What is the need of Pavement Sub Grade Compaction? Describe the quality control tests or check for the same

Subgrade Compaction In case to provide maximum structural support (as measured by MR, CBR, or R-value), subgrade soil must be compacted to an adequate density. If not, the subgrade will continue to compress, distort, or erode after construction, resulting in fractures and deformation in the pavement.

Quality Control checks:

1. The representative soil samples collected from the selected borrow pits for the construction of highway subgrade are subjected to relevant laboratory tests to check the values of LL, PL, Grain Size Distribution (by wet-sieve analysis), soluble sulphate content, organic matter, and the compacted density. If the material does not pass the tests, it shall not be used.
2. Lab Compaction test is carried out on the selected soil to determine the OMC and maximum dry density of the soil, to be specified during construction.
3. The CBR value of the soils selected for use in highway subgrade are determined in the laboratory, compacted at 97% MDD.
4. The field moisture content of the loose soil after spreading to the desired thickness is determined by any rapid test method, so as to estimate the extra quantity of water to be added to bring up to the OMC of the soil.
5. After adding additional water as required and mixing thoroughly, the moisture content is again checked by a rapid method to ensure that the OMC is achieved.
6. After the soil layer is adequately rolled, the field density and moisture content are determined by one of the approved methods.
7. Steps 1 to 4 shall be repeated for each subsequent layer.
8. The finished surface levels of the subgrade are checked with reference to desired longitudinal and cross profile of the road if these are found to exceed the permissible tolerance of (+ 20 mm and -25 mm), the appropriate corrections are made by removing or adding part of the soil on the surface and by further rolling.