Tunnels & Bridges

Why tunnels?

A tunnel is an underground passage made without removing the overlying rock or soil.

Advantages

• Shorter and straighter routes through hills and mountains; avoids long detours and steep gradients.
• Avoids deep open cuttings, which are costly and unstable (a tunnel becomes economical when the cutting depth exceeds a certain limit).
• Passes under rivers, sea channels and cities without disturbing surface activities.
• Protected from snow, avalanches and weather; strategic protection.
• Less land acquisition and less disturbance of the environment and ecology on the surface.

Disadvantages

High initial cost and construction time; requires specialised equipment and skilled labour; geological uncertainty and hazards; needs ventilation, lighting and drainage throughout life.

Classification

Shapes: circular sections best resist external pressure from all sides (soft ground, water tunnels under pressure, TBM tunnels); horseshoe and D-shaped sections are common in rock for road and railway tunnels (flat floor for traffic, arched roof resists vertical load); rectangular sections for cut-and-cover and immersed tunnels; egg-shaped for sewers.

Tunnel surveying

1. Surface survey — the tunnel alignment is established on the surface by triangulation, traverse, and now GNSS and total stations; portals and shaft positions are fixed and connected.
2. Transfer of alignment underground:
   • Through portals — directly by theodolite/total station along the alignment.
   • Through shafts — two plumb lines (heavy weights in oil to damp swinging) hung down the shaft on the alignment; the line through them is extended underground; optical plumbing, laser plumbing and gyro-theodolites are used in modern practice.
3. Transfer of levels through shafts by steel tapes or electronic distance measurement from surface benchmarks.
4. Underground control — traverses, laser guidance systems for TBMs, regular checks to ensure headings driven from opposite ends meet accurately ("breakthrough").

Shafts and pilot tunnels

Shafts (vertical or inclined openings from the surface) are provided to:

• Increase the number of working faces and speed up construction.
• Provide ventilation, drainage, lighting and access.
• Remove muck and bring in materials.
• Locate the alignment and check geology.
• Serve permanently for ventilation (road tunnels) or as surge shafts (hydropower).

Pilot tunnel — a small tunnel driven parallel to (or along) the main tunnel ahead of it to explore geology, drain water, provide ventilation and access, and allow the main tunnel to be enlarged from several points.

Tunnelling in hard rock

Methods of excavation

Drill-and-blast cycle

1. Marking the drilling pattern on the face.
2. Drilling holes with pneumatic/hydraulic drills (often mounted on drill jumbos).
3. Charging holes with explosives and connecting detonators (delay sequence).
4. Blasting.
5. Ventilation to remove fumes and dust.
6. Scaling — removal of loose rock from crown and walls.
7. Mucking — loading and hauling broken rock.
8. Temporary support — rock bolts, shotcrete, steel ribs as required.
9. Survey and marking for the next round.

Drilling patterns (cuts)

The first holes fired (cut holes) create a free face into which the rest of the rock breaks:

• Wedge (V) cut — pairs of angled holes forming a wedge.
• Pyramid (diamond) cut — holes converging to a point.
• Burn cut / parallel cut — closely spaced parallel holes, some left uncharged as relief holes; suitable for long rounds and narrow tunnels.
• Drag (fan) cut — holes inclined downward, for laminated rock.

Then easer holes (relievers), trim holes (contour/perimeter holes, sometimes with smooth blasting to reduce overbreak) and lifter holes (floor).

Tunnel boring machines (TBMs) in rock

Full-face rotary cutter heads with disc cutters; types: open (gripper) TBM (grips the tunnel walls — for competent rock), single-shield and double-shield TBMs (for fractured rock, installing segmental lining). Advantages: smooth circular profile, minimal overbreak and ground disturbance, high advance rates in suitable rock, safer; disadvantages: very high cost, long mobilisation, inflexible cross-section, difficulty in highly variable or squeezing ground.

New Austrian Tunnelling Method (NATM)

A design philosophy that uses the surrounding rock mass as the main load-bearing element:

• Controlled excavation (often sequential) with immediate flexible primary support — shotcrete, rock bolts, wire mesh or lattice girders — allowing controlled deformation so that the rock forms a load-bearing ring.
• Continuous monitoring of deformations (convergence) to adjust support.
• A final inner concrete lining is placed later.
• Suited to variable ground (rock to soft ground); widely used for road, rail and metro tunnels.

Tunnelling in soft ground

Rock mass classification

• Terzaghi's rock load classification — rock load on supports expressed as a function of tunnel dimensions for different rock conditions (historical basis of steel-rib support).
• Rock Mass Rating (RMR, Bieniawski) — sum of ratings for uniaxial compressive strength, RQD, spacing of discontinuities, condition of discontinuities, groundwater, adjusted for orientation of discontinuities; RMR 0–100 classifies rock from very poor to very good and suggests stand-up time and support.
• Q-system (Barton, NGI):

$J_n$ = joint set number; $J_r$ = joint roughness; $J_a$ = joint alteration; $J_w$ = joint water reduction; SRF = stress reduction factor. The three quotients represent block size, inter-block shear strength and active stress. Q ranges from about 0.001 (exceptionally poor) to 1000 (exceptionally good) and is used with charts to select support (bolt spacing, shotcrete thickness).

Components of a bridge

Classification of bridges

Structural forms — key features

• Slab bridges — solid or voided slabs for short spans.
• T-beam and box girder bridges — common RCC/PSC bridges for medium spans; box girders have high torsional stiffness (curved bridges, flyovers).
• Truss bridges — steel members in triangulated frames — railway bridges and medium to long spans.
• Arch bridges — carry loads mainly in compression; need strong abutments to resist thrust.
• Balanced cantilever bridges — built segmentally outward from piers.
• Cable-stayed bridges — deck supported by inclined cables from towers; efficient for medium to long spans.
• Suspension bridges — deck hung from main cables anchored at ends; longest spans.

Site selection and investigations

Ideal site: a straight reach with a narrow, well-defined channel and stable, high banks; firm foundation strata at reasonable depth; square crossing (flow perpendicular to the bridge); absence of whirls and cross currents; minimum training works; good approaches without costly embankments; proximity to the planned alignment; availability of materials and access.

Investigations: topographic survey (catchment and site plan, cross-sections upstream and downstream), hydrological data (rainfall, flood records, HFL, LWL, velocity, discharge), geotechnical investigations (boreholes, soil and rock properties, scour characteristics), navigational requirements, seismic zone, traffic and road/rail data.

Hydraulic design

Design discharge

Estimated by empirical formulas (Dickens, Ryves, Inglis), the rational method (small catchments), area–velocity method from flood marks (slope–area method with Manning's formula), unit hydrograph method, and flood frequency analysis. IRC:5 and IRC:SP:13 guide the design discharge and return periods; the maximum of several estimates is normally checked.

Linear waterway

• For alluvial streams, the regime waterway (Lacey's) $P=4.75\sqrt{Q}$ (m; $Q$ in m³/s) is taken as the effective linear waterway.
• For quasi-alluvial/rocky streams, the waterway is fixed by the natural stream width at HFL; contraction is limited to control afflux and scour.
• Effective linear waterway = total width between abutments minus the effective obstruction of piers.

Afflux

Afflux is the rise in water level upstream of a bridge due to contraction of the waterway.

Excessive afflux can flood upstream areas, cause high velocities through the bridge and deep scour.

Scour depth (IRC:78)

Foundations are taken sufficiently below the maximum scour level (with minimum grip lengths prescribed in IRC:78) and are protected by aprons, flooring or pitching where needed.

Clearance and freeboard

• Vertical clearance — between HFL (with afflux) and the lowest point of the superstructure, to pass floating debris; increases with discharge (IRC:5 values).
• Freeboard — the height of the formation level of approaches/guide bunds above HFL.
• Navigational clearance — for navigable waterways, as required by the waterway authority.

Economic span

For a bridge of a given total length with equal spans, the total cost is minimum when the cost of superstructure of one span ≈ cost of substructure (one pier with its foundation).

If the superstructure cost of a span varies as $k{L}^2$ and the cost of a pier with foundation is $P$ (independent of span), the economic span is:

Deep or costly foundations favour longer spans; shallow foundations and cheap piers favour shorter spans.