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Geotechnical Engineering-II [Lec #21: Lateral Earth Pressure)

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 #21: Lateral Earth Pressure)

  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 # 21 22-Nov-2017
  2. 2. 2 EARTH PRESSURE Pressure exerted by soil on structures.
  3. 3. 3 EARTH PRESSURE Pressure exerted by soil on structures. Wall moving towards soil
  4. 4. 4 EARTH PRESSURE – SIGNIFICANCE Purpose of this wall? Main load acting on wall? -- Earth pressure How to determine it?
  5. 5. 5 EARTH PRESSURE – SIGNIFICANCE
  6. 6. 6 WHAT AFFECTS EARTH PRESSURE? -- Soil type Strong soil  less pressure (e.g. OCC soil will have less EP) -- Imposed loading - Self weight of soil (overburden) - Surcharge load -- Soil-structure interaction More the soil-structure friction  less will be earth pressure
  7. 7. 8 v’(or o’) v’ h’h’ Case – I Rigid wall  No movement Earth pressure at rest LATERAL EARTH PRESSURE – CONCEPT z Pressure exerted on wall/structure with its lateral movement restrained How to determine Lateral Earth Pressure ? 𝐾0 = 𝜎ℎ ′ 𝜎𝑣 ′ Coefficient of earth pressure at rest; Using Earth Pressure Coefficients
  8. 8. 9 LATERAL EARTH PRESSURE – CONCEPT v v h Case – II  Wall moving away from soil  Active Earth pressure, Ka  Ka << Ko Ko Kp “Passive State” Failure due to compression “At Rest” - no strain Ka “Active State” Failure due to expansion Tension (-) Compression (+) Earth Pressure “Active State” Failure due to expansion/tension in soil “Passive State” Failure due to compression Case – III  Wall moving towards soil  Passive Earth pressure, Kp  Kp >> Ko
  9. 9. 10 EARTH PRESSURE -- REAL LIFE SCENARIO --
  10. 10. 11 LATERAL EARTH PRESSURE – CONCEPT Typical Values
  11. 11. 12 WALL MOVEMENT TO ACTIVATE ACTIVE/PASSIVE EARTH PRESSURE
  12. 12. 13 EARTH PRESSURE COEFFICIENT AT REST (Ko) Vertical effective stress = ’o = gz Horizontal effective stress = ’h = Kogz For loose coarse-grained soil, KO can be estimated by using the empirical relationship (Jacky, 1944) where f’ = drained friction angle For dense sand backfill, Jacky formula may underestimate the effect of compaction and thus KO. The following formula should be used gd = actual compacted unit wt. of the sand behind the wall gd(min) = dry unit wt. of sand in the loosest state 𝐾0 = 1 − sin 𝜙′
  13. 13. 14 EARTH PRESSURE COEFFICIENT AT REST (Ko) The increase of Ko in dense sands compared to Jacky formula is due to over-consolidation. For fine grained, normally consolidated soils 𝐾0 = 1 − sin 𝜙′ ∙ (𝑂𝐶𝑅)0.5 For OCC
  14. 14. 15 EARTH PRESSURE COEFFICIENT AT REST (Ko)
  15. 15. 16 EARTH PRESSURE ON A WALL AT REST
  16. 16. 17 EARTH PRESSURE ON A WALL AT REST Partially Submerged Soil gsub (or g’) = gsat - gw
  17. 17. 18 EARTH PRESSURE ON A WALL AT REST Partially Submerged Soil
  18. 18. 19 EARTH PRESSURE ON A WALL AT REST Partially Submerged Soil Force per unit length of the wall = Sum of areas of pressure diagram
  19. 19. 20 Practice Problem #1
  20. 20. 24 LATERAL EARTH PRESSURE – Review
  21. 21. 25 CONCLUDED REFERENCE MATERIAL Principles of Geotechnical Engineering – (7th Edition) Braja M. Das Chapter #13 Essentials of Soil Mechanics and Foundations (7th Edition) David F. McCarthy Chapter #17 Geotechnical Engineering – Principles and Practices – (2nd Edition) Coduto, Yueng, and Kitch Chapter #17

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