Roads, Bridges & Dams

Transportation and the role of roads

Modes of transportation: roads (highways), railways, waterways (inland and ocean), airways and pipelines.

Characteristics of road transport:

• Maximum flexibility — door-to-door service, routes and schedules changeable.
• The only mode that can serve every area (feeder to all other modes).
• Suitable for short and medium distances, perishable goods and passenger travel.
• Lower initial investment than railways; but higher energy use per tonne-km, accidents and pollution.

Roads promote economic development, agriculture and markets, social services (health, education), national integration and defence.

History of road construction

Development of roads in India

Twenty-year road development plans

Classification of roads

Rural roads (Nagpur classification, modified)

ODR and VR together are rural roads.

Urban roads (IRC)

Arterial roads (through traffic, limited access), sub-arterial roads, collector streets (collect traffic from local streets to arterials) and local streets (access to properties).

Road patterns

Highway planning

Objectives

To plan an overall road network for maximum utility, to fix priorities for phased development, to plan future development accounting for traffic growth, and to work out financing.

Planning surveys

1. Economic studies — population, agricultural and industrial products, existing facilities, per capita income.
2. Financial studies — sources of funds, revenue from vehicle taxes, tolls, betterment levies.
3. Traffic studies — volume, origin–destination, traffic flow, accidents, future growth.
4. Engineering studies — topography, soil, materials, drainage, road inventory, location surveys.

Results lead to a master plan — a long-term plan of the road network, and phasing of works.

Saturation system (maximum utility per unit length)

Used to choose between alternative road networks:

1. Assign utility units to villages/towns by population ranges (and to agricultural and industrial products per tonne).
2. For each alternative, total the utility units served.
3. Compute utility per unit length = total utility units ÷ road length.
4. The network with the highest utility per unit length is given priority.

Major programmes

• National Highways Development Project (NHDP, 1998) — phased upgrading of NHs; Golden Quadrilateral (about 5846 km linking Delhi, Mumbai, Chennai and Kolkata) and the North–South (Srinagar to Kanyakumari) and East–West (Silchar to Porbandar) corridors, port connectivity and further phases.
• Pradhan Mantri Gram Sadak Yojana (PMGSY, 2000) — all-weather road connectivity to eligible unconnected habitations (population 500+ in plains; 250+ in hilly, tribal and desert areas), later phases for upgrading and consolidation.
• Bharatmala Pariyojana (2017) — economic corridors, inter-corridor and feeder roads, border and international connectivity, coastal and port connectivity roads, expressways.
• Sagarmala (port-led development) and PM Gati Shakti National Master Plan (2021) for integrated multimodal infrastructure planning.

India has one of the largest road networks in the world (second largest by length).

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.

Reservoirs

A reservoir is a body of water impounded by a dam to store water for later use.

Storage zones and levels

Capacity

• Area–elevation curve from contour surveys; capacity–elevation curve by integration using the trapezoidal formula $\Delta V=\frac{h}{2}(A_1+A_2)$, the cone formula $\Delta V=\frac{h}{3}\left(A_1+A_2+\sqrt{A_1{A}_2}\right)$ or the prismoidal formula.
• Safe (firm) yield — maximum quantity that can be supplied during a critical dry period; secondary yield — water available in excess of safe yield in wet years.
• Required storage for a given demand: mass curve (Rippl) method or sequent peak algorithm (see Runoff).

Reservoir sedimentation

Rivers carry sediment; when flow enters a reservoir, velocity falls and sediment settles — coarse near the head (forming deltas), fine near the dam; density currents can carry fine sediment right up to the dam.

• Trap efficiency ${\eta }_T$ — percentage of incoming sediment retained; depends mainly on the capacity–inflow ratio (C/I); Brune's curves relate them — large storage reservoirs trap almost all sediment.
• Life of a reservoir — the time until sediment fills a specified part of its capacity (e.g. dead storage filled, or live storage reduced to the point the reservoir no longer serves its purpose). Life is computed stepwise because trap efficiency falls as capacity falls.
• Distribution of sediment with depth — empirical area reduction method.

Control of sedimentation:

1. Catchment treatment — afforestation, soil conservation, check dams (most effective long-term).
2. Proper selection of dam site (low sediment yield).
3. Sluicing / flushing — releasing sediment-laden flood water through low-level outlets before it settles; drawdown flushing.
4. Sediment bypass tunnels; venting density currents.
5. Dredging or excavation (costly).
6. Vegetal screens at the head of the reservoir; design with adequate dead storage.

Reservoir losses: evaporation (major in shallow, wide reservoirs), seepage, sedimentation.

Dams — classification

Site selection

Sound foundation rock at reasonable depth; narrow valley opening into a wide basin upstream (small dam, large storage); suitable site for spillway; availability of construction materials; minimum submergence of valuable land, forests, habitations and mineral deposits; low sediment yield; accessibility; watertight reservoir basin; geological stability (no active faults, landslides).

Gravity dams

A gravity dam resists external forces by its own weight. It is the most durable type, requires little maintenance, suits almost any height, and can accommodate an overflow spillway section, but needs a strong rock foundation.

Forces acting

Wave height (Molitor–Stevenson):

($V$ = wind velocity in km/h, $F$ = fetch in km, $h_w$ in m.) Freeboard is commonly taken as about 1.5 $h_w$ above the maximum water level.

Load combinations

The dam is checked for combinations such as: construction condition (empty reservoir, with and without earthquake); normal operating condition (full reservoir, normal uplift, silt, ice); flood discharge condition (MWL, full uplift, tailwater); normal operating + earthquake; drains choked (extreme uplift). Permissible factors of safety are lower for extreme combinations.

Modes of failure

Elementary profile

The theoretical profile of a gravity dam is a right-angled triangle with the vertical face upstream, height $H$ equal to the water depth and base $B$.

The practical profile adds a top width (roadway, commonly about 14% of the height with a minimum), freeboard, and upstream/downstream batters near the base.

Galleries, joints and foundation treatment

• Galleries — for drainage of seepage (drain holes), inspection, grouting and instrumentation.
• Contraction joints (transverse) with shear keys and water stops; longitudinal joints in very large blocks.
• Foundation treatment: excavation to sound rock; consolidation grouting (shallow, to strengthen the foundation); curtain grouting (deep, near the heel, to reduce seepage and uplift); drainage holes downstream of the curtain; treatment of faults and seams (dental concrete).