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FOUNDATION ENGINEERING - IBearing Capacity of Shallow FounationsCourse Instructor : Syed Zishan AshiqLecture No. 04August 2015Mirpur University of Science & Technology Mirpur AJKDepartment of Civil Engineering

General shearDense soils,Rock, NC clays

Defined failure surf.Fast failureBearing Capacity Failures

Local shear

Intermediate case+/- gradual failure

PunchingLoose sands,weak clays (dr.)

F. surf. not definedGradual failure

Bearing Capacity Failures(Vesic, 1963 and 1973)Deep foundationsSand Circular foundationsWe design for the general shear case (for shallow foundations)Bearing Capacity TheoryLIMIT EQUILIBRIUMDefine the shape of a failure surfaceEvaluate stresses vs. strengths along this surface

BC FactorBearing Capacity TheoryLIMIT EQUILIBRIUMUltimate bearing capacity = qult = ?(Bearing press. required to cause a BC failure)Moments about point A

Terzaghis Bearing Capacity TheoryAssumptions

D < 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

Terzaghis Bearing Capacity TheoryTerzaghis Bearing Capacity TheoryTerzaghi developed the theory for continuous foundations (simplest, 2D problem).

From model tests, he expanded the theory to:Nc = cohesion factorNq = surcharge factorN = self wt factorTerzaghis Bearing Capacity Theoryfn (f)Groundwater level effectsgroundwaterReduction in apparent cohesion - cap (sat. soil for lab tests)Decrease in sShear strengthaffectsby

Groundwater Level EffectsCase I:

Case II :Case III :

For total stress analysis:

regardless of the case(gw effects are implicit in cT and fT)Factor of SafetySafe BC = qs

FS = function ofsoil typeextent of site characterization soil variability structure typeuncertaintyBC of Shallow Foundations in practice (per Mayne 97)Undrained

The value of su is taken as the average within a depth = to 1B to 1.5B beneath the foundation baseNc* = 5.14 for strip footing= 6.14 for square or circular footing

(Mayne, 1980)Drained:

Consider gw cases (I, II, or III to determine g)Ng* = fn (foundation shape and f)BC of Shallow Foundations in practice (per Mayne 97)Sands:

Perform drained analysisClays:Perform bothBC of Shallow Foundations in Practice (per Mayne 97)

1. Find B so that FS = 3Get qGet q ult (by BC analysis)Set FS ratio and solve for BConsider (drained vs. undrained) and methods for obtaining OCR and f ---- CPT?

2. Find B and D so that FS = 3Get qGet q ult (by BC analysis)Set FS ratio and solve for BProblem Formulation BC DesignDetermine this for various D valuesImportant too:Foundation shape (cost and labor)Moment loads and eccentricityWeight of the foundationsAllowable Bearing Capacity

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