Unit II: Shallow Foundations

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Unit II Shallow Foundations
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Settlement
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Distribution of soil pressure
Soil pressure distribution in cohesionless soil
Soil pressure distribution in cohesive soil
RIGID FOUNDATION
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Distribution of soil pressure
Soil pressure distribution in cohesionless soil
Soil pressure distribution in cohesive soil
FLEXIBLE FOUNDATION
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Components of Settlement

• Se - Elastic or Immediate Settlement
• Sc - Consolidation Settlement
• Ss - Secondary Settlement

S Se Sc Ss
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Computation of Elastic Settlement

• Elastic settlement based on the Theory of
  Elasticity
• Janbu et al., (1956) method of determining
  settlement under an undrained condition
• Schmertmann's method of calculating settlement in
  granular soils by using CPT values.

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Computation of Modulus of Elasticity

• Laboratory Methods
• Triaxial tests on representative undisturbed
  samples
• Other tests to find undrained cohesion of soil
• A varies from 500 to 1500 for inorganic stiff
  clays
• A varies from 100 to 500 for organic or soft
  clays
• Field Methods
• Plate Load Test
• Standard penetration test (SPT)
• Static cone penetration test (CPT)

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Equations for computing Es using of SPT and CPT
values (in kPa)
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Based on Theory of Elasticity
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Elastic Settlement

• Net elastic settlement of flexible footing

µ varies from 0.1 to 0.5 For saturated clay, µ
0.5
If is a function of the L/B ratio of the
foundation, and the thickness H of the
compressible layer.
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Terzaghis method of calculating If from curves
derived by Steinbrenner (1934)
For µ 0.5, If F1, µ 0, If F1 F2. For
Intermediate Values of µ,

F1 and F2 versus H/B
Method of estimating settlement
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• Elastic Settlement of Rigid Footing
• The immediate settlement at the center is
  approximately 0.8times that obtained for a
  flexible foundation at the center.

Sef CrdfSe
Sef Final elastic settlement Cr Rigidity
Factor ( Highly rigid footing, Cr 0.8) Df
Depth factor Se Settlement for a surface
flexible footing
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Influence factor Bowles

• Bowles (1996) - Influence factor
• (for various shapes of rigid and flexible
  footings)

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JANBU, et al METHOD
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JANBU, BJERRUM AND KJAERNSLI'S METHOD
OFDETERMINING ELASTIC SETTLEMENT UNDER
UNDRAINEDCONDITIONS
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SCHMERTMANN'S METHOD OF CALCULATING SETTLEMENT IN
GRANULAR SOILS BY USING CPT VALUES
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Modified triangular vertical strain influence
factor distribution diagram
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SCHMERTMANN'S METHOD
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SCHMERTMANN'S METHOD

• C1 C2
• Modulus of Elasticity Es

t Time in years for which settlement is
required
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SCHMERTMANN'S METHOD

• Lunne and Christoffersen (1985)
• For normally consolidated sand
• For over consolidated sand (OCRgt2)

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Consolidation Settlement
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Equations for Computing Settlement

• Normally consolidated clay
• Over consolidated clay
• for
• for