Highway Geometric Design – Gradients

Vertical Alignment — Longitudinal Gradients:

• Gradient is the rate of rise or fall along the length of the road with respect to horizontal.
• Expressed as ‘1’ vertical unit to ‘N’ horizontal units.

>> Type of Gradients:

1. Ruling Gradient : Also known as ‘Design Gradient’.                                              During normal course, the Gradient on road should never exceed this value.                                The gradient is fixed in such a way that speed of the vehicle should not decrease while moving on the gradient.                                                                                                                              The gradient is also fixed in such a way that maximum power developed by the engine is equal to the power required to overcome the resistance to motion on the grade at design speeds.
2. Limiting Gradients: (Maximum gradient):                                                                      A gradient steeper than the ruling gradient which may be used in restricted length, where keeping with in the ruling gradient is not possible due to topography.
3. Exceptional Gradient: A gradient steeper than the limiting gradient which may be used in short stretches only (< 60 m in 1 km road length or should not exceed 100 m at a stretch) in exceptional situations.
4. Minimum Gradient: On the basis of satisfactory road drainage requirements. Generally about 0.5% or 1 in 200 in concrete drains and 1% or 1 in 100 in kutcha open drains.

>> Gradients for roads in different terrains:

>> Grade Compensation: As per limitation IRC Grade Compensation is not necessary for gradients flatter than 4% and therefore when applying grade compensation correction, gradients needs not be eased beyond 4%.

Grade Compensation = (30 + R) / R 
                                                          (or) 75 / R

which ever is less. 
Where R = radius in m