Strain softening.pdf

7/27/2019 Strain softening.pdf

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:Development & Innovation inDevelopment & Innovation inGeotechnical Research: A FewGeotechnical Research: A Few

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ExamplesExamples

06 Ap ril 2010 at06 April 2010 at HohaiHohai UniversityUniversity

Chu JianChu JianNanyang Technological University, Singapore

Outline

Present state and future needs

Common approaches for doing somethingdifferent

Exam les in fundamental research

Examples in technology development

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Present state and future need

Soil mechanics has not fully developed into aproper branch of science yet

There are new demands for new knowledge tosupport new development and new challenges

R&D works are still carried out by adoptingtraditional geotechnical engineering approaches

Need a multiple disciplinary approach forinnovation or development of new knowledge

New emphasis on sustainability development

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Approaches

Challenge established Be critical you must have sound

fundamentals ec a ternat ves

Be innovative Merge different technologies Be open minded

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STRAIN SOFTENING

Example 1: Challenge established

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Deviatorstress

Same 3

--Old conceptOld concept

Drainedbehaviour ofsand in triaxial

Dense sand

Loose sand

Strain softening

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Axial strain%

Volumetricstrain + compression

--dilation


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R = 1/3

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What it should be: There is no strain softening inThere is no strain softening intraixial CD tests even for dense sand.traixial CD tests even for dense sand.

er u an o(GTJ, 1993)

Is there strain softening?

Strain softening is referred to as a behaviour wherethe shear resistance (or shear stress) reduces withcontinuous development of plastic shear strains.

So we cannot use stress path tests to study strain.

Can we do strain path tests? How?

Can strain softening be observed in strain pathtests?

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Strain path tests

One way to do strain path test is to control the strainincremental ratio, e.g.,

It also offers a way to model drainage conditionsother than drained or undrained

dv/d1

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dv/d1

> 0, Compression

= 0, Undrained

< 0, Dilation

StrainStrainpathpath

testingtesting

Depending on thestrain path dv/d1,dense sanddense sand canbehave like loosesand.

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dv/ d1 = -0.67on dense sand

dv/d1 = -0.11 fordense sanddense sand

Strain softeningis controlled bystrain path

Strain softening in strain path testingStrain softening in strain path testing

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-1.0

-0.5

0.0

0.5

1.0

0.6 0.7 0.8 0.9 1.0

Void Ratio, ec

d/

d1

StrainHardening

StrainSoftening

Boundary

CriticalPoint

C0ecr=0.884

dv/d1 =0.0

Hardening Region

Softening Region

C1

C2

0.68

-0.4

pc' = 200 kPa

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Softening softeningSoftening softeningssurfaceurface

- .5

-1.5

-1.0

-0.5

0.0

0.5

1.0

0 200 400 600 800 1000

pc' (kPa)

d/

d1

StrainHardening

StrainSoftening

Boundary

Medium Dense Sand

(ec = 0.68 0.71)

HardeningRegion

SofteningRegion


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Exam le2: Chal len e establ ished

EFFECT OF TOTAL STRESSPATH ON UNDRAINEDBEHAVIOUR

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ntroduction

Type Axial stress, a Lateral stress, r dq/dp

Axial compression (AC) da >0 dr= 0 3 0

Lateral extension (LE) da = 0 dr< 0 -3/2 0

Axial extension (AE) da < 0 dr= 0 3 90

L at er al co mp re ss io n (L C) da = 0 dr> 0 -3/2 90

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Bishop and Wesley (1976

AE and LC Results on very looseMT specimens

Why? How to model?

Three possible reasons for the test results:

Wrong testing data

The effective stress principle may not be valid

There is no unique relationship between strain pathan e ec ve s ress pa , .e., o r a g v en s ra n pa ,the resulting stress path can be different (althoughthe asymptotic behaviour is still path dependent).

If so, what are the factors causing the differences?

How to model it using a constitutive model?


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Example 3: Be critical

INCREASE IN UNDRAINEDSHEAR STRENGTH OF SOILDUE TO CONSOLIDATION

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Methods of calculation

Method 1:

Method 2:

where:

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Two Sets of Failure Equations

f= c + (nf u) tanFor effective stress analysis:

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or o a s ress ana ys s:

f= cu + nftanuc & , or cu (& u=0) are shear strengthparameters of soil and need to be determined by either

lab or in-situ tests

Alternative method

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(cu/ v0 )OC = (cu/ v0 )NCOCRm

We can establish the above relationship based on the FVTbefore consolidation.

cu = (c/p)v

Example

uh

+ ue0

ue

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ueue

0

2

4

6

8

10

12Ele

vation(m)

Initial

30 days60 days

90 days

uo(z)

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14

16

18

20

-100 -50 0 50 100 150 200 250

Pore water pressure (kPa)

us(z)

cu = 75 x 0.22 = 16.5 kPa

v = 75 kPa


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EARTH PRESSURECALCULATION

Example 4: Check alternatives

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Retaining wall for layeredsoil

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Earth pressure calculation

Use = 25o for clayKa = 0.4D

= Kav = 29 kPa

E = Kav

= 42 kPa

Using cu: Pa=279 kN/m

D

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uE = w w = a

E

uEUsing :Pa = 329 kN/m

Pa can be even bigger!

or smaller

with drains!

BIOCEMENT & MICROBIALGEOTECHNOLOGY

Example 5: Be innovative

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Biocement

Sand grain

Slime bonding

Sand grain

Sand grain

Slime bonding

Sand grain

Scanning ElectronMicrograph (SEM) to showthe mineralization of calciteonto sandy grains.

Bonding of sandgrains by slime

Microbial Geotechnology

is a new branch of Geotechnical Engineeringaiming to improve the mechanical propertiesof soil so that it will be more suitable for

construction, environmental purposes, aswell as for ddisaster mitigation and coastalmanagement.


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Two Approaches

Bioclogging is the production of pore-filling

materials through microbial activity so thatthe porosity and hydraulic conductivity ofsoil can be reduced.

Biocementation is generation of particle-binding materials through microbial so thatthe strength and stiffness of soil can beincreased.

Some methods exert combined effects of thetwo approaches.

Applications

Bioclogging:

To form curtains to reduce the mitigation of

pollutants in soils To prevent piping of earth dams and dikes

To mitigate reservoir leakage

Biocementation:

To control erosion

To reduce liquefaction potential

To enhance stability of slopes and dams

To increase bearing capacity of foundations

Advantages of biocement It consumes much less energy and is more

environmentally friendly, as biocement made of naturally occurring microorganisms and could be usedto replace energy intensive cement and other chemicalproducts;

The construction processes can be much simplified, asthe biocement can be used in either liquid or powderform and the microorganisms can reproduce andspread themselves in-situ without using intensivemechanical mixing;

It is much more cost effective, as the biocement costsmuch less to produce and the construction processesis simpler.

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Biogrouts

The possible biogrouting methods include:

Ferrous/ferric- containing solution produced byiron-reducing bacteria from iron ore

xopo ysacc ar es pro uce y o gotrop cbacteria

Conventional biogrout containing calciumchloride, urea, and urease-producing bacteria

Increase in strength of sand bybiocementation

800

1000

1200

1400

1600

essiveStrength(kPa

0

200

400

600

0 2 4 6 8 10 1 2 14

Mass CaCl2/Mass Sand (%)

UnconfinedCompr

Wet Samples Dry Samples

using CaCO3 precipitation method

Pictures of the samples

Sand columns treated by microbial polysaccharides (left)and by ferrous salts produced by iron-reducing bacteriafrom iron ore (right)


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Hydraulic conductivity(Fe(OH)2 precipitation method)

Application-1: Water pond indesert

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ApplicationApplication--2: Mitigation of2: Mitigation ofliquefaction damagesliquefaction damages

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Reclimed sand

TA-102

Improved byvibroflotation method

7m

5m

0m

-10m

-20m

-30m

10m

Alluvialclay

Diluvialgravel

0m 50m 100m

cm64.av =S

The effect of partial saturation

Recent studies have shown that liquefactionpotential of sand can be largely reduced byintroducing a small portion of gas into the soil.

The presence of gas bubbles could reduce thepore water pressure generated, and henceimprove the stability against liquefaction.

One of the most convenient way to introducegas bubbles in sand is to use micro-organisms.

Denitrification is one of the processes adoptedby He Jia.

Denitrification process Denitri fying bacteria are used to produce nitrogen gas

from nitrate. Denitri fying bacteria are heterotrophicanaerobic microbes. The reaction equation is:

2 5 3 2 2 25 C H O H + 1 2 K N O = 6 N + 1 0 C O + 9 H O + 1 2 K O H

,chemicals need to be add into the media for the growthof the bacteria.

In both cultivation and sampling stage, anaerobiccondition should be ensured.

In both cultivation and sampling stage, one batch of cultivation lasts two days.

Use gas bubbles toincrease resistanceto liquefaction ofloose sand

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No morecompaction?


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This could be a result of bacteria effect

Microbiologically-influenced corrosion (MIC)

Pigeon poo blamed for deadly Minnesota bridge collapse.

A bridge collapse in America, which killed 13 people, has been

blamed on a build of pigeons' poo. Experts say that the birds'

droppings deposited over the bridge's framework helped the

steel beams to rust faster through bacterial formation ofammonium and its bacterial oxidation to nitric acid.

NH4+ + 2 O2 NO3- + 2H+ + H2O

DailyMail,08/27/2007Photo:AaronBecker

UNDERWATER CITY

Example 6: Be innovative

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Reclamation in deeper water

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Objectives

To develop a new space creation approach NEUSpace (NEwUnderwater Space) to make use of the sea space to constructunderwater infrastructure and at the same time use the top-side ofthe infrastructures as reclaimed land.


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Underwater City built using seawalls A underwater city in Bulgaria. Diameter = 459 m and depth = 22 mhttp://www.techeblog.com/index.php/tech-gadget/underwater-city)

Underwater hotels in Dubai(http://weburbanist.com/wp-content/uploads/2007/11/underwater-hotel-3d-diagram.jpg

Suction caisson method

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53After Anderson 2005

Construction

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Method forspace creation

Underwater structure Cavern ReclaimedLand for oil

tanks

Constructioncost S$/ m3

storage space

$82 (by assuming the total cost is 2times of material cost)

$11 to $34 /m3 (after subtractingthe cost for reclamation the land on top

$242(actual costin Spore)

$600 to $900(actual cost in

HK)

Cost Estimation for Oil Storage

SOIL IMPROVEMENTMETHODS

Example 7: Merge different techn ologies

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Use of jet-grouting layer forexcavation in soft clay

NicollHighwayfailureinSingapore

Jet mechanical mixing (JMM)method

Itcombinesjetgroutingwithcementmixingtoforagroutslabatthebottomandcement

mixedpileontop(OsborneandNg08)

JMM application in deepexcavation

ReconstructionoftheNicollHighwayStationinSingapore(OsborneandNg08)

Hybrid or Bi-modulus method

Ston e c olum ns top T SM dr il li ng toolCMC bottomCMC displacement aurger Stone columns top TSM drilling toolCMC bottom Stone columns top TSM drilling toolCMC bottomCMC displacement aurger

Thelowerpartofthecolumnisperformedbycontrolledmoduluscolumns(CMC)andtheupperpartbystonecolumns.

Peat


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USE OF PLASTIC WASTE

Example 8: Be open minded

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Can PVD be recycled?

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Plastic + Soil Specimens Unconfined compressive strength

Pros & Cons of using plastic waste

It is not cost effective to remove PVDs at themoment

It is not cost-effective to use plastic waste as

a construction materials However, the method offers a better solution

for fast repair of runways or roads.

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Strain softening.pdf