Settlement of shallow foundation
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Settlement of shallow foundation
- 2. 2 Total Settlement Total settlement scet SSSS ++= Se = immediate settlement (elastic deformation) Sc = primary consolidation settlement (due to dissipation of excess pore water pressure) Ss = secondary consolidation settlement (due to adjustment of soil fabric)
- 3. 3 Immediate Settlement: Occurs immediately after the construction. This is computed using elasticity theory (Important for Granular soil) Primary Consolidation: Due to gradual dissipation of pore pressure induced by external loading and consequently expulsion of water from the soil mass, hence volume change. (Important for Inorganic clays) Secondary Consolidation: Occurs at constant effective stress with volume change due to rearrangement of particles. (Important for Organic soils) ________________________________________________ For any of the above mentioned settlement calculations, we first need vertical stress increase in soil mass due to net load applied on the foundation
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- 7. 7 Stress Due to a Concentrated Load
- 8. 8 Stress due to a Circularly Loaded Area
- 9. 9 Distribution of Stress (from a vertical line load)
- 10. 10 Stress below a Rectangular Area
- 14. 14 Types of elastic settlement in foundation
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- 16. 16 Elastic settlement Based on the Theory of Elasticity
- 17. 17 If the foundation is perfectly flexible then, the elastic settlement (Se) may be If = depth factor (Fox, 1948) α = factor that depends upon the location of foundation where settlement is being calculated
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- 22. 22 F1 and F2 versus H/B ( After Steinbrenner, 1934)
- 23. 23 If = depth factor (Fox, 1948)
- 24. 24 α = factor that depends upon the location of foundation where settlement is being calculated α = 4, settlement at the center of foundation α = 1, settlement at the corner of foundation
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- 26. 26 A rigid foundation 1 mX 2m is shown in Fig. Calculate the elastic settlement at the center of the foundation.
- 27. 27 (a) Initial condition (b) At the moment of load Consolidation Process Valve closed S=0 ∆σ∆σ∆σ∆σ’=0 ∆∆∆∆u=0 Valve closed S=0 ∆σ∆σ∆σ∆σ’=0 ∆∆∆∆u=P/A PA
- 28. 28 Valve opened Consolidation Process (Continued) (c) At a time, t S=δδδδ(t) ∆σ∆σ∆σ∆σ’=kδδδδ(t) ∆∆∆∆u=P/A-kδδδδ(t) P δδδδ(t) Valve opened S=δδδδp ∆σ∆σ∆σ∆σ’=kδδδδp=P/A ∆∆∆∆u=0 P δδδδp (d) At completion of consolidation
- 30. 30 Consolidation Curve Time (log scale) Deformation Stage I: Initial compression Stage II: Primary consolidation Stage III: Secondary consolidation tp
- 31. 31 Over-Consolidation Ratio A Current ground surface Highest ground surface in the past γγγγ z h Preconsolidation stress (pressure) - the maximum effective stress the soil has experienced in the past pc (or σσσσp’) = γγγγ(h+z) OCR = pc/σσσσz’ OCR > 1 Overconsolidated soil OCR = 1 Normally-consolidated soil OCR < 1 Under-consolidated soil
- 32. 32 Pressure, p (log scale) VoidRatio,e pc a b c d e f g αααα αααα Determination of Preconsolidation Stress from Lab Results
- 33. 33 Pressure, p (log scale) VoidRatio,e e0 Field consolidation curve Lab consolidation curve Remolded specimen Consolidation curveDisturbance increases 0.42e0 Effect of Soil Disturbance
- 34. 34 Pressure, p (log scale) VoidRatio,e e0 Virgin consolidation curve Lab consolidation curve 0.42e0 Cc pc=σσσσz’ e - logp Curve for Normally Consolidated Soil Cc = Compression index
- 35. 35 Pressure, p (log scale) VoidRatio,e e0 Virgin consolidation curve Lab consolidation curve 0.42e0 Cc pcσσσσz’ Cr Lab rebound curve e - logp Curve for Overconsolidated Soil Cr = Recompression index
- 36. 36 VoidRatio,e ep ∆∆∆∆e Time, t (log scale) t1 t2 Cαααα=∆∆∆∆e/log(t2/t1) e - logt Curve for Secondary Consolidation
- 37. 37 Typical Compression Indices Cc = 0.1 to 0.8 and Cc = 0.009(LL-10) Cr = Cc/5 to Cc/10 Cαααα/Cc = 0.01 to 0.07 For soils
- 38. 38 Stress, σσσσ’ (log scale) VoidRatio,e pc = σσσσz’ ∆σ∆σ∆σ∆σ Primary Consolidation Settlement of Normally Consolidated Soil σ σ∆+σ + = ' z ' z o c c log e HC S 1 H = Thickness of soil layer
- 39. 39 Primary Consolidation Settlement of Overconsolidated Soil Stress, σσσσ’ (log scale) VoidRatio,e σσσσz’ ∆σ∆σ∆σ∆σ pc σσσσ Cr 1 Stress, σσσσ’ (log scale) VoidRatio,e σσσσz’ ∆σ∆σ∆σ∆σ pc Cr 1 Cc 1 σ σ∆+σ + = ' z ' z o r c log e HC S 1 σ∆+σ + + + = c ' zc o r c p log e HC )OCRlog( e HC S 011
- 40. 40 Rate of Consolidation For U<60% 2 v 100 U 4 T π = ( )U10093307811Tv −−= log.. For U>60% 2 dr v v H tC T = Clay Sand H Hdr
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- 43. 43 Elastic settlement Based on the Theory of Elasticity
- 44. 44 Elastic Settlement of Rectangular footings
- 45. 45 Elastic Settlement on Saturated Clay
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- 54. 54 Elastic Settlement Using the Strain Influence Factor: [Schmertman Method (1978)]
- 55. 55 Elastic Settlement Using the Strain Influence Factor: [Schmertman Method (1978)]
- 56. 56 Elastic Settlement Using the Strain Influence Factor: [Schmertman Method (1978)]
- 57. 57 Elastic Settlement Using the Strain Influence Factor: [Schmertman Method (1978)]
- 58. 58 Elastic Settlement Using the Strain Influence Factor: [Schmertman & Hartman Method (1978)]
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- 60. 60 Procedure for Schmertman Method (1978)
- 61. 61 Procedure for Schmertman Method (1978)
- 62. 62 Procedure for Schmertman Method (1978)
- 63. 63 Procedure for Schmertman Method (1978)
- 64. 64 Notes on Schmertmann Method
- 65. 65 Notes Cont…
- 66. 66 Notes Cont…
- 67. 67 Elastic Settlement Using the Strain Influence Factor: [Schmertman Method (1978)]
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- 76. 76 Primary Consolidation • Expulsion of water from soils accompanied by increase in effective stress and strength • Amount can be reasonably estimated based on lab data, but rate is often poorly estimated
- 77. 77 Consolidation Settlement • This method makes use of the results of the conventional oedometer test where the consolidation parameters of the soil are measured. • To compute the stress changes within the soil mass. The stress changes are computed using a Boussinesqtype approach assuming elasticity. • The important parameter for consolidation settlement calculation is the net effective stress change in the soil. • Usually the settlements are calculated for the soil divided into a number of sub-layers and the final total settlement is the sum of individual sub-layer settlements
- 85. 85 Average Vertical Stress Increases Due to a Rectangularly Loaded Area
- 86. 86 Average Vertical Stress Increases Due to a Rectangularly Loaded Area
- 87. 87 Average Vertical Stress Increases in a given layer
- 88. 88 Secondary Consolidation • At the end of primary settlement, settlement may continue to develop due to the plastic deformation (creep) of the soil. • The stage of consolidation is called secondary consolidation.
- 90. 90 Field Load Test • Ultimate load, allowable load, and settlement can be determined from a field load test (ASTM D- 1194-72). • The test plates are either square (12”or 18”) or circular (12” to 30”)
- 91. 91 Allowable Bearing Capacity for Sandy Soil based on Settlement
- 92. 92 Allowable Bearing Capacity for Sandy Soil based on Settlement
- 93. 93 Reliability of Settlement Computations • The predication is quite satisfactorily in general. • The predication is better for inorganic, insensitive clays than for others. • The time rate of consolidation settlement is not well-predicted.
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- 95. 95 Average Vertical Stress Increases Due to a Rectangularly Loaded Area
- 96. 96 Average Vertical Stress Increases Due to a Rectangularly Loaded Area
- 97. 97 Average Vertical Stress Increases in a given layer