Chapter 3: Soil Mechanics

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Matakuliah S2094 / Rekayasa Pondasi Tahun
2005 Versi 1.1
Pertemuan 10Penurunan Pondasi Dangkal
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Settlement Analysis Based on Laboratory Tests

• Approach used when good quality undisturbed
  samples can be obtained from soil
• Perform consolidation test
• Obtain Cc, Cr , e0, and ?m
• Perform settlement analysis

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Two Methods of Analysis

• Classical Method
• Based on Terzaghis Theory
• One dimensional compression
• Skempton and Bjerrum Method
• Considers distortion settlement
• Uses an adjustment factor for 3D compression

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Classical Method Foundation Rigidity Factor
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Classical Method
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Thickness of Soil Sub-layers
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Settlement PredictionsN.C. Clays
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Settlement PredictionsO.C. Clays Case I
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Settlement PredictionsO.C. Clays Case II
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Skempton Bjerrum Method

• Settlement,
• Distortion Settlement (?d)
• Consolidation Settlement (?c)

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Distortion Settlement
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3D Adjustment Factor, ?
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Influence Factors, I1 and I2
I1
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I2
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Example 7.3
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Example 7.4
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Settlement Analysis based on In-Situ Tests

• Techniques for estimating settlements in sands
  are nearly always based on in-situ test results
• In sands, settlement analysis is not performed
  based on consolidation analysis
• Instead, we use Equivalent Modulus of Elasticity,
  Es

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Es from SPT Data (Table 7.4)
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Es from CPT Data (Table 7.3)
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Schmertmanns Method
C1 depth factor C2 Secondary creep factor
C3 Shape factor
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Strain Influence Factor
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Simplified Schmertmann Method

• When Es is uniform with depth over the depth of
  influence
• For square/circular footings

• For continuous footings

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Settlements in Stratified Soils

• When the soil profile primarily consists of clays
  and silts use procedures described in Section
  7.4 to estimate settlement
• For clays/silts use laboratory consolidation
  tests to determine Cc/(1e0) and Cr/(1e0)
• For sands use Cc/(1e0) or Cr/(1e0) from Table
  3.7

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Settlements in Stratified Soils

• When the soil profile primarily consists of sands
  and silts estimate settlement based on
  Schmertmann Method
• Use in-situ test data to determine Es for sand
• Use following equation to determine Es for clays

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Settlements in Stratified Soils

• Alternative Approach
• Conduct separate analysis for clays/silts and
  sands
• Add the computed settlements

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Differential Settlement

• Total Settlement, ?
• Differential Settlement, ?D
• Rotation, ?

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Differential Settlement

• Variations in Soil Profile
• Variations in Structural Loads
• Design Controlled by Bearing Capacity
• Construction Tolerances

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Computing Differential Settlement

• Consider best case/worst case scenarios
• Use ?D/? ratios
• Clayey Soils. Figure 7.20
• Sands . Figure 7.21

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Total and Differential Settlement for Clays
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Total and Differential Settlement for Sands
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Design values of ?D/? Ratios
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Rate of Settlement

• In saturated soils rate of settlement depends
  largely on how fast the water drains away
• In Clays rates of settlement are slower than in
  sands
• Consolidation theory allows you to calculate rate
  of settlement (Chapters 19 and 20)
• Rate of settlement in sands vary depending upon
  the pattern of loading

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How Accurate are our Settlement Predictions?
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Computer Software SCHMERT (schmertmann.xls)