1D CONSOLIDATION Part 1 D A Cameron Barnes Chapter 6

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1D CONSOLIDATION Part 1D A Cameron Barnes
Chapter 6
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SCOPE

• 1D consolidation testing
• Consolidation parameters
• Settlement
• Rate of settlement
• Pre-consolidation pressure
• Excess pwp
• 1D consolidation
• Isochrones of excess pwp
• Geotechnical Practice!

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Introduction

• Non-recoverable or plastic settlement when loaded
• Saturated soils and time dependent consolidation
• REVISION
• The phases of soil
• gas (or air), water and solid

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Consider the response of DRY SOIL to vertical load

• Upon loading, particles pack down into closer
  contact
• reducing the air voids
• occurs immediately
• Upon unloading, some settlement may be recovered
• grain re-arrangement may minimize rebound

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Dry soil response
Sample Height (mm)
Applied Pressure (kPa)
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WET SOIL RESPONSE

• No air voids in the sample
• External loading cant produce any immediate
  change in volume of the soil!
• undrained loading of a saturated soil, or
  short term loading response
• HOWEVER with time, the water can
  escape
• soil drainage

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UNDRAINED LOADING

• Drainage is very slow
• permeability, k ? m/sec, Darcys Law? q kiA
• Settlement can only occur in the short term under
  external vertical pressure, if the soil can
  expand laterally
• (since ?V 0)
• In 1-D loading, the soil is completely restrained
  laterally

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Conditions for Undrained Loading Response

• Saturated soil
• Low permeability soil
• NOT free draining soils
• (sands and gravels)
• BUT fine-grained soils
• (silts and clays)

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The Long Term Response (constant load)

• Despite the low permeability, the soil will
  drain, over months, or maybe years
• reduction in void ratio, e
• Settlement!
• ?Vsoil ?Vwater
• sc ?Vw 1D consolidation

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Consolidation - definition

• Long-term settlement under constant external
  pressure
• through drainage of some pore water
• Equilibrium is reached, when the increase of
  effective stress in the soil the increase of
  total stress arising from the applied surface
  pressure, i.e.
• ??v ??'v

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CONSOLIDATION STRESS STATE

• EXAMPLE large loaded area, stress at 2 m depth
• ?v 35 kPa, due to soil dead weight of soil
• The pwp before load applied was uo 15 kPa
• the initial pwp, due to a water table the
  steady state pwp, or uss
• ? initial ?v? was ?vo? 20 kPa

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Example (contd)

• Applied surface pressure q 80 kPa
• So, the extra vertical total stress at 2 m for a
  large uniformly loaded area,
• ??v ? 80 kPa
• (Remember last lecture?)
• What initially supports this extra pressure?

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SOIL soil solids wateranalogy spring
water

• No load
• No drainage
• No volume change

• Load, but no drainage
• No volume change
• No stress in spring

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What takes the external pressure?
Ans the water! i.e. pwp rises above the steady
state pwp by the level of the extra stress caused
by the external load
Additional, or, EXCESS pwp, ue
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Tap open Drainage

• Load
• Slow drainage
• Small volume change
• Spring stressed!

• Equilibrium
• No more loss of water
• Consolidation ended
• u uss

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Equilibrium?
When the pwp returns to what it was before
external pressure was applied The spring then
takes all the extra stress, ??v
EXCESS pwp, ue, is lost through drainage
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Back to the Example

• ?vo 35 kPa
• uo 15 kPa
• ?vo? 20 kPa
• ??v ? 80 kPa (external loading)
• ue ? 80 kPa ?u (1D)
• ?v? _at_ t 1 sec?
  (35 80) (15 80)
  20 kPa
• ?v? _at_ t ?
  (35 80) (15)
• 100 kPa

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In Summary

• Consolidation in saturated, low permeability
  soils, under external loading, is about building
  up the effective vertical stress by dissipating
  the initial excess water pressure in the soil
  through drainage
• - or shedding of load back to the soil skeleton,
  so that ?v? ?v

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End Note

• A consolidated soil is a physically improved
  soil!
• Less void space
• Greater contact between soil particles
• Stronger and stiffer!

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Example of Consolidation Settlement

• In the 1960s
• Launceston levee bank
• Tamar River
• Design height - 3 m
• During construction, settlements were obvious

• levee bank at 2 m height
• after one year had reduced to 1.2 m
• 40 settlement!
• unable to reach the design height

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Part 2 Testing for Consolidation Parameters
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The 1-D Consolidation Test
Water level
Seating
Specimens ? 20mm thick, 50-75 mm dia. Loading
by a lever system hanging weights
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Just as load is applied,

• What is the corresponding pwp
• in the sample?
• Ans u uss ueo
• Since uss 0
• and ueo ?v the external pressure
• ? u ?v

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• Small seating pressure applied
• Initial swell or settlement ?
  saturation?
• Dial gauge re-zeroedand full pressure applied
• Height change monitored with time until stabilized

Ho
At equilibrium, u ue 0
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Void ratio and 1D Strain
Strain,
Void Ratio,
But for 1D,
Saturated soil, no air
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1D Consolidation
- 1D only
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Time, t, after load application but before
equilibrium

• Intermediate level of pore water pressure
• u 0 at drainage surfaces, any time!
• soil drains though top or bottom face of sample
• Maximum path length?
• h H/2

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Intermediate pwp distribution, i.e. before
equilibrium is reached

x 0 kPa
x ut lt ueo
x 0 kPa
Drainage faces, u 0
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Information from testing

• Each stage
• Time rate of consolidation
• All loading stages
• Compressibility of the soil
• Pre-consolidation history of the soil
• Unloading stages
• rebound or swelling index

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Time rate of consolidation
Height (mm)
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Stage 1, 12.5 to 25 kPa
Stage 2, 25 to 50 kPa
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100
1000
1
TIME (log units)
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coefficient of consolidation

• Is estimated from plots of settlement against
  EITHER
• SQRT(time)
• LOG10 (time)
• AND using Terzaghis 1D Consolidation theory

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Compressibility of the soil all stages
Height (mm)
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25 kPa
100 kPa
mv/Ho
200 kPa
1
15
10
500
??v at end of test (kPa)
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Terzaghis coefficient of volume compressibility,
mv
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Compressibility Index, Cc
Void ratio, e
log10??v at end of test (kPa)
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Compressibility Index, Cc
Definition
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Preconsolidation pressure
Void ratio, e
OC
NC
NCL or virgin consolidation line
log10??v at end of test
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Over Consolidation Ratio (OCR)

• The ratio of the maximum past effective stress
• to the current effective stress
• Casagrande construction demonstrated in
  practical

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Swelling index
Void ratio, e

log10??v at end of test
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Summary

• CONSOLIDATION TESTING
• provides
• Pre-consolidation ratio and hence OCR
• Compressibility
• from which estimates of settlements may be made
• The time rate of settlement