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岩土研究的发展与创新岩土研究的发展与创新::几个例子几个例子Development & Innovation in Development & Innovation in Geotechnical Research: A Few Geotechnical Research: A Few

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ExamplesExamples

06 April 2010 at 06 April 2010 at HohaiHohai UniversityUniversity

楚剑楚剑 Chu JianChu JianNanyang Technological University, Singapore新加坡南洋理工大学

Outline

• Present state and future needs• Common approaches for doing something

different• Examples in fundamental researchExamples in fundamental research• Examples in technology development

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Present state and future need

• Soil mechanics has not fully developed into a proper branch of science yet

• There are new demands for new knowledge to support new development and new challenges pp p g

• R&D works are still carried out by adopting traditional geotechnical engineering approaches

• Need a multiple disciplinary approach for innovation or development of new knowledge

• New emphasis on sustainability development

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Approaches

• Challenge established 挑战现有理论

• Be critical – you must have sound fundamentals 大胆怀异

Ch k lt ti 尝试不同方法• Check alternatives 尝试不同方法

• Be innovative勇于创新

• Merge different technologies 技术整合

• Be open minded 敢于接受

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STRAIN SOFTENINGExample 1: Challenge established

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Dev

iato

r str

ess

Same σ3’

--Old conceptOld conceptDrained behaviour of sand in triaxial tests

Dense sand

Loose sand

Strain softening

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Axial strain%

Volu

met

ric s

trai

n

tests

老教课书中的应变软化

+ compression

-- dilation

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Aft Ch d L

R = σ1’/σ3’

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

三轴排水试验下没有应变软化三轴排水试验下没有应变软化

After Chu and Lo (GTJ, 1993)

Is there strain softening?

• Strain softening is referred to as a behaviour where the shear resistance (or shear stress) reduces with continuous development of plastic shear strains.

• So we cannot use stress path tests to study strain softeningsoftening.

• Can we do strain path tests? How? • Can strain softening be observed in strain path

tests?

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Strain path tests

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

• It also offers a way to model drainage conditions other than drained or undrained

dεv/dε1

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dεv/dε1

> 0, Compression

= 0, Undrained

< 0, Dilation

Strain Strain path path

testingtesting

Depending on the strain path dεv/dε1, dense sanddense sand can behave like loose sand.

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dεv/ dε1 = -0.67 on dense sand

dεv/dε1 = -0.11 for dense sanddense sand

Strain softening is controlled by strain path

Strain softening in strain path testingStrain softening in strain path testing

1 5

-1.0

-0.5

0.0

0.5

1.00.6 0.7 0.8 0.9 1.0

Void Ratio, ec

d εν /

dε 1

Strain HardeningStrain SofteningBoundaryCritical Point

C0ecr = 0.884

dεv/dε1 = 0.0

Hardening Region

Softening Region

C1

C2

0.68

-0.4pc' = 200 kPa

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Softening softeningSoftening softeningssurfaceurface

-1.5

-1.5

-1.0

-0.5

0.0

0.5

1.00 200 400 600 800 1000

pc' (kPa)

d εν /

dε 1

Strain HardeningStrain SofteningBoundary

Medium Dense Sand(ec = 0.68 − 0.71)

Har

deni

ng R

egio

nSo

fteni

ng R

egio

n

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Example 2: Challenge established

EFFECT OF TOTAL STRESS PATH ON UNDRAINEDBEHAVIOUR

Example 2: Challenge established

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Intr

od

uct

ion

I

Type Axial stress, σa Lateral stress, σr dq/dp α

Axial compression (AC) dσa >0 dσr = 0 3 0

Lateral extension (LE) dσa = 0 dσr < 0 -3/2 0

Axial extension (AE) dσa < 0 dσr = 0 3 90

Lateral compression (LC) dσa = 0 dσr > 0 -3/2 90

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Bishop and Wesley (1976）

AE and LC Results on very loose MT 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 path

d ( ff ti ) t th i f i t i thand (effective) stress path, i.e., for a given strain path, the resulting stress path can be different (although the 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 SOIL DUE TO CONSOLIDATION

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Methods of calculation

Method 1:

Method 2:

where:

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Two Sets of Failure Equations

τf = c’ + (σnf – u) tanφ’ For effective stress analysis:

F t t l t l i

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For total stress analysis:τf = cu + σnf tanφu

c’ & φ’, or cu (& φu=0) are shear strength parameters of soil and need to be determined by either

lab or in-situ tests

Alternative method

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(cu/ σv0’ )OC = (cu/ σv0’ )NCOCRm

We can establish the above relationship based on the FVT before consolidation.

Δcu = (c/p’)Δσv

Example

uh + ue0

ue

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ueue

0

2

4

6

8

10

12Ele

vatio

n (m

)

Initial30 days60 days90 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)

Δcu = 75 x 0.22 = 16.5 kPaΔσv´ = 75 kPa

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EARTH PRESSURE CALCULATION

Example 4: Check alternatives

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Retaining wall for layered soil

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Earth pressure calculation

Use φ’ = 25o for clayKa = 0.4σD

’ = Kaσv’ = 29 kPa

σE’ = Kaσv

’ = 42 kPau = γ h = 60 kPa

Using cu: Pa=279 kN/m

σD’

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uE = γwhw = 60 kPa

σE’

uEUsing φ’: Pa = 329 kN/m

Pa can be even bigger!

or smaller with drains!

BIOCEMENT & MICROBIAL GEOTECHNOLOGY

Example 5: Be innovative

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Biocement

Sand grain

Slime bonding

Sand grain

Sand grain

Slime bonding

Sand grain

Scanning Electron Micrograph (SEM) to show the mineralization of calcite onto sandy grains.

Bonding of sand grains by slime

Microbial Geotechnology

• is a new branch of Geotechnical Engineering aiming to improve the mechanical properties of soil so that it will be more suitable for construction, environmental purposes, as well as for ddisaster mitigation and coastal management.

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Two Approaches

Bioclogging is the production of pore-filling materials through microbial activity so that the porosity and hydraulic conductivity of soil can be reduced.Biocementation is generation of particle-binding materials through microbial so that the strength and stiffness of soil can be increased.Some methods exert combined effects of the two approaches.

Applications

Bioclogging:To form curtains to reduce the mitigation of pollutants in soilsTo prevent piping of earth dams and dikesTo mitigate reservoir leakageg g

Biocementation:To control erosionTo reduce liquefaction potentialTo enhance stability of slopes and damsTo increase bearing capacity of foundations

Advantages of biocement• It consumes much less energy and is more

environmentally friendly, as biocement made ofnaturally 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 by iron-reducing bacteria from iron oreE l h id d d b li hiExopolysaccharides produced by oligotrophic bacteriaConventional biogrout containing calcium chloride, urea, and urease-producing bacteria

Increase in strength of sand by biocementation

800

1000

1200

1400

1600

essi

ve S

tren

gth

(kPa

)

0

200

400

600

0 2 4 6 8 10 12 14

Mass CaCl2/Mass Sand (%)

Unc

onfin

ed C

ompr

e

Wet Samples Dry Samples

using CaCO3 precipitation method

Pictures of the samplesSand columns treated by microbial polysaccharides (left) and by ferrous salts produced by iron-reducing bacteria from iron ore (right)

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Hydraulic conductivity(Fe(OH)2 precipitation method)

Application-1: Water pond in desert

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ApplicationApplication--2: Mitigation of 2: Mitigation of liquefaction damages liquefaction damages

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Reclimed sand

TA-102

Improved byvibroflotation method

7m

5m

0m

-10m

-20m

-30m

10m

Alluvial clay

Diluvial gravel

0m 50m 100m

cm64.av =S

The effect of partial saturation

Recent studies have shown that liquefaction potential of sand can be largely reduced by introducing a small portion of gas into the soil.The presence of gas bubbles could reduce the pore water pressure generated, and hence improve the stability against liquefaction.One of the most convenient way to introduce gas bubbles in sand is to use micro-organisms.Denitrification is one of the processes adopted by He Jia.

Denitrification process• Denitrifying bacteria are used to produce nitrogen gas

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

• Besides the chemicals mentioned above some other

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↑

• Besides the chemicals mentioned above, some otherchemicals 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 ofcultivation lasts two days.

Use gas bubbles to increase resistance to liquefaction of loose sand

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No more compaction?

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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 of ammonium and its bacterial oxidation to nitric acid.NH4

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

Nitric acidNitric acid

Daily Mail , 08/27/2007Photo: Aaron Becker

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 (NEw Underwater Space) to make use of the sea space to construct underwater infrastructure and at the same time use the top-side of the 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 for space creation

Underwater structure Cavern Reclaimed Land for oil

tanksConstruction cost S$/ m3

storage space

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

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

$242(actual cost in S’pore)

$600 to $900(actual cost in

HK)

Cost Estimation for Oil Storage

SOIL IMPROVEMENT METHODS

Example 7: Merge different technologies

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Use of jet-grouting layer for excavation in soft clay

Nicoll Highway failure in Singapore 

Jet mechanical mixing (JMM) method

It combines jet grouting with cement mixing to for a grout slab at the bottom and cement 

mixed pile on top (Osborne and Ng 08)

JMM application in deep excavation

Reconstruction of the Nicoll Highway Station in Singapore (Osborne and Ng 08)

Hybrid or Bi-modulus method

Stone columns top TSM drilling toolCMC bottomCMC displacement aurger Stone columns top TSM drilling toolCMC bottom Stone columns top TSM drilling toolCMC bottomCMC displacement aurger

The lower part of the column is performed by controlled modulus columns (CMC) and the upper part by stone columns. 

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 the moment

• 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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谢谢！

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