Why do we need sand filling under foundation footings?

Generally, 150mm thickness of sand filling is provided beneath the PCC bed.

You may be confused!.

The concrete is a stable element then why do we need sand filling beneath PCC in footing!.

There are five possible reasons behind sand filling under footings. Let’s see each other briefly.

Reason 1 – After completion of the consolidation, the earth may still look like it has more undulations. So to provide uniformity for the PCC bed, the sand will be filled and compacted.
Reason 2 – Another major purpose of a sand filling is to restrict the structured settlement from sudden impact under footings. If the footing is laid over directly on the bedrock or soil surface, the structure may be exposed to partial settlements. Even minor impacts on the bedrock will affect structural stability.
Reason 3 – Even though the anti-termite treatment prevents the structure from termite attacks, the sand filling acts as an extra layer to protect the termite entrance.
Reason 4 – Foundation soil may contain some contaminants which are harmful and corrode the concrete. The sand filling is provided to help avoid the concrete directly touching the soil surface. We could also use crushed sand instead of river sand.
Reason 5 – Sometimes, the water table level is equal to the depth of footing. So the soil looks very wet and clayey. If the concrete was poured directly on the wet soil surface, there is a chance for concrete ingredients segregation. It is also very critical for the compaction of concrete. So sand is filled to avoid those issues.