Chapter 6 Bearing Capacity

7/29/2019 Chapter 6 Bearing Capacity

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Priyantha Jayawickrama, Ph.D.

Associate Professor

Chapter 6:Bearing Capacity of ShallowFoundations

Texas Tech UniversityDepartment of Civil and Environmental Engineering


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CE 4321: Geotechnical Engineering Design

Shallow FoundationsBearing Capacity

The problems of soil mechanics can

be divided into two principal groups -stability problems and elasticityproblems

- 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 ElevatorCanada (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 canwithstand 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 ( Dr > 67%)

-general shear

Footings in Loose to Medium dense

(30%< Dr < 67%) - Local Shear

Footings in Very Loose Sand (Dr < 30%)-

punching shear


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Bearing Capacity Formulas

zDucult sNq


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Terzaghi Bearing CapacityFormulas


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Terzaghi Bearing Capacity Formulas

BNNNcq qzDcult 5.0

For Square foundations:

For Continuous foundations:

BNNNcq qzDcult 4.03.1

For Circular foundations:

BNNNcq qzDcult 3.03.1


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Terzaghi Bearing Capacity Factors

07.5 whenNc

)2/45(cos2 2

2

aNq

tan)360/75.0(expa

1

cos2

tan2

pKN

0tan

1

whenNN qc


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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-infinitemass

general shear failure

footing is very rigid compared to soil


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See Extra Handout

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 6.26Ground Inclination Factors.Eq. 6.27 and 6.28

Bearing Capacity Factors . Eq. 6.29, 6.30 and 6.31

gbidsBNgbidsNgbidsNcq qqqqqqzDccccccult 5.0


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Vesic Formula Shape Factors

c

q

cN

N

L

Bs 1

tan1

L

Bsq

L

Bs 4.01


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Vesic Formula Depth Factors

B

Dk

1tan

2)sin1(tan21 kdq

1d

kdc 4.01


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Bearing Capacity ofShallow 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 CapacityAnalyses

6.9 Bearing Capacity Spreadsheet


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Groundwater Table Effect


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Groundwater Table Effect;Case I

1. ModifyzD

2. Calculate as follows:

wb


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Groundwater Table Effect;Case II

1. No change inzD

2. Calculate as follows:

B

DDww 1


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Groundwater Table Effect;Case III

1. No change inzD

2. No change in


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Allowable Bearing Capacity

F

qq ulta

.. Allowable Bearing Capacity

F. Factor of safetyaq


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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 StrengthParameters

Use Saturated Strength Parameters

Use Undrained Strength in clays (Su)

Use Drained Strength in sands,

Intermediate soils that where partiallydrained conditions exist, engineers

have varying opinions; UndrainedStrength can be used but it will beconservative!

andc


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Accuracy of Bearing CapacityAnalysis

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 constructioncost much

Large footings in Sands Bearing capacity

estimates were reasonable but design wascontrolled by settlement


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Accuracy; Bearing Capacity Analysis


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Bearing Capacity Spreadsheet

Can be downloaded fromhttp://www.prenhall.com/coduto

See Appendix B (page 848) forfurther instructions

Documents

Chapter 6 Bearing Capacity