Cross Slope or Camber is the slope provide to the road surface in the transverse direction to drain of the rain water from the road surface. Drainage and quick disposal of water from the pavement surface by providing cross slope is consider important because of the following reasons :
Why camber is provided ?
- To prevent the entry of surface water into the pavement layers and equally importantly, the sub grade soil through pavements the stability, surface condition. Likewise, The life of the pavement get adversely affect if the water enters in to the sub grade and the soil gets soaked
- As well as, Prevent the entry of water into the bituminous pavement layers, as continued contact with water causes stripping of bitumen from the aggregates and results in deterioration of the pavement layer.
- Similarly, To remove the rain water from the pavement surface as quickly as possible and to allow the pavement to get dry soon after the rain.
Requirements of Cross Slope or camber depends on :
- Type of pavement surface
- The amount of rainfall
A flat camber of 1.7 to 2.0 % is sufficient on relatively impervious pavement surface likewise cement concrete or bituminous concrete. In pervious surface ( likewise water bound macadam, gravel or earth road ) which may allow surface water to get into the sub grade soil, steeper camber is require. Steeper camber is provide in areas of heavy rainfall to enable the surface water to drain off at a rapid rate.
Recommended values of camber
A range of values are given with a view that in localities with lower rainfall, a flatter camber and also in places with high rainfall, a steeper camber can be adopt.
The cross slope for shoulders should be 0.5 % steeper than the cross slope of adjoining pavement, subject to a minimum of 3.0% and a maximum value of 5.0% for earth shoulders.
The cross slope suggest for the carriageway, paved shoulders and edge strip of expressways with bituminous surface as well as cement concrete surface is 2.5% in regions with annual rain fall exceeding 1000 mm and 2.0% in places with less than 1000 mm rain fall.
Shape and Types of Cross Slope or camber
In the field of course, camber of the pavement cross section is provide with a suitable shape. Different shapes that are commonly adopt are parabolic, straight line or as well as straights with parabolic curve at the top. In cement concrete pavements, straight line camber is usually adopt as it is easy to lay the same during construction and also as the camber require is relatively flat.
Providing camber in the field
In order to provide the desire amount and shape of camber, templates or camber boards are prepare with the chosen shape and specify cross slope. These templates are use to check the lateral profile of finish pavement during construction. Forming a straight line camber is very simple. In the case of parabolic camber the general equation, y = x^2 / a may be adopt.
Here, a = nW/2 for a pavement of width W and cross slope 1 in n.
y = 2.X^2 / nW
In a direct where the rainfall is heavy, two types of road pavement are to be construct. (a) Two lane State Highway with bituminous concrete surface and moreover (b) major district road of WBM pavement, 3.8 m wide. What should be the height of the crown with respect to the edges in these two cases, assuming straight line camber ?
Straight line type camber has be suggest.
(a) For SH width of two lane pavement = 7.0 m
For bitumen concrete surface in heavy rain fall area, provide a cross slope of 1 in 50. Rise of crown with respect to the edges = 7/2 * 1/50 = 0.07m.
(b) For WBM road in heavy rain fall area, provide a cross slope of 1 in 33 or 3.0 %
Rise of crown in with respect to edges on a pavement of width 3.8 m
= ( 3.8 x 1 ) / ( 2 x 33 ) = 0.058 m