Under-Reamed Pile

An under-reamed pile is a bored cast-in-situ pile with one or more enlarged cylindrical/conical bulbs ('under-reams') formed along its shaft or at its base using an under-reaming tool. IS 2911 Part 3 governs its design and construction. The bulbs (typically 2-3× shaft diameter) anchor the pile below the active (seasonal moisture) zone, mobilising bearing on the bulb tops and shaft resistance to resist both downward load and the uplift from swelling.

It is the workhorse foundation for expansive black-cotton soils across central/western India — by founding and anchoring the bulb below the depth of seasonal volume change, it prevents the heave/settle cracking that destroys shallow footings on such soils. Bulb spacing, depth below active zone (≥1.5 m), reinforcement and load capacity follow IS 2911 Part 3 + the safe load is confirmed by pile load tests (IS 2911 Part 4).