Test: Horizontal Alignment - Civil Engineering (CE) MCQ

IRC recommends a value of 0.9m for two lane pavement for radius of curve 100m, this value decreases with increase in the length of curve.

• As per IRC73-1980, Clause 9.1.1, uniformity of design standard is one of the essential requirements of a road alignment.
• For a given section there must be consistent application of a design element to avoid unexpected situation being created for the drivers.
• For mountainous terrain, the drainage of the road surfaces is very essential, so as to achieve consistency, any unnecessary break in horizontal alignment at cross-drainage structures should be avoided.
• Thus the horizontal alignment in mountainous country is conditioned by drainage structures.

The relationship between centrifugal force and weight of the vehicle is called impact factor or centrifugal ratio.

Transition curve is a curve in plan which is provided to change the horizontal alignment from straight to circular curve gradually means the radius of transition curve varies between infinity to R or R to infinity.
IRC recommends Spiral or clothoid as the ideal transition curve due to following reasons:
(i) It satisfies that rate of change of centrifugal acceleration is constant i.e., L_s.R = constant.
Where L_s = length of transition curve R = radius of curve.
(ii) The calculation and field implementation of spiral curve is simple and easy.
(iii) It enhances aesthetics also.

The transverse inclination to the pavement surface is called as super elevation or cant banking which is equal to tanθ.

Transition curve

• A curve that is provided to change the horizontal alignment from straight to circular curve gradually is known as transition curve.

Transition curve is provided in horizontal alignment to

• Introduce gradually the centrifugal force between the tangent point and the beginning of the circular curve, avoiding a sudden jerk on the vehicle.
• Enable the driver turn the steering for his own comfort and security.
• Enable gradual introduction of the designed super elevation and extra widening of pavement at the start of the circular curve.
• Improve the aesthetic appearance of the road.

The safe allowable speed is Va = √(27.94R)
Va = √(27.94*120)
Va = 60kmph.

e+f = v2/127R
e+f = 3600/(127*150)
e+f = 0.188
e = 0.188-0.15
e = 0.038.

The raise of outer edge is given by
E = Be,
E = 7*0.07 = 0.49m

The extra widening is the sum of mechanical widening and physiological widening, the mechanical widening is done for the safety of vehicles and psychological widening is done for the comfort of passengers.