Slope Stability Analysis of London Slope Stability Analysis of London Clay Rail EmbankmentsClay Rail Embankments

Rob Gilbert

Talk OutlineTalk Outline

nn Brief outline of problemBrief outline of problemnn History of embankment constructionHistory of embankment constructionnn Research so farResearch so farnn Discussion of the projectDiscussion of the projectnn FindingsFindingsnn ReviewReview

Area of embankments concernedArea of embankments concerned

High Plasticity

Non-plastic ground

M25

LONDONArup 2006

Early Embankment constructionEarly Embankment constructionnn Constructed quickly using local Constructed quickly using local

materialsmaterialsnn Moved by horse or locomotive Moved by horse or locomotive nn Tipped in place with little compactionTipped in place with little compactionnn Ash used to top the embankmentAsh used to top the embankment

Arup 2006

Current Problems and External Current Problems and External ProcessesProcesses

Ballast

Arup 2006

Problems site?Problems site?Dense mature trees

Grass and shrubs

Gravel covering

Very close land boundary

Management action planManagement action plan

Arup 2006

How to tackle the problemHow to tackle the problem

nn Construct a generic embankment profileConstruct a generic embankment profilenn Vary height / slope angle / crest widthVary height / slope angle / crest widthnn Other factorsOther factors

nn Pore pressurePore pressurenn Strength of rootsStrength of rootsnn Train LoadingTrain Loadingnn UndercuttingUndercutting

nn Soil parameters Soil parameters –– Sensitivity analysisSensitivity analysis

Standard profileStandard profile

Description: London ClayUnit Weight: 19Cohesion: 2Phi: 21Ru (added): 0.155

Description: RootsUnit Weight: 19Cohesion: 7Phi: 21Ru (added): 0.155

Description: Embankment f ill: cohesiv eUnit Weight: 19Cohesion: 2Phi: 21

Description: AshUnit Weight: 11Cohesion: 0Phi: 35

Description: BallastUnit Weight: 18Cohesion: 0Phi: 40

Description: RootsUnit Weight: 19Cohesion: 7Phi: 21

0 2 4 6 8 10 12 14 16 18 20-4

-3

-2

-1

0

1

2

3

4

5

6

7

8

9

10

Shoulder pressure 10kN/m2

Track pressure 50kN/m2

Mature treepressure 1kN/m2

Height 6m

Piezometricsurface

Cess Width

Pore Pressures Pore Pressures –– Using Using rru u CoefficientsCoefficients

Description: BallastUnit Weight: 18Cohesion: 0Phi: 40

Description: AshUnit Weight: 11Cohesion: 0Phi: 35

Description: Embankment fill: cohesiveUnit Weight: 19Cohesion: 2Phi: 21

Description: London ClayUnit Weight: 19Cohesion: 2Phi: 21Pore-Air Pressure: 0

-4 -2 0 2 4 6 8 10 12 14 16 18 20 22-3

-2

-1

0

1

2

3

4

5

6

7

8

9

Computed pore pressure

Required pore pressure

Unit weight of water set to 70%

ru coefficient used in London Clay

Grid of pore pressuresGrid of pore pressures

Grid of Factors of SafetyGrid of Factors of Safety

2 4 6 81:1.5

1:2

1:2.5

1:3

1:3.5

1:4Factor of Safety

Height (m)

Slope

Height vs. Slope

1.15-1.301.00-1.150.85-1.000.70-0.850.55-0.700.40-0.55

Standard Profile with no vegetation

FoS 1.0

Target FoS1.15

MatrixMatrix

Previous grid modelled with different cess widths for each geometry

LUL vegetation envelopeBare slope profile Terrace Gravels

Internal geometryInternal geometryAsh Internal Profile

0.8

0.9

1

1.1

1.2

1.3

0 0.5 1 1.5 2 2.5

thickness (m)

Fact

or o

f saf

ety

6m

2m

Ballast Internal Profile

0.8

0.9

1

1.1

1.2

1.3

0 0.5 1 1.5 2 2.5

thickness (m)

Fact

or o

f saf

ety

6m

2m

Difference in train loadsDifference in train loads

0.1

LUL Network Rail

0.05

0.02Height

FindingsFindings

nn Link back to estimates Link back to estimates nn Sensitivity analysisSensitivity analysisnn Correlation with case studiesCorrelation with case studiesnn It was possible to rule out some It was possible to rule out some

embankments of being at risk from deep embankments of being at risk from deep seated failureseated failure

AdviceAdvicenn Recommend BRecommend B--bar method to represent bar method to represent

pore pressurespore pressuresnn MorgensternMorgenstern--Price method of slip surface Price method of slip surface

analysis should be usedanalysis should be used

LimitationsLimitationsnn Huge possible range of embankment Huge possible range of embankment

profilesprofilesnn Unusual ground conditions Unusual ground conditions –– shear planesshear planesnn Lack of data for large embankmentsLack of data for large embankments

ReferencesReferencesnn Arup. 2006. London Clay Earth Works. Reference Document for Arup. 2006. London Clay Earth Works. Reference Document for

Embankments Unpublished Report. (Produced for Network Rail Embankments Unpublished Report. (Produced for Network Rail LNET)LNET)

nn CiriaCiria C592. Infrastructure and Embankments C592. Infrastructure and Embankments –– condition appraisal condition appraisal and remedial treatment. and remedial treatment. ContrustionContrustion Industry Research and Industry Research and Information AssociationInformation Association, London 2003., London 2003.

nn SkemptonSkempton, A.W. 1996. Embankments and Cuttings on the early , A.W. 1996. Embankments and Cuttings on the early Railways. Railways. Construction History, Construction History, 11, pp. 3311, pp. 33--4949

nn Vaughan, P.R. 2003. Historic and recent studies of the stabilityVaughan, P.R. 2003. Historic and recent studies of the stability of of cutting and embankment slopes for roads and railways and the cutting and embankment slopes for roads and railways and the potential for improved design and maintenance procedures. potential for improved design and maintenance procedures. NewslopsskenbackgrndNewslopsskenbackgrnd 154.03 Imperial college154.03 Imperial college

nn Ground Engineering May 2006, Feat of Clay, p 22, (author unknownGround Engineering May 2006, Feat of Clay, p 22, (author unknown))