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

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

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

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

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

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

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

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

8 SIVA Copyright©  H -  e Relation saturated clay GL q kPa saturated clay GL q kPa HoHo Time = 0 + e = e o HH Time =  e = e o -  e average vertical strain =

9 SIVA Copyright©  H -  e Relation Consider an element where V s = 1 initially. ee 1 eoeo Time = 0 + Time =   average vertical strain =

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

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

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

13 SIVA Copyright© Consolidation Test H1H1 q1q1 eo- e1eo- e1 eoeo HoHo q2q2 loading in increments allowing full consolidation before next increment

14 SIVA Copyright© Consolidation Test unloading

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

16 SIVA Copyright© Compression and recompression indices log  v ’ void ratio 1 CcCc C c ~ compression index C r ~ recompression index (or swelling index) 1 CrCr 1 CrCr

17 SIVA Copyright©2001 Preconsolidation pressure log  v ’ void ratio p’p’ preconsolidation pressure is the maximum vertical effective stress the soil element has ever been subjected to

18 SIVA Copyright© Virgin Consolidation Line log  v ’ void ratio virgin consolidation line p’p’  vo ’ eoeo e o,  vo ’ original state

19 SIVA Copyright©2001 Overconsolidation ratio (OCR) log  v ’ void ratio virgin consolidation line p’p’  vo ’ eoeo original state Field  vo ’

20 SIVA Copyright©2001 Overconsolidation ratio (OCR) log  v ’ void ratio OCR=1 OCR=2 OCR=13 ~current state VCL Normally consolidated clay Slightly overconsolidated clay Heavily overconsolidated clay

21 SIVA Copyright© More to come…

22 SIVA Copyright©2001 Settlement computations e o,  vo ’, C c, C r,  p ’, m v -oedometer test  =q q kPa H Two different ways to estimate the consolidation settlement: (a) using m v (b) using e-log  v ’ plot settlement = m v  H next slide

23 SIVA Copyright©2001 Settlement computations ~ computing  e using e-log  v ’ plot initial  vo ’ eoeo  vo ’+  ee If the clay is normally consolidated, the entire loading path is along the VCL.

24 SIVA Copyright©2001 Settlement computations ~ computing  e using e-log  v ’ plot  vo ’ initial eoeo  vo ’+  If the clay is overconsolidated, and remains so by the end of consolidation, ee VCL note the use of C r

25 SIVA Copyright©2001 Settlement computations ~ computing  e using e-log  v ’ plot  vo ’ initial eoeo  vo ’+  If an overconsolidated clay becomes normally consolidated by the end of consolidation, VCL p’p’ ee

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

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

28 SIVA Copyright© Preloading Prefabricated Vertical Drains to Accelerate Consolidation Installation Cross section of PVD

29 SIVA Copyright© Prefabricated Vertical Drains Installation of PVDs


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