The design speed for a two-way traffic on a two way lane road is 50
kmph. What is
the value of safe stopping sight distance, if co-efficient of friction
is 0.37 and reaction
time of driver as 2.5 sec?
Consider the following statements related to economic design of
modern track:
1. Axle loading, possible
weight reduction of the rolling stock and increase in number
of axles should be rationalized.
2. Rail to sleeper
fastenings for different type of sleepers at high speeds should have
definite standards.
3. The weight of the
sleeper and the rail may not be related to each other.
Which of the above statements are correct?
(a) 1 and 2 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3
(A)
A modern track has to be rationalized in view of the following to achieve over-all economy
on Indian Railways.
i. Rail requirements, like quality of steel, design of rail-section and manufacturing
process, should be rationalized so that the rail section can sustain the expected
stresses due to rolling loads.
ii. Wheel diameter in relation to qualities of rail steel and steel of the wheel tyre should
also be rationalized so as to keep Hertzian stresses within the endurance limits of
the steel.
iii. Axle loading, possible weight reduction of the rolling stock and increase in number
of axles should be rationalized.
iv. Rail-to-sleeper fastenings i.e., elastic fastenings for different types of sleepers at
high speeds should have definite standards.
v. The weight of the sleeper and the rail should be correctly related to each other
Camber is the slope provided in the transverse direction of the road to drain off the rain water from the road surface. Usually camber is provided in the straight roads by raising the centre of the carriage way with respect to the edges forming highest point at the centre.
Straight slope of camber is easier to be cast and maintained in case of concrete pavement. Parabolic shaped camber is difficult to construct and maintain. It has more steeper edges which are inconvenient to use. However, it is kept in bituminous pavements for better drainage of water.
Pavement and its Shape of camber
Cement concrete= pavementStraight
Bitumen pavement=Parabolic
Type of Explosive
Suitability
Blasting Powder
In large quarrying blocks
Dynamite
Small boreholes and quarries
Cordite and Gelignite
Under water
Lithofracteor
Tunnels
Kerbs
Kerb and its Height abovethe pavement
1.Mountain kerb= 10 cm
2.Semi-barrier type kerb= 15 cm
3.Barrier type kerb= 20 cm
Off Tracking Formula
The off-tracking on the road is given by:
Eoff = nLsquare/2R
where
n = number of lanes
L = wheel base
R = Radius of the curve
Eoff = Off-tracking
Temperature stresses and Variations
Temperature stresses are developed in rigid pavements due to variations in temperature. This is caused by the following two factors:
1. Seasonal variation in temperature and
2. Daily Variation in temperature
Statement I – False
Daily variation in temperature:
It causes warping stress due to the temperature gradient across the slab thickness.
During the day, the top surface will try to expand but the weight of the slab will resist and hence, compression will be developed on the top surface and tension will develop at the bottom surface.
During the night, the bottom surface will be at a higher temperature and hence, has a tendency to expand, but the weight of the slab will resist it, generating compression at the bottom surface and tension at the top surface.
During Day Time:
During Night Time:
During the day the top of the pavement slab gets heated under the sunlight when the bottom of slab still remains relatively colder.
Seasonal Variation in temperature
It causes frictional stresses due to the overall change in the temperature of the slab. During summer season as the mean temperature of the slab increases, the concrete pavement expands towards the expansion joints and in winter season opposite phenomena occurred as shown in the figure:
Frictional Stresses:
Durring summer
Durring winter
Floating gradient:- The gradient on which a motor vehicle moving with a constant speed continues to descend with the same speed without any application of power brakes is called floating gradient i.e the vehicle does not require any tractive effort to maintain the specific speed.
Minimum gradient:- The minimum desirable slope essential for effective drainage of rainwater from the road surface is called minimum gradient
Exceptional gradient:- The gradient steeper than the limiting gradient which may be used in short length of road, only in an extraordinary situation is called exceptional gradient.
Ruling gradient:- The gradient usually adopted while making the road alignment is called the ruling gradient. It is the maximum gradient within which the designer attempts to design the vertical profile of a road. It is also known as design gradient.
Nagpur road plan or First 20 year road plan (1943 - 63):
In this plan road network in the country was classified in to:
i) National Highways
ii) State highways
iii) Major District roads
iv) Other District roads
v) Village roads
In the Nagpur conference, recommendations were made for the geometric standards of roads, bridge specifications and highway organisations and two plan formulae were finalised. This two plan formulae assumed star and Grid pattern of road network.
Note:
1. The rectangular or the block pattern has been adopted in the city roads of chandigarh.
2. Radial and circular pattern of road network is adopted in Connaught place (New Delhi).
Flexible Pavement
A pavement which consist of a mixture of asphaltic or bituminous material and aggregate placed on a good quality and compacted granular material is termed as flexible pavement. The flexible pavement has low or negligible flexural strength. Example-Water bound macadam roads.
Water bound macadam roads:
The roads whose wearing course consist of clean crushed aggregates, mechanically interlocked by rolling and bound together with filler material and water and laid on a well compacted base course, is called water bound macadam road.
California Bearing Ratio (CBR) test:
California Bearing Ratio (CBR) test is a method of classifying and evaluating soil-subgrade and base course materials for flexible pavements.
CBR test, an empirical test, has been used to determine the material properties for pavement design.
This test measures the strength of the material and is not a true representation of the resilient modulus.
It is a penetration test wherein a standard piston, having an area of 3 in 2 (or 50 mm diameter), is used to penetrate the soil at a standard rate of 1.25 mm/minute.
The pressure up to penetration of 12.5 mm and it’s ratio to the bearing value of a standard crushed rock is termed as the CBR.
Periodic Volume Counts – are used to calculate expansion factors needed to estimate the annual traffic volume.
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