Designing the foundation for a piled retaining wall involves considerations for stability, bearing capacity, and soil-structure interaction. Below is a detailed checklist with explanations and examples for the design of a piled retaining wall foundation:
1. Site Investigation:
- Explanation: Conduct a thorough site investigation to understand soil conditions.
- Example: Perform soil tests (SPT, CPT) to determine soil properties, bearing capacity, and potential sliding or settlement.
2. Load Analysis:
- Explanation: Analyze and calculate loads acting on the piled retaining wall.
- Example: Consider the weight of the wall, backfill soil pressure, surcharge loads, water pressure, and seismic loads to determine the total lateral load.
3. Bearing Capacity:
- Explanation: Ensure that the soil bearing capacity is sufficient for the applied loads.
- Example: If the total lateral load is 200 kN/m and the soil bearing capacity is 300 kN/m², the foundation is considered suitable.
4. Sliding Stability:
- Explanation: Check and ensure the foundation resists sliding.
- Example: Calculate the sliding force and compare it to the resisting force provided by the foundation. Use appropriate safety factors.
5. Overturning Stability:
- Explanation: Verify the foundation’s resistance to overturning.
- Example: Assess the moments and forces acting on the piled wall and ensure they are counteracted by the resisting moments and forces provided by the foundation.
6. Foundation Geometry:
- Explanation: Determine the appropriate dimensions and shape of the foundation.
- Example: Design a piled foundation with the required number and spacing of piles to distribute the loads evenly along the length of the retaining wall.
7. Pile Selection:
- Explanation: Select appropriate pile types based on soil conditions and design requirements.
- Example: Choose driven piles or bored piles depending on the soil profile and load-carrying capacity needed.
8. Pile Spacing and Arrangement:
- Explanation: Determine the optimal spacing and arrangement of piles for stability.
- Example: Space the piles evenly along the wall length and provide additional reinforcement where required.
9. Pile Length:
- Explanation: Calculate the required length of the piles to reach load-bearing strata.
- Example: If the competent soil layer is 10 meters below the ground surface, design piles with lengths exceeding 10 meters to ensure adequate embedment.
10. Pile Cap Design:
- Explanation: Design the pile cap to distribute loads to individual piles.
- Example: Include a reinforced concrete pile cap with appropriate dimensions and reinforcement to transfer loads from the retaining wall to the piles.
11. Reinforcement:
- Explanation: Specify reinforcement in the foundation and pile cap, if required.
- Example: Include reinforcing bars in the pile cap to enhance its tensile strength and improve overall stability.
12. Pile Installation:
- Explanation: Specify procedures for pile installation.
- Example: Require proper driving or boring techniques and procedures to achieve the desired pile lengths and capacities.
13. Testing and Monitoring:
- Explanation: Include provisions for pile load testing and monitoring during construction.
- Example: Conduct a static load test on selected piles to verify their capacity and assess the performance of the foundation.
14. Drainage:
- Explanation: Implement drainage measures to reduce hydrostatic pressure.
- Example: Install drainage pipes around the pile caps to prevent water accumulation and reduce the risk of soil liquefaction.
15. Backfill Compaction:
- Explanation: Specify compaction requirements for the backfill soil.
- Example: Compaction should be done in lifts with specified moisture content to achieve adequate density.
16. Construction Sequence:
- Explanation: Develop a construction sequence that ensures stability during and after construction.
- Example: Install piles, construct the pile cap, and then build the retaining wall while monitoring for settlement or other issues.
17. Quality Control:
- Explanation: Implement quality control measures during construction.
- Example: Monitor pile installation for proper alignment and penetration, check reinforcement placement, and verify the dimensions of the pile cap.
18. Code Compliance:
- Explanation: Ensure compliance with local building codes and standards.
- Example: Adhere to relevant codes such as AASHTO LRFD Bridge Design Specifications or local geotechnical standards.
19. Professional Engineer Involvement:
- Explanation: Engage a qualified geotechnical and structural engineer to oversee the design.
- Example: Have licensed engineers review and approve the piled retaining wall foundation design to ensure it meets safety and code requirements.
20. Record Keeping:
- Explanation: Maintain detailed records of the piled retaining wall foundation design process.
- Example: Document all design calculations, soil test results, construction activities, and as-built drawings for future reference and audits.
By following this checklist, engineers can ensure a well-designed piled retaining wall foundation that effectively resists lateral loads, provides stability, and minimizes potential failure modes.