Introduction:
Under reamed piles are a type of foundation piles that are bored into the ground to provide support for structures. They have a unique design feature called under reaming, which creates a bulb-shaped enlargement at the base of the pile. This enlargement increases the bearing capacity of the pile and enhances its load-carrying capacity.

Explanation:
Under reamed piles are normally bored as cast-in-situ piles. This means that they are constructed on-site by drilling a hole into the ground and then filling it with concrete. There are several reasons why cast-in-situ piles are preferred for under reamed piles:

1. Flexibility of Design: Cast-in-situ piles allow for flexibility in design. The size, shape, and depth of the under reamed bulb can be customized based on the specific soil conditions and load requirements of the structure. This flexibility is not easily achievable with pre-cast or steel piles.

2. Quality Control: Cast-in-situ piles offer better quality control. The entire installation process can be closely monitored to ensure that the pile is constructed in accordance with the design specifications. Any issues or deviations can be addressed and rectified immediately, resulting in a more reliable foundation system.

3. Seamless Connection: Cast-in-situ piles provide a seamless connection between the under reamed bulb and the shaft of the pile. This ensures a smooth transfer of loads from the structure to the underlying soil. In contrast, pre-cast or steel piles may have joints or connections that can be potential weak points and compromise the overall stability of the foundation.

4. Cost-effectiveness: Cast-in-situ piles are generally more cost-effective compared to pre-cast or steel piles. The materials required for cast-in-situ piles, such as cement, aggregates, and reinforcement, are readily available and relatively inexpensive. The construction process is also simpler and requires less specialized equipment.

5. Adaptability: Cast-in-situ piles can be easily adapted to suit site-specific conditions. The borehole can be adjusted as necessary to accommodate any encountered obstructions or changes in soil properties. This adaptability is crucial in situations where unforeseen challenges may arise during the construction process.

In conclusion, under reamed piles are normally bored as cast-in-situ piles due to their flexibility of design, quality control, seamless connection, cost-effectiveness, and adaptability. These advantages make cast-in-situ piles the preferred choice for constructing under reamed piles in various soil conditions and structural requirements.