Test 3

Question 1

Shallow foundations vs Deep Foundations

Answer 1

Shallow foundations: located just below structure
 Typically referred to as “Footings”

Deep foundations: extend down into earth
 Typically referred to as “Piles”

Question 2

Three general criteria to be satisfied by Footings

Answer 2

 Must be located properly (horizontally & vertically)
 Not to adversely affect other elements
 Not to be adversely affected by other elements
 Must be safe from bearing capacity failure
 Must be safe from excessive settlement

Question 3

Basic design steps footing

Answer 3

1. Calculate loads acting on footing
2. Obtain soil information from field & lab tests & results
3. Determine depth & location of footing
4. Evaluate bearing capacity of soil
5. Determine size of footing
6. Compute footing contact pressure & check stability
   1. against sliding
   2. against overturning
7. Estimate settlement (total & differential)
8. Design footing structure

Question 4

Design Loads

Answer 4

Dead loads: overall weight of structure itself

Live loads*: weights of everything that is not permanent

Wind loads*: worst case combinations

Snow loads*: based on accumulations
*(usually based on building codes)

Earth pressure: lateral force
due to earth below ground
(usually treated as a Dead Load)

Water pressure: lateral
pressure or hydrostatic uplift

Earthquake forces: lateral,
vertical, & torsional forces

Question 5

Depth & Location Considerations

Answer 5

 Adjacent Structures
 The deeper the new footing & the closer the old structure, the greater
the potential for damage
 Use 45° rule for adjacent structures

 Property Lines
 Footing extends beyond building line
 Potential cave-in of adjacent land

 Water
 Groundwater Considerations
 Construction below water table is difficult & expensive
 Presence of water can reduce strength of soils
 Potential hydrostatic uplift problems
 Waterproofing issues for floors / walls

 Frost
 Alternate expansion & contraction (lifting & dropping the
footing)
 Place footing below max. frost depth
 Building Codes usually specify min. depth

 Soil Volume Change
 Shrinking & swelling w/ changes in water content of soil
 Greatest at surface –reduces w/ depth.
 Usually insignificant below 10ft
 Worse at edges of footings
 Does not occur below water table

 Underground Anomalies
 Caves
 Faults
 Old tunnels, utility services, sewers, etc…

 Building Codes

Question 6

Bearing Capacity Special Cases

Answer 6

 Effect of Water Table on Bearing Capacity
 Inclined Loads
 Eccentric Loads
 Footings on Slopes