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Under Reamed Pile

by Swati

Under Reamed Pile:

  • Short bored under-reamed pile foundations are particularly suitable for black cotton soil. 
  • The principle of under-reamed pile foundations is the anchoring of the superstructure at a depth where the volume changes of soil are negligible.
  • One achieves the anchoring by enlarging the pile by under-reaming at this zone. Then One carries the superstructure on capping beams, which they kept clear of the ground.
  • The size of the pile depends on the safe load bearing capacity of pile and loading.
  • A pile diameter varies from 20 cm but for piles larger than 30 cm boring by manual labour becomes difficult.
  • The ratio of the diameter of under-ream to that of the stem may vary from 2 to 3, depending on the bearing capacity of the soil.
  • One bores the hole by auger and when it reaches required level, they enlarge the base to suitable diameter by a special tool.
  • They lowers the Reinforcement and fills the concrete in the hole.
  • Recently these short bored piles were provided with another enlarged rim at some height besides the under reaming at base.
  • At C.B.R.I Roorkee, they developed these double under-reamed pile. They suggested double reamed pile for two or three storeyed buildings on deep layers of black cotton soils.

Construction of Under Reamed Pile:

  1. Boring
  2. Under-reaming
  3. Concreting


  • This is usually done by manual earth auger, handled by a tripod hoist and winch. 
  • One ensures verticality of hole by a guide frame.
  • They may use Casing pipe in loose soil. 
  • They use extension rods for deep borings.
  • Drill bits or chopping bits can be used for hard soils.


  • After reaching desired depth, they expands bore hole 0to form a ream or bulb with under reaming tool consisting of two collapsible cross blades with a bucket at the bottom to collect scraped soil.
  • Scraped soil collected in bucket is lifted up and disposed off from time to time. 
  • One should check the spread diameter before concreting.


  • Soon after the boring operation, concreting is required to be done. 
  • Prefabricated reinforcement cage is to be first inserted in hole are concrete of required grade is poured or pumped in the hole.

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