HORIZONTAL CURVE S

Horizontal curve joins the two intersecting lines which lie in a horizontal plane. e.g. Curve provided to connect two roads meeting at an angle. Horizontal curves are further classified as:

(a) Simple circular curve
(b) Compound curve
(c) Reverse curve
(d) Transition curve
(e) Combined curve
(f) Broken-back curve

Note: Compound and reverse curves are provided for low speed, roads and railways.

Simple Circular Curve

• A simple circular curve has the property that it connects two straight lines with a curve of constant radius at all the points on the curve and connects the two straight lines tangentially. In Figure, T₁TT₂ is a simple circular curve.

Compound Curve

• Many a times it is not possible to provide a curve of constant radius to connect the two straight lines. In that case, we provide more than one curve of different radii to connect them.
• When all the curves turn in the same direction then the resulting curve is called as compound curve. In Figure, T1T and TT2 are the two curves of different radii R1 and R₂ respectively which take turn in the same direction.

Reverse Curve

• In this type of curve, the two curves of different radii R1 and R2 take turn in opposite directions as shown in Figure
• These types of curves are also called as serpentine curve or curve the S curve because of their shape.

Transition Curve

• While going through a straight road, if a curve is suddenly encountered, we feel a jerk. In order to avoid such a sudden jerk, it is essential that the transition from the straight line to the curve takes place gradually and NOT suddenly. Thus a curve of varying radius is provided which takes off from the straight line, turns gradually and finally meets the curve i.e. attains the same radius as that of the curve. This type of curve is called as transition curve as shown in Figure. It is also called as easement curve.

Combined Curve

• It is a combination of simple and transition curves and is invariably provided in all types of highways and railways.

Broken-back Curve

• This is a very old type of curve used in the past wherein a straight line was provided in between the circular curves as shown in Figure. This type of curve is not suitable for high speed maneuvering and is not used these days.