Foundation Types
Foundation selection depends on soil conditions, structural loads, and site constraints. Here's a comprehensive guide to common foundation types:
- Strip Foundation Continuous foundation beneath walls, typically 1.5-3x wall width. Depth usually 2-6 feet depending on soil and frost line. Cost-effective for residential construction.
- Raft Foundation Single reinforced concrete slab supporting entire building footprint. Thickness 8-24 inches based on loads. Ideal for weak soils (bearing capacity <1500 psf).
- Pad Foundation Square or rectangular isolated footings, typically 2-6x column width. Depth usually 1-1.5x width. Economical for frame structures with well-spaced columns.
- Pile Foundation Deep foundations extending 20-200 feet. Used when bearing soil is deep or loads are high. Types include driven, bored, and CFA piles.
Soil Considerations
Soil Testing
Conduct SPT tests for bearing capacity. Minimum N-value >30 for shallow foundations. Test for organic content (<5% acceptable) and plasticity index.
Water Table
Keep foundation minimum 2ft above water table. Install drainage if water table is high. Consider dewatering needs during construction.
Settlement
Maximum allowable differential settlement 1:500. Monitor consolidation in clay soils. Consider preloading for problematic soils.
Soil Improvement
Options include soil replacement, compaction (95% Proctor), chemical stabilization, or ground anchors for special cases.
Design Considerations
- Load Distribution Design for uniform pressure distribution. Maximum eccentricity <1/6 of foundation width. Consider dynamic loads for machinery foundations.
- Frost Protection Minimum depth below frost line (varies by location). Typical range 2-5 feet in cold climates. Use frost-protected shallow foundations where applicable.
- Drainage Install perimeter drains with filter fabric. Slope grade away from foundation (min. 2%). Waterproof below-grade walls.
- Reinforcement Minimum steel ratio 0.25% of gross area. Use epoxy-coated bars near grade. Provide adequate cover (3" for soil contact).
Construction Methods
Excavation
Proper depth and width. Consider access.
Formwork
Strong and accurate. Check dimensions.
Concrete Pour
Proper mix and curing. Weather considerations.
Waterproofing
Essential for below-grade foundations.
Quality Control
- Soil Compaction Test before and after excavation.
- Concrete Testing Slump and strength tests required.
- Steel Placement Verify cover and spacing.
- Inspections Required at key construction stages.