MatakuliahMatakuliah : S2094 / Rekayasa Pondasi: S2094 / Rekayasa PondasiTahunTahun : 2005: 2005VersiVersi : 1.1: 1.1

Pertemuan 05Pertemuan 05Daya Dukung TanahDaya Dukung Tanah

Shallow Foundations Shallow Foundations Bearing CapacityBearing Capacity

• The problems of soil mechanics can The problems of soil mechanics can be divided into two principal groups - be divided into two principal groups - stability problemsstability problems and and elasticity elasticity problemsproblems

- Karl Terzaghi, 1943- Karl Terzaghi, 1943

Karl Terzaghi (1883-1963)Karl Terzaghi (1883-1963)

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

Karl Terzaghi at Harvard, Karl Terzaghi at Harvard, 19401940

Bearing Capacity FailureBearing Capacity Failure;; Fargo Grain Elevator, N. DakotaFargo Grain Elevator, N. Dakota

Transcosna Grain Elevator, CanadaTranscosna Grain Elevator, CanadaOct. 18, 1913Oct. 18, 1913

West side of foundation sank 24-ft

Stability ProblemStability ProblemBearing Capacity FailureBearing Capacity Failure

• Bearing Capacity AnalysisBearing Capacity Analysis• How do we estimate the How do we estimate the maximum maximum

bearing pressurebearing pressure that the soil can that the soil can withstand before failure occurs?withstand before failure occurs?

Bearing Capacity FailuresBearing Capacity Failures

Types/Modes of FailureTypes/Modes of Failure general shear failuregeneral shear failure local shear failurelocal shear failure punching shear failurepunching shear failure

General Shear FailureGeneral Shear Failure

Punching Shear FailurePunching Shear Failure

Model Tests by Vesic (1973)Model Tests by Vesic (1973)

General GuidelinesGeneral Guidelines

Footings in clays - Footings in clays - general sheargeneral shear Footings in Dense sands ( > 67%)Footings in Dense sands ( > 67%)

--general sheargeneral shear Footings in Loose to Medium dense Footings in Loose to Medium dense

(30%< < 67%) - (30%< < 67%) - Local ShearLocal Shear Footings in Very Loose Sand ( < 30%)-Footings in Very Loose Sand ( < 30%)-

punching shearpunching shear

rD

rD

rD

Bearing Capacity FormulasBearing Capacity Formulas

zDucult sNq

Terzaghi Bearing Capacity Terzaghi Bearing Capacity FormulasFormulas

Terzaghi Bearing Capacity Terzaghi Bearing Capacity FormulasFormulas

BNNNcq qzDcult 5.0

For Square foundations:

For Continuous foundations:

BNNNcq qzDcult 4.03.1

For Circular foundations:

BNNNcq qzDcult 3.03.1

Bearing Capacity FactorsBearing Capacity Factors

Terzaghi Bearing Capacity Terzaghi Bearing Capacity FormulasFormulas

D D B B No sliding between footing and soilNo sliding between footing and soil soil: a homogeneous semi-infinite soil: a homogeneous semi-infinite

massmass general shear failuregeneral shear failure footing is very rigid compared to soilfooting is very rigid compared to soil