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Geotechnical Engineering-II [Lec #11: Settlement Computation]

Class notes of Geotechnical Engineering course I used to teach at UET Lahore. Feel free to download the slide show.
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Geotechnical Engineering-II [Lec #11: Settlement Computation]

  1. 1. 1 Geotechnical Engineering–II [CE-321] BSc Civil Engineering – 5th Semester by Dr. Muhammad Irfan Assistant Professor Civil Engg. Dept. – UET Lahore Email: mirfan1@msn.com Lecture Handouts: https://groups.google.com/d/forum/geotech-ii_2015session Lecture # 11 11-Oct-2017
  2. 2. 2 FOUNDATION TYPES 1. Shallow Foundations a. D/B ≤ 1 (Terzaghi, 1943); later researchers said D/B can be up to 3-4. b. Depth generally less than 3m 2. Deep Foundations Focus of this course
  3. 3. 3 TYPES OF FOUNDATION FAILURE 1. Due to excessive settlement 2. Due to shear failure in soil Focus of this chapter Shall be discussed in Chapter titled “Bearing Capacity of Soil”
  4. 4. 4 SOIL SETTLEMENT Pisa Tower, Italy The total vertical downward deformation at the surface resulting from the applied load is called settlement.
  5. 5. 5 TYPES OF SOIL SETTLEMENT (A) Types w.r.t. Permanence (i) Permanent/Irreversible Settlement • Caused by sliding/rolling of soil particles under applied stress • Reduction of void ratio • Crushing of soil particles • Consolidation settlement (ii) Temporary Settlement • Settlement due to elastic compression of soil • Generally very small in soils TYPES OF SOIL SETTLEMENT
  6. 6. 6 TYPES OF SOIL SETTLEMENT (B) Types w.r.t. Uniformity (i) Uniform Settlement • All the points settle by equal amount • Generally occur under rigid foundations loaded with uniform pressure and resting over uniform soil • Minimal risk to structural stability • Risk to serviceability (eg. utility lines, etc.) (ii) Differential Settlement • Different parts of the structure settle by different magnitude
  7. 7. 7 (C) Types w.r.t. Mode of Occurrence (i) Immediate/Elastic Settlement: • Caused by elastic deformation of dry/moist/saturated soil • No change in moisture content • Occurs immediately after construction • Computed using elasticity theory • Important for Granular soils (ii) Primary Consolidation Settlement: • Due to expulsion of water from the soil mass • Dissipation of pore pressure => Increase in effective stresses • Important for Inorganic clays (iii) Secondary Consolidation Settlement: • Volume change due to rearrangement of particles • Occurs at constant effective stress (i.e. no drainage) • Important for Organic soils • Similar to creep in concrete TYPES OF SOIL SETTLEMENT
  8. 8. 9 MAGNITUDE OF SETTLEMENT CALCULATION Consolidation Settlement
  9. 9. 11 SETTLEMENT TYPES Si  Granular Soils Time Settlement Sc, Sc(s)  Cohesive Soils Elasticity Theory Consolidation Theory Empirical Correlations
  10. 10. 12 MAGNITUDE OF SETTLEMENT CALCULATION Consolidation Settlement Already covered in Geotech-I Quick Revision in Geotech-II
  11. 11. 13 Before Consolidation Solids Water After Consolidation Soil volume reduction due to expulsion of water upon application of external load/stress. fully saturated soil, so all voids filled with water only (no air) Solids Water CONSOLIDATION OF SOIL Saturated Fine-grained Soil
  12. 12. 14 CONSOLIDATION PARAMETERS Magnitude of consolidation settlement dependent on compressibility of soil (i.e. the stiffness of the spring)  expressed in term of compression index (Cc) Rate of consolidation/settlement dependent on i. permeability, & ii. compressibility of soil.  expressed in term of co-efficient of consolidation (Cv) Quick Revision in Geotech-II
  13. 13. 15 CONSOLIDATION TEST Interpretation of Test Results         VC HT t 2 Magnitude of settlement → compression index (Cc) Rate of consolidation → co-efficient of consolidation (Cv) Time required for consolidation (Consolidation Time) → 1. Time ~ Deformation curve i. Cv (Coefficient of consolidation) 2. Pressure ~ Deformation curve i. Cc (Compression index) ii. Cr (Recompression index) iii. aV (Coefficient of compressibility) iv. mV (Coefficient of volume change) SOIL Porous Stones
  14. 14. 16 CONSOLIDATION TEST Pressure ~ Deformation Curve p e aV    e ~ p plot e p Δe Δp aV = coefficient of compressibility Cc = compression index mV = coefficient of volume change Δe log (p2/p1) e log p 1 2log p p e CC   e ~ log p plot e a m V V   1 Strain p Δe Δp p mV    e e ~ p plot
  15. 15. 17 CLAY 100,000 years ago 80,000 years ago 30,000 years ago 10,000 years ago 5,000 years ago 1,000 years ago Today STRESS HISTORY Normally Consolidated Soil If the present effective stress (σv0’) in the clay is the greatest stress it has ever experienced in its history. i.e., pre-consolidation pressure (σp’) ≈ present effective stress (σv0’) (σp’) ≈  10% of (σv0’) ≈ σVO’
  16. 16. 18 STRESS HISTORY Over Consolidated Soil If the present effective stress (σv0’) in the clay is smaller than the effective stress experienced in the past. i.e., present effective stress (σv0’) < re- consolidation pressure (σp’) σVO’ CLAY 100,000 years ago 80,000 years ago 30,000 years ago ICE AGE 20,000 years ago 18,000 years ago 15,000 years ago 5,000 years ago Today
  17. 17. 19 STRESS HISTORY Over Consolidation Ratio (OCR) v0 p σ' σ' OCR  σv0’= present effective overburden pressure σp’= pre-consolidation pressure (maximum pressure in past) Normally consolidated soils Over-consolidated soils Under-consolidated soils → OCR = 1 → OCR < 1 → OCR > 1 - Under-consolidated soils are the ones which are undergoing consolidation settlement, i.e. the consolidation is not yet complete and the equilibrium has not yet been reached under the overburden load. - Pore water pressure are in excess of hydrostatic pressure.
  18. 18. 20 SETTLEMENT COMPUTATIONS ' '' log vo vo cCe     If the clay is normally consolidated, the entire loading path is along the VCL. initial vo’ eo vf’= vo’+ ’ e final 1 Cc H e e S o c    1 VCL                ' '' log 1 vo vo o c c e C HS   ’vf ' )'( log vo vo C e C      CASE I: ’p ≈ ’vo < ’vf p’
  19. 19. 21 SETTLEMENT COMPUTATIONS If the clay is over-consolidated, and remained so by the end of consolidation. CASE II: ’vo < ’vf < ’p initial vo’ eo vf’= vo’+  e final 1 Cc VCL 1 Cr p’ ' '' log vo vo rCe     H e e S o c    1                ' '' log 1 vo vo o r c e C HS   ’vf ' )'( log vo vo e Cr     
  20. 20. 22 SETTLEMENT COMPUTATIONS If the over-consolidated, soil becomes normally consolidated by the end of consolidation. CASE III: ’vo < ’p < ’vf initial vo’ eo vf’= vo’+  e final 1 Cc VCL 1 Cr p’ ' '' log ' ' log p vo c vo p r CCe       H e e S o c    1                                ' '' log 1 ' ' log 1 p vo o c vo p o r c e C H e C HS     ’vf
  21. 21. 23 CONSOLIDATION – SUMMARY H e e Ssettlement o c    1  = ’ + u         VC HT t 2 %60; 1004 2        ufor u T  %60 );100(log933.0781.1 10   ufor uT AG W H wS S S    S SwS W WAGH e   )( 0  1 2log p p e CC          HHVV mV                ' '' log 1 vo vo o c c e C HS                  ' '' log 1 vo vo o r c e C HS                                 ' '' log 1' ' log 1 p vo o c vo p o r c e C H e C HS     For NCC For OCC If OCC is loaded beyond σp’ )10(009.0  LLCC Cr CC  1.0 Terzaghi & Peck (1948)
  22. 22. 25 CONCLUDED REFERENCE MATERIAL An Introduction to Geotechnical Engineering (2nd Ed.) Robert D. Holtz & William D. Kovacs Chapter #8 & 9

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