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Effects of head plate size on nailed CDG steep slope behaviour subjected to rising

groundwater

Supervisor: Prof Charles W W NgPhD Candidate: Robin Z B Zhou

The Hong Kong University of Science and Technology

Contents

IntroductionLiterature reviewObjectivesCentrifuge modelling testsNumerical modelling analysisConclusions

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Introduction

Load transfer mechanism of soil nailed structure (after Shiu & Zhang 2004)

Literature review

Effect of head plate on distribution of reinforced tensile force (after Muramatsu et al 1992)

Tmax is developed close to the nail head for the grating crib

Excavation: experiment

Head end

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Locations of maximum tensile force in soil nail (after Shiu& Zhang 2004) (Numerical, strength reduction)

Normalised distance from the nail head

Different conclusion:

Tmax locates further away from the slope surface for larger nail heads

Objectives

Effects of nail head size on steep slope behaviour subjected to rising groundwater

Failure mechanism

Tensile force distributionLocation of maximum tensile force

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Centrifuge modelling

Nailed (400 mm wide grillage)CGNGr65_30

Nailed (600 mm wide head )

Nailed (400 mm wide head )

Nailed (no head)

No nail

Reinforced conditions

CGNH165_30

CGNH65_30

CG65_30

CGN65_30

I.D.

Test programme

Nailed model package (CGN65_30)

LVDT1PPT

Model box

Downstream drainage board

Upstream drainageboard

Outlet hole

Upstream temporary reservior

Downstream temporary reservior

Inlet hole

Unit:mm

Coarse CDG block

PPT4

LVDT2

Nail B

R.C. 95%30g

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Instrumented Soil Nails (font view)

45 Nails

5 columns

9 rows

Test Results

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Observed failure modes

cracks

CGN65_30 (no head)

0.33H

crack

Local failure

CG65_30 (no nail)

Local failures were observed in nailed slopes with different dimensions of head plate

Localized failure

CGNH165_30 (600 wide H)CGNH65_30 (400 wide H)

Local failure Local failure

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Local failures were observed in nailed slopes with grillage

Local failures

CGNGr65_30

(400 wide G)

Measured Nail axial force distributions with uw contour

X (m)

0 5 10 15 20 25 30

Y (m

)

0

5

10

15

20

CGN65_30 CGNH65_30 CGNH165_30 CGNGr65_30

-100

0

+100 kN

A quasi-steady state was reached+ ve Tension- ve Compressionuw = -20kPa

100

60

100

8070

50

20

4030

-10

8

0

1

2

3

4

5

00.20.40.60.81Location of maximum nail force (a/l)

Inst

rum

ente

d na

il la

yer

CGN65_30CGNH65_30CGNH165_30CGNGr65_30

l a

Measured maximum nail tensile force locus

Locations of Tmax move towards slope surface with grillage

No obvious trend for cases with different nail head size

Three Dimensional Numerical Analysis

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Used software: ABAQUS

Soil model: Mohr Coulomb

Mesh for 3D-model

35.0,2500 == υkPaE00 20,7.38',5' === ψφkPac

Soil element: Coupled 3D pore fluid/stress elementNail/head/grillage model: Linear elastic

-40kPa-200

2040

6080

100120140

Calculated pore water pressure contour at a quasi-steady state

Unreinforced slope (CG65_30)

Equivalent plastic strain contour

Incremental displacement vector

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Calculated nail axial stress distributions with uwcontour

X (m)

0 5 10 15 20 25 30

Y (m

)

0

5

10

15

20

CGN65_30 CGNH65_30 CGNH165_30 CGNGr65_30

-2

0

+2 MPa

A quasi-steady state was reached+ ve Tension- ve Compressionuw=-40kPa-20

020

40

60

80

100

120140

160

Calculated locus of maximum nail force

0

1

2

3

4

5

00.20.40.60.81Location of maximum nail force (a/l)

Inst

rum

ente

d na

il la

yer

CGN65_30CGNH65_30CGNH165_30CGNGr65_30

l a

No obvious trend for Tmax locus with larger head plate

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Summary (1)

For the slope without soil nail, a continuous slip surface was triggered by rising groundwaterInclusion of soil nails (without nail head) didprevent the formation of an obvious continuous slip surface but not crack at the crest and local failure near the toeFurther inclusion of nail heads is beneficial to prevent the failure with the active zone sliding off the upper soil

Summary (2)

Greater nail axial tensile forces (especially for lower nails) were mobilised close to slope surface with larger nail head plate/grillageUsed numerical model can simulate experimental nail force distribution reasonably if there is no crack was observed in the testFrom these models (experimental, numerical), Tmax locus does not lie further away from or closer to the head for larger nail heads

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Thank You!

Q & A