International Geosynthetics SocietyIGS Education Committee

Training Course on Geosynthetics8IGG, Yokohama, 2006

Geosynthetics in Embankments on Soft Soils

Prof. J. OtaniKumamoto University, Japan

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Training Course on Geosynthetics8th ICG - Yokohama

Geosynthetics in Embankments on Soft Soils

Jun Otani, Kumamoto University, Japan

Contents1. Introduction

(current situation, classification, idea of the method)

2. Stability analysis of embankment on soft soil(concept of analysis, case studies)

3. Reinforcement with other methods(idea of the method, behavior, design

calculation)4. Concluding remarks

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1. Introduction

The earth reinforcement technique is one of effective solutions in geotechncail engineering.

The applications to retaining walls and slopes have relativelypopular on this technique.

This is because only stability or failure is main issue.

Current situation

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However,

Basal reinforcement is rather complicated.

Because,

1) Stability, and the same time,2) Deformation

Basal Reinforcements (1)

Geotextiles(a)Reinforcement beneath

foundation

Geotextiles

Geotextiles(b) Reinforcement beneath

any kinds of structures

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Basal reinforcement (2)

Embankment Geotextiles

Soft ground

Embankment

Geo-cell

Classification of the method

(1) Improvement of trafficability

(2) Basal reinforcement

(3) Combined technology with

1) Sheet method, 2) Net method, 3) Bamboo method

1) Plane (geotextiles and geogrids)2) 3-D (mattress and geo-cell)

1) Piling2) Improved soil pile

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Embankment on soft soils

The use of geosynthetics or geotextiles to improve deformation and stability of embankment over soft soils is one of the effective solutions in geotechnical engineering. Here, not only stability of embankment but also

controlling the ground settlement are expected.

1) To reduce settlement or

displacement of soft soils

geosyntheticsSoft ground

geosyntheticsSoft ground

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2) To prevent failure of total

embankment and soft soils

geosyntheticsbreak

failure surfacesoft ground geosynthetics

breakfailure surface

soft ground

3) To prevent sliding failure at

the geosynthetics surface

sliding forcefriction

geosynthetics soft ground

sliding forcefriction

geosynthetics soft ground

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2. Stability analysis

1) For overall stability

where MD: driving moment of soil (kN m/m)MR: resisting moment of soil (kN m/m)∆MR: reaction moment by geosynthetics

(kN m/m)

3.1~2.1typicallyM

MMF

D

RRs ≥

∆+=

Geotextiles

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PA: active thrust due to embankment ongeosynthetics (active earth pressure) (kN/m)

PR: friction along the fill-reinforcement interface force (kN/m)

5.1typicallyPP

FA

Rs ≥=

Active earth pressure

Geotextiles

2) For stability against slip failure

The efficiency of the geosynthetics

End of construction

reinforced

unreinforced

Soft soils

Reinforcement1.0

Safety factor

End of construction

reinforced

unreinforced

Soft soils

Reinforcement

by Jewell, 1996

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Case history (plane)

Embankment

Silty soil Geogrids

Case history (3-D)

1：1.5

1.5m

0.3m

3.0m Embankment

Soft ground

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Case history (construction process)

During placing reinforcing materials

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During construction at the slope face

Construction of embankment

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Final photos of the structure

3. Reinforcement withother methods

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Reinforcement with pile

Geotextiles

Soft soil

Foundation groundArching

Membrane effect

Pile

Concept of this technique

Embankment load is shared with geotextiles and pile.Reduce of settlement, stability of the ground, and lateral displacement can be expected.Not only mechanical stability but also economical advantage are expected.

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System of reinforcement with pile

Piles

Basalgeotextilereinforcement

Concrete connectingbeam

Geogridreinforcement Grassed

side slopesPavement

Pavement

by Lawson, 2003

Importance of mechanism of the system

There are two or more techniques combined.Behavior is complicated because of combined nature of the system.Stress re-distribution due to combining has to know for the design

We try to evaluate real behavior of the system using X-ray CT !

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Behavior of reinforcement with pile using X-ray CT

45mm45mm

Jack

Settlement plate

ToyouraSand Pile

Geogrid

126mm

100mm

200mm

Overburdenpressure

X-ray CT room

X-ray

Test apparatus

Test apparatus

Reinforcing materials

Grid-A Grid-B0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 2 4 6 8 10 12 14 16 18 20

Strain (%)

Tens

ile F

orce

(kN

/m)

Grid-A

Grid-B

Force-stain relationship

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Test cases and test method

overburden

100mm

30mm

Soils

CT scanning

6mm

16mm

2mm pich

CT scanning

Case 1: without geogridCase 2: with Grid-ACase 3: with Grid-B

Test cases

CT images

2mm 4mm 6mm 8mm 10mm

12mm

14mm

2mm 4mm 6mm 8mm 10mm

12mm

14mm

CASE1 (without geogrid)2mm 4mm 6mm 8mm 10m

m12mm

14mm

CASE2 (with Grid-A)

CASE3 (with Grid-B)

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高

Vertical CT images

Pile Pileθ

CASE1

Densityhigh

low

Grid-A

Pile Pile

θ1θ2

CASE2

Densityhigh

low

θ

Pile Pile

Grid-B

CASE3

Densityhigh

low

Arching behavior in 3-D

Pileθ

杭Pile3-D reconstruction image for the case of without reinforcementfrom X-ray CT

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Summary of the method with piles(one of Japanese practice)

Embankment on soft soils(reduce of settlement and differential settlement)

Geotextiles Pavement

Geotextiles after settlement

Settlement of pileSettlement of soilDifferential settlement

Improved soil pile

Two types of piling (low improved soil pile)

Clay

(a) Floating type (b) End bearing type

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Applications of piles

1:1.8 H=8m

foundations

(a) End bearing type Floating type

Model of load at the ground due to pile

Pile

pile

pile

pile

d: pile diameterλ: pile spacing

from CT image Load model at the ground due to pile(stress redistribution due to piles)

Total volume of this area is the load distributionfor the ground in between 4 piles.

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Load redistribution

load for pile

load for the ground

pile

Depth ofembankment

Load redistribution for one pile

Calculation of settlement

PPSS C

C ∆∆

= 0

LEPS P

P∆

=

S0: settlement without reinforcement

∆P: total embankment load

∆Pc: embankment load for the ground

∆Pp: embankment load for the pile

L : pile length

E : Young’s modulus for the pile

Settlement of the ground

Settlement of pile

Differential settlement: ∆S = Sc - Sp

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Important notice

1. Spacing of 2 piles 2. Strength of improved soil pile

D

H

Soft ground

embankment

Check for punching failurein the embankment

Stress on pile

Spacing (m)

⊿PP

Check for the strength in terms of spacing

Case history (with soil cement pile, Japan)

1.2m

2.0m10.25m

5.5m

2.5m

Soft Clay

GeogridW = 8m

3.7m

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Cont.

Cont.

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4. Concluding remarksEarth reinforcement technique is well established even for basal reinforcement.This technique for embankment on soft soils also has been used successfully.Arching effect in the soil was clearly investigated in

3-D using X-ray CT.The combined technology with reinforcement will become more popular.Earth reinforcement will be used to the countermeasure for natural disaster more.

(heavily rain, earthquake, tsunami and else)

Thank you and

Merci beaucoup

Acknowledgement

Some the data and photos in this lecture were originally made by the last TC9/ISSMGE members in Japan and those are the contents of the book entitled “New Applications of Earth Reinforcement Technique” in Japanese under Japanese Geotechnical Society. All the contributions are highly appreciated.