What are under reamed piles? What is the construction procedure and advatages?

Under reamed piles are bored cast-in-situ concrete piles having one or more number of bulbs formed by enlarging the pile stem. These piles are best suited in soils where considerable ground movements occur due to seasonal variations, filled up grounds or in soft soil strata

Under reamed piles are developed by C.B.R.I.( Central Building Research Institute). These are made from RCC.The special feature of these piles is that an enlarged portion are given at bottom or at midpoint which is known as ‘under reamed ‘or ‘bulb’.If the pile has one bulb, it is called single reamed, if the pile has two bulbs then it is called double reamed, and if there are more than two bulbs, it is called multiple under reamed piles.

ADVANTAGES AND DISADVANTAGES OF UNDER-REAMED PILES[4]

Advantages –

• It decreases the vertical settlement and also differential settlement.
• It is used when soil tends to swell and shrink due to moisture variation or expansive nature of the soil.
• Provision of under-reams or bulbs has the advantage of increasing the bearing and uplift capacities.
• When the number of bulbs are increased from one to two, the load carrying capacity of the Under-Reamed Pile is increased.
• The provision of bulbs is of special advantage in Under-Reamed Piles to resist uplift and they can be used as anchors.
• The cost advantages of Under-Reamed Piles are due to the reduced pile shaft diameter, resulting in less concrete needed to replace the excavated material.

Disadvantages –

• At a depth, where nature of soil varies with a climatic condition, Under-Reamed Piles are not suitable for waterlogged soil, as they take load by friction.
• These piles need strict quality control and regular supervision during the construction.
• Most of the times, Under Reamed Piles are driven manually with hand operated machine. Hence maintaining plumb of pile is very essential, because if they are not in plumb whole load transfer mechanism would change.