If soil is of expansive type, like in Black Cotton Soil, n best foundation type is -

Explanation:

Black cotton soil is an expansive soil having clay mineral (montmorillonite) which is responsible for the excessive swelling and shrinkage characteristics of the soil.

Note:

To construct the structure under this type of soil, an under-reamed pile is been provided. These piles are taken to depths below the zone of seasonal variation in moisture content.

Under-reamed pile

• An under-reamed pile is a special type of bored pile which is provided with a bulb at the end.
• The usual size of such piles is 150 to 200 mm shaft diameter and 3 to 4 m long.