Introduction
As we embarked on constructing the dual carriage bridge at Aviation Village, off Bill Clinton Drive, Airport Road, Abuja, establishing a robust foundation was our primary focus. A strong foundation ensures the bridge can withstand traffic, wind, and other forces over decades. However, during the construction of foundation for abutment C1, we faced an unexpected challenge that required us to adapt our approach. This article shares that experience, detailing how we anchored the pile cap directly to bedrock and why it will work.
The Original Plan
Our initial design relied on 450mm diameter bored piles to support the bridge’s abutments and piers. Each pile was to be drilled at least 15 meters deep into stable soil strata until it reaches a sound rock layer, providing the necessary strength for the structure. These piles, made of reinforced concrete with a minimum strength of 40 N/mm² and reinforced with high-yield steel bars (410 N/mm²), were engineered to handle significant loads, including HA + 45 units of HB vehicle load and wind speeds up to 35 m/s [1]. This approach followed standard bridge design practices outlined in BD 37/01 and BS5400, ensuring safety and durability.
The Challenge
While excavating for abutment C1, we hit a roadblock—literally. We encountered sound rock, or bedrock, at a shallow depth. This hard, unyielding layer was too tough for our piling equipment to penetrate. This isn’t uncommon in construction, especially in regions with shallow bedrock, where traditional piling becomes impractical [2]. We needed a solution that would maintain structural integrity without derailing the project timeline or budget.
The Solution
After assessing the situation, we opted to anchor the pile cap directly to the bedrock, bypassing the need for deep piles at this location. Here’s how we did it:
- Drilling: Using a Diesel Air Compressor and Jack Hammer, we drilled holes approximately 5 feet deep into the bedrock.
- Reinforcement: We inserted Y25 reinforcement bars into these holes and grouted them with cement to secure them in place.
- Spacing: The bars were spaced about 2 feet apart across the pile cap area, ensuring even load distribution.
This method effectively turned the bedrock into a natural anchor for the pile cap, which would then support the abutment above.
Why This Approach Works
Anchoring to bedrock is a proven technique in construction when piling isn’t feasible. According to industry insights, this method leverages the inherent strength of the rock, reducing the need for extensive concrete and steel [3]. By grouting reinforcement bars into the bedrock, we created a secure bond that transfers the bridge’s loads directly to this stable layer. It’s a practical, cost-effective solution that has been used successfully in similar projects, such as foundations for wind turbines and other structures built on rocky terrain [3]. Discussions among engineers also highlight its reliability for pile caps in bedrock conditions [4].
Ensuring Stability and Compliance
To confirm this adaptation met engineering standards, we consulted our partners at IAAI Engineering Consultants Nig. Ltd. They reviewed the method against BD 37/01 and BS5400, ensuring it could support the bridge’s design loads—such as the 1,230 kN capacity originally planned for each pile [1]. The approval validated that anchoring to bedrock would provide equivalent stability, keeping the project on track and compliant with regulations.
Conclusion
This experience at abutment C1 underscores the importance of flexibility in construction. Encountering sound rock forced us to pivot from our original piling plan, but by anchoring the pile cap directly to the bedrock, we have secured a solid foundation for the bridge. It’s a practical fix that balances engineering rigour with efficiency, and I am excited to share more updates as we progress.
References
- “BRIDGE DESIGN BILL CLINTON DRIVE ABUTMENT DECK AND REINFORCEMENT DETAILS.pdf” – Project documentation detailing foundation specs and design standards.
- Fondation on Bedrock – GreenBuildingAdvisor – Insights on building foundations on shallow bedrock.
- Rock Anchor Foundation – CTE WIND – Overview of rock anchoring as a foundation technique.
- Pile Foundations in Bedrock – Eng-Tips Forums – Engineering discussion on adapting pile foundations in bedrock.
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