Shallow Foundations Bearing Capacity

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Shallow Foundations Bearing Capacity

• The problems of soil mechanics can be divided
  into two principal groups - stability problems
  and elasticity problems
• - Karl Terzaghi, 1943

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Karl Terzaghi (1883-1963)

• Father of modern soil mechanics
• Born in Prague, Czechoslovakia
• Wrote Erdbaumechanick in 1925
• Taught at MIT (1925-1929)
• Taught at Harvard (1938 and after)

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Karl Terzaghi at Harvard, 1940
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Bearing Capacity Failure
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Transcosna Grain Elevator Canada (Oct. 18, 1913)
West side of foundation sank 24-ft
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Stability ProblemBearing Capacity Failure

• Chapter 6. Bearing Capacity Analysis
• How do we estimate the maximum bearing pressure
  that the soil can withstand before failure occurs?

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Bearing Capacity Failures

• Types/Modes of Failure
• general shear failure
• local shear failure
• punching shear failure

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General Shear Failure
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Punching Shear Failure
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Model Tests by Vesic (1973)
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General Guidelines

• Footings in clays - general shear
• Footings in Dense sands ( gt 67)
• -general shear
• Footings in Loose to Medium dense
• (30lt lt 67) - Local Shear
• Footings in Very Loose Sand ( lt
  30)- punching shear

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Bearing Capacity Formulas
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Terzaghi Bearing Capacity Formulas
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Terzaghi Bearing Capacity Formulas
For Continuous foundations
For Square foundations
For Circular foundations
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Terzaghi Bearing Capacity Factors
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Bearing Capacity Factors
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Terzaghi Bearing Capacity Formulas

• D ? B
• No sliding between footing and soil
• soil a homogeneous semi-infinite mass
• general shear failure
• footing is very rigid compared to soil

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Further Developments

• Skempton (1951)
• Meyerhof (1953)
• Brinch Hanson (1961)
• De Beer and Ladanyi (1961)
• Meyerhof (1963)
• Brinch Hanson (1970)
• Vesic? (1973, 1975)

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Vesic? (1973, 1975) Formulas
Shape factors. Eq. 6.14, 6.15 and 6.16 Depth
Factors . Eq. 6.17, 6.18 and 6.19 Load
Inclination Factors . Eq. 6.20, 6.21 and
6.22 Base Inclinations factors .. Eq. 6.25 and
6.26 Ground Inclination Factors. Eq. 6.27 and
6.28 Bearing Capacity Factors . Eq. 6.29, 6.30
and 6.31
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Vesic? Formula Shape Factors
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Vesic? Formula Depth Factors
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Bearing Capacity of Shallow Foundations

• 6.3 Groundwater Effects
• 6.4 Allowable Bearing Capacity
• 6.5 Selection of Soil Strength Parameters
• 6.6 Local Punching Shear Cases
• 6.7 Bearing Capacity on Layered Soils
• 6.8 Accuracy of Bearing Capacity Analyses
• 6.9 Bearing Capacity Spreadsheet

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Groundwater Table Effect
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Groundwater Table Effect Case I

1. Modify ?'zD
2. Calculate ?' as follows

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Groundwater Table Effect Case II

1. No change in ?'zD
2. Calculate ?' as follows

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Groundwater Table Effect Case III

1. No change in ?'zD
2. No change in ?'

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Allowable Bearing Capacity

• .. Allowable Bearing Capacity
• F . Factor of safety

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Factor of Safety

• Depends on
• Type of soil
• Level of Uncertainty in Soil Strength
• Importance of structure and consequences of
  failure
• Likelihood of design load occurrence

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Minimum Factor of Safety
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Selection of Soil Strength Parameters

• Use Saturated Strength Parameters
• Use Undrained Strength in clays (Su)
• Use Drained Strength in sands,
  
• Intermediate soils that where partially drained
  conditions exist, engineers have varying
  opinions Undrained Strength can be used but it
  will be conservative!

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Accuracy of Bearing Capacity Analysis

• In Clays ..Within 10 of true value (Bishop and
  Bjerrum, 1960)
• Smaller footings in Sands. Bearing capacity
  calculated were too conservative but
  conservatism did not affect construction cost
  much
• Large footings in Sands Bearing capacity
  estimates were reasonable but design was
  controlled by settlement

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Accuracy Bearing Capacity Analysis
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Bearing Capacity Spreadsheet

• Can be downloaded from http//www.prenhall.com/cod
  uto
• See Appendix B (page 848) for further instructions