Architects Don't Design Foundations, But They Must Understand Soil Reports
The structural engineer sizes foundations; the geotechnical engineer determines soil properties; but the architect coordinates both and is responsible for ensuring that geotechnical investigations are commissioned as an owner-furnished service (per AIA B101 Article 5). On the ARE PPD, architects are expected to understand basic soil terminology, be able to interpret information from a geotechnical report, and understand how soil conditions drive fundamental design decisions. Mastering these concepts makes exam questions about foundation selection and site constraints much more approachable.
Allowable Soil Bearing Capacity
Allowable bearing capacity is the maximum load per unit area that a foundation can apply to the soil without causing shear failure or excessive settlement. It is expressed in pounds per square foot (psf) or kips per square foot (ksf) and is reported by the geotechnical engineer based on soil testing and analysis. The allowable capacity already includes a factor of safety (typically 2–3) against the ultimate bearing failure load.
Typical allowable bearing capacities: bedrock: 50+ ksf; dense gravels: 8–12 ksf; dense sands: 4–8 ksf; stiff clays: 2–4 ksf; medium clays: 1–2 ksf; soft clays: less than 0.5 ksf. The actual design value depends on site-specific testing, not general tables.
Common Soil Types and Their Characteristics
| Soil Type | Bearing Capacity | Drainage | Settlement Risk |
|---|---|---|---|
| Bedrock | Very high | Variable | Negligible |
| Dense gravel | High | Good | Low |
| Sand (dense) | Moderate-high | Good | Low |
| Sand (loose) | Low | Good | Liquefaction risk (seismic) |
| Clay (stiff) | Moderate | Poor | Moderate (consolidation) |
| Clay (soft) | Low | Poor | High (long-term consolidation) |
| Fill (engineered) | Varies | Varies | Verify with testing |
| Fill (uncontrolled) | Very low | Variable | Very high, unpredictable |
Settlement: Immediate vs. Consolidation
When load is applied to a soil, settlement occurs. Two types: immediate settlement occurs quickly as the soil grains rearrange under load. Consolidation settlement is a long-term process in clay soils where water is slowly squeezed from the soil pores over months or years. Clay soils can continue to settle for years after construction. Differential settlement - unequal settlement between different parts of the building - causes structural distress and cracking even when total settlement would be acceptable.
Expansive Soils
Expansive soils (typically clay minerals like montmorillonite) swell when wet and shrink when dry. Buildings on expansive soils can experience significant damage from heave (uplift) during wet seasons. Design strategies include: deep foundations that extend below the zone of moisture variation, moisture barriers to prevent soil wetting, and post-tensioned slab foundations designed to resist differential movement.
Liquefaction
Liquefaction occurs in loose, saturated sandy soils during earthquakes. The seismic shaking causes the soil to temporarily behave like a liquid, losing all bearing capacity. Buildings on liquefiable soils can sink or tilt dramatically. Mitigation strategies include ground improvement (densification, drainage) or deep foundations that reach through the liquefiable layer to stable soil or rock.
Key Exam Points
- Allowable bearing capacity: maximum load per unit area to avoid failure; already includes factor of safety.
- Soft clays: lowest bearing capacity; highest settlement risk.
- Dense gravels and bedrock: highest bearing capacity; low settlement.
- Consolidation settlement in clays: long-term, water-driven, years to complete.
- Liquefaction: seismic phenomenon in loose saturated sands.
Study PPD on AREprep
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