Consolidation of Clays

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

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

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

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

5 During consolidation Due to a surcharge q applied at the GL, the stresses and pore pressures are increased at A. q kpa σ u A σ saturated clay GL..and, they vary with time. SIVA Copyright

6 During consolidation σ remains the same (=q) during consolidation. u decreases (due to drainage) while σ increases, transferring the load from water to the soil. q kpa σ u A σ saturated clay GL σ u σ q SIVA Copyright

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

8 H - e Relation average vertical strain = H H o q kpa GL H q kpa GL saturated clay e = e o H o saturated clay e = e o - e Time = 0 + Time = SIVA Copyright

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

10 H - e Relation Equating the two expressions for average vertical strain, consolidation settlement H H o = e 1+ e o change in void ratio initial thickness of clay layer initial void ratio SIVA Copyright

11 Coefficient of volume compressibility ~ denoted by m v ~ is the volumetric strain in a clay element per unit increase in stress no units = change in volume original volume i.e., m v V = V σ kpa -1 or MPa -1 kpa or MPa SIVA Copyright

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

13 Consolidation Test loading in increments allowing full consolidation before next increment q 1 q 2 H 1 H o e o e o - e 1 H1 e = (1 + e H 1 o o ) e 2 = SIVA Copyright

14 Consolidation Test unloading SIVA Copyright

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

16 Compression and recompression indices void ratio C r 1 C c ~ compression index C c C r ~ recompression index (or swelling index) 1 C r 1 log σ v SIVA Copyright

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

Overconsolidation ratio (OCR) e o original state virgin consolidation line

20 Overconsolidation ratio (OCR) VCL ~current state OCR=1 void ratio OCR=13 OCR=13 OCR=2 Normally consolidated clay Heavily overconsolidated clay OCR=2 SIVA Copyright 2001 Slightly overconsolidated clay log σ v

21 More to come SIVA Copyright

22 Settlement computations Two different ways to estimate the consolidation settlement: H q kpa σ=q e o, σ vo, C c, C r, σ p, m v -oedometer test SIVA Copyright 2001 (a) using m v settlement = m v σ H (b) using e-log σ v plot settlement next slide e = H 1 + e o

23 Settlement computations ~ computing e using e-log σ v plot If the clay is normally consolidated, the entire loading path is along the VCL. e o initial e e = C c σ ' + σ ' log vo σ ' vo σ vo σ vo + σ SIVA Copyright 2001

24 Settlement computations ~ computing e using e-log σ v plot If the clay is overconsolidated, and remains so by the end of consolidation, initial e = C r σ ' + σ ' log vo σ ' vo e o e VCL note the use of C r σ vo σ vo + σ SIVA Copyright 2001

25 Settlement computations ~ computing e using e-log σ v plot If an overconsolidated clay becomes normally consolidated by the end of consolidation, initial e = σ ' C log p + C r σ ' vo c log σ vo ' + σ p ' σ ' e o e VCL σ vo SIVA Copyright 2001 σ p σ vo + σ

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

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

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

29 Prefabricated Vertical Drains Installation of PVDs SIVA Copyright

Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay

Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay 46 Module 3: Lecture - 8 on Compressibility and Consolidation Contents Stresses in soil from surface loads; Terzaghi s 1-D consolidation theory; Application in different boundary conditions; Ramp loading;