CROSS SECTION OF ROAD AND IT'S ELEMENTS | HIGHWAY ENGINEERING

CROSS SECTION OF ROAD AND IT'S ELEMENTS 


The cross section of the road is as shown in the below figure :

Cross section of road in embankment

Figure : Cross section of road in embankment 

Various elements of highway cross section are as follows :

1) Carriage Way :

The width of pavement way on which vehicles travel is called carriage way width or pavement width.

The carriage way intended for one line of traffic movement is called a traffic lane. The width of carriageway depends upon the width of traffic lane and the number of traffic lanes.

The width of traffic lane depends upon the width of vehicle and the minimum side clearance between two vehicles.

Width of carriage way for :

Single Lane = 3.75m

Two Lanes (without raised kerbs) = 7 m

Two Lanes (with raised kerbs) = 7.50m

Multi Lane Pavements = 3.50m

2) Formation Width :

Formation width (road way) is the top width of the highway embankment or the bottom width of cutting excluding the side drains. 

Formation width = Width of carriage way + Width of shoulders

3) Right Of Way :

Right of way is the area of land acquired for the road, along its alignment. It is the distance between the boundary stones on either side of the road.

4) Road Shoulders :

Shoulders are provided along the road edge to serve as an emergency lane for vehicles. As per IRC, the minimum width of shoulders should be 2.5 m.

Shoulders are used for :

  • Repair of broken down vehicles.
  • Slow moving vehicles. 
  • Overtaking operations.
  • To act as an emergency lane.
  • For future widening of roads.
  • For temporary diversion of traffic during road repair.
  • To lay water supply pipes, drainage pipes, electric cables, gas lines, telephone lines,etc.
  • For temporary parking of vehicles.

The shoulders should have sufficient load bearing capacity to support loaded truck even in wet weather. 

The surface of the shoulder should be rougher than the traffic lanes so that vehicles are discouraged to use the shoulder as a regular traffic lane. 

The colour of the shoulder should preferably be different from that of the pavement so as to be distinct. 

5) Side Slope : 

The slope of earthwork in filling (embankment) or in cutting is called side slope. Side slope imparts stability to the earthwork. 

It should be as flat as possible for the purpose of safe traffic movement and also for asthetic reasons. 

But in case of flatter slopes amount of earthwork increases on the other side in steeper slopes erosion of soil will be more.

Side Slope For Embankment = 1:2

Side Slope For Cutting :

  • For Ordinary soil = 1:1 to 1:1/2
  • For Broken rock = 1:1/2 to 1:1/4 
  • For Soft rock = 1:1/4 to 1:1/8
  • For Hard rock it should be approximately perpendicular

6) Berm :

The distance between the road toe and the inner edge of borrow pit is called berm.

It prevents the erosion of embankment soil. 

7) Boundary stone :

To indicate the boundary of land acquired for road, stones are driven into the ground at about 30 m distance on either side from the centre line of the road and these stones are known as boundary stones.

8) Side drain :

For the drainage of rain water, drains are provided on either side of the road. Normally, side drains are required for road in cutting. For road in embankment, side drain is not necessary.

9) Building line (B.L.) : 

The distance from the centre line of road on either side, within buildings is not permitted is called building line. 

The purposes of building line are :

  • For future widening of road. 
  • To reduce the chane of accidents.
  • To relieve the residents from noise pollution. 
  • To prevent disturbance to the traffic by the nearby residents.

10) Control line (C.L.) :

At the locations like bank, hospital, factory, theatre, etc. on the road, where more people gather disturbance to the traffic will be more.

The distance from the centre line of road to such building is called control line.

11) Spoil Bank :

The banks constructed from surplus excavated earth on the side of road cutting parallel to its alignment, are known as spoil banks.

The soil from spoil bank can be used for the repair of shoulders.

12) Borrow Pits :

The pits dug along the road alignment for using excavated earth in the construction of embankment, are known as borrow pits.

Borrow pits should be dug at least 5m away from the toe of embankment.

The small portion of earth left undug in a borrow pit to measure the depth of excavation is known as deadman.

13) Kerbs :

The boundaries along the edge of the pavement and shoulders or footpaths are known as kerbs.

The kerb may also be provided between the pavement and traffic separator. 

Functionally they could be of three types:

i) Low or mountable kerbs : 

They are also called class-I kerbs.

These kerbs are indicators between the boundry of a road and shoulders. 

The height of the kerb is such that the drivers find no difficulty in crossing these kerbs and use the shoulders in case of emergency.

Normally the height of low road kerbs is kept only 10 cm above the pavement edge. These kerbs channelize the traffic and also allow longitudinal drainage of road. 

ii) Semi-barrier kerbs :

This is also called class-II kerbs.

It prevent encroachment of slow speed or parking vehicles to the foot path. But at acute emergency vehicles can climb over it and can be parked on footpaths or shoulders. 

It has a height of 15 to 20 cm and 25 mm better to prevent scraping of tyres.

iii) Barrier type kerbs : 

These kerbs are also known as high speed barriers.

The height varies from 23 cm to 45 cm w.r.t. the edge of the pavement. 

They are mainly provided to cause obstructions to the vehicles leaving the carriage way under emergency.

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