Lecture 7 - Pre Loading

5/19/2010

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Behzad FatahiPhD, MEng, BEng (Hons), CPEng, MIEAust, NPER

University of Technology Sydney (UTS), and Coffey Geotechnics Pty Ltd, Sydney Office

Soft Ground Improvement Soft Ground Improvement

Using PreloadingUsing Preloading(Including Surcharge, Vertical Drain, and Vacuum)(Including Surcharge, Vertical Drain, and Vacuum)

49119

Problematic Soils and Ground Improvement Techniques

Preloading Preloading

Preloading and Vertical DrainsPreloading and Vertical Drains

Preloading with VacuumPreloading with Vacuum

Selected ExamplesSelected Examples

OUTLINEOUTLINE

Process of loading the ground with surcharge equal or more than future structure load and then removing the

surcharge after end of the required degree of consolidation

Ground Improvement Using PreloadingGround Improvement Using Preloading

Pressure (kPa)

e1.0

0.9

0.8

0.7

0.61 10 100 1000

Slope Cr

Re-compression

or swelling line

Slope Cr

Compression lineSlope Cc

σ′o Pressure (kPa)

e1.0

0.9

0.8

0.7

0.61 10 100 1000

Pressure (kPa)

e

Pressure (kPa)

e1.0

0.9

0.8

0.7

0.61 10 100 1000

1.0

0.9

0.8

0.7

0.6

1.0

0.9

0.8

0.7

0.61 10 100 10001 10 100 1000

Slope Cr

Re-compression

or swelling line

Slope Cr


Re-compression

or swelling line

Slope Cr


σ′oσ′o

Real Soil Consolidation Behaviour

Pressure (kPa)

e1.0

0.9

0.8

0.7

0.61 10 100 1000

Re-compression

or swelling line

Slope Cr


Ground Improvement Using Preloading Ground Improvement Using Preloading -- continuedcontinued

Pressure (kPa)

e1.0

0.9

0.8

0.7

0.61 10 100 1000

Slope Cr

Re-compression

or swelling line

Slope Cr


σ′o

Ideal Soil Consolidation Behaviour


Pressure (kPa)

e1.0

0.9

0.8

0.7

0.61 10 100 1000

Ideal Soil Consolidation Behaviour

Normally consolidated

(on the line)

Over-consolidated

Soil is normally consolidated if the current stress in the soil is the maximum ever experienced by the soil. Soil is over-consolidated if it has been subjected to a larger stress than the current stress.


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ConsolidationExample 1 :

84 kPa

4 m

2 m

γt=16 kN/m3

γt=18 kN/m3

cv=4 m2/year

Normally consolidated:

Cc=0.2, Cr=0.05, eo=1.1

Sand

Clay

a) Calculate the settlement of the clay layer 584 days after application of 84kPa surface loading just before it removal?

b) Calculate the long term settlement of the clay layer after removal of 84 kPa load and application of 50kPa surface loading?

ConsolidationPart a)

84kPa

4 m

2 m

Settlement 584 days after loading?

γt=16 kN/m3

γt=18 kN/m3

cv=4m2/year


Cc=0.2, Cr=0.05, eo=1.1

Initial stress:

σ = 2×18+ 2×16 = 68 kPa

u = 4 × 9.8 = 39.2 kPa

σ′i = 68 -39.2 = 28.8 kPa

∆e=0.2×log(112.8/28.8)=0.119

σ′f = 28.8 + 84 = 112.8 kPa

Stf=(0.119/2.1) ×4 = 0.227 m

Sand

Clay

4.02

=×

=

p

vv

d

tcT

U (

%)

Dimensionless time, Tv

00

40

60

20

80

100

Calculation of Settlement

0.0 0.2 0.4 0.6 0.8 1.0

0.7

U = 0.70

Stt = U × Stf

= 0.70×0.227=0.159m

Time for 100% settlement?

Assume Tv=3.0

v

2v

c

DPTt

×= years12

4

43 2

=×

=

ConsolidationPart a)

84kPa

4 m

2 m

Settlement 584 days after loading?

γt=16 kN/m3

γt=18 kN/m3

cv=4m2/year


Cc=0.2, Cr=0.05, eo=1.1

Initial stress:

σ = 2×18+ 2×16 = 68 kPa

u = 4 × 9.8 = 39.2 kPa

σ′i = 68 -39.2 = 28.8 kPa

∆e=0.2×log(112.8/28.8)=0.119

σ′f = 28.8 + 84 = 112.8 kPa

Stf=(0.119/2.1) ×4 = 0.227 m

Sand

Clay

4.02

=×

=

p

vv

d

tcT

years44

41 2

=×

=

ConsolidationPart b)

50 kPa

4 m

2 m

Long term settlement after reloading?

γt=16 kN/m3

γt=18 kN/m3

cv=4m2/year

Initially over consolidated:

Cc=0.2, Cr=0.05, eo=1.1

Initial stress:

σ = 2×18+ 2×16 = 68 kPa

u = 4 × 9.8 = 39.2 kPa

σ′i = 68 -39.2 = 28.8 kPa

∆e=0.05×log(78.8/28.8)=0.022

σ′f = 28.8 + 50 = 78.8 kPa

Stf=(0.022/2.1) ×4 = 0.042 m

Sand

Clay

σ′pc = 28.8+ 0.7×84=87.6 kPa

If the soil was not initially loaded

∆e=0.2×log(78.8/28.8)=0.087

Stf=(0.087/2.1) ×4 = 0.167 m

Soil is normally consolidated

Preloading (method 1)Preload soft soil equal to the eventual structural load

Wait until primary consolidation is complete – how long?

Remove the preload, allow soil to swell

Construct structure – OC consolidation settlement is expected

Heavy Preloading (method 2)Preloading much greater than the eventual structural load

Wait until settlement is equal to the expected total settlement

that would have occurred if the structure load had been applied

Remove the total surcharge and Construct structure

Ground Improvement Using PreloadingGround Improvement Using Preloading -- continuedcontinued

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Method 1

Method 2

The average effective stress just before unloading in both methods are equal, therefore, the settlement values are the same.

Methods 2 is more time effective but more surcharge fill material is required (time-cost analysis).

Example 2 :

A very wide road should be constructed on top of 10m deep soft clay overlaying a sandstone layer. The total pressure induced by road embankment and traffic is 60kPa. Calculate the consolidation settlement of the road for the following two options.

Option 1) placing 4m of surcharge with unit weight of 20 kN/m3 for 36months

Option 2) Placing 7.5m of surcharge with unit weight of 20 kN/m3 for 10months

(for simplicity conduct the consolidation settlement calculations for one layer).

60kPa

Cc=0.3, Cr=0.05, eo=1.0

cv=4 m2/year

γγγγt=15 kN/m3


• Low bearing capacity

• Excessive and differential settlements

Disadvantages of Excessive

Preloading

Preload and Surcharge Preload and Surcharge -- ContinuedContinued

Preloading Technique

Time

Fill

Heig

ht

fH

3t

Upper Bound

2t ct1t

2H

1H

0H

Lower Bound

Time

Fill

Heig

ht

fH

3t

Upper Bound

2t ct1t

2H

1H

0H

Lower Bound

Preload and Surcharge Preload and Surcharge -- ContinuedContinued

Staged-construction is required for high surcharge

embankments to prevent failure

Some difficulties associated with Some difficulties associated with surcharging onlysurcharging only

Preloading method sometimes may not work alone due to a thick soft clay layer

Preloading method sometimes may not work alone

because of very low permeability of the clay layer

which makes the consolidation process very long and

not practical

Sometime the required surcharge will be very high which has cost consequences.

Sometimes rate of undrained shear strength gain is very small so the rapid placement of the surcharge embankment will cause foundation failure (very important).

Preloading and Vertical Drain Preloading and Vertical Drain SystemSystem

Decrease the length of the drainage path.

Horizontal permeability of soil is normally greater than vertical permeability

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Vertical DrainsVertical Drains

Shorten the length of the drainage path

Accelerate the rate of pore water pressure dissipation

Accelerate the rate of consolidation / settlement

Preloading and Vertical DrainsPreloading and Vertical Drains Combination of vertical and Radial Combination of vertical and Radial ConsolidationConsolidation

The average degree of consolidation due to vertical and radial consolidation can be estimated using Carillo (1942) equation:

(1-Uave)=(1-Uv)(1-Ur)

where,

Uave is the average degree of consolidation

Uv is the vertical degree of consolidation

Ur is the radial degree of consolidation

Sand Drains

They were widely used between 1930 -1980 with

diameter changing between 20 - 60 cm and with 1.5m

to 6m spacing.

Prefabricated Vertical Drains (PVD)

PVDs consist of a plastic core and a filter all around.

The filter material can be paper, fibrous material or

porous plastic.

Different Types of Vertical DrainsDifferent Types of Vertical Drains

Sand DrainSand Drain

Closed Mandrel Method: soil is displaced by pushing a closed

end tube and filling it with sand

Open Mandrel Method: soil is removed after an open end tube is

pushed into the ground and then filled with sand

Sand DrainsSand Drains

Prefabricated Vertical Drains (PVDs)Prefabricated Vertical Drains (PVDs)

Composed of plastic core with a longitudinal channel wick functioning as drain, and a sleeve of fibrous material as a filter protecting the core

PVD InstallationPVD Installation

Installation RigDrain Delivery Arrangement

Cross section of

mandrel and drain

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Wick DrainWick Drain Potential benefits of vertical drainsPotential benefits of vertical drains

Soft groundconsolidates under load

Surcharge Surcharge

Without drains With Vertical Drains

vertical drains with surcharge

Without vertical drains

Time

Set

tlem

ent

Disturbance of soft soil because of installation methods of PVDs. Permeability of soil next to the PVD decreases, and also pore water pressure is generated in the soil during installation (Smear Zone).

PVD wall can be damaged and disturbed and the drain permeability reduces (drain is assumed as free drainage boundary).

Main Factors Influencing PVD performanceMain Factors Influencing PVD performance

Smear Zone

Equivalent PVD

(Equilibrium Equation)

Square pattern Triangular pattern

Influence zone of drains (dInfluence zone of drains (dee))

Equivalent Drain DiameterEquivalent Drain Diameter

Drain size Drain diameter (mm, dw)w (mm) t (mm) Eq. (1) Eq. (2) Eq. (3)

95 5 63.6 50.0 51.098 4 64.9 51.0 51.898 5 65.5 51.5 52.594 4 62.3 49.0 49.893 4 61.7 48.5 49.3

Eq. (1)

Eq. (2)

Eq. (3)

Smear Zone ParametersSmear Zone Parameters(after Xiao, 2001)

rs = radius of smear zonerm=equivalent radios of mandrel

Kh & kv = horizontal and vertical permeability of soilks= permeability of smear zone

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Surcharge

Tensile Fabric Tencate WX600kN/m

Soft Clay

0.6m 2

1

PVDs

watertable

Berm

2

1 Berm Berm

20m

Surcharge


Soft Clay

0.6m 2

1

PVDs

watertable

Berm

2

1 Berm Berm

Surcharge


Soft Clay

0.6m 2

1

PVDs

watertable

Berm

2

1 Berm Berm

Surcharge


Soft Clay

0.6m 2

1

PVDs

watertable

Berm

2

1 Berm Berm

Surcharge


Soft Clay

0.6m 2

1

PVDs

watertablewatertable

Berm

2

1 Berm Berm

20m

For accurate design of ground improvement using

surcharge and PVD, to consider the smear zone effects,

numerical analysis using computer codes is required.

As the 3D simulation of embankment plus PVDs on the

soft ground is very time consuming, simplified 2D

equivalent plan-strain analysis is very common in practice.

Example Surcharge Embankment 3D section of PVD and soil

Simulation of Surcharge and PVDsSimulation of Surcharge and PVDs Converting 3D Axisymmetric Vertical Drain Converting 3D Axisymmetric Vertical Drain Parameters to PlanParameters to Plan--Strain ParametersStrain Parameters

(Indraratna and Redana,1997)

Simplified Simplified DDesign Method For Preload esign Method For Preload with Vertical Drainswith Vertical Drains

For simplicity it is assumed that each vertical drain is independent and is located in centre of a soil cylinder.

t

u

r

u

rr

uch

∂

∂=

∂

∂+

∂

∂ 12

2

The governing Equation for the radial consolidation is:

u=u0 at t=0 at all placeu=u0 In the draIn at any tIme

The governing Equation for the vertical consolidation is:

t

u

z

ucv

∂

∂=

∂

∂2

2u=u0 at t=0 at all place

u=u0 In the drainage boundaries at any tIme

Simplified Simplified DDesign Method For Vertical esign Method For Vertical DrainsDrains

By solving the horizontal consolidation equation the radial degree of consolidation can be calculated as:

F

T

h

h

eU

8

1

−

−= 2

.

e

h

hd

tcT =where,

According to Hansbo(1979);F = F(n) + Fs + Fr

where,F(n) : Due to spacing of drains, n=re/rd

Fs : Due to smear effectFr : due to well resistance

There are graphs and equation to calculate F, and Uh


VarIatIon Of Uh and Uv with tIme factor for varIous n values

A road embankment is constructed on top of a 9.2m thick

layer of clay, sandwiched between silty sand at top, and

dense sand at the bottom. The required degree of

consolidation before the embankment consolidation is

90% within 9 months. For this purpose, sand drains of

450mm diameter, need to be installed in a square

arrangement. From the laboratory tests, assume that

ch=0.288 m2/month and cv=0.187 m2/month.

Estimate the spacing of the drains.


Example 2

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Vacuum ConsolidationVacuum Consolidation Vacuum Consolidation Vacuum Consolidation -- ContinuedContinued

Initial conditionsInitial conditions Geotextile on top of clayGeotextile on top of clay

Sand mat installationSand mat installation Vertical Drain installationVertical Drain installation

Vacuum Consolidation Vacuum Consolidation -- ContinuedContinued

Geomembrane installationGeomembrane installationSealing TrenchesSealing Trenches

Membrane WeldingMembrane Welding Pumping StationsPumping Stations

Conditions of Application of the MethodConditions of Application of the MethodConditions of Application of the MethodConditions of Application of the MethodConditions of Application of the MethodConditions of Application of the MethodConditions of Application of the MethodConditions of Application of the Method :Compressible Saturated Clay, Silt, PeatLow permeabilityWatertable close to surfaceNo or few sand contents or pockets ( air and water leakage )Light to medium loads

Vacuum Load equivalent to 4m of surcharge (Saving in Earthmoving)

Isotropic load

No risk of slope failure: high embankments built up in reduced period on soft soils

Time Saving

BenefitsBenefitsBenefitsBenefitsBenefitsBenefitsBenefitsBenefits :


vertical drains with surcharge

without vertical drains

Time

Se

ttle

me

nt

vertical drains with surcharge and vacuum preloading


Classical SurchargeClassical Surcharge Vacuum MethodVacuum Method

4 m


4 m

Failure SurfaceFailure Surface

No FailureNo Failure


Failure SurfaceFailure Surface




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If H > H limit , FAILURE

H


If H > Hlimite , FAILUREIf H > H limit , FAILUREIf H > Hlimite , FAILUREIf H > H limit , FAILURE


Vacuum Method

No limitation : High Surcharge built up in limited period

Vacuum Pressure

Vacuum Method

No limitation : High Surcharge built up in limited period

Vacuum Pressure

H+


H+



Surcharge Fill

Surcharge with PVDs

Bridge Piles

2.5m

(Design Height)

DSM Zone

X?

50m

6.2m for FRB1-24.8m for FRB3

1m

(zone 1)(zone 2)Surcharge Fill

Surcharge with PVDs

Bridge Piles

2.5m

(Design Height)

DSM Zone

X?

50m

6.2m for FRB1-24.8m for FRB3

1m

(zone 1)(zone 2)

Design of transition zone between different ground Design of transition zone between different ground improvement areasimprovement areas

Transition Zone Design for Bridge Abutments

Vacuum Preloading – Ballina Bypass

Australia’s first use of vacuum preloading

•More cost effective than piles, stone columns or deep soil mixing

25m deep clay deposit, at the Southern Abutment of Emigrant Creek

crossing

•Low bearing strength

•Unsuitable for surcharge preloading

Poor groundwater quality

•Treatment required


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Settlement

•5mm/day before vacuum preloading

•22mm/day during vacuum preloading

•1.2mm/day after vacuum preloading

•5m (20%) total settlement at deepest clay deposit

System stability and full vacuum pressure maintained throughout

operation

Vacuum preloading suitable

for sites with deep deposits

of soft clay

Electro-Osmosis

Removal of excess pore pressure from soil through electric currents

Cathodes and anodes inserted into soil

•Pore water attracted to cathodes

•Electro-osmotic conductivity much greater than hydraulic

conductivity

Inefficient and costly so rarely used

Electro-osmosis

cell

Water flows from anodes

to cathodes

Sand layer

Clay layer

Cathodic PVD System

Preload (surcharge)

Anodes system

in soil

Conclusions

Ground improvement techniques allowing previously unworkable sites to

be utilised

Traditional preloading, preloading with vertical drains and vacuum

preloading all viable consolidation techniques. Electro-osmosis still

needs further development to be cost effective and viable.

•Traditional preloading – slowest, accurate, less monitoring

•Preloading with vertical drains – faster, more monitoring, research

still being conducted

•Vacuum preloading – fastest, constant monitoring, still needs a lot of

research

Thorough geological and geotechnical investigations needed before

making decision.

Documents

Lecture 7 - Pre Loading