UNIT 1 Bearing Capacity of Shallow Foundation

7/27/2019 UNIT 1 Bearing Capacity of Shallow Foundation

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Bearing Capacity Theory

T.Sandeep

Asst.Prof,Department of Civil EngineeringMalla Reddy Engineering College

Dept of CE,MREC-T.Sandeep


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Bearing Capacity Failure

a)General Shear Failure Mostcommon type of shear failure;occurs in strong soils and

rocks

b) Local Shear FailureIntermediate between generaland punching shear failure

c) Punching Shear FailureOccurs in very loose sandsweak clays



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Bearing Capacity Failure

General shear failure

Local shear failure

Punching shear failure



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Soil Conditions and Bearing

Capacity Failure



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Load Displacement

Curves(after Vesic(1973))

a) General Shear Failure

b) Local Shear Failure

c) Punching Shear Failure



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Comments on Shear Failure

Usually only necessary to analyze general shearfailure.

Local and punching shear failure can usually beanticipated by settlement analysis.

Failure in shallow foundations is generallysettlement failure; bearing capacity failure mustbe analyzed, but in practical terms is usuallysecondary to settlement analysis.



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Development of Bearing

Capacity Theory Application of limit equilibrium methods first done by

Prandtl on the punching of thick masses

of metal.

Prandtl's methods adapted by Terzaghi to bearingcapacity failure of shallow foundations.

Vesicand others improved on Terzaghi's originaltheory and added other factors for a more completeanalysis



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Assumptions for Terzaghi's Method

Depth of foundation is less than or equal to itswidth

No sliding occurs between foundation and soil(rough foundation)

Soil beneath foundation is homogeneous semiinfinite mass

Mohr-Coulomb model for soil General shear failure mode is the governing

mode (but not the only mode)



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Assumptions for Terzaghi's Method

No soil consolidation occurs

Foundation is very rigid relative to the soil

Soil above bottom of foundation has no shear

strength; is only a surcharge load against the

overturning load

Applied load is compressive and appliedvertically to the centroid of the foundation

No applied moments present



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Failure Geometry for Terzaghi's Method



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Notes on Terzaghi's Method Since soil cohesion can be difficult to quantify,

conservative values of c (cohesion) should be used.

Frictional strength is more reliable and does notneed to be as conservative as cohesion.

Terzaghi's method is simple and familiar to many

geotechnical engineers; however, it does not takeinto account many factors, nor does it consider casessuch as rectangular foundations.



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The General Bearing Capacity Equation.



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Other Factors



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Other Factors

For continuous footing,s = 1

For perpendicular load,

i = 1 For level foundation,b =1

For level ground,

g =1 Need to compute factors- Bearing Capacity FactorN,- Depth Factord



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Groundwater Effects



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Groundwater Effects

Shallow groundwater affects shear strength in two ways:

Reduces apparent cohesion that takes place when soils

are not saturated; may necessitate reducing thecohesion measured in the laboratory

Pore water pressure increases; reduces both effective

stress and shear strength in the soil (same problem asis experienced with unsupported slopes)



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Groundwater Effects



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FOOTINGS WITH ECCENTRICORINCLINED LOADINGS

Eccentricity

InclinationDept of CE,MREC-T.Sandeep


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FOOTINGS WITH One WayEccentricity

In most instances, foundations are subjected to moments in addition to thevertical load as shown below. In such cases the distribution of pressure by thefoundation upon the soil is not uniform.



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Note that in these equations, when the eccentricity ebecomes B/6, qminis zero.

For e> B/6, qminwill be negative, which means that tension

will develop. Because soils can sustain very little tension, there will be a

separation between the footing and the soil under it. Also note that the eccentricity tends to decrease the load

bearing capacity of a foundation.

In such cases, placing foundation column off-center, asshown in Figure is probably advantageous. Doing so in effect, produces a centrally loaded foundation

with a uniformly distributed pressure.



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Footing with Two-way Eccentricities

Consider a footing subject to a vertical ultimate load Qult and a moment M asshown in Figures a and b. For this case, the components of the moment Mabout the x and y axis are Mx and My respectively. This condition is equivalentto a load Q placed eccentrically on the footing with x = eB and y = eLas shown

in Figure d.



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Footing with Two-way Eccentricities



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Example 1



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Example 1



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Example 2


Example 2


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Example 2



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Footings with Inclined Loads



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1. Compute the inclination factors using the equations given below:

inclination of load with respect to vertical

2. Use the inclination factors just computed to compute Hansenshape factors as



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3. These are used in the following modifications of the "edited

Hansen bearing capacity equation:

Use the smaller value ofqu\tcomputed byeither of Equations.



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The Bearing Capacity ofMulti-Layered Soils



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The Bearing Capacity of Layered Soils



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In layered soil profiles, the unit weight of the soil, the angle offriction and the cohesion are not constant throughout the depth.The ultimate surface failure may extend through two or more of thesoil layers.

Consider the case when the stronger soil is underlain by a weakersoil. IfH, the thickness of the layer of soil below the footing, isrelatively large then the failure surface will be completely located inthe top soil layer, which is the upper limit for the ultimate bearingcapacity.

If the thickness H is small compared to the foundation width B, apunching shear failure will occur at the top soil stratum, followed bya general shear failure in the bottom soil layer.

IfH is relatively deep, then the shear failure will occur only on thetop soil layer.



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Meyerhof and Hanna (1978) and Meyerhof(1974)



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Meyerhof andHannas

punching shearcoefficient Ks



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Variationofca/c1

withq2/q1

based on the

theory ofMeyerhof andHanna (1978)


E ample on la e ed soils


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Example on layered soils



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Ground Factors



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Base Factor

For footings with angled foundation bases

When footing is level, b = 1Dept of CE,MREC-T.Sandeep

Rigi ity


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Rigi ityFactors



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Bearing Capacity from Field Tests



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Bearing Capacity from SPT



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B i C i f SPT


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Bearing Capacity using CPT



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Bearing Capacity for FieldLoad Tests PLT



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Bearing Capacity for Field Load Tests PLT

For Granular Soils:

For Cohesive Soils:



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Correction of Standard penetration number

It has been suggested that the SPT be standardized to someenergy ratio Erwhich should be computed as

Note that larger values ofErdecreasethe blow count Nnearlylinearly, that is, Er45gives N = 20and Er90gives N = 10;

Example ofNfor Er45=20 we obtain for the arbitrarily chosenEr=70, (Er70):

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UNIT 1 Bearing Capacity of Shallow Foundation