Compaction vs Consolidation

Most of the construction sites have some part of backfilled soil along the building periphery. The density of these backfilled soil plays a major role in supporting and transferring forthcoming structure's load safely. Higher the density, safer the forthcoming structures. To achieve maximum density we follow compaction and or consolidation in backfilling process.

3 Parts of Soil mass

By removing the water and air from the soil mass we can able to achieve higher density in backfilled soils.

Loosen soil to dense soil

Compaction is the process of expelling air from soil mass by applying dynamic load to increase the density of backfilled soil. Compaction requires soil to,

• have OMC - Optimum Moisture Content at which soil density can be achieved to its maximum
• be filled and compacted in layers. The thickness of layers is depended on the compaction machine used

Soil Compaction

Consolidation is the process of expelling water from the soil mass with static load. Consolidation requires

• Soil to be in saturated level - so we need to add water up to saturation level manually
• Backfilling area to be left undisturbed under static load for long time period (in days/weeks)

Soil Consolidation

Now, the practical applications: major difference is explained as below,

Compaction has to be done in layer by layer with OMC. By selecting suitable compaction machine for layer thickness of 200mm, a 1 meter depth of backfilling can be compacted in two days for an area of 50 sq.mt to achieve 95% of maximum dry density. For the same area it will take a week or two for consolidation to happen. Also in some cases, it takes longer time period to achieve 95% of maximum dry density.

So compaction has to be done at all the backfilling soils wherever a useful surface/structure will be put on.

The consolidation can be done only where there will not be any useful surface/structure coming on it, example: a simple soft landscape.

For all the backfilled area soil compaction test is mandatory for the entire depth of filling. we will discuss about the tests for backfilling in further articles.