Under Reamed Piles

Under Reamed Piles – Construction, Uses & Application

Under Reamed Piles – Construction, Uses & Application


  • This Pile (Under reamed piles) type developed by C.B.R.I.( Central Building Research Institute).
  • The under reamed piles are made from RCC.
  • This pile type special feature is 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.

Under reamed pile primly developed for an area where black cotton soil could cause structural instability. Many times, during, soils undergo volumetric changes due to moisture variation underneath the ground surface. The black cotton soil excess shrinkage and expansion property can cause distress which is very dangerous and critical as far as the bearing of the foundation is concerned. It is a fact that after the introduction of under reamed pile for it is considered as the safest and economical foundation for such black cotton soils or expansive soils.


Features and Details of Under Reamed Piles:

  • Diameter of the pile = 20 to 50cm
  • Bulb Diameter  = 2 to 3 times of dia. of the pile
  • Pile Length = 3 to 8m
  • Space between 2 piles = 2 to 4m
  • Spacing between 2 bulbs = 1.25 to 1.5 times of bulb dia.
  • The load-carrying capacity = 20 to 40 ton
Under Reamed Piles
Under Reamed Piles

These are bored cast-in-situ concrete piles having one or more bulbs formed towards the bottom by enlarging the borehole of the pile stem. The enlargements help in providing substantial bearing or anchorage. Such piles have been found to be useful in expansive soils like black cotton soil as the bulbs provide anchorage against uplift due to swelling pressures. The diameter of the under reamed bulbs may be of the order of 2 to 3 times the stem diameter. The spacing of the bulbs is 1.25 to 1.5 times the stem diameter. The topmost bulbs should be at a minimum depth of 2 times the bulb diameter.

Boring for the piles is carried out in the usual way. Thereafter the bulbs are formed by means of an under reamer rotated by the drill rod. The buckets are used to remove excavated soil. The reinforcement cage is then lowered and the pile concreted.  The cement quantity and concrete slump shall be as recommended bored cast-in-situ piles.

Increasing load-carrying capacity:

1. By putting more bulbs

2. By increasing the dia. of the bulb

3. By increasing the length of the pile

4. By providing proper reinforcement


Construction of Under Reamed Piles:

  • The machinery for under reamed pile is developed by C.B.R.I. Rurki.
  • The machine is easy to use and it is light weighted.
  • It consists of three tools :

1. Spiral Auger

Under Reamed Piles

2. Under – reamer

Under Reamed Piles

3. Boring guide

Under Reamed Piles
  • The hole or bore for under reamed pile is done by using a spiral auger.
  • The spiral auger consists of a special cutter at the bottom to dig the bore.
  • The soil that is dug is taken out by the bucket.
  • Special cutters are used for making the bulb. The dia. of the bulb can be increased by applying excess pressure. The auger is set up on tripod after the digging is done. So the digging is done in the vertical direction.
  • As reaching sufficient depth auger is pulled out. The reinforcement is then inserted into the hole. Concreting is done. If more than one pile, the beam is constructed which connects the piles and make them act like a single unit. The wall is then constructed on the beam.


  • As black cotton soil has a property that gets expand as it comes in contact with moisture or water and it gets contracted as it is dried out. Because of this reason, there may be a chance of cracks in the structure.
  • To avoid damage caused by soil volume change, the under reamed pile used.
  • Sometimes, when foundation soil does not have sufficient bearing capacity, under reamed piles are used to increase the capacity.
  • The under reamed piles are suitable when the water level in sandy soil is high.
  • It should be used. When there are lifting forces acting,

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