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Ground Improvement for Ultra-Soft Hydraulic Fill

Using A New Combined Method of Electro-osmosis,

Vacuum and Surcharge Preloading

Hanlong LIU / Wengang Zhang Presenter: Wengang Zhang School of Civil Engineering Chongqing University

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Overview of Presentation

Background

Research and Development

Field Test and Result Analyses

Engineering Applications

Summary and Conclusions

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“National Strategy” demand

Background

As Chinese State Council approved

the national strategy of 12

provinces’ coastal development, a

mega-scale coastal development

scheme, comprising mainly of the

Bohai Sea, the Yangtze River Delta

and the Pearl River Delta, is being

carried out along the 18 000 km

long coastline.

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Building land demand

Land resource supply

Harbor

Industrial arichitecture

Equipment manufacturing

Traffic

Residence

National defense construction

How to solve the contradiction

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The most effective solution is coastal reclamation.

Yang Shan Deep-draft Port,

Shanghai

Maca International Airport

Lin Gang Industrial District,Tianjing

Industrial District,Caofeidian

Nan Sha,Guangxi Province

Reef Construction

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However: Most of the land to be reclaimed by dredging offshore marine clay has super low permeability and bearing capacity and ultra high water content and compressibility. shortage of sand, vacuum pre-loading or surcharge preloading without sand cushion was rather challenging Mechanical equipment or construction personals can’t enter site due to its low bearing capacity.

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It is urgent to develop a fast, efficient and economical method to treat the

large areas of hydraulic fill ground.

The techniques of elelctro-osmosis or the combination of electro-

osmosis and other methods began to be taken into considered as a

potential treatment method.

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Research and Development

Electro-osmosis, by Casagrande (1949); Verified by quantities of lab experiments and field tests; .accelerates the seepage speed in soft clay; .promotes the consolidation; .significantly enhances both SS and the bearing capacity; .effective in saturated fine soil with thick bound water layers. However, some limitations: .produces large amounts of cracks in the ground; .increased effective stress by lowering the GWL cannot compact the voids introduced by electro-osmosis effect; Soil will be wet again when the water level rises after electro-osmosis is completed or subjected to precipitation.

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Vacuum preloading and surcharge preloading .not efficient when applied on soft clay with very low permeability .long treatment time with poor treatment quality It is difficult to drain the loosely bound water out of fine soils by vacuum preloading or surcharge preloading. Combination? Shang (1998), Micic et al. (2001) : electroosmosis with surcharge; Wu and Wu(2011), Wang and Vu (2010) : electroosmosis with vacuum; However, no finding has been reported to combine electro-osmosis with both vacuum preloading and surcharge preloading followed by field testing.

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Plastic PVD

Air/water

cut-off

wall

ePVD

Peripheral

trench

EmbankmentWater or soil

Geotechnical fabric

Geomembrane

Electrical wireHorizontal drainage pipeSand mat

Schematic view of a combined method of electro-osmosis, vacuum preloading and surcharge preloading to strengthen soft clay ground.

ePVD: electrical prefabricated vertical drains, functioning as both electrodes and vertical drains; plastic PVD: complementary vertical drains; Vacuum formed under geomembrane by pumping; Surcharge applied by placing water or soil above the geomembrane.

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ePVD and mechanism of the proposed combined method

Insulated copper wire

Flexible drainage pipe

Perforated Galvanized

steel pipe wrapped by

filtering fabric

Newly invented compressible electrical prefabricated vertical drain (ePVD)

Steel pipe, perforated and wrapped in filtering fabric, functions as both vertical drainage and an electrode; Insulated copper wire: prevent the electro-osmotic system from corroding; Electric vertical drain (EVD) Electrokinetic geosynthetics (EKG) More compressible, yet too expensive.

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Vacuum applied first , pore water gathers at vertical drains and is drained out through both the ePVDs and plastic PVD;

Meanwhile, a DC voltage is applied to the soil via ePVDs, speeding up water flow to the cathodes, water flow caused by elelctro-osmosis can be expressed by a similar equation with the water flow caused by vacuum;

Application of voltage to soil with appropriate drainage conditions, generating negative pore pressure with the soil mass (Jones et al. 2010):

Surcharge preloading exerts an additional stress, compacting the voids induced by water being drained out by electro-osmosis and vacuum.

Q h hk i A

Q e ek i A

= /e w hu k k

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' '= - - +v e su u

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Field Test and Result Analyses

15

10

5

020 25 30 35 40 45 50 20 30 40 50 2.2 2.4 2.6 2.8 3.0 2 3 4 5 6 7 8

Note:LL=liquid limit, PL=plastic limit, w=water content, cu=undrained vane shear strength, E

s=compression modulus

kh(10

-8m/s)E

s(MPa)c

u(kPa)(%)wPLLL

Soil Description

(184.7)

Soft silty clay

with fine sand

Soft silty clay

Fine sand

Soft silty clay

Dep

th (

m)

Dredged fill

Soil profile and soil properties in test site

Dredged fill layer was approximately 3 m, very soft sludge without any strength.

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Cathode

Anode

Drainage pipe

Electrical wire

(a)

Anode

Unit：m

Cathode

Voltage probe

(b)

Electrodes arrangement

Before designing the electro-osmotic system of the proposed combined method, the electrical resistivity of the soil should be measured to determine the output of the power supplies, the cross-sectional area of the electrical wires, and the measurement range of the measuring instruments.

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Plastic PVD

ePVD

Plastic PVD

Embankment

Water

Area A Area B

Dredged fill

Soft silty clay

Fine sand

Soft silty clay

Soft silty clay

with fine sand

0

10

15

5

Surface settlement gauge

Piezometer

Embankment

Inclinometer

Unit: m

Piezometer

Inclinometer

Air/watercut-offwall

proposed combined method

conventional vacuum combined with surcharge preloading

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Surface settlement gauge

Piezometer

Vane shear test

Static loading test

(a) Area A (b) Area B

ePVD

Plastic PVD

V1

V2V3

V4

Unit: m

Plan of ePVDs and PVDs installation, field instrumentation

Area A was also equally divided into three sub-sections, known as Area A1, Area A2, and Area A3

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Horizontaldrainage pipe Insulated

copper wire

Insulatedaluminum wire

Insulatedconnecter

BA1

A2

A3

The electro-osmotic drainage system and the detailed connection

All the ePVDs were connected to DC power supplies by insulated aluminum wires to make up the electro-osmotic drainage system.

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Testing procedures: .A vacuum load of 85 kPa had been applied in both Area A and B within 10 days by pumping; .DC power supplies started right after application; .DC paused for 2 hours for every 4-hour operation; .After the vacuum preloading and electro-osmosis had been working for 15 days, surcharge preloading of 2m depth of water; .Working together for 30 days before the treatment finished

0 10 20 30 40 50

0

20

40

60

80

100

120

Vacuum load

Surcharge load

Load

(kP

a)

Time (day)

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0 10 20 30 40 50 60 70 80 90 100 110 120

600

500

400

300

200

100

0

B: V+SB: V

A: V+E+SA: V+E

Stage 1

Sett

lem

ent

(mm

)

Time (day)

Area A1 (V+E+S)

Area A2 (V+E+S)

Area A3 (V+E+S)

Area B (V+S)

Prediction of V+S

Average of V+E+S

V: Vacuum preloading

E: Electroosmosis

S: Surcharge preloading

Stage 2

Surcharge applied

Ground surface settlements

Settlements of areas treated with proposed method are 20% greater. The settlement of Area B was still increasing rapidly when Area A almost completed at Day 45.

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FEM prediction shows a reasonably good match with the observed data. Settlement contour at Day 100 simulated by the finite element analysis

Vacuum combined with surcharge preloading method needs 100 days for equal settlement brought by proposed method in 45 days. Saving approximately half of the treatment time.

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Vane shear tests performed to investigate the reinforcement effect.

14

12

10

8

6

4

2

00 20 40 60 80 100 120 140

Active electrode

depth

Soft silty clay

Vane shear strength (kPa)

Dep

th (

m)

Before treated

V4 (Area B)

V3 (Area A)

V2 (Area A)

V1 (Area A)

Average

Dredged fill

Soft silty clay

Fine sand

Silty soft clay with fine sand

Area B: undrained vane shear strength of clay layers increased approximately from 17-25 kPa to 32-40 kPa; Area A: increased more significantly up to 50-58 kPa.

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Soft ground in Wenzhou

Reinforced area is 2.79

km2. Compared with the

fill slag reclamation, it

saved time of more than

2 years. The undrained

shear strength is

improved from 0 to 60

kPa in 3 months.

Engineering Application #1

Engineering Applications

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Nansha port, Guangzhou.

The treatment volume is

about 29 million m3, and the

treatment area is about 860

thousand m2.

Engineering Application #2

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Summary and Conclusions

Effectiveness of the proposed combined method and its

applicability validated through field tests:

(1) Combined method induced a 20% greater settlement given

the same treatment time;

(2) For the same treatment effect, the proposed method can save

approximately half treatment time;

(3) The proposed method can more significantly increase the

undrained vane shear strength of the soft soils.

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Thank you for your attention!