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**Consolidation of Clays**

N. Sivakugan Duration: 17 minutes

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**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).

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

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

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**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

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**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

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**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

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**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

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**H -e Relation Consider an element where Vs = 1 initially.**

eo Time = 0+ Time = average vertical strain =

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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

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**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

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**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

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**Consolidation Test loading in increments eo Ho eo- e1**

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

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Consolidation Test unloading

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

void ratio loading v’ increases & e decreases unloading v’ decreases & e increases (swelling)

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**Compression and recompression indices**

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

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**Preconsolidation pressure**

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

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**Virgin Consolidation Line**

log v’ void ratio original state virgin consolidation line vo’ eo eo, vo’ p’

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**Overconsolidation ratio (OCR)**

log v’ void ratio original state virgin consolidation line vo’ eo Field vo’ p’

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**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

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More to come…

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**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

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**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’+

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**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’+

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**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’

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

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

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**Preloading Installation**

Cross section of PVD Installation Prefabricated Vertical Drains to Accelerate Consolidation

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**Prefabricated Vertical Drains**

Installation of PVDs

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