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

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

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

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

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

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

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

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

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

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

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

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

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

14. Consolidation Test
unloading

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

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

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

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

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

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

21. More to come…

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

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

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

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

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 Installation
Cross section of PVD
Installation
Prefabricated Vertical Drains to Accelerate Consolidation

29. Prefabricated Vertical Drains
Installation of PVDs