Roads, Bridges and Transportation Engineering
Roads, Bridges and Transportation Engineering covers highway classification and geometric design, pavement layers, drainage and bridge components, with numericals on camber, gradient and superelevation.
Road classification
National Highways connect major cities; State Highways connect district headquarters; Major District Roads link production and market areas; Other District Roads serve rural areas; Village Roads link villages to the nearest district road. Responsibility increases from village roads up to national highways.
Geometric design
Camber drains the surface: about 2.5-3 percent for bituminous, 1.5-2 percent for cement concrete and 4 percent for earthen roads. Ruling gradient is the design gradient; limiting and exceptional gradients are steeper and used only in difficult terrain.
Worked example: superelevation
For a design speed of 80 km/h and radius 300 m, e = V squared / (127 R) = 6400 / (127 x 300) = 0.168, that is 16.8 percent. This exceeds the IRC limit of 7 percent for plain terrain, so the design uses e = 7 percent and friction carries the rest: f = 0.168 - 0.07 = 0.098, which is within the 0.15 limit.
Pavements
A flexible pavement has subgrade, granular sub-base, wet mix macadam, bituminous base and wearing course, and is designed from the CBR of the subgrade. A rigid pavement is a cement concrete slab on a base, with expansion, contraction and construction joints and dowel bars at joints.
Bridges
Components are deck, girder, abutment, pier, wing wall, bearing and approach. IRC Class A and Class 70R are the standard live loads. Waterway comes from the design flood, and scour depth governs foundation depth.
Traps
- Using the overtaking sight distance formula where stopping sight distance is asked.
- Stopping at the computed e without checking the 7 percent limit and the friction share.
- Confusing an abutment, which ends the bridge, with a pier, which is intermediate.
- Designing the pavement on the average CBR instead of the 95th percentile value.