HIGHWAY ALIGNMENTAlignment

The position or the layout of the central line of the highway on the ground is called the alignment.

Horizontal alignment includes straight and curved paths.

Vertical alignment includes level and gradients.

Alignment decision is important because a bad alignment will enhance the construction,

maintenance and vehicle operating costs.

Short + Straight

Easy construction + maintain

Easy gradient + smooth

Curve

Safe construction + operation

Safe geometric

design

Economical

The requirements of an ideal alignment are

Factors That Control The Alignment

1. Obligatory points: These are the control points governing the highway alignment.

These points are classified into two categories. Points through which it should pass and points through which it

should not pass.

bridge site:

Mountain

intermediate town:

Obligatory points through which the alignment should not pass are:

religious places:

very costly structures: lakes/ponds etc:

2. Traffic

3. Geometric Design

4. Economy

5. Other Considerations are

Drainage considerations, Hydrological factors, Political considerations and Monotony.

Special considerations while aligning roads on hilly areas

Stability Drainage

Geometric standard of hill roads

L0

Resisting Length

The resisting length of the alignment should kept as low as possible

h

The total work done in moving a load W from station A to Station B along the Length, L0

upto height, h

= W f L0 + W h = W f (L0 + h/f) = W f Lr

Where Lr is the resisting length f is the coefficient of frictional resistance

Map Study

To have an idea of several possible alternate routes so that further details of these may be studied later at the site.

Following details are collected from the map. Alignment avoiding valleys, ponds or lakes.

When the road has to cross a row of hills, possibility of crossing through a mountain pass

Approximate location of bridge site for crossing rivers, avoiding bend of the river, if any,

When a road is to be connected between two stations, one of the top and the other on the foot of the hill, then alternate routes can be suggested keeping in view the permissible gradient;