These are bored cast in-situ concrete piles having one or more bulbs created towards the bottom through enlarging the bore hole of the pile stem. The enlargements help in giving substantial bearing or anchorage. Like piles have been found to be meaningful in expansive soils like black cotton soil as the bulbs give anchorage against uplift because of swelling pressures. The diameter of the underreamed bulbs might be of the order of 2 to 3 times the stem diameter. The spacing of bulbs is 1.25 to 1.5 times the stem diameter. A top most bulb should be at a minimum depth of 2 times the bulb diameter.
Boring for the piles is carried out within the common way. Thereafter the bulbs are created through means of an underreamer rotated through the drill rod. The excavated soil is erased through means of buckets. The reinforcement cage is then lowered and the pile concreted. The cement content and slumps shall be as denoted for bored cast in-situ piles.
Bored compaction pile is a changed form of underreamed pile whereas, after the concrete is poured, the reinforcement assembly, along with a cone welded at the bottom is driven by the fresh concrete along with the help of a driving pipe, thereby compacting the concrete.
φ1 = 45o approx, φ2 = 30 to 45o approx, Du = Normally 2.5 D ( All dimensions in mm )
Figure : Section of Underreamed Piles
While you have a situation when everthe upper portion has to project above water, steel and concrete or wood and concrete piles could be used. Lower portion below water could be either of timber or steel although the upper portion can be of concrete.