_l} $$

Where:

• $\mu$ = Coefficient of adhesion (depends on rail/wheel condition, ~0.25 for dry, ~0.15 for wet)

• $$\displaystyle W_l $$ = Weight on driving wheels of locomotive (in kN)

[!TIP] Exam Focus: Derive TE considering all components. Remember: Adhesion limit ($$\displaystyle \mu W_l $$) is the fundamental upper bound for TE. Gradient and curve resistances reduce the effective hauling capacity.

Factors Affecting Tractive Effort:

1. Adhesion Coefficient ($\mu$): Primary factor. Increases with clean, dry rails; decreases with moisture, grease, or leaves.

2. Locomotive Weight on Drivers ($$\displaystyle W_l $$): Directly proportional. More weight on powered wheels increases maximum TE.

3. Speed: Adhesion coefficient generally decreases with speed. TE from adhesion drops at high speeds.

4. Gear Ratio: For diesel/electric locos, lower gear ratios provide higher TE at low speeds (for freight), higher ratios for higher speeds (for passenger).

Rail Sections, Fastenings & Sleepers

A. Rails

• Types by Section: Double-headed, Bull-headed, Flat-footed (most common now).

• Types by Material: Carbon steel (high tensile strength, wear-resistant).

• Types by Length: Standard lengths (e.g., 12m, 13m, 22m, 26m in India). Longer rails reduce number of joints.

• Rail Wear: Loss of metal from rail head due to:

  • Abrasive wear (from wheel-rail contact).

  • Fatigue wear (from repeated stresses - shelling, cupping).

  • Corrosive wear.

B. Rail Fastenings

C. Sleepers (Sleepers)

D. Rail Creep

• Definition: Longitudinal movement of rails relative to sleepers.

• Causes: Repeated wheel thrust (especially during acceleration/braking), temperature changes (thermal expansion if not fully restrained), inadequate fastenings.

• Effects: Buckling (in hot weather), misalignment of points & crossings, gap closure at joints.

• Prevention: Adequate fastenings (spikes, clips), proper consolidation of ballast, anti-creep devices (like crab anchors).

Ballast

• Requirements of Good Ballast:

  1. Hard & Angular: To resist abrasion and provide interlocking.

  2. Durable & Weather-resistant: Against frost, rain.

  3. Porous & Draining: To allow free drainage of water.

  4. Clean: Free from dust, clay, organic matter.

  5. Proper Gradation: Mix of sizes (typically 25-65 mm) for good load distribution and drainage.

• Common Materials & Performance:

  • Broken Stone (Grit): Best - hard, durable, excellent drainage. (Granite, quartzite).

  • Gravel: Good, but may have rounded particles (less interlock).

  • Sand: Poor - low interlock, gets displaced easily, frost heave risk.

  • Slag: Fair - but may be chemically reactive/expansive.

  • Coal Cinders: Poor - soft, dusty, acidic.

• Functions of Ballast:

  1. Load Distribution: Transmits wheel load from sleeper to sub-grade.

  2. Drainage: Provides a porous medium.

  3. Resilience & Elasticity: Absorbs shocks/vibrations.

  4. Stability: Holds track in position (lateral & longitudinal).

  5. Adjustability: Allows easy raising/lowering of track for maintenance.

Wheel-Rail Interface & Track Geometry

A. Coning of Wheels

• Principle: Wheels are conical (tapered) with a slope of about 1:20. The effective rolling diameter changes as the wheel moves laterally on the rail.

• Advantages:

  1. Self-centering: On straight track, the wheel-set centers itself.

  2. Facilitates Curve Negotiation: Outer wheel uses larger diameter, inner wheel uses smaller diameter, allowing different travel distances without slipping.

  3. Reduces Wear: Minimizes flange contact on curves.

B. Gauge & Widening on Curves

• Gauge: Distance between inner faces of rails (measured 14 mm below top).

  • Standard Gauge (Broad Gauge in India): 1676 mm.

• Widening of Gauge on Curves:

  • Need: Due to coning of wheels, flange clearance, and rigid wheelbase, the actual wheel-set occupies more width than the nominal gauge on a curve. Without widening, rails would be forced outward, increasing stress.

  • Calculation (Empirical - Indian Practice):

$$ \text{Widening } (W) = \frac{(L^2 + R^2)^{1/2} - R}{R} \times \text{Gauge} \approx \frac{L^2}{8R} \text{ (for small angles)} $$

    Where $L$ = wheelbase (m), $R$ = curve radius (m).

*   **Importance**: Prevents rail binding, reduces derailment risk, ensures smooth running.

C. Types of Gradients

II. RAILWAY TRACK GEOMETRIC DESIGN

Horizontal Curves

• Types:

  1. Simple Curve: Single constant radius.

  2. Compound Curve: Two or more simple curves of different radii on same side of tangent (no straight transition).

  3. Reverse Curve: Two simple curves of opposite curvature meeting at a common tangent point (PT/PC coincide). Requires transition curves for safety.

  4. Transition Curve (Spiral): Gradually varying radius from straight (infinite) to full curve radius. Essential for high-speed safety, provides gradual steering, reduces lateral jerk.

Super Elevation (Cant)

• Definition: Raising the outer rail above the inner rail on a curve to counteract the effect of centrifugal force.

• Purpose/Necessity: To provide a comfortable, safe ride by balancing part of the centrifugal force, reducing lateral pressure on rails and wheels, and allowing higher speeds.

• Derivation of Equilibrium Cant ($e$):

  At equilibrium, component of centrifugal force balanced by cant:

$$ e = \frac{GV^2}{127R} \quad \text{or} \quad e = \frac{V^2}{gR} \quad (\text{in consistent units}) $$

Where:

*   $e$ = Super elevation (m)

*   $G$ = Gauge (m) (1.676 m for BG)

*   $V$ = Speed of train (km/h)

*   $R$ = Radius of curve (m)

*   $g$ = Acceleration due to gravity (9.81 m/s²)

• Cant Deficiency ($D$):

  • Concept: The shortfall in super elevation for a given speed. When actual cant ($$\displaystyle e_a $$) < equilibrium cant ($e$), deficiency $$\displaystyle D = e - e_a $$.

  • Significance: Represents unbalanced lateral acceleration felt by passengers. Codes limit maximum $D$ (e.g., 75 mm for BG in India) for comfort and safety.

• Negative Super Elevation:

  • Definition: Outer rail is lower than inner rail. Occurs on curves where slow-speed trains run on a curve designed for higher-speed trains.

  • Advantages (with sketch):

    1. Prevents slow trains from exerting excessive inward lateral force on outer rail (which would happen if full cant were provided).

    2. Reduces wear on outer rail flange for slow trains.

    3. Allows sharing of curve by trains of vastly different speeds.

  • Sketch Concept: Show a curve with outer rail lower than inner rail, indicating slow train direction.

[!TIP] Exam Key: Always write the equilibrium cant formula in standard form: $$\displaystyle e = \frac{GV^2}{127R} $$. Remember: Cant Deficiency = Equilibrium Cant - Actual Cant.

Effect of Curve Radius on Train Speed & Safety:

• Smaller Radius → Higher centrifugal force → Requires more super elevation for same speed.

• For a given cant, smaller radius limits maximum safe speed ($$\displaystyle V_{max} \propto \sqrt{R} $$).

• Very small radii require transition curves and impose severe speed restrictions, increasing journey time.

• Excessive speed on small-radius curves causes wheel climb (derailment risk) and rail/wheel wear.

III. RAILWAY YARDS, SIGNALING & INTERLOCKING

Railway Stations & Yards

Marshaling Yard (Classification Yard):

• Purpose: To sort incoming wagons from different trains and form new outgoing trains (rake formation).

• Layout Types: Gravity yard (uses gravity for sorting), Flat yard (uses shunting engines), Hump yard (uses a small hill - hump - for gravity sorting; most efficient).

Signaling Systems

Classification:

1. By Function:

   • Stop Signals (Danger/Red): Prohibit entry.

   • Distant Signals (Caution/Yellow): Warn of stop signal ahead.

   • Shunting Signals: Control movements within yards.

2. By Location:

   • Station Signals: Protect station limits.

   • Block Signals: Protect sections between stations (block sections).

3. By Aspect (Number of lights/position):

   • Two-Aspect: Red/Green.

   • Three-Aspect: Red/Yellow/Green.

   • Four-Aspect: Red/Yellow/Green + Yellow-Green (or Lunar for shunting).

Types in Stations/Yards:

• Home Signal: First stop signal at station entrance.

• Starter Signal: Allows train to leave station.

• Advanced Starter: Protects exit from station to block section.

• Shunting Signals: For movements within yard limits.

Interlocking Systems

• Principle: Mechanical/Electrical locking of points, signals, and tracks to ensure safe sequence of operations. A signal cannot be set to "clear" unless the corresponding route (points set correctly, track section clear) is secured.

• Mechanical Interlocking:

  • Layout: Tappet, lever, and connecting rods in a signal cabin.

  • Working: Levers for points and signals are interconnected via tappets and locking bars. A lever cannot be moved unless the required locking condition is satisfied (e.g., a signal lever cannot be pulled to "off" unless the point lever is already locked in correct position).

  • Components: Levers, locking frames, tappets, connecting rods, signal wires/rods.

• Electrical Interlocking (EI):

  • Working: Uses relays and electrical circuits. Position of points and signals are sensed electrically. Logic is implemented in relay circuits. A signal can clear only if all necessary relays (for route, track clearance) are energized.

  • Comparison:

    | Feature | Mechanical Interlocking | Electrical Interlocking | | :--- | :--- | :--- | | Mechanism | Physical levers, rods, tappets | Relays, circuits, switches | | Speed | Slow (manual operation) | Fast (electrical operation) | | Complexity | Limited for complex layouts | Can handle very complex layouts | | Maintenance | Mechanical wear | Electrical faults | | Modern Use | Heritage/less busy | Standard for modern stations |

Turnouts (Points & Crossings)

• Components:

  1. Switch (Points): Pair of tapered rails (stock rail & tongue rail) that guide wheels from one track to another.

  2. Crossing (Frog): Unit where two tracks cross. Has a nose and wing rails to guide wheel flanges.

  3. Check Rails: Guard rails placed opposite the nose to prevent wheel from striking the point of the frog.

  4. Lead/Straight: Track between switch and crossing.

  5. Closure Rails: Connect switch to crossing.

• Working Principle: The tongue rail is moved (by lever/motor) to make contact with one of the stock rails, thereby defining the route. The wheel flange is guided by the tongue rail through the frog onto the desired track.

IV. BRIDGE ENGINEERING: PLANNING, INVESTIGATION & LOADING

Bridge Planning & Site Selection

• Factors Affecting Site Selection:

  1. Hydrological: Flood level, discharge, scour depth, stream flow pattern.

  2. Geological: Foundation soil/rock bearing capacity, fault lines, slope stability.

  3. Topographic: Valley/river width, approach road alignment, terrain.

  4. Economic: Cost of foundation, approach roads, land acquisition.

  5. Navigational: Clearance (horizontal & vertical) for river traffic if applicable.

  6. Structural: Economical span length (minimizes total cost).

• Bridge Investigations & Surveys:

  • Preliminary: Topographic survey, hydrological study (flood data), reconnaissance.

  • Detailed: Geotechnical investigation (bore logs, SPT, lab tests), hydrological calculations (design discharge), scour analysis.

  • Construction: Setting out, material testing.

• Key Concepts:

  • Economical Span: Span length where total cost (superstructure + substructure + foundations) is minimum. Increases with better materials (steel > concrete).

  • Clearance:

    • Vertical Clearance (HFL): Minimum height from highest flood level or navigation requirement to bottom of superstructure.

    • Horizontal Clearance: Width between abutments/piers to allow water flow and debris passage.

  • Afflux / Backwater:

    • Definition: Rise in upstream water level (flood level) due to obstruction by bridge piers/abutments.

    • Effects: Increases flood level, may cause flooding/erosion upstream, increases pressure on structure, affects approach road design. Must be minimized by streamlining piers.

Bridge Loading Standards

• Indian Railway Loading Standards (IRS):

  • Broad Gauge (BG): Loading Class 70R (or updated). Represents a distributed load of 70 kN/m (approx 7.1 t/m) over a loaded length. For concentrated loads, uses "Bogie Load" (e.g., 22.5 t per axle for 2-axle bogie, 30.5 t for 3-axle).

  • Meter Gauge (MG): Lower classes (e.g., 35R).

  • Code: IRS Bridge Rules (latest).

• IRC Loading for Highway Bridges (IRC 6, IRC 21):

  • IRC 6: Standard loads and stresses. Primary loads:

    • Class A: 70R (similar to railway) for major bridges.

    • Class B: 30R (30 kN/m) for minor bridges.

    • IRC 21: Provides Load and Resistance Factor Design (LRFD) format. Uses IRC 6 loads with load factors.

  • Also considers: Impact factor (speed factor), vehicle impact (collision), seismic loads (IS 1893), wind loads (IS 875), temperature effects, erection loads.

• Types of Loads & Forces:

  1. Dead Load (DL): Self-weight of structure (permanent).

  2. Live Load (LL): Moving vehicular/train loads (IRC/IRS classes).

  3. Impact Load: Dynamic effect due to moving load. Given by Impact Factor (I) = A/(B+L) (A,B constants, L = loaded length).

  4. Wind Load (WL): Lateral pressure (IS 875).

  5. Seismic Load: Earthquake forces (IS 1893).

  6. Temperature Load: Expansion/contraction stresses.

  7. Braking Force: Longitudinal force from vehicle braking.

  8. Centrifugal Force: On curved bridges.

  9. Water Current & Scour Pressure: On piers.

  10. Earth Pressure: On abutments/retaining walls.

  11. ** Erection Loads**: During construction.

V. BRIDGE SUB-STRUCTURE: FOUNDATIONS & SUPPORTING WORKS

Bridge Foundations

• Shallow Foundations (Depth < Width):

  • Spread Footing: Individual footings under piers/abutments.

  • Combined Footing: Under multiple columns.

  • Raft/Mat Foundation: Thick slab covering entire area for poor soil/high loads.

• Deep Foundations (Depth > Width):

  • Piles: Vertical/ inclined members (concrete, steel, timber). Transfer load to deep strata.

  • Well Foundations (Caissons): Open-bottom, sunk by excavation. Used for deep, sandy/scour-prone rivers.

    DiagramSEARCH: "well foundation sinking process"

  • Caissons: Large watertight chambers (floating/sunken). Used for deep water.

  • Cofferdams: Temporary watertight enclosure to pump out water and work in dry conditions.

Well Foundations - Procedure for Sinking:

1. Sinking by Excavation: Remove soil from inside well curb using ** grabs, buckets, or manual labor**.

2. Sinking by Water Jet: Use high-pressure water jets to loosen soil (in sand/silt).

3. Sinking by Percussion: Drop heavy chisel/dolly inside well to break hard strata/rock.

4. Sinking by Cutting Edge: As soil is removed, well sinks under its own weight + kentledge (added weight).

5. Control: Use tremie concrete for plug at bottom, steel/wooden diaphragms to prevent tilting.

6. Reaching Final Depth: Place well cap (RCC) above well top.

Precautions During Well Sinking:

1. Tilting: Monitor constantly; correct by excavating from high side, loading low side.

2. Heave: Prevent by maintaining adequate depth of excavation below plug.

3. Sand Boils: Control by reducing differential head (lowering water inside well).

4. Seepage: Use tremie concrete under water.

5. Obstruction: Remove boulders/obstructions carefully.

6. Pressure Relief: Provide pressure relief holes in well curb to prevent excessive bottom pressure (see Scour section).

Cofferdams:

• Definition: Temporary structure to exclude water from an area, creating a dry work environment.

• Types:

  • Earthfill Cofferdam: Simple, for shallow water, low head.

  • Sheet Pile Cofferdam: Single-wall (for low head), Double-wall (for higher head, with intermediate fill).

  • Braced Cofferdam: For deep excavation in soil.

• Usage: Foundation construction for piers/abutments in rivers/lakes, repair works.

Sheet Piles:

• Types:

  • Wooden: Temporary, low durability.

  • Steel: Common (Z-section, straight web). Reusable, strong.

  • Concrete: Precast, durable, for permanent structures.

• Applications: Cofferdams, retaining walls, foundation enclosures, erosion protection.

Scour & Foundation Safety

• Procedure for Estimating Scour Depth:

  1. Maximum Flood Discharge ($$\displaystyle Q_{max} $$): From hydrological data (e.g., 50-year flood).

  2. Lacey's Formula (for alluvial rivers):

$$ D_s = 0.47 \left( \frac{Q}{f} \right)^{1/3} \quad \text{(for mean scour)} $$

$$ D_{s(max)} = 1.75 \times D_s \quad \text{(for maximum scour during flood)} $$

    Where:

    *   $$\displaystyle D_s $$ = Scour depth (m) below HFL

    *   $Q$ = Design discharge (m³/s)

    *   $f$ = Lacey's silt factor = $$\displaystyle 1.76 \sqrt{d_{mm}} $$ ($$\displaystyle d_{mm} $$ = mean silt size in mm)

3.  **Considerations**: Account for **contraction** (if bridge width < river width), **local scour** (around piers - more severe), **afflux**.

• Criticality in Bridge Safety:

  • Scour is the #1 cause of bridge failures (piers/abutments undermined).

  • Underestimation leads to foundation settlement, tilting, collapse.

  • Design must ensure foundation base is below maximum probable scour depth with adequate safety.

  • Requires regular inspection after floods.

Pressure Relief Phenomenon:

• Context: In well foundations or deep foundations in sandy soils.

• Issue: As water flows around the well curb, ** Bernoulli's principle** causes a low-pressure zone on the upstream side of the well bottom. This reduces effective pressure on soil, potentially causing piping or boiling (soil particles carried away).

• Prevention: Provide pressure relief holes (vents) in the well curb at the bottom. These allow water to enter the well, equalizing pressure and preventing suction.

Ancillary Structures

• Wing Walls:

  • Function: Retain approaches/backfill behind abutments; protect embankment from scour.

  • Types: Splayed (at 45° to abutment), Parallel (for wide bridges), Return (U-shaped).

• Return Walls:

  • Function: Short walls perpendicular to wing walls, connecting them to the abutment or embankment. Provide additional support and define the opening.

VI. BRIDGE SUPER-STRUCTURE: TYPES, MATERIALS & FLOORING

Bridge Superstructure & Substructure Components

Factors Influencing Choice of Superstructure:

1. Span Length: Slab/beam (short), truss/arch (medium), cable-stayed/suspension (long).

2. Material Availability & Cost: Concrete (common), steel (long spans), composite.

3. Site Conditions: Deep water (prefabricated segments), seismic zone (ductility), soil type.

4. Traffic Type & Volume: Highway, railway, pedestrian.

5. Aesthetic & Maintenance Requirements.

Bridge Flooring Systems

Selection Criteria:

• Railway: Ballasted or trough with ballast preferred for track support and damping.

• Highway: Solid slab or grid (for weight reduction). Trough used for composite construction.

• Consider: Dead load, live load distribution, drainage, maintenance access, construction speed.

Bridge Materials

VII. BRIDGE MAINTENANCE, FAILURE & STRENGTHENING

Bridge Inspection & Maintenance

• Types of Inspections:

  1. Routine/Periodic: Visual checks (monthly/quarterly) for obvious defects (cracks, scour, bearing condition).

  2. Detailed/In-depth: Annual/biennial. Close inspection, testing (hammer sounding, half-cell potential for corrosion), measurement of deflections/cracks.

  3. Special/Investigative: After events (flood, earthquake, accident), or for specific suspected issues. Uses NDT (ultrasonic, radiography).

• Importance of Systematic Data Collection:

  • Creates asset inventory and condition database.

  • Enables prioritization of maintenance/repair funds.

  • Tracks deterioration rate for life-cycle prediction.

  • Forms basis for bridge management systems (BMS).

  • Ensures safety by monitoring critical elements.

Bridge Failures

• Common Causes:

  1. Scour & Foundation Failure (most common).

  2. Overloading (beyond design capacity).

  3. Material Deterioration: Corrosion (steel), alkali-silica reaction (concrete), fatigue.

  4. Design/Construction Defects: Errors, poor workmanship.

  5. Impact: Ship/vessel collision, vehicle collision.

  6. Seismic Events.

  7. Fire.

• Remedial Measures & Prevention:

  • Regular scour monitoring and countermeasures (riprap, guide banks).

  • Enforce load limits (posting).

  • Protective coatings for steel, cathodic protection.

  • Seismic retrofitting.

  • Impact protection (fenders, dolphins).

  • Strict quality control during construction.

Strengthening of Bridges

• Methods:

  1. External Post-Tensioning: Adding external tendons to increase moment capacity (common for concrete girders).

  2. Jacketing: Adding reinforced concrete/steel jacket around columns/pier to increase section and confinement.

  3. Adding Plates/Struts: Welding steel plates to beams (increasing section modulus), adding external struts/ties.

  4. FRP Wrapping: Carbon/Glass Fiber Reinforced Polymer wraps for confinement of columns or shear strengthening of beams.

  5. Replacing Components: Replacing severely corroded beams, decks, or bearings.

  6. Increasing Support: Adding intermediate supports (piers) to reduce span.

• Selection Based on Deficiency:

  • Increased Moment Capacity → Post-tensioning, adding plates.

  • Shear Deficiency → FRP wrapping, adding stirrups.

  • Column Confinement/Seismic → Jacketing, FRP wrapping.

  • Foundation/Scour → Add piles, riprap, deepen foundation.

  • Deck Deterioration → Replace deck slab.

VIII. TUNNEL ENGINEERING: PLANNING, CONSTRUCTION & SUPPORT

Tunnel Planning & Preliminary Works

• Tunnel Shafts:

  • Purpose: Provide access for construction (men, materials, muck), ventilation during construction, emergency egress.

  • Location: At strategic points (near portals, mid-point for long tunnels).

  • Construction: Sunk like well foundations (circular/rectangular). Lined after reaching final depth.

• Pilot Shaft (Drift):

  • Purpose: Small-diameter tunnel driven ahead of main tunnel to:

    1. Investigate ground conditions (geology, water).

    2. Drain water from face.

    3. Ventilate main tunnel face.

    4. Provide access for main tunneling.

  • Construction: Smaller size, driven first along the tunnel axis.

Tunnel Construction Methods in Different Ground

Drift Method for Hard Rock:

• Procedure:

  1. Drive a small pilot drift (1.8m x 1.8m) ahead of main tunnel.

  2. Crown Drift: Enlarge top part of pilot to form an arch.

  3. Bench Drift: Excavate lower part (bench).

  4. Full Face: Remove remaining rock between crown and bench.

  5. Support: Install rock bolts, shotcrete, steel sets immediately after each step to prevent rock fall.

• Advantage: Allows inspection of rock mass before full-face excavation, reduces risk of collapse in poor rock.

[!TIP] Exam Sketch: For drift method, draw cross-section showing sequence: (a) Pilot drift, (b) Crown drift, (c) Bench drift, (d) Full face with support.

Tunnel Support & Lining

• Why Lining is Done?

  1. Structural Support: Provides permanent load-bearing capacity, prevents rock fall.

  2. Waterproofing: Prevents seepage (especially in water-bearing strata).

  3. Durability: Protects against weathering, chemical attack.

  4. Smooth Finish: Provides finished surface for drainage, aesthetics, and reduces air resistance (for rail/road tunnels).

• Types of Lining:

  • Timber: Temporary, in good rock.

  • Brick/Stone Masonry: In fair ground, historical.

  • Concrete: Most common - cast-in-situ, precast segments.

  • Shotcrete (Gunite): Sprayed concrete, often with steel fibers. Used with rock bolts (NATM - New Austrian Tunneling Method).

  • Steel Lining: In very poor ground or under high water pressure (e.g., underwater tunnels).

• Methods of Lining Installation:

  1. Immediate (Primary) Support: Rock bolts, wire mesh, shotcrete applied soon after excavation (NATM).

  2. Permanent Lining: Cast-in-situ concrete using forms, or precast segmental lining (erected by TBM/special equipment).

  3. Timber Lagging: Temporary between steel sets.

Tunnel Utilities & Safety

• Drainage:

  • Importance: Prevent water accumulation, seepage, ice formation, corrosion.

  • Methods: Side drain (along tunnel invert), central drain, pumping (if gravity flow not possible), waterproofing membrane.

• Ventilation:

  • Purpose: Remove exhaust fumes (vehicles), dust, heat; supply fresh air.

  • Methods:

    1. Natural Ventilation: Using shafts/pressure difference (short tunnels).

    2. Forced Ventilation: Using fans (long tunnels).

    3. Jet Fans: Installed in tunnel, create air flow.

    4. Shaft Ventilation: Using dedicated ventilation shafts.

• General Safety Precautions:

  • Ground Support: Prompt installation of support.

  • Ventilation & Lighting: Continuous fresh air, adequate lighting.

  • Emergency Exits: Refuge chambers, cross-passages (for long twin tunnels).

  • Monitoring: Deformation monitoring, gas detection (methane, CO).

  • Training: Workers trained in emergency procedures.

Special Tunneling Phenomena & Case Studies

• Pressure Relief Phenomenon (in Rock Tunneling):

  • Issue: In squeezing ground (weak rock), the in-situ stress is higher than the rock's strength. When tunnel is excavated, stress redistributes, causing rock to creep inward (squeeze), leading to heavy pressure on support.

  • Mitigation: Use yielding support (steel sets with sliding joints), thick shotcrete, pre-support (spiles), reduce tunnel size if possible.

• Bridge Action in Tunnels:

  • Refers to the design of portal structures and invert.

  • Portal: Acts like a bridge abutment retaining the approach embankment. Must be designed for earth pressure, seismic loads.

  • Invert: The tunnel floor. In poor ground, may need a concrete invert slab (like a bridge deck) to distribute loads and prevent uplift.

• Brief Notes on Two Important Tunnels (Example - Choose any two):

  1. Pir Panjal Railway Tunnel (India, 11.2 km):

     • Longest in India, part of USBRL (Udhampur-Srinagar-Baramulla).

     • Constructed by TBM in complex geology (fault zones, water inflows).

     • Features: India's first cable-suspended TBM, emergency cross-passages.

  2. Channel Tunnel (Eurotunnel, 50.5 km):

     • Under English Channel, connects UK-France.

     • Three bores: Two rail tunnels, one service tunnel.

     • Constructed by TBMs from both ends, met under sea.

     • Immersed tube method for undersea section? (No, actually TBM through chalk marl).

  3. Seikan Tunnel (Japan, 53.9 km):

     • World's longest undersea tunnel.

     • Under Tsugaru Strait. Dual-gauge (Shinkansen & freight).

     • Faced severe water inflows (tectonic zone), used multiple TBMs and freezing.

  4. Atal Tunnel (India, 9.0 km):

     • Longest single-tube highway tunnel above 10,000 ft.

     • In Himalayan geology (fault zones, water). Used Drill & Blast.

     • Features: Ventilation system, emergency tunnel within main tube.

[!TIP] Exam Focus: Be ready to sketch drift method and well foundation sinking. Know Lacey's formula and pressure relief concept. For tunnel questions, always mention support/lining and ventilation/drainage as integral parts.