Consolidation of Clays

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Consolidation of Clays
N. Sivakugan Duration: 17 minutes

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).

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

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

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

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

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

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

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

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

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

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

allowing full consolidation before next increment q1 q2 H1 eo Ho eo- e1

e – log v’ plot - from the above data loading v’ increases &

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

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

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

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

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

More to come…

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

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’+ 

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’+ 

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’

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