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Geotechnical Engineering-II [Lec #13: Elastic Settlements]

Class notes of Geotechnical Engineering course I used to teach at UET Lahore. Feel free to download the slide show.
Anyone looking to modify these files and use them for their own teaching purposes can contact me directly to get hold of editable version.

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Geotechnical Engineering-II [Lec #13: Elastic Settlements]

  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 # 13 18-Oct-2017
  2. 2. 2 Practice Problem #2 Bore holes at a building site show the following strata (levels in meters measured from ground surface) Top Soil 0 to -1 Sand -1 to -5 (Water table at -3.5) Clay -5 to -9 There is impervious rock below -9 m. The bulk density of the top soil and sand is 19.62 kN/m3 and that of clay is 18.83 kN/m3. A building is constructed on a concrete raft 324 m2 at 3 m below the surface. The total load is 90x103 kN. Assuming a spread of the load of 2 vertical to 1 horizontal, calculate the final settlement. The coefficient of volume compressibility (mV) is 434x10-6 m2/kN. In a consolidation test, a specimen of the clay reached 90% settlement in 4 hours. The specimen was 20 mm thick. Estimate the time in years for the building to reach 90% of its final settlement.
  3. 3. 3 MAGNITUDE OF SETTLEMENT CALCULATION Consolidation Settlement Today’s Discussion
  4. 4. 4 • Caused by elastic deformation of dry/moist/saturated soil • No change in moisture content i.e. “Undrained Settlement” • Occurs immediately after the construction • Important for Granular soil IMMEDIATE (ELASTIC) SETTLEMENT
  5. 5. 5 Main parameters for elastic settlement computation • Modulus of elasticity (Es) • Poisson’s ratio (ν) Determination of Elastic Parameters • Laboratory tests  expensive, time consuming • In-situ tests  expensive, reliability? • Commonly determined through empirical correlations • Use of empirical correlations depend upon limitations of correlations, experience, engineering judgement, etc. ELASTIC PARAMETERS
  6. 6. 6 POISSON’S RATIO Found. Analysis & Design Bowles (5th ed.)
  7. 7. 7 MODULUS OF ELASTICITY Found. Analysis & Design Bowles (5th ed.)
  8. 8. 8 MODULUS OF ELASTICITY Found. Analysis & Design Bowles (5th ed.)
  9. 9. 9 1. Methods Based Upon Theory of Elasticity a. Timoshenko and Goodier (1951) b. Mayne & Poulos (1999) Method 2. Methods Based Upon Strain Influence Factor a. Schmertmann (1978) Method b. Terzaghi et al. (1996) Method 3. Methods Based Upon SPT-N Values a. Modified Mayerhof (1996) Method b. Peck and Bazaraa Method c. Burland and Burbridge (1985) Method 4. Elastic Settlement of Saturated Clays a. Janbu, Bjerrum, and Kjaernsli (1956) Method IMMEDIATE (ELASTIC) SETTLEMENT COMPUTATION Mainly SANDS
  10. 10. 10 ASSUMPTIONS • Strictly applicable to flexible bases on half-space. • The half-space may either be cohesionless materials of any water content or unsaturated cohesive soils. • The soils may either be inorganic or organic; however, if organic, the amount of organic material should be small, because both Es and s are markedly affected by high organic content. • In practice, most foundations are flexible. Even very thick ones deflect when loaded by the superstructure loads. • If the base is rigid, the settlement will be uniform, and the settlement factor IS will be about 7 % less than computed by equations. If footing base is considered rigid, ISR = 0.931IS TIMOSHENKO AND GOODIER (1951) METHOD
  11. 11. 11 Settlement at the corner of uniformly loaded flexible rectangular footing of dimensions B’xL’ from Timoshenko and Goodier (1951) is given as; TIMOSHENKO AND GOODIER (1951) METHOD Found. Analysis & Design Bowles (5th ed.) P-303
  12. 12. 12 IF Square foundation; L/B = 1 Strip foundation; L/B ≥ 5
  13. 13. 13 Settlement at the corner of uniformly loaded flexible rectangular footing of dimensions B’xL’ from Timoshenko and Goodier (1951) is given as; TIMOSHENKO AND GOODIER (1951) METHOD Found. Analysis & Design Bowles (5th ed.) P-303
  14. 14. 14 TIMOSHENKO AND GOODIER (1951) METHOD Found. Analysis & Design Bowles (5th ed.) P-303 H = thickness of compressible layer
  15. 15. 15 ADD a Slide for description of Es i.e. Hard layer for next year. Use Bowles & Das for reference. Also for Round Bases, Convert into Equivalent Square TIMOSHENKO AND GOODIER (1951) METHOD
  16. 16. 16
  17. 17. 17 PRACTICE PROBLEM #3 Estimate the elastic settlement at the center of the raft (or mat) foundation for a building with the given data; qo = 134 kPa; B x L = 33.5 x 39.5 m The strata comprises of a 6.0 m thick dense sand deposit (ES = 42.5 MPa) overlying a hard clay stratum (ES = 60 MPa) extending to a depth of 14.0 m below NSL. A sandstone deposit (ES ≥ 500 MPa) exists below 14.0 m depth. The foundation is placed at a depth of 3.0 m below NSL.
  18. 18. 18 IF Square foundation; L/B = 1 Strip foundation; L/B ≥ 5
  19. 19. 19 PRACTICE PROBLEM #4
  20. 20. 21 IF Square foundation; L/B = 1 Strip foundation; L/B ≥ 5
  21. 21. 22
  22. 22. 23 CONCLUDED REFERENCE MATERIAL Foundation Analysis and Design (5th Ed.) Joseph E. Bowles Chapter #5 Principles of Geotechnical Engineering (7th Ed.) Braja M. Das Chapter #11 Essentials of Soil Mechanics and Foundations (7th Ed.) David F. McCarthy Chapter #10 (Schmertmann Method) (Timoshenko & Goodier Method) (Modified Mayerhof Method)

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