Short Answer:

An under-reamed pile foundation is a special type of deep foundation used in expansive or weak soils, where piles have bulb-like enlargements (called under-reams) at the bottom to increase load-bearing capacity and resist uplift forces. These bulbs are created by boring tools during construction.

Under-reamed piles are mostly used in black cotton soil areas or where the ground swells and shrinks due to moisture changes. They provide better stability, reduce settlement, and prevent damage caused by soil movement, making them ideal for buildings in unstable ground conditions.

Detailed Explanation:

Under-reamed pile foundation

An under-reamed pile foundation is a type of bored cast-in-situ concrete pile that includes one or more bulb-shaped enlargements (reams) at its lower part. These enlargements increase the bearing surface area and provide additional anchorage to resist upward forces caused by swelling soil, especially in areas with expansive clay like black cotton soil.

Unlike straight piles that transfer loads only through skin friction and end bearing, under-reamed piles offer better grip and performance in problematic soils. The concept was introduced by the Central Building Research Institute (CBRI), Roorkee, and is widely used across India.

Where and why under-reamed piles are used

1. Expansive Soil Areas

Soils like black cotton soil expand when wet and shrink when dry. This swelling-shrinking behavior can push shallow foundations upward and cause cracks in walls and floors. Under-reamed piles anchor the structure below the active zone and prevent uplift.

2. Weak Surface Soils

When the topsoil has low bearing capacity, under-reamed piles are used to reach a deeper, firmer layer and provide greater bearing strength.

3. Earthquake Zones

These piles offer better lateral stability, especially in seismic areas, where ground movement can displace traditional footings.

4. Light to Medium Load Structures

Under-reamed piles are suitable for residential buildings, schools, boundary walls, and machine foundations, especially in poor soil conditions.

Features of Under-Reamed Piles

• Bulb Size: The bulb is usually 2 to 3 times the diameter of the pile shaft.
• Number of Bulbs: Depending on soil condition and load, one or more bulbs can be provided.
• Spacing: Piles are spaced 2 to 3 meters apart and connected with a pile cap.
• Diameter and Depth: Common diameters range from 150 mm to 300 mm, and depths can go beyond 3 meters, depending on soil investigation.

Advantages

• Resists uplift and swelling pressure in clayey soils.
• Reduces differential settlement in the building.
• Requires less concrete compared to straight bored piles for the same capacity.
• Economical and easy to construct using manual or mechanical augers.
• Provides long-term stability to lightweight structures.

Construction Process

1. Drilling the pile bore up to the required depth.
2. Forming the under-reamed bulb using a reaming tool.
3. Inserting reinforcement cage into the pile hole.
4. Pouring concrete to form the pile.
5. Connecting piles with a pile cap to distribute load.

Proper site investigation and supervision are necessary to ensure correct bulb formation and alignment.

Conclusion:

An under-reamed pile foundation is a special deep foundation designed for expansive or weak soils. Its bulb-shaped enlargements provide extra anchorage and reduce movement caused by soil swelling or shrinkage. These piles are ideal for light structures in unstable soil and offer a cost-effective and durable solution for ensuring building stability.