Lane distribution factor, D

The transverse distribution of heavy vehicles across the width of the carriageway along both the directions (in the case of two-way traffic movement) is to be taken into account. In case this cannot be assessed from actual field studies, the following recommended guidelines on lane distribution factors, D may be followed:

a] On undivided roads with single lane carriageway the total number of heavy vehicles along both the directions are taken or the lane distribution factor D = 1.0.

b] On undivided roads with two lane carriage way, D = 0.75 and the total number of heavy vehicles along both the directions is to considered.

c] On undivided roads with four lane carriageway, D = 0.40 and the total number of heavy vehicles along both the directions is to considered.

d] On roads with divided carriage way with lanes each, D = 0.75 and the number of heavy vehicles along each direction is considered.

e] On roads with divided carriage way with three lanes each, D = 0.60 and the number of heavy vehicles along each direction is considered.

f] On roads with divided carriageway with four lanes each, D = 0.45 and the number of heavy vehicles along each direction is considered.

Basic principle of C.B.R. Method.

The basic principle of CBR method of flexible pavement design is based on the concept that the total thickness of flexible pavement required mainly depends upon two factors, namely (1) CBR value of the soil sub-grade over which the pavement is to be laid and (2) The magnitude of the wheel load or intensity of traffic loads expected.

A weaker soil sub-grade with lower CBR value will need a flexible pavement of higher thickness. Also a sub-grade soil with any particular CBR value will require thicker flexible pavement structure to cater for higher magnitude of design wheel load or for higher intensity of traffic loads.

Advantages of CBR method.

One of the main advantages is the simplicity of conducting the CBR test in the laboratory as well as the method of [pavement design using simple design charts. The CBR method of flexible pavement design was being extensively used in different countries of the world for quite a long period of time. However based on the local design requirements (such as traffic, climatic and other environmental factors) each country developed their own design chart. For example the United Kingdom (UK) and several states in the USA, developed their own empirical pavement design charts making use of the CBR value of the sub-grade soil.

Limitations of CBR method.

There are several limitations of both the CBR test the design method. Some of these are given below.

a] It is important to understand the limitations of the CBR test itself on sub-grade soil, which is an empirical penetration test for assessing the strength characteristic. The CBR values does not represent any of the basic strength properties of the soil.

b] The punching sear under the CBR test condition does not in any way represent the stress on the subgrade through the flexible pavement layers due to traffic wheel loads.

c] The specified four days soaking period may not be sufficient in some highly clayey soils to represent the worst field moisture content in areas with water logging.

e] The general limitations of empirical pavement design method as given in Ar. 7.5.1 above are applicable in this case also.