Lecture 05 Soil Compression - KOCWcontents.kocw.net/KOCW/document/2014/hanyang/parkduhee/... ·...

CIE_3008 토질역학 및 실험 Lec.5 Soil Compression 1

Lecture 05Soil Compression

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Soil CompressionSoil compression: what is it?

Total volume decreases what is reduced?

Soil compression: what causes it? Stress increase and/or self internal reaggrangement of soil particles

Types of soil compressionElastic settlement (also termed immediate settlement)

Sands

Consolidation Clays

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Elastic settlementElastic settlement

Caused by the stress incrementCauses no change in the pore pressure pore water flows out!Occurs in dry soils and saturated soils with high permeabilityCan solve with elastic stress – strain relationshipCan solve for the settlement if stress increments are known (e.g. provide in previous lecture)Integrate over the depth of influence

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Elastic settlementA special case: 1D compression

Radial displacement not allowed Displacement only allowed in vertical direction

Determine vertical stress – strain relationshipDetermine vertical stress – radial stress relationship

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ConsolidationConsolidation: used to describe time dependent settlement that occurs in claysRemember in sands, compression occurs due to air / pore water flowing out of the soil structureIn clays, due to very low permeability, it takes a lot of time for the pore water to flow outThink of the case belowIf in sand, the pore water is

not allowed to drain,would settlement occur?

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ConsolidationWhen valve is closed

When valve is open

Note:

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Time dependent variation of stresses and pressure

isochrone

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1D Laboratory consolidation test - OedometerTubular steel mold size: 64 mm in diameter and 25 mm thickTwo porous stones at the boundariesApply load in increments (usually loaded for 24 hrs)

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1D Laboratory consolidation test - OedometerDeveloping void ratio – vertical stress plot

Step 1: calculate Hs

Step 2: calculate Hv

Calculate initial void ratio e0

Calculate subsequent changes in void ratios and effective vertical stresses

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Normally vs. Overconsolidated clays

Effect of Stress history

Overconsolidation ratio

More common to use σ’p

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Test Results

B

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Methods to determine ’p1. Casagrande

1. Choose the point of maximum curvature (point A)2. Draw horizontal line from point A3. Draw a line tangent to point A4. Bisect the angle made by steps 2 and 35. Extend the straight line portion of the virgin curveThe point of intersection of these two lines is the ’p

A

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Field curve Schmertmann (1953)

1. Select ’v02. Calculate e0. Draw a horizontal line from e0. Select Point A.3. From the point 0.42e0, draw a horizontal line. The pointthat intersects the lab virgin curve is point B.4. Connect points A and B. The line is the field virgin compression line.

Due to disturbance, the lab compression curve has a slope less than that of the field virgin curve.

NC Clay

’v0

e0

0.42e0

A

B

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Field curve Schmertmann (1953)

1. Select ’p using Casgrande’s method2. Calculate e0. Draw a horizontal line from e0 up to ’v0. Then drawA line parallel to the lab rebounding curve.Select Point A. This the field recompression curve.3. From the point 0.42e0, draw a horizontal line. The pointthat intersects the lab virgin curve is point B.4. Connect points A and B. The line is the field virgin compression line.

Due to disturbance, the lab compression curve has a slope less than that of the field virgin curve.

OC Clay

’p

e0

0.42e0

A

B

Lab rebounding curve

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Settlement PredictionsN.C. Clays

S Cc

1 eo H log

zf

z0

Cc: compression indexav: coefficient of compressibilitymv: coefficient of volume change

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Settlement PredictionsN.C. Clays

V=1+eo

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Settlement PredictionsO.C. Clays…… Case I

r zf

0 z0

CS H log1 e

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Settlement PredictionsO.C. Clays…… Case II

r c c zf

0 z0 0 c

C CS H log H log1 e 1 e

Final

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Cc and Cr

Undisturbed ClayCc = 0.009(LL-10)

Disturbed ClayCc = 0.007(LL-10)

Cr = 1/5 – 1/10 Cc

Cr

Cc

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Secondary compression

0

0

/ log1

c rs

p

C C C tS He t

settlement

u=0

0

0

log1s

p

C tS He t

logeC

t

C/Cc = constantC/Cc = 0.01 – 0.07

average: 0.04

0

0

/ log1

c cs

p

C C C tS He t

w0

C

Mesri, 1973Inorganic clays and silts: 0.04 0.01Organic clays: 0.05 0.01Peat : 0.06 0.01

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Meaning of Consolidation Curve

e1

2

v

EOP curve (End of Primary Curve)

12

settlement

u=0

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Uniqueness of EOP curve

e

v

EOP curve (End of Primary Curve)

tp for laboratory test is very shorttp in the field is very longAre the curves from laboratory identical to the curve from field?

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Secondary compressionIf tp is small, then secondary compression can be large.Suppose the clay is NC.

How can we reduce the secondary compression?

Surcharge

QuizHow can we plot the secondary compression in an EOP curve?

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Preconsolidation Pressure’p Largest pressure that the soil has experienced? Is it true?

Pressure at which major structural changes including the break-down of Interparticle bonds and interparticle displacement begin to occur.

-Boundary between stiff and soft deformation response of a soil to loading-Expressed as the overconsolidation ratio (OCR)-Mostly 1.2 – 3-Mechanism contributing to ’p

-Geological loading and unloading-Freezing and thawing-Secondary compression-Weight of ice

Sketch the preconsolidation pressure of a soil that has undergone secondary compression