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))