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 = 50-75 mm Height = 20-30 mm metal ring (oedometer) field lab

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

Consolidation Test unloading

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

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

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

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

Prefabricated Vertical Drains Installation of PVDs