Posted by Deep foundations transfer heavy structural loads to stable strata beneath weak surface soils. Under reamed piles are a specialized form of bored cast-in-situ piles featuring one or more enlarged bulbs near the base. These bulbs boost bearing capacity and anchor the foundation against uplift in expansive soils.
What Is an Under Reamed Pile Foundation?
Under reamed piles consist of a vertical concrete shaft with one or more under-reamed bulbs formed at the bottom by mechanically enlarging the borehole. The bulb’s increased cross-section provides enhanced end-bearing capacity and resistance to uplift caused by soil swelling.
Why Use Under Reamed Piles?
- Increased end-bearing capacity due to the enlarged bulb area.
- Resistance to uplift pressures from volumetric soil changes in expansive soils.
- Cost-effective solution for soft and black cotton soils where shallow foundations may fail.
- Reduced settlement compared to straight-shaft piles.
Design Considerations of Under Reamed Piles
- Pile diameter: 20 cm to 50 cm.
- Bulb diameter: 2 × to 3 × pile diameter.
- Effective pile length: 3 m to 8 m.
- Spacing between piles: 2 m to 4 m center-to-center.
- Spacing between bulbs (in multi-reamed piles): 1.25 × to 1.5 × bulb diameter.
- Load-carrying capacity: 20 ton to 40 ton per pile.
- Applicable code: IS 2911 (Part III) – 1980 for pile foundation design and construction in India.
Construction Process
- Drill a vertical borehole to the required depth using an auger or rotary drilling rig.
- Engage the under-reaming tool at the bottom of the hole to cut the bulb to the specified diameter.
- Remove loose spoil and flush the hole to ensure a clean cavity.
- Lower the reinforcement cage into the borehole if structural reinforcement is needed.
- Pour concrete, maintaining a slump of 10 cm–15 cm for unlined holes and 15 cm–20 cm for lined holes to prevent segregation and ensure complete bulb filling.
- Allow the concrete to cure before proceeding with cap beams or ground beams.
Types of Under Reamed Piles
- Single-reamed piles: one bulb at the pile base.
- Double-reamed piles: two bulbs along the pile shaft.
- Multiple-reamed piles: more than two bulbs for enhanced uplift resistance or heavier load support.
Applications of Under Reamed Pile Foundations
Under reamed pile foundations find their niche in:
- Residential and light commercial structures on black cotton or expansive soils.
- Foundations in regions with significant groundwater fluctuation.
- Anchoring structures like retaining walls, water tanks, and mast foundations against uplift forces.
Advantages and Disadvantages of Under Reamed Pile Foundation
Advantages
- Provides high end-bearing capacity in weak soils.
- Anchors against uplift from moisture-induced soil expansion.
- Economical in challenging soil conditions compared to alternative deep foundations.
- Yields minimal settlement under service loads.
Disadvantages
- Requires specialized drilling rigs and skilled operators.
- Limited to soil strata that can be under-reamed without collapse.
- Risk of borehole instability if proper flushing and support are not maintained.
- Not suitable for hard rock or very dense granular soils where under-reaming tools cannot penetrate effectively.
Conclusion
Under reamed pile foundations offer a targeted, economical approach for building on expansive and soft soils. By strategically sizing and spacing bulbs, engineers can tailor bearing capacity and uplift resistance to project requirements. Awareness of design codes, accurate construction practices, and site-specific soil conditions ensures the longevity and safety of structures supported on under reamed piles.
Read Also-
IS 2950 [Code of Practice for Design and Construction of Raft Foundation]
Types of Pile Foundation (Explained in Detail)
Compaction of Soil vs Consolidation of Soil
Types of Foundation for Building Construction and Their Applications