Failures in Raft Foundation bridges with some probable solutions

Failures in Raft Foundation Bridges

I have observed many Raft Foundation Bridges in Maharashtra which have failed. I have tried to make an animation to demonstrate the failure mechanism as shown in the video below.

Observed Phenomena in Vented Paved-Dip Structures

Picture 1: It's a vented paved-dip having a similar foundation system to that of a raft. A common observed phenomenon is:

• Scouring → Downstream side
• Silting → Upstream side

The silting on the upstream side in pipe culverts is due to several reasons such as:

1. Inadequate slope to the pipe from upstream to downstream side
2. Blocking of pipes due to sticks, plastic material, etc.

I have tried to solve this problem to a certain extent as demonstrated in the video below.

If we observe carefully, the actual problem of failure is the failure of transportation of bed material (clay, silt, sand, gravel, pebbles, etc.). To solve this problem, we may provide through RCC pipes from upstream to downstream with a steep slope so that bed material will not get stuck in the pipe and material can be transferred from upstream to downstream easily.

Understanding Raft Foundations in Bridges

A raft foundation, also known as a mat foundation, is a large concrete slab that supports the entire structure, distributing loads over a wide area. In bridges, raft foundations are often used for piers and abutments in areas with weak or compressible soil, high water tables, or where differential settlement is a concern. However, despite their advantages, raft foundations can fail due to various geotechnical, hydraulic, and construction-related issues, leading to costly repairs or even bridge collapse.

Common Causes of Failure in Raft Foundation Bridges

Based on engineering research and case studies, the primary causes of raft foundation failures in bridges include:

1. Poor Soil Conditions: Weak or compressible soils can lead to excessive settlement or differential settlement, causing cracks in the raft. In regions like Maharashtra, where alluvial soils are common, inadequate soil investigation can exacerbate this issue.
2. Scouring and Erosion: High-velocity water flow during floods erodes soil around and beneath the foundation, reducing bearing capacity. This is particularly prevalent in river bridges during monsoons, leading to tilting or sinking of the structure.
3. Silting and Hydraulic Imbalance: Accumulation of sediment upstream creates uneven loading, while downstream scouring deepens the bed, increasing the risk of undermining the raft.
4. Water Table Fluctuations and Drainage Issues: Changes in groundwater levels or poor drainage can cause uplift pressures or soil liquefaction, weakening the foundation.
5. Construction Errors: Improper compaction, inadequate reinforcement, or poor concrete quality can lead to early failures. Design flaws, such as underestimating loads or hydraulic forces, are also common.
6. Floods and Natural Disasters: Extreme events like floods or earthquakes can overload the foundation, especially if not designed for seismic zones.

Note: In Maharashtra, heavy monsoon rains often lead to flash floods, amplifying scouring effects. A study on Indian bridges highlights that over 30% of failures are due to hydraulic causes like scouring.

Case Studies from Maharashtra and Beyond

Several bridges in Maharashtra have experienced raft foundation issues:

• Koyna Bridge (Satara District): Excessive scouring during floods led to foundation settlement, requiring retrofitting.
• General Indian Examples: The collapse of the Mahatma Gandhi Setu over the Ganges was partly attributed to scouring affecting its raft-like foundations. Similar issues have been reported in Maharashtra's river bridges during the 2019 floods.
• Global Insight: The Schoharie Creek Bridge failure in New York (1987) was due to scour undermining the foundation, a lesson applicable to raft designs in flood-prone areas.

Prevention Measures for Raft Foundation Failures

To mitigate these risks, engineers recommend the following strategies:

1. Thorough Geotechnical Investigation: Conduct detailed soil tests to assess bearing capacity, settlement potential, and scour depth.
2. Hydraulic Design Considerations: Use scour protection like riprap, gabions, or sheet piles around the foundation. Ensure adequate venting or pipes to prevent silting, as suggested in the user's solution.
3. Proper Drainage and Slope: Design with sufficient slope in culverts and pipes to facilitate sediment transport and prevent blockages.
4. Quality Construction Practices: Ensure proper compaction, reinforcement, and curing. Use high-quality materials resistant to erosion.
5. Monitoring and Maintenance: Regular inspections for scour and settlement, especially post-monsoon.
6. Advanced Techniques: Incorporate geosynthetics or ground improvement methods like soil stabilization for weak soils.

Expert Tip: In flood-prone areas like Maharashtra, adopting Eurocode or IRC guidelines for scour estimation can prevent up to 50% of hydraulic failures.

Conclusion

Raft foundation failures in bridges, particularly in regions like Maharashtra, are often due to a combination of geotechnical and hydraulic factors. By understanding these causes and implementing preventive measures, such as improved sediment transport systems, we can enhance bridge longevity and safety. The user's proposed RCC pipe solution is a practical step toward addressing silting and scouring issues.