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1 Consolidation of Clays N. Sivakugan Duration: 17 minutes

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SIVA Copyright©2001 2 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).

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SIVA Copyright©2001 3 What is Consolidation? This leads to settlements occurring over a long time, which could be several years. time settlement

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SIVA Copyright©2001 4 In granular soils… Granular soils are freely drained, and thus the settlement is instantaneous. time settlement

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SIVA Copyright©2001 5 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.

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SIVA Copyright©2001 6 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,

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SIVA Copyright©2001 7 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

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SIVA Copyright©2001 8 H - e Relation saturated clay GL q kPa saturated clay GL q kPa HoHo Time = 0 + e = e o HH Time = e = e o - e average vertical strain =

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SIVA Copyright©2001 9 H - e Relation Consider an element where V s = 1 initially. ee 1 eoeo Time = 0 + Time = average vertical strain =

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SIVA Copyright©2001 10 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

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SIVA Copyright©2001 11 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

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SIVA Copyright©2001 12 Consolidation Test ~ simulation of 1-D field consolidation in lab. field GL lab undisturbed soil specimen Dia = 50-75 mm Height = 20-30 mm metal ring (oedometer) porous stone

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SIVA Copyright©2001 13 Consolidation Test H1H1 q1q1 eo- e1eo- e1 eoeo HoHo q2q2 loading in increments allowing full consolidation before next increment

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SIVA Copyright©2001 14 Consolidation Test unloading

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SIVA Copyright©2001 15 e – log v ’ plot log v ’ void ratio loading v ’ increases & e decreases unloading v ’ decreases & e increases (swelling) - from the above data

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SIVA Copyright©2001 16 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

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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

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SIVA Copyright©2001 18 Virgin Consolidation Line log v ’ void ratio virgin consolidation line p’p’ vo ’ eoeo e o, vo ’ original state

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

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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

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SIVA Copyright©2001 21 More to come…

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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

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

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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, ee VCL note the use of C r

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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’ ee

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SIVA Copyright©2001 26 Preloading Preloading at West Kowloon Expressway, Hong Kong. (5-10 m embankments for 2-5 months)

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SIVA Copyright©2001 27 Preloading Piezometers measure pore pressures and thus indicate when the consolidation is over.

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SIVA Copyright©2001 28 Preloading Prefabricated Vertical Drains to Accelerate Consolidation Installation Cross section of PVD

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SIVA Copyright©2001 29 Prefabricated Vertical Drains Installation of PVDs

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