Showing posts from January, 2014

Factors affecting design of pavements

Design of pavement consist of two parts:

Design of the mix to be used as the material for different layers.Design of the thickness of the different pavement layers. In this post I will discuss here the various factors which are considered while designing the pavement thickness.

Design Wheel LoadType of sub-gradeClimatic conditionsProperties of the pavement layersEnvironmental conditionsOther special conditions.Design Wheel Load: Design wheel load is the main factor which decides the thickness of the pavement. Of course higher the design wheel load larger will be the pavement thickness. There are other factors related with the design wheel load.           Total Design wheel load, Magnitude of contact pressure, repetition of load and dynamics of            load is analyzed. Total thickness is governed by the total design wheel load and the quality of         the surface material is governed by the magnitude of the contact pressure.           Higher speed of the load will result in le…

Sight Distance at Intersections

Hello there,
Stopping site distance, Overtaking sight distance and Sight distance at intersections  are important to study and check for the design of the highways. Sight distance at the intersections: IRC recommends that at non-controlled intersections, sufficient visibility should be provided such that sight distance on each road is at-least equal to SSD corresponding to the design speed of the road.

IRC recommends that a minimum visibility distance of 15 m along the minor road and a distance of 220, 180, 145 and 110 m along the major road corresponding to design speeds of 100, 80, 65 and 60 kmph. respectively may be provided.

Apart from three situations above, following sight distances are considered by IRC in highway design.

Intermediate sight distance(ISD) = 2.SSD When overtaking sight distance cannot be provided, intermediate sight distance is provided to give limited overtaking opportunities to fast vehicles.

Head light sight distance This is the distance along the road, visible…

Traffic signs and Rotary intersection - (Traffic Engineering)

Traffic Signs: These should be mounted on sign posts pointed alternatively with 25 cm black and white band.

Regulatory signs: These are used to inform road users of certain laws, regulations and prohibitions.

Warning signs: These are used to warn the road users of certain hazardous conditions that exist on or adjacent to the roadway. Shape of these are equilateral triangles with its open pointing upwards.
Informatory signs:  These signs are used to guide the road users along to make travel easier, safe and pleasant. Informative signs are grouped under following sub-heads: Direction and place identification signs.Facility information signs.Other useful information signs.Parking signs.Flood gauge.Conditions when traffic rotary is justified: American Association of State highway officials(AASHO) have suggested that lowest limit of traffic volume when a traffic rotary is justified is about 500 vehicles per hour or all intersecting roads put together and maximum limit beyond which the rotary …

One liners - Transportation Engineering for GATE, PSUs - Part 10

Hello there,
How have you been? Here is another round of one liners for your preparation for the GATE and PSUs examinations.

Ratio of width of the car parking area required at kerb for 30 degree parking relative to 60 degree parking is approximately 0.5Load carrying capacity of rigid pavements depends more on the properties of concrete than strength of sub-grade.Compared to flexible pavements, rigid pavements are more affected by temperature variations.Collision diagrams are used to study accident pattern and determine remedial measures.Emergency escape ramp is provided in down grades of a highway.Plasticity index of the fraction passing 425 micron IS Sieve in case of sub-base or base course would be less than 6.If the CBR value obtained at 5 mm penetration is higher than that at 2.5 mm, then the test is repeated for checking; and if the check test reveals a similar trend, then the CBR value is to be reported as higher value obtained at 5 mm penetration.Road construction/design in chr…

One liners - Transportation Engineering for GATE, PSUs - Part 9

Hello there,
How have you been? Here is another round of one liners for your preparation for the GATE and PSUs examinations.

For Sandy soils, most common method of stabilization is Soil bitumen stabilization.For bituminous or asphalt concrete, proportioning of materials is determined from laboratories.Limiting gradient on highways is associated with the requirement of maximum tractive effort for a short distance.In group index method of flexible pavement design, daily volume of commercial vehicles decides the thickness of base and surface course.In bituminous pavement, alligator cracking is mainly due to fatigue arising from repeated stress application.The main function of prime coat is to improve bond between the existing base and surfacing of new construction.The design of horizontal transition curve length for highways should take into account three factors namely, allowable values of rate of change of centrifugal acceleration and superelevation and the minimum length formula given…

One liners - Transportation Engineering for GATE, PSUs - Part 8

Hello there,
How have you been? Here is another round of one liners for your preparation for the GATE and PSUs examinations.

The aggregates required for one kilometers length of water bound macadam road per meter width and for 10 mm thickness is 12 cubic meter.The camber of shoulders in water bound macadam roads is equal to the cross slope of pavement.The binder normally used in flexible pavement construction is bitumen.In highway construction rolling starts from sides and proceeds to center.For the construction of water bound macadam roads, the correct sequence of operations after spreading coarse aggregates is dry rolling, application of screening, wet rolling and application of filler.For the penetration macadam construction, the bitumen is sprayed after the aggregates are spread and compacted.When bituminous surfacing is done on already existing black top road or over existing cement concrete road, the type of treatment to be given is tack coat.A bituminous primer is a high viscos…

One liners - Transportation Engineering for GATE, PSUs - Part 7

Hello there,

How have you been? Here is another round of one liners for your preparation for the GATE and PSUs examinations. Bottom-most layer of a pavement is called Sub-grade.Flexible pavement distribute the wheel load through a set of layers to the sub-grade.The number of repetitions, which the pavement thickness designed for a given wheel load should be able to support during the life of a pavement is 1000000.Group Index Method of design of flexible pavement is an empirical method based on physical properties of sub-grade soil.Minimum and maximum values of a Group Index can be 0 and 20 respectively.If the Group Index value of sub-grade is between 5 to 7, then sub-grade is treated as poor.Tyre pressure influences the quality of the surface course.Rigidity factor for a tyre pressure greater than 7 kg/cm2 is less than 1.In a dual wheel assembly if P is equal to each wheel load, S is centre to centre distance of dual wheels and d is the distance between walls of wheels, then total str…

One liners - Transportation Engineering for GATE, PSUs - Part 6

Hello there,
How have you been? Here is another round of one liners for your preparation for the GATE and PSUs examinations. A traffic rotary is justified where number of intersecting roads is between 4 to 7.When a number of roads are meeting at a point and only one of the roads is important, then the suitable shape of rotary is tangent.Maximum number of vehicles can be parked with 90 degree parking.When the width of kerb parking space and width of street are limited, generally preferred parking system is parallel parkingAs per IRC recommendations, the average level of illumination on important roads carrying fast traffic is 30 lux.The most economical lighting layout which is suitable for narrow roads is single side lighting.The direct interchange ramp involves diverging to the right side and merging from right.In soils having same plasticity index, if liquid limit is increased, then compressibility and permeability increases but dry strength decreases.Aim of the Impact test is to che…