What is Under reamed piles|| construction procedure || Advantages and Disadvantages.

Under reamed piles are one type of deep foundation. Under reamed piles are generally used in black cotton soil. Here we explain what is Under reamed piles, Advantages and disadvantages, Types, Construction procedure.

What is Under reamed piles:

Under reamed piles are special types of piles manufactured by the Central Building Research Institute (C.B.R.I).  Such piles are made of R.C.C. This type of pile specially design for expansion soil (Black cotton soil).

  1. The bottom end of the pile has a spherical enlarged portion called an under rim or bulb. 
  2. The diameter of the pile is 20 cm to 50 cm and the diameter of the bulb is 2 to 3 times the diameter of the pile. 
  3. Under reamed Pile lengths range from 3 m to 8 m. 
  4. The spacing between the two piles is 2 m to 4 m. 
  5. The safe load on the pile is 20 to 40 tons. 
  6. The vertical spacing between two bulbs is 1.25 to 1.50 times the diameter of the bulb.  More than one bulb is placed if the load on the pile is high. 

IS Code for Under reamed piles:

IS 2911-3 (1980): Code of practice for design and construction of pile foundations, Part 3: Under-reamed
piles – Download Here

Under reamed piles are used in which type of soils?

Under read piles foundation is used for the following soils. 

1. Under reamed piles are generally used in black cotton soil:

This type of soil expands when it comes in contact with water and contraction occurs when water is removed.  So that cracks appear in the construction done on such clay.  An under reamed pile is used in the foundation to remove this defect. 

2. Under reamed piles are used in low bearing capacity Outdated soil (filled soil)

3.Under reamed piles are used in sandy soil when water table is high. 

4. Under reamed piles are used, Where lifting forces appear at the base of foundation.

Types of Under reamed pile:

  1. Single-bulb cast-in-situ pile,
  2. Multiple-bulb pile. 

A single bulb attached  at the bottom end of a pile is called a single bulb under reamed pile.  The pile with two bulbs is called a double bulb under reamed pile. And the pile with more than two bulbs is called a multiple bulb under reamed pile.

Also read: Caisson Foundation | Types | Advantages and Disadvantages

How to increase load bearing capacity of Under reamed pile?

The load carrying capacity of the under-read pile can be increased in the following ways. 

  • Increasing the number of bulbs
  • Increasing the diameter of the bulb
  • Increasing the length of pile
  • By proper designing and placing of reinforcements in under reamed piles.

Construction  procedure of under reamed piles :

Under reamed piles are mostly artificially constructed.  Tool for Such piles of construction developed by C.B.R.T. Roorkee.  This tool is fairly simple and light in weight.  This tool has the following below. 

  • Spiral Auger
  • Under – reamer
  • Boring guide
auger used in under reamed piles construction.

A spiral auger is used to drill a hole in the ground for an Under reamed piles. Cutter is attached at the end of Auger to easily dig ground. Below the spiral auger, the filling bucket is hung to remove soil. Auger handles are also used to increase depth. 

A special type of cutter is used to make the bulb, the diameter of the bulb can be increased by applying pressure on the handle of the auger.

Excavation is carried out from the auger only after passing the handle of the auger through a special design made on the head of a tricycle placed on the ground so that the digging of the pile hole is done in the vertical direction only. 

After digging the pile hole and the bulb to the required depth, the auger is taken out and the Case of reinforcement is inserted in the hole. Then Concreting is done. 

All the piles are connected to each other by forming a beam at the head of the pile.  The wall is constructed over the beams. 

Advantages and Disadvantages of under reamed pile:


  • Such piles are 15 to 20% cheaper than strip footing. 
  • Less material is required for such a pile. 
  • No heavy digging is required, so operations can be carried out even in the rainy season. 
  • There is no need for back filling in such piles. 
  • Shoring is not required. 
  • Dewatering is not required.


  • Required great workmanship.
  • Skills required for placing of such type of piles.
  • Maintaining verticality in ground is difficult. Because some time such pile are driven by hand-operated machine and it is very difficult.
  • Required strict and regular supervision with great quality control.

Under Reamed Piles Pdf

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1 thought on “What is Under reamed piles|| construction procedure || Advantages and Disadvantages.”

  1. IS CODE REVISION 2911 2010 2015 shall be also incorporated use of M25 concrete essential. Length of the pile shaft to its diameter is to be checked & kept below 12 so that it behaves as elastic short column


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