Foundation Types

Why This Matters

Foundations are where geotechnical engineering meets structural design—they're the critical interface between a building and the earth beneath it. You're being tested on your ability to match foundation types to specific soil conditions, load requirements, and site constraints. This means understanding load transfer mechanisms, bearing capacity principles, and settlement control strategies. Every foundation choice reflects a geotechnical problem being solved.

Don't just memorize names and depths. Know why a mat foundation works on weak soil while a spread footing doesn't. Understand how piles transfer load differently than drilled shafts. When you see an exam question about foundation selection, you should immediately think: What's the soil doing? What's the load doing? How does this foundation solve both problems? That conceptual framework will serve you far better than rote facts.

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Shallow Foundations: When Surface Soils Can Handle the Load

Shallow foundations work when competent bearing soil exists near the surface—typically within 3 meters (10 feet) of grade. The principle is simple: spread the structural load over enough area that soil bearing pressure stays within safe limits. These are economical choices when conditions allow.

Spread Footings

• Isolated pads beneath individual columns—the most basic foundation type, sized to keep bearing pressure below the soil's allowable capacity
• Load distribution through geometric spreading—the footing's width determines how much area shares the column load, following the $q = P/A$ relationship
• Settlement control through adequate sizing—undersized footings cause excessive or differential settlement, a common design failure mode

Strip Footings

• Continuous concrete strips supporting load-bearing walls—linear elements that distribute wall loads along their length
• Standard choice for residential and low-rise construction—economical for structures where walls carry loads rather than columns
• Width determined by wall load and soil bearing capacity—wider strips needed for heavier walls or weaker soils

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Mat and Raft Foundations: Spreading Load Across Problem Soils

When soil bearing capacity is low or variable, the solution is often to spread the load across the entire building footprint. Mat foundations turn the whole structure into one giant footing, dramatically reducing bearing pressure and controlling differential settlement.

Mat Foundations

• Single thick slab supporting all columns and walls—essentially one massive footing that treats the entire building as a unit
• Reduces bearing pressure by maximizing contact area—if $q = P/A$, then maximizing $A$ minimizes $q$
• Controls differential settlement—rigid mat forces the structure to settle uniformly rather than tilting or cracking

Raft Foundations

• Functionally identical to mat foundations—terms are often used interchangeably, though "raft" emphasizes the floating behavior on soft soil
• Ideal for large footprint structures on weak or compressible soils—warehouses, industrial buildings, and structures on fill
• Thickness designed for structural rigidity—must resist bending moments from column loads and soil reactions

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Pile Foundations: Transferring Load to Depth

When surface soils can't support the structure, piles bypass weak material entirely by transferring load to competent strata below. This happens through two mechanisms: end bearing (resting on rock or dense soil) and skin friction (resistance along the pile shaft).

Pile Foundations

• Long, slender elements driven or cast into the ground—typically 10-60+ meters deep depending on soil profile
• Load transfer via end bearing, skin friction, or both—design depends on whether a hard layer exists and how much shaft resistance develops
• Materials include concrete, steel, and timber—selection based on load magnitude, corrosion environment, and installation method

Drilled Shafts

• Cast-in-place concrete piles constructed by drilling—hole is excavated, reinforcing cage inserted, then filled with concrete
• Larger diameters than driven piles—typically 0.5-3 meters, allowing enormous load capacity per element
• Preferred for heavy loads and variable soil conditions—drilling allows inspection of bearing stratum before concrete placement

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Caissons and Piers: Specialized Deep Foundation Solutions

Some sites require deep foundations that go beyond standard piles—particularly where construction occurs through water or where massive loads demand oversized elements. Caissons and piers fill these specialized roles.

Caissons

• Large hollow structures sunk into place—constructed at surface, then excavated beneath to sink to bearing depth
• Essential for bridge piers and marine construction—allows work through water by creating a dry workspace at depth
• Can be open, pneumatic, or box type—selection depends on groundwater conditions and depth requirements

Pier Foundations

• Vertical columns extending to deeper bearing strata—similar concept to drilled shafts but often constructed differently
• Used where surface soils are inadequate—transfers load past weak layers to competent material below
• Often combined with grade beams—piers support beams that then support walls, creating a hybrid system

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Quick Reference Table

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Self-Check Questions

1. A structure with heavy column loads sits on a site with 5 meters of soft clay over dense sand. Which foundation types could work, and what load transfer mechanism would each use?

2. Compare mat foundations and spread footings: under what soil conditions would you choose one over the other?

3. Both driven piles and drilled shafts are deep foundations—what are two key differences in their construction that might influence selection on a given project?

4. A residential building with load-bearing walls needs a foundation on moderately strong soil. Which shallow foundation type is most appropriate, and why?

5. If an FRQ asks you to explain how a friction pile differs from an end-bearing pile, what soil profile characteristics would make each the preferred choice?