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Consolidation of Clays

Consolidation of Clays. N. Sivakugan. Duration: 17 minutes. GL. saturated clay. What is Consolidation?. When a saturated clay is loaded externally,. the water is squeezed out of the clay over a long time (due to low permeability of the clay). settlement. time. What is Consolidation?.

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Consolidation of Clays

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  1. Consolidation of Clays N. Sivakugan Duration: 17 minutes

  2. GL saturated clay What is Consolidation? When a saturated clay is loaded externally, the water is squeezed out of the clay over a long time (due to low permeability of the clay).

  3. settlement time What is Consolidation? This leads to settlements occurring over a long time, which could be several years.

  4. settlement time In granular soils… Granular soils are freely drained, and thus the settlement is instantaneous.

  5. q kPa GL  u ’ A saturated clay During consolidation… Due to a surcharge q applied at the GL, the stresses and pore pressures are increased at A. ..and, they vary with time.

  6.  u ’ q kPa GL  u ’ A saturated clay q During consolidation…  remains the same (=q) during consolidation. u decreases (due to drainage) while ’ increases, transferring the load from water to the soil.

  7. GL q kPa saturated clay water squeezed out One Dimensional Consolidation ~ drainage and deformations are vertical (none laterally) ~ a simplification for solving consolidation problems reasonable simplification if the surcharge is of large lateral extent

  8. GL GL q kPa q kPa H saturated clay saturated clay Ho e = eo e = eo - e Time = 0+ Time =  H -e Relation average vertical strain =

  9. eo 1 H -e Relation Consider an element where Vs = 1 initially. e Time = 0+ Time =  average vertical strain =

  10. change in void ratio consolidation settlement initial void ratio initial thickness of clay layer H -e Relation Equating the two expressions for average vertical strain,

  11. no units kPa-1 or MPa-1 kPa or MPa Coefficient of volume compressibility ~ denoted by mv ~ is the volumetric strain in a clay element per unit increase in stress i.e.,

  12. GL porous stone undisturbed soil specimen Dia = 50-75 mm Height = 20-30 mm metal ring (oedometer) Consolidation Test ~ simulation of 1-D field consolidation in lab. field lab

  13. q1 q2 H1 Ho eo Consolidation Test loading in increments allowing full consolidation before next increment eo-e1

  14. Consolidation Test unloading

  15. void ratio log v’ e – log v’ plot - from the above data loading v’ increases & e decreases unloading v’ decreases & e increases (swelling)

  16. Cr 1 void ratio Cc 1 Cr 1 log v’ Compression and recompression indices Cc ~ compression index Cr ~ recompression index (or swelling index)

  17. void ratio log v’ preconsolidation pressure Preconsolidation pressure is the maximum vertical effective stress the soil element has ever been subjected to p’

  18. original state virgin consolidation line eo void ratio log v’ vo’ Virgin Consolidation Line eo, vo’ p’

  19. original state virgin consolidation line eo void ratio vo’ Field log v’ vo’ Overconsolidation ratio (OCR) p’

  20. VCL ~current state OCR=1 void ratio OCR=2 OCR=2 OCR=13 OCR=13 Slightly overconsolidated clay log v’ Overconsolidation ratio (OCR) Normally consolidated clay Heavily overconsolidated clay

  21. More to come…

  22. q kPa =q H next slide Settlement computations Two different ways to estimate the consolidation settlement: (a) using mv settlement = mv H (b) using e-log v’ plot eo, vo’, Cc, Cr, p’, mv -oedometer test

  23. initial eo e vo’ vo’+  Settlement computations ~ computing e using e-log v’ plot If the clay is normally consolidated, the entire loading path is along the VCL.

  24. initial eo e note the use of Cr VCL vo’ vo’+  Settlement computations ~ computing e using e-log v’ plot If the clay is overconsolidated, and remains so by the end of consolidation,

  25. initial eo e VCL vo’ p’ vo’+  Settlement computations ~ computing e using e-log v’ plot If an overconsolidated clay becomes normally consolidated by the end of consolidation,

  26. Preloading Preloading at West Kowloon Expressway, Hong Kong.(5-10 m embankments for 2-5 months)

  27. Preloading Piezometers measure pore pressures and thus indicate when the consolidation is over.

  28. Preloading Cross section of PVD Installation Prefabricated Vertical Drains to Accelerate Consolidation

  29. Prefabricated Vertical Drains Installation of PVDs

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