Question

Which of the following corresponds to the recommendations of IRC for pavement thickness determination by CBR method?

1. CBR tests are to be conducted in-situ
2. Static compression is best adopted
3. The top 50 cm of subgrade should be compacted to as near the proctor density as possible

Answer 1

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

(a) The CBR tests should be performed on remoulded soils in the laboratory. In-situ tests are not recommended for design purposes. The specimens should be prepared by static compaction wherever possible and otherwise by dynamic compaction. The standard test procedure should be strictly adhered to.

(b) For the design of new roads, the subgrade soil sample should be compacted at OMC to proctor density whenever suitable compaction equipment is available to achieve this density in the field. In the case of existing roads, the sample should be the field density of subgrade soil (at OMC or at a field moisture content).

(c) In new constructions the CBR test samples may be soaked in water for four days before testing.

(d) At Least three samples should be tested on each type of soil at the same density and moisture content. The specified limits of maximum variation in CBR are 3% for CBR values upto 10% 5% for values 10 to 30 and 10% for values 30 to 60%.

(e) The top 50 cm of subgrade should be compacted at least upto 95 to 100 percent of Proctor density.