bearing capacity - 1.ppt

1.0 Bearing Capacity*Bearing Capacity Mr. Jjuuko Samuel, NDEJJE*

Ultimate Bearing Capacity*Bearing Capacity Mr. Jjuuko Samuel, MUKThe load per unit area of the foundation at which shear failure in soil occurs is called the ultimate bearing capacity.*

Bearing Capacity Mr. Jjuuko Samuel, MUK

Principal Modes of FailureBearing Capacity Mr. Jjuuko Samuel, MUK*General Shear Failure:Sudden or catastrophic failureWell defined failure surfaceBulging on the ground surface adjacent to foundationCommon failure mode in dense sand*


Principal Modes of FailureBearing Capacity Mr. Jjuuko Samuel, MUK*Local Shear Failure:Common in sand or clay with medium compaction.Significant settlement upon loading.Failure surface first develops right below the foundation and then slowly extends outwards with load increments.Foundation movement shows sudden jerks first (at qu1) and then after a considerable amount of movement the slip surface may reach the ground.A small amount of bulging may occur next to the foundation.*


Principal Modes of FailureLocal Shear Failure:

Bearing Capacity Mr. Jjuuko Samuel, MUK**


Principal Modes of FailureBearing Capacity Mr. Jjuuko Samuel, MUK*Punching Failure:Common in fairly loose sand or soft clayFailure surface does not extends beyond the zone right beneath the foundationExtensive settlement with a wedge shaped soil zone in elastic equilibrium beneath the foundation. Vertical shear occurs around the edges of foundation.After reaching failure load-settlement curve continues at some slope and mostly linearly.*


Principal Modes of FailurePunching Failure:



Principal Modes of FailureBearing Capacity Mr. Jjuuko Samuel, MUK**


Terzaghis Bearing Capacity TheoryBearing Capacity Mr. Jjuuko Samuel, MUK*AssumptionL/B ratio is large plain strain problemDf BShear resistance of soil for Df depth is neglectedGeneral shear failureShear strength is governed by Mohr-Coulomb Criterion*


Terzaghis Bearing Capacity TheoryBearing Capacity Mr. Jjuuko Samuel, MUK**


Terzaghis Bearing Capacity TheoryBearing Capacity Mr. Jjuuko Samuel, MUK*due to only self weight of soil in shear zonedue to soil cohesion only (soil is weightless)due to surcharge only*


Terzaghis Bearing Capacity TheoryBearing Capacity Mr. Jjuuko Samuel, MUK*Weight termCohesion termSurcharge termTerzaghis bearing capacity equationTerzaghis bearing capacity factors*


Terzaghis Bearing Capacity TheoryLocal Shear Failure:Bearing Capacity Mr. Jjuuko Samuel, MUK*Modify the strength parameters such as:Square and circular footing:For squareFor circular*


Terzaghis Bearing Capacity TheoryBearing Capacity Mr. Jjuuko Samuel, MUK*Effect of water table:Case I: Dw DfCase II: Df Dw (Df + B)In bearing capacity equation replace by:Case III: Dw > (Df + B)No influence of water table.*


Terzaghis Bearing Capacity TheoryBearing Capacity Mr. Jjuuko Samuel, MUK*Another recommendation for Case II:Rupture depth:*


Skemptons Bearing Capacity Analysis for cohesive SoilsBearing Capacity Mr. Jjuuko Samuel, MUK*~ For saturated cohesive soil, = 0 Nq = 1 and N = 0For strip footing:with limit of Nc 7.5 For square/circularfooting:with limit of Nc 9.0 For rectangular footing:for Df 2.5 for Df > 2.5 qu = c.Nc + qNet ultimate bearing capacity, qnu = qu .Dfqu = c.Nc *


Effective Area Method for Eccentric LoadingBearing Capacity Mr. Jjuuko Samuel, MUK*In case of Moment loadingIn case of Horizontal Force atsome height but the column iscentered on the foundation*


General Bearing Capacity Equation:(Meyerhof, 1963)Bearing Capacity Mr. Jjuuko Samuel, MUK*Shape factorDepth factorInclination factorEmpirical correction factors[By Hansen (1970):[By Vesic (1973):Ground factorBase factor*


Meyerhofs Correction Factors:Bearing Capacity Mr. Jjuuko Samuel, MUK*ShapeFactorsFor 10oFor lower valueDepthFactorsFor 10oFor lower valueInclinationFactors*


Hansens Correction Factors:Bearing Capacity Mr. Jjuuko Samuel, MUK*InclinationFactorsDepthFactors*


Hansens Correction Factors:Bearing Capacity Mr. Jjuuko Samuel, MUK*ShapeFactorsHansens Recommendation for cohesive saturated soil*


Bearing Capacity Mr. Jjuuko Samuel, MUK*Notes:1. Notice use of effective base dimensions B, L by Hansen but not by Vesic.2. The values are consistent with a vertical load or a vertical load accompanied by a horizontal load HB.3. With a vertical load and a load HL (and either HB=0 or HB>0) you may have to compute two sets of shape and depth factors si,B, si,L and di,B, di,L. For i,L subscripts use ratio L/B or D/L.4. Compute qu independently by using (siB, diB) and (siL,diL) and use min value for design.*

Bearing Capacity Mr. Jjuuko Samuel, MUK*Notes:1. Use Hi as either HB or HL, or both if HL>0.2. Hansen (1970) did not give an ic for >0. The value given here is from Hansen (1961) and also used by Vesic.3. Variable ca = base adhesion, on the order of 0.6 to 1.0 x base cohesion.4. Refer to sketch on next slide for identification of angles and , footing depth D, location of Hi (parallel and at top of base slab; usually also produces eccentricity).Especially notice V = force normal to base and is not the resultant R from combining V and Hi..*

Bearing Capacity Mr. Jjuuko Samuel, MUK*Note:1. When =0 (and 0) use N = -2sin() in N term.2. Compute m = mB when Hi = HB (H parallel to B) and m = mL when Hi = HL (H parallel to L). If you have both HB and HL use m = (mB2 + mL2)1/2. Note use of B and L, not B, L.3. Hi term 1.0 for computing iq, i (always).*

Suitability of MethodsBearing Capacity Mr. Jjuuko Samuel, MUK**

UseBest forTerzaghiVery cohesive soils where D/B1 or for a quick estimate of qult to compare with other methods. Do not use for footings with moments and/or horizontal forces or for tilted bases and/or sloping groundHansen, Meyerhof, VesicAny situation that applies, depending on user preference or familiarity with a particular method.Hansen, VesicWhen base is tilted; when footing is on a slope or when D/B> 1.


IS:6403-1981 RecommendationsBearing Capacity Mr. Jjuuko Samuel, MUK*Net Ultimate Bearing capacity:For cohesive soilsNc, Nq, N as per Vesic(1973) recommendationsShapeFactorsFor rectangle,For square and circle, sc = 1.3 sq = 1.2 s = 0.8 for square, s = 0.6 for circle *


IS:6403-1981 RecommendationsBearing Capacity Mr. Jjuuko Samuel, MUK*DepthFactorsInclination Factors The same as Meyerhof (1963)*


Bearing Capacity Correlations withSPT-value: Peck, Hansen, and Thornburn (1974) & IS:6403-1981 RecommendationBearing Capacity Mr. Jjuuko Samuel, MUK**


Bearing Capacity Correlations with SPT-valueBearing Capacity Mr. Jjuuko Samuel, MUK*Teng (1962):For Strip Footing:For Square andCircular Footing:For Df > B, take Df = BWater Table Corrections:Rw 1R/w 1*


Bearing Capacity Correlations with CPT-valueBearing Capacity Mr. Jjuuko Samuel, MUK*IS:6403-1981 Recommendation:Cohesionless SoilSchmertmann (1975):*


Bearing Capacity Correlations with CPT-valueBearing Capacity Mr. Jjuuko Samuel, MUK*IS:6403-1981 Recommendation:Cohesive Soil*

Soil TypePoint Resistance Values (qc) kgf/cm2Range of Undrained Cohesion (kgf/cm2)Normally consolidated claysqc < 20qc/18 to qc/15Over consolidated claysqc > 20qc/26 to qc/22


Bearing Capacity of Footing on Layered SoilBearing Capacity Mr. Jjuuko Samuel, MUK*Depth of rupture zoneor approximately taken as BCase I: Layer-1 is weaker than Layer-2Design using parameters of Layer -1Case II: Layer-1 is stronger than Layer-2Distribute the stresses to Layer-2 by 2:1 method and check the bearing capacity at this level for limit state.Also check the bearing capacity for original foundation level using parameters of Layer-1Choose minimum value for design*


Bearing Capacity of Footing on Layered SoilBearing Capacity Mr. Jjuuko Samuel, MUK*Another approximate method for c- soil:For effective depthFind average c and and use them for ultimate bearing capacity calculation*


Bearing Capacity of Stratified Cohesive SoilBearing Capacity Mr. Jjuuko Samuel, MUK*IS:6403-1981 Recommendation:*


Bearing Capacity of Footing on Layered Soil: Stronger Soil Underlying Weaker SoilBearing Capacity Mr. Jjuuko Samuel, MUK*Depth H is relatively smallPunching shear failure in top layerGeneral shear failure in bottom layerDepth H is relatively largeFull failure surface develops in top layer itself*


Bearing Capacity of Footing on Layered Soil: Stronger Soil Underlying Weaker SoilBearing Capacity Mr. Jjuuko Samuel, MUK*Ca = caH ca = adhesionB = width of the foundationCa = adhesive forcePp = passive force per unit length of the faces aa and bbqb = bearing capacity of the bottom soil layer = inclination of the passive force Pp with the horizontal*


Bearing Capacity of Footing on Layered Soil: Stronger Soil Underlying Weaker SoilBearing Capacity Mr. Jjuuko Samuel, MUK*The punching shear coefficientBearing capacities of continuous footing of width B under vertical load on the surface of homogeneous thick bed of upper and lower soil*


Bearing Capacity of Footing on Layered Soil: Stronger Soil Underlying Weaker SoilBearing Capacity Mr. Jjuuko Samuel, MUK*For Strip Footing:Where, qt is the bearing capacity for foundation considering only the top layer to infinite depthFor Rectangular Footing:Special Cases:1. Top layer is strong sand and bottom layer is saturated soft clayc1 = 0 2 = 02. Top layer is strong sand and bottom layer is weaker sandc1 = 0 c2 = 03. Top layer is strong saturated clay and bottom layer is weaker saturated clay1 = 0 2 = 0*


Eccentrically Loaded FoundationsBearing Capacity Mr. Jjuuko Samuel, MUK**


Determination of Effective Dimensions for Eccentrically Loaded foundations (Highter and Anders, 1985)Bearing Capacity Mr. Jjuuko Samuel, MUK*Case I:*


Determination of Effective Dimensions for Eccentrically Loaded foundations (Highter and Anders, 1985)Bearing Capacity Mr. Jjuuko Samuel, MUK*Case II:*


Determination of Effective Dimensions for Eccentrically Loaded foundations (Highter and Anders, 1985)Bearing Capacity Mr. Jjuuko Samuel, MUK*Case III:*


Determination of Effective Dimensions for Eccentrically Loaded foundations (Highter and Anders, 1985)Bearing Capacity Mr. Jjuuko Samuel, MUK*Case IV:*


Determination of Effective Dimensions for Eccentrically Loaded foundations (Highter and Anders, 1985)Bearing Capacity Mr. Jjuuko Samuel, MUK*Case V:Circular foundation*


Meyerhofs (1953) area correction based on empirical correlations: (American Petroleum Institute, 1987)Bearing Capacity Mr. Jjuuko Samuel, MUK**


Bearing Capacity of Footings on Slopes Meyerhofs (1957) SolutionBearing Capacity Mr. Jjuuko Samuel, MUK*Granular Soilc = 0*


Bearing Capacity of Footings on SlopesMeyerhofs (1957) SolutionBearing Capacity Mr. Jjuuko Samuel, MUK*Cohesive Soil = 0*


Bearing Capacity of Footings on Slopes Graham et al. (1988), Based on method of characteristicsBearing Capacity Mr. Jjuuko Samuel, MUK**


Bearing Capacity of Footings on Slopes Bowles (1997): A simplified approachBearing Capacity Mr. Jjuuko Samuel, MUK**


Soil Compressibility Effects on Bearing Capacity Vesics (1973) ApproachBearing Capacity Mr. Jjuuko Samuel, MUK*Use of soil compressibility factors in general bearing capacity equation.These correction factors are function of the rigidity of soil.Rigidity Index of Soil, Ir:Critical Rigidity Index of Soil, Icr:*


Soil Compressibility Effects on Bearing Capacity Vesics (1973) ApproachBearing Capacity Mr. Jjuuko Samuel, MUK*Compressibility Correction Factors, cc, c, and cqFor Ir Irc cc = cq = c = 1For Ir < Irc *


bearing capacity - 1.ppt

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