Foundation Selection Depends on Soil Conditions and Structural Loads
Every building transfers its loads to the earth through a foundation. The choice of foundation type is driven by two factors: the loads the building imposes and the capacity of the soil to receive those loads. When soil near the surface can adequately support the building loads, a shallow foundation (spread footing or mat) is appropriate. When surface soils are inadequate, deep foundations (piles or drilled piers) must transfer loads to stronger soil or rock at depth. The ARE PPD tests foundation type selection in the context of soil conditions, building loads, and cost considerations.
Shallow Foundations
Spread Footings (Isolated Column Footings)
Spread footings are the simplest and most common shallow foundation type. A spread footing is a pad of reinforced concrete beneath a single column that distributes the column load over a large enough area to keep soil bearing pressure within the allowable capacity. The footing size is determined by dividing the column load by the allowable soil bearing pressure. If soil bearing capacity is 3,000 psf and a column carries 300 kips, the required footing area is 300,000 lb / 3,000 psf = 100 sf, so a 10×10 footing would work. Spread footings are appropriate when soil has adequate bearing capacity and minimal differential settlement risk.
Continuous (Strip) Footings
Continuous footings run continuously under walls or closely spaced columns. They distribute load from bearing walls over a long strip of soil and are standard under all load-bearing walls in residential construction. Width is determined by wall load and soil bearing capacity.
Mat (Raft) Foundation
A mat foundation is a single large concrete slab that covers the entire building footprint, distributing the building's total load over the entire site area. Mat foundations are used when: soil bearing capacity is very low (so spread footings would need to be so large they would overlap), there is significant variation in column loads that would cause differential settlement with individual footings, or basement construction is needed (the mat serves as the basement slab and foundation). Mat foundations are expensive due to the large volume of concrete and reinforcement required but are the right solution for difficult soil conditions.
Deep Foundations
Pile Foundations
Piles are long, slender structural elements driven or cast into the ground to transfer building loads to soil or rock at depth. Two types of pile action:
- End-bearing piles: Transfer load primarily through their tip, which rests on a strong bearing stratum (rock or dense soil) at depth. The pile is essentially a column in the soil.
- Friction (skin friction) piles: Transfer load through frictional resistance along the entire length of the pile, when no firm bearing stratum is within reach. Load is distributed along the pile surface.
Common pile types: driven steel H-piles, concrete-filled steel pipe piles (cased), precast concrete piles, and cast-in-place concrete piles. Pile caps connect groups of piles to the columns above.
Drilled Piers (Caissons)
Drilled piers are large-diameter bored holes (typically 18 inches to 6 feet in diameter) filled with concrete and reinforcement. They can be belled (enlarged at the bottom) for greater bearing area. Drilled piers are advantageous in urban environments where pile driving vibrations and noise are problematic, and in rock bearing conditions where drilling is feasible.
Foundation Selection Summary
| Foundation Type | Best When... |
|---|---|
| Spread footing | Adequate soil capacity, uniform loads, no basement |
| Mat | Poor or variable soils, need to minimize differential settlement |
| End-bearing pile | Firm bearing stratum reachable at depth |
| Friction pile | No firm stratum, load transferred by skin friction |
| Drilled pier | Urban site, rock bearing, large loads |
Key Exam Points
- Shallow foundations when soil has adequate bearing capacity.
- Deep foundations when surface soils are inadequate.
- Mat foundation for very low bearing capacity or high differential settlement risk.
- End-bearing pile: load to tip on firm stratum. Friction pile: load along pile surface.
- Drilled pier (caisson): bored hole; belled bottom increases bearing area.
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