HIGHWAY ENGINEERING: GRADIENT AND ITS TYPE

 GRADIENT

The ratio of rising or fall to horizontal length is called gradient or it may be also defined as the rate of rising or fall along the length of road with respect to horizontal.

It is expressed as 1 in n ( which means 1 vertical: n horizontal) or x% ( which means x vertical units: 100 horizontal units). It may also be expressed as tanα where α is slope angle.

Note: ascending gradients are treated as positive. 
         descending gradients are treated as negative.

      

Types of gradient:

1. Ruling gradient
2. Limiting gradient
3. Exceptional gradient
4. Minimum gradient

1. Ruling gradient

        The gradient usually adopted while making the road alignment is called the ruling gradient. It is the maximum gradient within which the designer attempts to design the vertical profile of a road. It is also known as design gradient. As per, the recommended value of ruling for plain or rolling terrain is 1 in 30 or 3.3%

2. Limiting gradient

        The gradient steeper than the ruling gradient, which may be used for a limited road length is called the limiting gradient or maximum gradient. It is used where the topography of a place compels adopting a steeper gradient than the ruling gradient to minimize the cost of road construction.

3. Exceptional gradient

        The gradient steeper than the limiting gradient which may be used in a short length of the road, only in an extraordinary situation is called exceptional gradient. This type of gradient is adopted only in a very difficult situation and for a short length not exceeding 100m at a stretch.

4. Minimum gradient

        The minimum desirable slope essential for effective drainage of rainwater from the road surface is called minimum gradient.