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Flexible, Rigid & Semi Rigid Pavement Design

Pavement design is the major component in the road construction. Nearly one-third or one-half of the total cost of construction goes into laying pavement. Naturally, Careful design of pavement becomes a very important part of Road Design. Pavement design is the major component in the road construction. Nearly one-third or one-half of the total cost of construction goes into laying pavement. Naturally, Careful design of pavement becomes a very important part of Road Design. 

Type of pavements:
• Flexible pavement

• Rigid pavement

• Semi-rigid pavement

Factors which affect the selection of these pavements
• Initial cost

• Availability of good materials

• Cost of maintenance

• Environmental conditions

• Availability of industrial wastes

• Traffic intensity

• Design parameters

Criteria for determining the flexible pavement

Pavement thickness experienced vertical compressive strain which has a influence up to the sub-grade due to the standard axle laden of magnitude 8.17 kN (8170 kg), and more than this causes permanent deformation in form of rutting. The maximum rutting can be accepted in village road as 50 mm before maintenance and the analytical evaluation can be done according to IRC:37. For rigid and semi-rigid pavement tensile stress is taken as the design criteria. To find the characteristic deflection of the pavement BBD(Benkelmen beam deflection) tests are conducted along the pavement’s most stressed path as per IRC:81 Specifications. During the test, several other tests and readings are recorded to find the deflection. They are CBR of the subgrade, Plasticity Index, Moisture content (Soil samples are collected and the results are obtained from the Soil laboratory), Atmospheric Temperature and pavement temperature are collected at a frequent interval during the day. With all these data, deflections are calculated using the graphs as given in the IRC:81. This is a tedious job and can take considerable time as we need to read the graph to get the values between moisture content and correction factor. These graphs are also classified based on the soil classification. The corrections for temperature also form the part of the analysis.

The VDF (Vehicle damage factor) and MSA (Million standard axles) are also to be anlaysed. VDF results are obtained by conducting the axle load surveys and MSA from traffic surveys. With these results the pavement thickness for overlay or new construction can be established using IRC:37.

Computation of design traffic: 

a=p(1+r)^(n+x)

Where,

  •     a=no. of commercial vehicles/day for design
  •     p=no. of commercial vehicles/day at last count
  •     r=annual growth rate of commercial traffic
  •     n=no. of years between last count and year of completion of construction
  •     x=design life in years

Using this formula we can get the CVPD(Commercial vehicles per day)

Pavement components

Subgrade:

To provide support to the pavement as its foundation. A minimum of 100% of standard proctor compaction should be attained  in the subgrade. For clayey soil 95% and moisture content of 2% in excess of optimum value. The soil below subgrade should be compacted to 97% of standard proctor compaction.

 

The test should be done per km or as specified by the engineer-in-charge depending  on the soil type.

Sub-base course:

Selected materials are placed on subgrade which is compacted to 98% of IS heavy compaction. Function of sub-base is to distribute the stresses over a wide area of the subgrade imposed by the traffic.

Materials:

Base course: To withstand high stress concentrations which develop due to traffic under the wearing surface. Different types of base course used are:

WBM

Crusher-run macadam

Dry lean concrete

Soft aggregate base course

Lime-fly ash concrete

Surface course:

Thickness of surface course depends upon the traffic volume and type of material used to it. For gravel roads extra thickness should be provided because of loss in thickness due to the traffic action regularly.

Bituminous wearing courses must be made up of good quality aggregate with an aggregate impact value not exceeding 30 % in order to reduce degradation of aggregates by crushing.            

Pavement design Module of ESurvey Civil Tools attempts to give a simple solution for achieving all these design parameters and Pavement thickness. The major advantage is it eliminates the tedious job of reading various graphs in the BBD Analysis.