05/10/2012

1

Preloading & Vertical drains (VD)

Currently practiced to improve very compressible (soft) soilsy p ( )

1. Preloading

2. Vertical drains (VD): sand & prefabricated drains, installation, constitutive material

3. Design of vertical drains networks

4 Vertical drains project in Tunisia

Soil Improvement Techniques, MB 1

4. Vertical drains project in Tunisia

5. Control and follow up of VD projects

Preloading

• Saturated compressible soil: low bearing capacity

• Embankment load: very simpleEmbankment load: very simple

• Increase undrained shear strength (Cu), partial primary consolidation

• Prediction from UC triaxial test

• Reduction of settlement

Soil Improvement Techniques, MB 2

Reduction of settlement

• Time constraint (vertical consolidation too long)

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Principle of preloadingP0 : real projected foundation

P1 : allowable loading

Soil Improvement Techniques, MB 3

)( 001u

fuuu CCUCC

Preloading Limitation:Staged construction: soft clays & (or) large height embankment, delayed consolidation

Real need: accelerate primary consolidation

The use of vertical drains: horizontal consolidation

Soil Improvement Techniques, MB 4

much more effective for reducing time consolidation

Too reduced drainage path & bigger permeabilty (kh > kv) of drain material

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Vertical Drainsassociated with preloading

• Sand Drains: may contribute in settlement reduction• better option for depthbetter option for depth

• Geodrains = Prefabricated Vertical Drains (PVD): rapid installation & higher drainage properties

• Limited depth (Equipment)

Focus: Drains installation designing drains

Soil Improvement Techniques, MB 5

Focus: Drains installation, designing drains Main criteria: Spacing between drains

networks, case histories: projects accomplished

Sand drainsDrain efficiency

• Constitutive material : drained coarse sandConstitutive material : drained coarse sand

Obeying to filter condition which depends on the gradation curve of the initial soil to improve.

Grain size distribution

Soil Improvement Techniques, MB 6

• Installation method

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Sand drains

• Efficiency: ensuring drainage

Good choice of grain size

• Installation: it sometimes needs substitution of initial soil (Several procedures)

Soil Improvement Techniques, MB 7

• Filter condition (Drain) grain size distribution of soil to consolidate

Filter condition: experiments

• Earth dams (Terzaghi ) :• Cohesive material containing a minimum of 15 % clay

No clogging of a filter D15 ≈ 0,1 mm.o c ogg g o a te 15 0,

• Purely frictional medium :

• Kérisel : complementary condition (form of grain size curve)

• D100 : Maximum diameter of grains filter

15

85

( )9

( )

D filter

D soil

Soil Improvement Techniques, MB 8

• Particles of diameter D :• Validity of filter conditions (Terzaghi) :

Laboratory experiment (Bertram, 1940)

(%)3

100D

D

15

85

( )4 5

( )

D filterto

D soil 15

15

( )

( )

D filter

D soil

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As conclusion:

• Sand drain has a limited duration of life.

risk of clogging by surrounding soil:

• Sand Drain « well installed »:

Works during all primary consolidation of initial soil.

Soil Improvement Techniques, MB 9

Soil Improvement Techniques, MB 10

Method of capped casing with recoverable tip, Magnan (1983).

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Prefabricated Vertical Drains (PVD)

• Origin: geo-composites

• Performances to attain:• Power full drainage

• Role of filter & prevents fines transportation of soil to improve.

Soil Improvement Techniques, MB 11

• Steps of installationInstallation effect: disturbance of initial soil

PVD’s installation

Soil Improvement Techniques, MB 12

Installation of card board drains, Magnan (1983).

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Shapes of shoe used for PVD embedment, Magnan (1983).

Soil Improvement Techniques, MB 13

PVD Installation Effects1) Smear and disturbance: soil displaced during

installation of drain (penetration).

* Disturbance of soil around the drain

* Shear strains and displacement

* Increase of total stresses and pore pressure

Drain performance is affected.

2) Well resistance: relates the degree of horizontal

Soil Improvement Techniques, MB 14

2) Well resistance: relates the degree of horizontal consolidation with drain’s length.

Horizontal consolidation decreases with depth

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Case histories (experienced projects)

Rate of settlement

Mandrel Smeared zonesettlement

Fast Small area Reduced

Soil Improvement Techniques, MB 15

Slow Large area Large

Design of vertical drains: acceleration of consolidation

1) Barron’s Theory (1948): unit cell model:Equivalent diameter: (drains regular network or mesh)

Assumptions: • Horizontal drainage (one dimensional consolidation) :• Constant vertical stress (applied load)• Equal vertical strain: non-uniform vertical stress.

eD

Soil Improvement Techniques, MB 16

q

2

2

1h

u u uC

t r r r

u: excess pore pressure

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Horizontal consolidation

Coefficient of horizontal of consolidation: 1 5h

v

C

C h oed

hw

k EC

hC tT Time factor 2h

e

TD

Time factor

Charts : ( , )h hU f T nd

Dn e

d Drain’s diameter (equivalent for PVD)

Design: needed time to reach a given horizontal consolidation rate?

Soil Improvement Techniques, MB 17

Design: needed time to reach a given horizontal consolidation rate?

n ; Cr and De : Data (Parameters)

Methodology applicable for all types of vertical drains

Carillo’s Theory (1942): same framework

3D consolidation (Ch and Cv)

Combined solutions of Terzaghi & Barron :

1 1 1U U U Charts (three) 1 1 1h vU U U

Ur and Uv U : global degree f consolidation

Charts (three)

.Ch and t : data Point A , draw vertical line

Draw horizontal Point B : Choice of U

Soil Improvement Techniques, MB 18

Diameter of drain and Diameter of influence, Fix the type of mesh

Spacing between drains

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Horizontal consolidation chart, Barron (1947).

Soil Improvement Techniques, MB 19

3D consolidation

Soil Improvement Techniques, MB 20

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Full design of PVD network

Soil Improvement Techniques, MB 21

Design of Prefabricated Vertical Drains

(Hansbo, 1979) 81 exp h

h

TU

F

F = F(n) + Fs + Fr

Barron

Smear Well resistance

U f(T ) U t T

Soil Improvement Techniques, MB 22

U = f(Th , n) : Uh et n Th

To calculate t

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Designing PVD (1)• Equivalent diameter of drain

• (Hansbo, 1979):2

badw

T8F = F(n) + Fs + Fr

Barron

Smear effect (disturbance during installation):

F

TU h

h

8exp1

4

3)(

w

e

d

DLnnF

w

s

s

hs d

dLn

k

kF 1

Soil Improvement Techniques, MB 23

Diameter of disturbed zone around the drain

Horizontal permeability of soil in disturbed zone

Horizontal permeability in undisturbed soil

sd

sk

hk

Designing PVD (2)

Well-resistance effect: Limited discharge capacity of drain

Drainage occurs at one end drain: L= Twice length of drain

Drainage occurs at both ends: L= length of drain

w

hr q

kzLzF

Drainage occurs at both ends: L length of drain

Discharge capacity of the drain at hydraulic gradient of 1:

Distance from the drainage end of the drain: Z

Time to obtain a given degree of consolidation:Use charts of Bergado et al (1991).

wq

hrs

h

e

ULnFFnF

C

Dt

1

1))((

8

2

Soil Improvement Techniques, MB 24

Coefficient of horizontal consolidation:

Use charts of Bergado et al (1991)

Ratio charts of Rixner et al (1986)

vv

hh C

k

kC

v

h

k

k