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1 Consolidation of Clays N. Sivakugan Duration: 17 minutes
SIVA Copyright© 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).
SIVA Copyright© What is Consolidation? This leads to settlements occurring over a long time, which could be several years. time settlement
SIVA Copyright© In granular soils… Granular soils are freely drained, and thus the settlement is instantaneous. time settlement
SIVA Copyright© 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.
SIVA Copyright© 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,
SIVA Copyright© 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
SIVA Copyright© 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 =
SIVA Copyright© H - e Relation Consider an element where V s = 1 initially. ee 1 eoeo Time = 0 + Time = average vertical strain =
SIVA Copyright© 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
SIVA Copyright© 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
SIVA Copyright© Consolidation Test ~ simulation of 1-D field consolidation in lab. field GL lab undisturbed soil specimen Dia = mm Height = mm metal ring (oedometer) porous stone
SIVA Copyright© Consolidation Test H1H1 q1q1 eo- e1eo- e1 eoeo HoHo q2q2 loading in increments allowing full consolidation before next increment
SIVA Copyright© Consolidation Test unloading
SIVA Copyright© e – log v ’ plot log v ’ void ratio loading v ’ increases & e decreases unloading v ’ decreases & e increases (swelling) - from the above data
SIVA Copyright© 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
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
SIVA Copyright© Virgin Consolidation Line log v ’ void ratio virgin consolidation line p’p’ vo ’ eoeo e o, vo ’ original state
SIVA Copyright©2001 Overconsolidation ratio (OCR) log v ’ void ratio virgin consolidation line p’p’ vo ’ eoeo original state Field vo ’
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
SIVA Copyright© More to come…
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
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.
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
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
SIVA Copyright© Preloading Preloading at West Kowloon Expressway, Hong Kong. (5-10 m embankments for 2-5 months)
SIVA Copyright© Preloading Piezometers measure pore pressures and thus indicate when the consolidation is over.
SIVA Copyright© Preloading Prefabricated Vertical Drains to Accelerate Consolidation Installation Cross section of PVD
SIVA Copyright© Prefabricated Vertical Drains Installation of PVDs
Consolidation Oedometer Test CLAY SAMPLE disc-shaped loading piston “floats” water squeezed from sample drains freely into chamber.
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1 Settlement Immediate settlement – Caused by elastic deformation of dry and moist soil without any change in moisture content Primary Consolidation Settlement.
Settlement and Consolidation CHAPTER 4. §4 Settlement and Consolidation § 4.1 General § 4.2 Oedometer test § 4.3 Preconsolidation pressure § 4.4 Consolidation.
Consolidation Theory Examples. Example 1 The Following results were obtained from an oedometer test on a specimen of saturated clay. A layer of this clay.
Consolidation Introduction and andTerminology. Although consolidation was not the culprit here, this is what happens when there’s differential settlement.
Consolidation I (CH11). Consolidation Process by which an in ’ causes a deformation of a soil mass by reducing its e (water seeps out of the voids).
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Log (p) Void Ratio (e) W.T. G.S. sand = 96 pcf clay = 110 pcf w c = ft 4 ft 16 ft PoPo Sand Clay PoPo 1.Soil sample was obtained from the clay.
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