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Highway Engineering

Definition of Geometric Design

  • Geometric design of highways deals with the dimensions and layout of visible features of the highway.
  • Geometric design fulfills the requirements of the driver and the vehicle, such as comfort, efficiency and safety.
  • Proper geometric design will help in the reduction of accidents and their severity.

Goals of Geometric Design

  • Maximize the comfort, safety and economy of facilities.
  • Provide efficiency in traffic operation.
  • Provide maximum safety at reasonable cost.
  • Minimize the environmental impacts.

Affects affecting geometric design

  • Design speed
  • Topography
  • Traffic
  • Environmental factors
  • Economical factors
  • Vehicle properties
  • Humans (the physical, mental and psychological characteristics of the driver and pedestrians like the reaction time).

Road alignment

  • Alignment is an arrangement in a straight line or in correct relative positions.
  • The position or the layout of the central line of the highway on the ground is called the alignment.
  • Horizontal alignment includes straight and curved paths.
  • Vertical alignment includes level and gradients.

Types of alignment

  • Horizontal Alignment
  • Vertical Alignment

Horizontal Alignment

  • Horizontal alignment in road design consists of straight sections of road, known as tangents. Connected by circular horizontal curves.
  • It is the design of the road in the horizontal plane.
  • Consists of a series of tangents, circular curves and transition curves.
  • Should provide safe travel at a uniform design speed.

Vertical Alignment

  • Vertical alignment is the longitudinal section.
  • Vertical alignment specifies the elevations of points along the roadway.

Carriage way

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

Road Shoulders

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

Formation width

  • Formation width is the top width of the highway embankment or the bottom width of cutting excluding the side drain.
  • Formation width = Width of Carr. Way + Width of shoulder.

Right of way

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

Berm

  • The distance between the road toe and inner edge of borrow pit is called Berm.
  • It prevents the erosion of embankment soil.

Side drain

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

Building line

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

Central line

  • At the location 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 to such building is called line.

Borrow pits

  • The pits dug along the road alignment for using excavated earth in construction of embankment are known as borrow pits.
  • Borrow pit should be dug at least 5m from toe of embankment.
  • The small portion left undug in a borrow pit to measure the depth of excavation is called deadman.

Kerbs

  • The boundaries between pavement and shoulders or footpath are known as kerbs.

Pavement design

  • A highway pavement is a structure consisting of superimposed layers of processed materials above the natural soil sub-grade.
  • The pavement must provide an acceptable riding quality, adequate skid resistance, favorable light reflecting characteristics, and low noise pollution.

Camber

  • Camber or cross slope is the slope provided to the road surface in the transverse direction to drain off rain water from the road surface.
  • The rate of camber is usually designated by 1 in n (1 vertical to n horizontal).

Super elevation

  • It is the slope across pavement surface and is fully developed in the circular curve.
  • Super elevation helps the vehicle to over come the centrifugal force on the curves on pavements.
  • The need for super-elevation on road curves, to ensure safety against skidding and over turning with the advent of fast moving traffic.
  • In the past, roads were constructed without any regard to super elevation on curves and had generally a cambered section for drainage purposes. It was little realized then that a vehicle moving on a curve had to overcome a centrifugal force to enable it to follow the curved path instead of a straight line, but, in justice to the early designers of roads, it must be said that there was no fast traffic in those days.

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