What are Road Curves ?
Road Curves play a vital role in the geometric design of road and railway alignments. Hence, it must be properly studied and designed so as to provide safety, comfort and convenience at the time of driving the vehicles or train on road curves.
The geometrical arc provided at change in alignment or gradient of road are known as curves.
Necessity of Road Curves :
Straight route of road or track is always desirable, since provides economy in the cost of construction, transportation and finally maintenance.
But when there is a change in alignment or gradient of road or railway track, then it becomes a need to provide curves under following circumstances
- Excessive cutting or filling can be prevented by providing the change in alignment by road curve.
- The obstruction like natural or artificial which comes in the way of straight alignment can be made easier by providing the by-pass with the help of curves.
- In the straight route, gradients are made more comfortable and easy by providing diversions with the help of road curves.
- In a straight route, if costly land comes in the way, then can be avoided by providing diversions with the help of curves.
- Track or road is made stable and safe side of the hill by changing the alignment.
Functions of Curves on Road and Track Alignment
Following are the various types of functions of the road curves which takes place in the alignment of road or railway track.
- Gradual change in direction or orientation in the alignment can be made by providing the curves.
- Road curves are provided so as to get comfort to the passengers.
- Gradual change in the direction or orientation in the alignment can be made by providing the curves.
- Curves are provided so as to get easy turning in case of road and track.
Types of Road Curves
Horizontal Curves :
The road curves which are provided at turning points so as to get gradual change in the direction of alignment of a road or a track are termed as horizontal curves. Generally these types of curves are in the horizontal plan.
In railway track, the minimum radius of a horizontal curve should be 175 m for broad gauge.
Horizontal as well as vertical, curves in case of road must ensure safety, comfort and convenience of the traffic.
1. Simple Road Curve :
The curve which consists of a single arc of a circle of which two straight tangents further connects and brings about a deflection of the road through an angle ‘θ’ is called as simple curve. See the diagram given below for better understanding.
Simple curve is normally represented by the length of its radius or by the degree of curve. This type of curve is provided at every change in alignment of the road or railway track in a plain and in hilly areas.
2. Compound Road Curves :
A curve of having the series of two and more simple curves of different radii curving in the same direction is called as compound curves. Refer the below figure for better understanding.
In compound curves, the two adjacent curves will have a common tangent ‘BC’ as shown in above figure. The centers of two adjacent curve lie on the same side of the curve as shown in above figure.
To avoid the cutting through hard rocks, heavy cutting or filling in the alignment of road or track, compound curves are provided.
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3. Reverse Curve :
The curve which consists of two simple curve having equal or different radii turning in opposite direction is called as reverse curve. The two centers of curves are on opposite sides of a common tangent ‘BD’.
Reverse curves are necessary on hill roads where frequently changes in the direction of travel is required. Reverse curves are also necessary for cross-overs in station yards and in the alignment of the railway tracks in hilly areas.
4. Transition Curve :
The transition curve is defined as the curve in which radius varies gradually from infinity to a finite value equal to that of the circular curve to be connected and vice versa is termed as transition curve.
These curves are commonly used in railway tracks between the circular curve and a tangent so as to provide easy and gradual change in direction.
Minimum length of transition curve should be determined from the following two consideration and greater of two values be adopted for design purposes.
(i) The rate of change of centrifugal acceleration should not causes discomfort to driver.
Where, L = length of transition in meter, V = Speed in km/h, R = Radius of circular curve in meter, C = 80/ (75 + V)
(ii) The rate of change of super-elevation should be such that it should not cause any discomfort to passage. On this base 10 minimum length of transition is found our from the relation.
For plain and rolling terrain,
For mountainous and steep terrain,
Vertical Curves
The curves which are provided in the alignment of road or track at change of gradient is termed as vertical curves.
Necessity of Vertical Road Curves :
- It gives adequate visibility and safety to the traffic.
- Vertical curves give gradual change in grade or slope.
- It gives adequate comfort to the passengers.
1. Summit Curves :
The curves which are having convex surface on upward side is termed as summit curves. Vertical curves are used mainly when rising gradient intersects a falling gradient or when rising gradient meets another rising gradient or when rising gradient meets a horizontal road or when the falling gradient meets at a steeper falling gradient.
2. Valley Curves :
The road curves which are having the convex surface on downward side is termed as valley curve. Above figure shows the different situations where valley curves are provided.