This part of IS 2911 covers the design, construction, and installation of bored cast-in-situ under-reamed piles. It is particularly crucial for designing foundations in expansive soils like black cotton soil to prevent uplift and settlement, and is widely used for transmission towers and lightly loaded structures.
Deals with the design and construction aspects of under-reamed piled foundations.
Quick Reference Values
ratio of under ream to shaft diameter2 to 3 (usually 2.5)
minimum spacing between piles2 times under-ream diameter (2Du)
minimum grade of concreteM 20
minimum longitudinal reinforcement0.4 percent of the gross cross-sectional area of the pile shaft
minimum clear cover to longitudinal reinforcement40 mm
vertical spacing between two under reams1.25 to 1.5 times the under-ream diameter
Key Formulas
Qu = Ap * Nc * cp + α * c * As + ... (Static formula for ultimate bearing capacity of under-reamed pile in clay)
Safe Load = Ultimate Bearing Capacity / Factor of Safety (Usually 2.5)
Practical Notes
Under-reamed piles are highly effective in expansive soils to counteract heave due to moisture changes.
Ensure proper cleaning of the borehole base and under-reamed bulbs using a bucket auger before placing concrete.
If high groundwater table is encountered, concrete must be placed using a tremie pipe to prevent cement wash-out and segregation.