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Some of the important points recommended by IRC for the CBR method of design (IRC:37 – 1970) are given below:

  • The CBR tests should be performed on remoulded soils in the laboratory. In – Site tests are not recommended for design purpose. The specimens should be prepared by static compaction where ever possible and other wise by dynamic compaction. The standard tesst procedure should be strictly adhered to.
  • For the design of new roads the sub grade soil sample should be compacted at OMC to proctor density whenever suitable compaction equipment is available to achieve this density in the fields; otherwise the soil sample may be compacted to the dry density expected to be achieved in the field. In the case of existing roads, the sample should be compacted to field density of sub graded soil (at OMC or at a field moisture content.)
  • In new constructions the CBR test samples may be soaked in water for four days period before testing. However in areas with arid climate or when the annual rain fall is less than 50 cm and the water table is too deep to affect the sub grade adversely and when thick and impermeable bituminous surfacing is provided, it is not necessary to soak the soil specimen before carrying out the CBR test. Wherever possible the most adverse moisture condition of the sub grade should be determined from the field study.
  • At least three samples should be tested on each 1 type of soil at the same density and moisture content. If the maximum variation in CBR valves of the three specimens exceeds the specified limits, the design CBR should be the average of atleast six samples ( The specified limits of maximum variation in CBR are 3 for CBR values up to 10,5 for values 10 to 30 and 10 for values 30 to 60%)
  • The top 50-cm of sub grad should be compacted at least up to 95 to 100 percent of proctor density.
  • An estimate of the traffic to be carried by the road pavement at the end of expected life should be made keeping in view the existing traffic and probable growth in traffic due to change in the land use. Pavements of major roads should be designed at least for 10days life period and the following formula may be used in such cases for traffic prediction.

A = P(1+r)(n+10).

Where A = Number of heavy vehicles per day for design (laden Weight>3 tonnes)

P = number of heavy vehicles per day at least count

r = annual rate of increase of heavy vehicles.

n = number of years between the last count and the year of completion of construction.

  • The value of P in the formula should be the seven day average of heavy vehicles found from 24 hour counts. If a reliable value of growth factor r is not available, a value of 7.5% may be assumed for roads in rural areas.
  • The traffic for the design is considered in units of heavy vehicles (of laden weight exceeding 3 tons) per day in both directions and are divided into seven categories A to G. The suitable design curve should be chosen from the table given in the design chart (fig). The design thickness is considered for single axle loads upto 8,200 kg and random axle loads upto 14,500 kg. For higher axle loads the thickness values should be further increased. (This is improvement over earlier mentioned values of 8160 kg and 4080 kg)
  • When sub-base course materials contain substantial proportion of aggregates of size above 20 mm, the CBR value of these materials would not be valid for the design of subsequent layers above them. This layers of wearing course such as surface dressing or open graded premixed carpet up to 2.5 cm thickness should not be counted towards the total thickness of pavement as they do not increase the structural capacity as the pavement.