sub base in road construction

granular sub base
  • Granular Sub Base is laid below the Base layer and above the Subgrade layer. The material used in sub-grade is Natural sand, moorum, Gravel, crushed stone, crushed slag.
  • Use of materials like brick material, kankar and crushed concrete shall be permitted in the lower sub Base.
  • Natural sand can not fulfill the grading requirement of granular sub Base hence natural sand can be used with a combination of other material.
granular sub base

Function of subbase

  • Sub-Base provides structural support to the base course and surface courses.
  • It improves the road surface drainage.
  • It prevents the sub-grade soil to enter into the pavement layer.


Grading requirements  of granular sub base
Grading requirements (percent by weight passing)
  • Grading I and Grading II should be preferred in the top layer of Sub-Base.
  • Grading III and Grading IV should be preferred in the lower sub-base.
  • Grading V and Grading VI should be preferred in the sub-base cum drainage layer.
  • When sub-base is laid in two layers ( upper sub base and lower sub-base ), in such cases each layer shall not be less than 150 mm.
  • If two grading is used in sub-base, place the grading of larger particle size at the bottom and the grading of smaller particle size above that.
  • Wet aggregate impact value should be determined if the water absorption is greater than 2 percent. Soft aggregate shall be tested for wet aggregate impact values.
  • If the liquid limit and the plastic limit of materials ( passing through IS sieve 425 microns) is greater than the permissible value, it means that there are more cohesive particles like clay. and sub-base materials have a tendency to absorbs moisture, which fails our requirement from sub-base that it should act as a free-draining layer.

Sources of Granular Sub Base

  • There are two sources of GSB.
  • Quarries from the river bed and removing the oversized particles ( size of 75 mm )from it
  • Quarries from rocks.
  • If we check the grading of GSB, and it has been found out that the material does not meet our grading requirements then it should be mixed/blended with missing grading with the required proportions.

Constructional requirement

  • The sub-base material shall be mixed with water up-to required water content. The water shall be mixed upto 6 percent approximately.
  • The moisture content of the mixed shall be from1 to 2 percent below from the optimum moisture content.
  • water shall be mixed mechanically by a suitable mixer equipped with provision for controlled addition of water. to ensure a homogenous and uniform mix.
  • Laying thickness shall be increased by 25 to 30 percent to achieve the desired design thickness.
  • a single layer up to a maximum of 125 mm thick shall be compacted to achieve 100 mm thick compacted layer by smooth wheel roller.
  • A single layer up to a maximum of 225 mm thick shall be compacted by a vibratory roller.
  • For carriageway having cross fall on both sides, rolling shall commence at the edges and progress towards the crown
  • Rolling shall commence at the lower edge and proceed towards the upper edge longitudinally for portions having unidirectional cross fall or on superelevation.
  • Rolling shall be continued till the density achieved is at least 98 percent of the maximum dry density.
  • After compaction, the compacted layer shall be free from movement under compaction equipment and from compaction planes.