Methods of test for soils, Part 7: Determination of water content-dry density relation using light compaction
1980 Edition

The light compaction test requires a 2.6 kg rammer that is dropped from a height of 310 mm, conforming to IS 9198-1979 standards. The mould used is a cylindrical metal container with a baseplate, generally having a volume of 1000 ml for typical soils. For coarser soils with particles up to 40 mm, a larger 2250 ml mould is used. The soil is compacted in three equal layers with 25 blows per layer for the 1000 ml mould or 55 blows for the 2250 ml mould. The mould is placed on a firm surface such as a concrete floor during compaction, and the soil surface is leveled with an extension which is removed prior to the final finishing. The mould with the compacted soil is weighed to the nearest gram to determine density.

To find the optimum moisture content (OMC), soil samples are prepared at various moisture levels, typically at least five, spanning a range around the expected OMC. Each sample is compacted and its dry density measured. These values are plotted on a graph with moisture content (%) on the x-axis and dry density (g/cm³) on the y-axis. The peak point on this curve indicates the maximum dry density (MDD), and the corresponding moisture content at this peak is the OMC. Precision in reporting OMC varies with moisture content ranges, with finer precision for lower moisture contents.

Yes, the method described in Clause 5.2 of IS 2720 Part 7 is specifically designed for soils prone to crushing, such as soft limestones or sandstones. This approach uses a larger sample size—about 15 kg passing the 20 mm sieve—and applies adjusted moisture content ranges and compaction procedures to minimize particle breakage. While originally intended for such soils, this procedure may be applied to any soil type if desired, offering flexibility to ensure accurate determination of dry density and moisture relationships.

This test is primarily optimized for fine to medium-grained soils and is not ideally suited for highly permeable soils such as clean gravels or coarse sands. The large particle sizes and high permeability of such soils hinder uniform compaction with the standard 2.6 kg rammer method. Additionally, preparing representative samples is challenging due to particle segregation and difficulty in achieving uniform moisture distribution. Consequently, laboratory results may not accurately reflect field compaction characteristics, often overestimating optimum moisture content and underestimating maximum dry density. Alternative testing methods or field density measurements are recommended for such soils.

Coarse particles retained on the 20-mm IS sieve must be separated from the soil sample before compaction testing. The proportion of these coarse fractions should be recorded to the nearest 1%. Prior to sieving, aggregates should be broken down so that individual particles remain, ensuring accurate separation. Only the soil passing through the 20-mm sieve is used for the compaction tests as specified in the standard. This protocol helps maintain uniformity and reliability in compaction test results.