Bearing Capacity of Shallow Foundations

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Bearing Capacity of Shallow Foundations

• Ch. 6.

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B.C. Failures
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B.C. Failures
Sand Circular foundations
(Vesic, 1963 and 1973)
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We design for the general shear case (for shallow
foundations)
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Bearing Capacity Theory LIMIT EQUILIBRIUM

• Define the shape of a failure surface
• Evaluate stresses vs. strengths along this surface

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Bearing Capacity Theory LIMIT EQUILIBRIUM
Ultimate bearing capacity qult ? (Bearing
press. required to cause a BC failure)
Moments about point A
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Terzaghis Bearing Capacity Theory

• Assumptions
• D lt or B
• Homogenous and isotropic s c stan(f)
• level ground
• rigid foundation
• full adhesion between soil and base of footing
• general shear failure develops

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Terzaghis Bearing Capacity Theory
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Terzaghis Bearing Capacity Theory

• Terzaghi developed the theory for continuous
  foundations (simplest, 2D problem).

From model tests, he expanded the theory to
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Terzaghis Bearing Capacity Theory
Nc cohesion factor Nq surcharge factor N?
self wt factor
fn (f) See table 6.1 for values
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Groundwater level effects
groundwater
by

• Reduction in apparent cohesion - cap (sat. soil
  for lab tests)
• Decrease in s

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Groundwater level effects
D
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Groundwater level effects
Case I
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Groundwater level effects
Case II
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Groundwater level effects
Case III
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Groundwater level effects
For total stress analysis
regardless of the case (gw effects are implicit
in cT and fT)
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FS for BC
Allowable BC qa
FS function of
soil type
structure type
soil variability
extent of site characterization
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BC of shallow foundations in practice (per Mayne
97)
Undrained
Nc 5.14 for strip footing 6.14 for square
or circular footing
The value of su is taken as the ave. within a
depth to 1B to 1.5B beneath the foundation base
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BC of shallow foundations in practice (per Mayne
97)
Drained
Ng fn (foundation shape and f)
Consider gw cases (I, II, or III to determine g)
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BC of shallow foundations in practice (per Mayne
97)
Sands
Perform drained analysis
Clays
Perform both
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Problem formulation BC design
1. Find B so that FS 3
Get q Get q ult (by BC analysis) Set FS ratio and
solve for B
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Problem formulation BC design
2. Find B and D so that FS 3
Get q Get q ult (by BC analysis) Set FS ratio and
solve for B
Determine this for various D values