Methods of test for soils, Part 8: Determination of water content-dry density relation using heavy compaction
1983 Edition

According to the standard, prepare at least five soil samples, each around 2.5 kg in weight, consisting of air-dried soil that passes through a 19 mm IS sieve. For soils containing coarser particles up to 37.5 mm, use a larger mould of 2250 cm³ capacity and sample passing the 37.5 mm sieve. Each sample should be mixed thoroughly with varying amounts of water to represent a range of moisture contents, including the anticipated optimum moisture. For cohesive soils, allow at least 16 hours in a sealed container for moisture equilibration. Compact the soil in a 1000 cm³ or 2250 cm³ mould in five layers, applying 25 blows per layer for the smaller mould or 55 blows for the larger mould, using a 4.9 kg rammer dropped from 450 mm. Ensure the soil fills the mould leaving no more than 6 mm excess to be struck off, then weigh the mould empty and with compacted soil to the nearest gram.

To find the optimum moisture content (OMC), prepare multiple soil samples with varying moisture levels around the expected OMC and compact each using the prescribed heavy compaction method. Measure the dry density and moisture content for every sample. Plot a graph with moisture content on the x-axis and dry density on the y-axis, then draw a smooth curve through the points. The peak of this curve indicates the maximum dry density, and the corresponding moisture content at this peak is the OMC. Reporting precision varies by moisture content range: nearest 0.2% for moisture below 5%, 0.5% for 5 to 10%, and 1% for values above 10%.

The test employs a 4.9 kg rammer dropped from a height of 450 mm to provide the compaction energy. Soil is compacted in five layers, with 25 blows per layer in a standard 1000 cm³ mould, or 55 blows per layer in a larger 2250 cm³ mould for coarse soils. The rammer must conform to IS 9189-1979 specifications. The moulds must meet the dimensions and material requirements specified in the standard. It is important to keep the rammer tube unobstructed to ensure free fall, and the compacted soil should be struck off leaving no more than 6 mm excess.

For soils susceptible to particle crushing, such as those containing soft granular components like certain limestones or sandstones, the standard recommends increasing the sample size to approximately 15 kg passing through a 19 mm sieve, compared to smaller samples for non-crushing soils. Additionally, a modified compaction procedure is applied to account for particle breakage during the test, ensuring the resulting data on dry density and moisture content remain reliable. This method can also be optionally used for all soil types to provide a consistent approach to fragile materials.

Heavy compaction involves a 4.9 kg rammer dropped from 450 mm height, compacting soil in five layers with 25 blows each (or 55 for larger moulds), simulating more intense field compaction usually for cohesive and some cohesionless soils. Light compaction uses a 2.6 kg rammer dropped from 310 mm height, with three layers and 25 blows per layer, suitable for lighter compaction scenarios. Heavy compaction typically results in higher maximum dry density and lower optimum moisture content, whereas light compaction produces lower dry densities and higher optimum moisture contents. Heavy compaction may cause crushing in fragile soils, which light compaction minimizes.