A combined foundation typically involves designing a single foundation system to support multiple columns or loads. This could include a combination of isolated footings, strip footings, or raft foundations. Here’s a detailed checklist with explanations and examples for the design of a combined foundation:
1. Site Investigation:
- Explanation: Conduct a comprehensive site investigation to understand soil conditions.
- Example: Perform soil tests (SPT, CPT) to determine soil properties, bearing capacity, and potential settlement.
2. Load Analysis:
- Explanation: Analyze and calculate all loads acting on the foundation.
- Example: Consider dead loads (structural weight), live loads, wind loads, and seismic loads to determine the total load on the foundation.
3. Soil Bearing Capacity:
- Explanation: Ensure that the soil bearing capacity exceeds the applied loads.
- Example: If the total load is 800 kN and the soil bearing capacity is 1200 kN/m², the foundation is considered suitable.
4. Settlement Analysis:
- Explanation: Analyze and limit potential settlement within acceptable limits.
- Example: If the allowable settlement is 30 mm, design the foundation to minimize settlement.
5. Footing Dimensions:
- Explanation: Determine the appropriate dimensions of individual footings (isolated, strip) or raft.
- Example: Design a combined foundation with individual footings, each with dimensions suitable for load distribution.
6. Reinforcement:
- Explanation: Specify the type and amount of reinforcement in individual footings or raft, as required.
- Example: Include reinforcing bars (rebar) in each footing or raft to enhance tensile strength.
7. Depth of Foundation:
- Explanation: Determine the optimal depth of individual footings or raft for stability.
- Example: If a particular column is over weak soil, increase the depth of its footing to reach more stable soil layers.
8. Column Layout:
- Explanation: Plan the layout of columns and distribute loads uniformly.
- Example: Arrange columns in a grid pattern, ensuring that individual footings or the raft distribute loads effectively.
9. Edge and Corner Details:
- Explanation: Address edge and corner details to prevent differential settlement.
- Example: Provide thickened slab sections or use a drop panel at column locations in the raft foundation to enhance load distribution.
10. Waterproofing and Drainage:
- Explanation: Implement waterproofing measures and ensure proper drainage.
- Example: Apply a waterproofing membrane on the foundation surface, and include drainage systems for each individual footing or the entire raft.
11. Construction Sequence:
- Explanation: Develop a construction sequence that ensures the stability of the combined foundation.
- Example: Excavate each footing area, compact the soil, pour and cure concrete in sections, and ensure proper curing to prevent cracking.
12. Quality Control:
- Explanation: Implement quality control measures during construction.
- Example: Conduct inspections at key construction stages, such as rebar placement and concrete pouring, to ensure compliance with design specifications.
13. Code Compliance:
- Explanation: Ensure compliance with local building codes and standards.
- Example: Adhere to relevant codes such as ACI 318 for concrete design or Eurocode 7 for geotechnical design, depending on the project location.
14. Professional Engineer Involvement:
- Explanation: Engage a qualified structural engineer to oversee the design.
- Example: Have a licensed structural engineer review and approve the combined foundation design to ensure it meets safety and code requirements.
15. Record Keeping:
- Explanation: Maintain detailed records of the combined 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 combined foundation that efficiently supports multiple columns, minimizes settlement, and provides stability for the entire structure.