Wednesday, August 21, 2013

Highway Geometrics- (Camber, Land Width, Carriage way, Sight Distance, Super Elevation, Gradient)


  • Highway Geometric: This is the branch of the highway engineering which deals with the geometrical elements of the roads like, land of width, formation width, carriage way, side slopes, shoulders, kerbs, sight distance, superelevation and highway curves.
  • Land Width: It is total width of the land acquired by the Govt. along the road for the construction and maintenance of the roads. No other buildings by public can be constructed on the land width. Land width depends upon the following factors:

(a) It depends on the type of the road to be constructed, like NH require more width as compared to the lower level highways.
(b) It depends on the anticipated future increase in the traffic on the route or the economic or industrial development of the areas which it aligns along its route.
  • Carriage Way: 
    Carriage way and formation width
 It is the width of the road which is used by the traffic for moving on it. It is generally central portion of the total land width and is paved and surfaced with the bituminous concrete for service to the road users. Width of the carriage way depends on the number of the lanes in the road which again depends on the class of the highway. If it is higher level road like NH then it will need more numbers of lanes and therefore the carriageway width will be more.
  • Camber: 
Camber is the transverse slope provided to the road surface for the drainage of the rainwater for the better performance of the road. Camber can be written as 1 in n or x%.

Drainage of the rainwater is necessary
(1) To maintain the safe value of the friction between the road surface and the tyres
(2) To maintain the strength and durability of the surface concrete
(3) To maintain the durability and strength of the sub-grade soil which can be harmed if the infiltration of the water takes place to it.
There are generally three types of the cambers: (a) Straight Camber  (b) Parabolic Camber  (c) Mixed Camber.
Types of Camber

(a) Straight Camber: This type of camber is provided by meeting two straight surfaces at the crown. Crown is the central and top most point on the surface of the road. The edge shape produces inconvenience to the traffic so it is not used in general.

(b) Parabolic Camber: Parabolic camber is provided by providing a parabolic shape to the surface of the road. It is also not used in general because it has steep slopes towards the edges, which can create the outward thrust to the vehicles.

(c) Mixed Camber: Mixed camber is formed by use of the straight surfaces at the edges but parabolic surface at the centre. It is mostly used for the road construction because both the problem of the earlier two are solved if we use this camber.

  • Gradient: It is the slope provided to the surface of the road in the longitudinal direction for the vertical alignment of the road. There are three kinds of gradients:
    A vehicle on ascending gradient

(a) Ruling Gradient  (b) Limiting Gradient (c) Exceptional Gradient  (d) Minimum Gradient.

  • Ruling gradient is the design gradient, so it is used to design the road in the vertical alignment.
  •   Limiting and exceptional gradients are provided in the limited stretch of the roads where necessary and in case of the emergencies or exceptional cases when such need arises respectively.
  • Minimum gradient is the gradient which is required as the minimum from the drainage point of view in case of  the plane areas.
  • Sight Distance: Sight distance at any instance is the distance along the centerline of the road which is visible to the eye of a driver at an height of 1.2 m from the road surface such that an obstruction of height 0.15 m is visible to him. The heights of the eye of the driver and the obstruction is standardized by the Indian Roads Congress.
    Sight Distance
Most important sight distance which are necessary to be studied here in the design point of view are:
(a) Stopping Sight distance
(b) Overtaking sight Distance
  • Stopping sight distance(SSD): SSD is the sight distance which is necessary for a driver to stop a vehicle from the design speed to the 0 speed without any collision with the obstruction on the road. It is also known as the absolute minimum sight distance so this much sight distance is provided at all the cross section of the road.
  • Overtaking Sight Distance(OSD): OSD is the sight distance which is necessary for a vehicle running at the design speed to overtake a slower moving vehicle without collision with the vehicles coming from the opposite direction. Generally It is not possible to provide the OSD at every cross section of the road so, it is provided after a stretch of the road.
  • Super- Elevation: The outer edge of the road with respect to the inner edge of the road is raised in case of the horizontal curves, this is called super-elevation. Super-elevation is necessary to counter-act the centrifugal force due to the radius of the curve and speed of the vehicle.
 e+f = v^2/ gR
where e= superelevation
           f = value of the friction
           v = Design speed in m/sec
            R = Radius of the horizontal curve in meters.

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