Overview
Lecture covers foundations, their purpose, failure causes, and types of shear failure in soils affecting shallow foundations.
Foundations: Definition and Purpose
- Foundation is the connecting link between superstructure and ground.
- Superstructure cannot be built directly on ground; foundation constructed first.
- Foundation transmits structural loads safely to the supporting soil.
- Key purpose: distribute load over larger area to keep soil pressure below allowable.
- Provides vertical support and maintains acceptable settlement of the structure.
Foundation Failure: Causes
- Shear failure of soil beneath the foundation; soil particles move from original positions.
- Excessive settlement beyond permissible limits for the structure.
Types of Shear Failure
- Three types: general shear failure, local shear failure, punching shear failure.
- Occurrence depends on soil density/consistency and load–settlement behavior.
General Shear Failure
- Occurs in dense sand or stiff clay under the foundation.
- Load–settlement curve shows sudden increase in settlement at ultimate load QU.
- At QU, a fully developed failure plane forms and reaches ground surface.
- Heaves appear on the sides of the foundation due to soil movement upward.
- Failure surface extends to ground, indicating complete shear failure.
Local Shear Failure
- Occurs in medium dense sand or clays of medium consistency.
- Load–settlement curve shows sudden jerks at a load QU1.
- Failure surface extends outward toward ground but does not reach surface at QU1.
- No heaves observed initially; failure is limited and not visible externally.
- With further load increase, settlement rate increases markedly after QU1.
- At higher load QU, failure surface reaches ground; heaves become observable.
Punching Shear Failure
- Occurs in loose sand or soft clay beneath the footing.
- Load–settlement curve shows sudden jerks at a load QU1.
- With further loading to QU, footing experiences sudden, steep settlement.
- Settlement curve becomes steeper; small load increases cause large settlement.
- No heaves appear because loose soil allows particle readjustment internally.
Load–Settlement Behavior Summary
- Settlement increases with load for all failure types; character differs by soil.
- General shear: distinct sudden settlement at QU with heaves at ground.
- Local shear: jerks at QU1; later at QU, surface expression and heaves occur.
- Punching shear: jerks at QU1; steep settlement at QU without surface heaves.
Key Terms & Definitions
- Foundation: Structural element transferring loads from superstructure to ground.
- Allowable bearing pressure: Maximum soil pressure without unsafe settlement or failure.
- QU (ultimate load): Load intensity where rapid settlement indicates shear failure.
- Heaves: Upward bulging of ground adjacent to foundation due to soil displacement.
- Failure surface: Shear plane along which soil mass moves during failure.
Comparative Table: Shear Failure Types
| Failure Type | Soil Conditions | Load–Settlement Behavior | Failure Surface | Surface Heaves | Notable Points |
|---|
| General shear | Dense sand; stiff clay | Sudden large settlement at QU | Fully developed; reaches ground | Present | Clear, well-defined failure plane |
| Local shear | Medium dense sand; medium consistency clay | Jerks at QU1; higher settlement rate after QU1; further failure at QU | Extends outward; reaches ground only at higher load QU | Absent at QU1; present at QU | Initially limited, not visible externally |
| Punching shear | Loose sand; soft clay | Jerks at QU1; steep settlement at QU | Predominantly vertical under footing | Absent | Soil readjusts internally; no surface expression |
Action Items / Next Steps
- Recognize soil type to anticipate likely shear failure mode in design.
- Ensure foundation area provides pressure below allowable bearing pressure.
- Monitor load–settlement response to identify QU and adjust loading accordingly.