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:
- 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.