Factors affecting design of pavements

Hi,
Design of pavement consist of two parts:

1. Design of the mix to be used as the material for different layers.
2. 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.

1. Design Wheel Load
2. Type of sub-grade
3. Climatic conditions
4. Properties of the pavement layers
5. Environmental conditions
6. Other 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 lesser contact time and thus the deflection of the pavement will be less but if the pavement is undulated then the impact will be higher when speed is higher so, dynamics plays an important role.

• Type of Subgrade : Subgrade quality is determined by various tests such as CBR test, Tri-axial test, plate load test etc. When this possess high strength the thickness required will be less and vice verse.

• Climatic conditions: Amount of rainfall and subsequently the moisture affects the moisture in the sub-grade. The daily and seasonal variation of the temperature also affect the design of flexible and rigid pavements. 

If freezing occurs in the winter then there are chances of frost action on the subgrade soil and so this should be considered while the design of the pavement.

• Properties of the pavement layers: Performance of the pavement layers depends on the properties of the materials used in the layers. The fatigue and their performance under the adverse environmental conditions should be tested beforehand and should be given due considerations.

• Environmental Conditions: Environmental factors such as, height of the embankment,  foundation details, depth of cutting and the level of the water table affects the design of pavements. Variation in temperature daily or seasonal induces the warping stresses in case of rigid pavements.

• In case of semi-rigid pavement materials, the formation of the hair cracks, their propagation has to studied before arriving at a specific method of design of the semi-rigid pavement.

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 to a driver during the night driving under the illumination of the vehicle head lights. This sight distance is critical at up-gradients and at the ascending stretch of the valley curves.

Thank you for your visit.

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:

1.  Direction and place identification signs.
2. Facility information signs.
3. Other useful information signs.
4. Parking signs.
5. 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 may not efficiently function is about 5000 vehicles per hour. IRC suggests that maximum volume of traffic that a rotary can efficiently handle is 3000 vehicles per hour entering from all the legs of the intersection. Traffic rotary angles may be provided where intersecting motor traffic is about 50% or more of the total traffic or all intersecting roads or where the fast traffic turning right is at-least 30% of the total traffic.

What is traffic rotary?

Traffic rotary is an enlarged road intersections where all converging vehicles are forced to move round a large central island in one direction before they can weave out of traffic flow into their respective directions radiating from the central island.

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 0.5
• Load 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 chronological sequence : Tresaguet, Telford, Macadam & C.B.R.
• The general requirement in constructing a reinforced concrete road is to place a single layer of reinforcement near the top of the slab.
• It is a common practice to design a highway to accommodate the traffic volume corresponding to 30th hour.
• The amount of mechanical energy imposed on the aggregate during the aggregate impact test is of the order of (15*14*38 kg-cm) 7980 kg-cm.
• For wet and cold climates emulsions are recommended to be used as binders.
• With reference to Marshall mix design criteria for highways, Stability value = 340 min.; Flow value = 8-16 & % air voids = 3-5.
• For the design of valley vertical curve, the design criteria used are rider comfort, headlight sight distance and drainage.
• Bird baths may be formed as the deformations which may be caused by localized or variable sub-grade failure.
• Pot holes A steep sided, bow-shaped cavity caused by the loss of surfacing as well as base course erosion.
• Ravelling is the removal of larger surface aggregates leaving craters.
• Subsidence is abrupt lowering of the road surface due to poor drainage.
• For carrying out bituminous patch work during the rainy season, the most suitable binder is bituminous emulsion.
• The characteristic rebound deflection on a pavement, using Benkelman beam study is used by IRC to suggest the equivalent overlay design thickness.

Do you have better resource to crack the competitive examinations like GATE and PSUs?

Please share here.

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 by the IRC.
• Penetration test is performed for the gradation of the asphalt cement.
• Marshall test is performed for design of bituminous concrete mix.
• Ring and ball test is performed to determine the softening point.
• Benkelman beam test is performed for overlay design.
• Reflection cracking is observed in bituminous overlay over cement concrete surface.
• General landslide prevention works on the hill roads are retaining walls, catch water drains and breast walls.
• In the Los Angeles Abrasion Test on aggregate, if the speed of the drum is increased to 50 rpm, then the abrasion value will increase.
• Low bitumen content in bituminous pavement causes ravelling in bituminous pavement.
• In desire-line diagram width of desire-line is proportional to the number of trips in both directions.

Check the reference book:

Thanks for visit!

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 viscosity cut-back.
• The suitable surfacing material for a bridge deck slab is mastic asphalt.
• The thickness of bituminous carpet varies from 20 to 25 mm.
• Sheet asphalt is a sand-bitumen mix without coarse aggregates.
• In highway construction on super-elevated curves, the rolling shall proceed from lower edge towards the upper edge.
• Expansion joints in cement concrete pavements are provided at an interval of 18 m to 21 m.
• Contraction joints are spaced closer than the expansion joints.
• Longitudinal joints are provided in cement concrete roads to prevent longitudinal cracking.
• Warping joints are provided to relieve stresses induced due to warping.
• A hill road is one which passes through a terrain with a cross slope of 25 to 60%.
• In hill roads if several alternate alignments are surveyed which fulfill the geometric standards, then the preferred alignment is the one which has resisting length as minimum.
• The camber is hill roads in case of bituminous surfacing is adopted as 2.5%.
• The super-elevation to be provided in horizontal curves of radius R in hill roads is V^2/(225R).
• The minimum design speed for hairpin bends in hill roads is taken as 20 kmph.
• The drain which is provided parallel to the roadway to intercept and divert the water from hill slopes is known as catch-water drain.
• The walls which are necessary on the hill side of roadway where earth has to be retained from slipping is known as breast wall.
• In hill roads the side drains are provided only on the hill side of the roads.
• Emulsions are used for stabilizing desert sand.
• Soil lime stabilization is very effective for stabilizing black cotton soils.
• Soil lime stabilization is quite effective in warm regions.

Check the reference book:

Thanks for visit!

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 stress due to the dual wheels at any depth greater than S is equivalent to a single wheel load of magnitude 2P.

• Main drawback of CBR method is that it gives the total thickness which remains same irrespective of the quality of materials used in the component layers.

• Critical load positions in a rigid pavement design is taken as interior, edge and corner loading.

Check the reference book:

 Thanks for visit!