Shallow Foundations Flashcards
2 modes of failure for shallow foundations
Settlement and Bearing Capacity
Types of Shallow Foundations
-Isolated Spread Footing: L/B < 10
-Continuous/Strip: most commonly used for buildings. L/B > 10
-Spread Footings w Cantilevered Wall: Resist lateral loads, useful for bridge abutments
-Retaining Structures (T-walls)
-Combined Footings
-Mat: can be founded on weak soil
Differences b/w General, Local and Punching Shear
General:
-associated with bearing failure
-increase in settlement with increasing load until certain load it rebounds
-C’ and phi’ are reduced by 1/3 for all footings except spread
Local
-Failure surface DOES NOT extend to the ground surface like general does
-No rebound with increasing load
-Associated with bearing failure
-C’ and phi’ are reduced by 1/3 for all footings except spread
Punching
-Failure only beneath footing where local and general extend beyond foundation footprint
-Foundation settles vertically with no upheaval of surrounding foundation
Water,Depth, Embedment Effects on Bearing Capacity in different soils
Cohesionless soils:
-As water table rises, BC decreases
-As width of foundation inc, BC increases
-As depth of embedment inc, so does BC
Cohesive
-No effect on BC as water table rises
-No effect on foundation width
-As depth of embedment inc, so does BC
Water,Depth, Embedment Effects on Bearing Capacity in different soils
Cohesionless soils:
-As water table rises, BC decreases
-As width of foundation inc, BC increases
-As depth of embedment inc, so does BC
Cohesive
-No effect on BC as water table rises
-No effect on foundation width
-As depth of embedment inc, so does BC
Eccentricity Equation
e = Moment/Load Pressure
Strain
change in void ratio / (1+ initial void ratio)
Active vs Passive Pressure
Active - moving away from soil/backfill. Requires less rotation/movement to mobilize
Passive - moving towards backfill. Requires more movement to mobilize
