What do you mean by gradient? What are its different types? Also give the recommended values of Gradient for different types of terrain
The gradient is the rate of rising or fall of road surface along its length with respect to the horizontal. It is the longitudinal slope provided to the road surface along its length. It is expressed as a ratio of 1 in x (one unit vertical to x units horizontal) and also expressed as a percentage, i.e., x in 100. 1 in 30 gradient shows 1 m rise or fall of road level in 30 m horizontal distance. When expressed in percentage it will be 3.33%, Gradient is also expressed in angles.
The gradient in the road should not be very steep. Steep grades are not only difficult to climb but also
the vehicle operating cost is increased.
Types of Gradients
Gradients are of the following types.
1. Ruling gradient
2. Ruling Gradient
3. Exceptional gradient
4. Minimum gradient
1. Ruling Gradient It is the maximum gradient within which the designer attempts to fix the vertical profile of the road. The ruling gradient is also known as the design gradient. This is the gradient normally and adopted in the design of vertical alignment and accordingly, the quantities of cut and fill are balanced.
During the selection of the ruling gradient for the purpose of design following factors are kept in mind:
(i) Length of grade
(ii) Type of terrain
(iii) Speed of vehicle
(iv) Pulling power of the vehicle.
Ruling gradient values of 1 in 30 on a plain and rolling terrain, 1 in 20 on mountainous terrain, and 1 in 16.7 on steep terrain has been recommended by I.R.C.
2. Limiting Gradient: It is steeper than the ruling gradient and is provided at the place where topography compels to adopt a steeper gradient to avoid an enormous increase in cost in a gentler gradient. The length of limiting grades should not be continuous but limited in short length. On hill roads, it may be frequently necessary to exceed the ruling gradient and adopt a limiting gradient; bur care should be taken to separate such stretches of steep gradients by providing either a road with an easier grade or a level road.
3. Exceptional Gradient: Exceptional gradient is steeper than ruling gradient and may be provided in short lengths of the road in some extraordinary situations. It should not exceed 60 meters in one kilometer of road length.
Values of gradients recommended by the I.R.C. are given in Table 2.4
4. Minimum Gradient: It is the minimum desirable slope essential for effective drainage of rainwater from the road surface. The minimum gradient would depend on rainfall, runoff, type of soil topography, and other site conditions. A gradient of about 1 in 500 may be sufficient to drain water in concrete drains while a kutcha open drain may require 1 in 250 slopes or even more.
Define stopping sight distance in highway engineering. What are the various factors on which it depends? How it is calculated?
Stopping Sight Distance: Stopping sight distance is the minimum sight distance available on a stop vehicle without collision. This is also sometimes called non-passing sight distance.
Sight distance available on a road to a driver at any instance depends on (i) features of the road ahead, ie, the horizontal alignment and vertical profile of the road, traffic condition and obstructions, (i) object above the road surface (ii) height of the driver’s eye above the road surface I.R.C has suggested the height or eye level of the driver as 1.2 m and the height of the object as 0.15 m above the road surface for the purpose of measuring stopping sight distance.
The stopping sight distance depends upon the following factors:
1. Total reaction time of the driver
2. Speed of vehicle
3. Efficiency of brakes
4. Slope of road surface
5. Frictional resistance between the road and the tyres.
The gradient on roads | Gradient in highway engineering
What is overtaking sight distance? Discuss in Details.
Passing or Overtaking sight Distance: It is the minimum distance open to the view of the driver of a vehicle intending to overtake a slow vehicle ahead with safety against the traffic of opposite direction. Passing sight distance is the distance measured along The center of the road which a driver with his eye level at 1.2 m above the road surface can see the top of an object 1.2 m above the road surface.
The minimum passing sight distance depends upon the following factors:
(a) Speeds of the overtaking, overtaken and the vehicle coming from the opposite direction.
(b) Slope of the road.
(c) Rate of acceleration of overtaking vehicle.
(d) Spacing between vehicles.
(e) Skill and reaction time of the driver.
Refer to Figure 2.3 the relative position of three vehicles A, B and C is shown by suffixes. Vehicle A running at design tries to overtake a slow-moving vehicle B on a two-lane road while the third
vehicle C comes from the opposite direction, its overtaking maneuver is shown in Figure 2.3. Then the passing sight distance required for vehicle A = d, +d, +d, Where di is the distance traveled by overtaking vehicle A during the reaction time from position A, to Ag. da is the distance traveled by oncoming vehicle C from C, to C, during overtaking operation da is the distance traveled by oncoming vehicle C from C, to C, during overtaking operation of vehicle A. The passing or overtaking sight distance for various speeds recommended by the I.R.C.
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