RSSB 1498-T679 The effects
of railway traffic loading on
embankment stability
Presented by
Graham Taylor/Eifion Evans
4 November 2015
Case History – Mottingham, Kent
2
Research Programme Objectives
 Measure damage to embankments of clay core construction
due to railway traffic loading
 Develop an analytical model for damage assessment due to
change in axle load or frequency
 Provide a simple and reliable assessment methodology for
prediction of damage to clay fill embankments and which
can be used for route classification
3
RSSB1498-T679 The
effects of railway traffic
loading on embankment
stability
4 November 2015
Research Programme Structure
Stage
Description
1A
Enhanced desk study
1B
Laboratory testing
2A
Embankment monitoring at prototype site
2B
Report on Stages 1A, 1B and 2A;
development of business case for subsequent
stages
3
Embankment monitoring at several sites
4
Final analytical model development
4
RSSB1498-T679 The
effects of railway traffic
loading on embankment
stability
4 November 2015
Stage 1
Initial Development of Analytical Model
 Preliminary analysis of NR records
– Maintenance records for embankment and track
– Track ballast thickness
 Laboratory testing programme
– Small-scale element tests
– Large-scale physical model tests
 Numerical modelling
 Development of analytical model
5
RSSB1498-T679 The
effects of railway traffic
loading on embankment
stability
4 November 2015
Stage 1
Measurement of Damage
 Model embankment and laboratory testing undertaken to determine the
required material characteristics and their response to vehicle loading
6
RSSB1498-T679 The
effects of railway traffic
loading on embankment
stability
4 November 2015
Stage 1
Measurement of Damage
 Model embankment and laboratory testing undertaken to determine the
required material characteristics and their response to vehicle loading
7
RSSB1498-T679 The
effects of railway traffic
loading on embankment
stability
4 November 2015
Stage 1
Measurement of Damage
 Model embankment and laboratory testing undertaken to determine the
required material characteristics and their response to vehicle loading
8
RSSB1498-T679 The
effects of railway traffic
loading on embankment
stability
4 November 2015
Clay Material Characteristics
Most important factors:
 Degree of clay plasticity, which is a measure of the way in which clay
deforms under loading
and
 The change in deviatoric stress
9
RSSB1498-T679 The
effects of railway traffic
loading on embankment
stability
4 November 2015
Effect of Plasticity
High plasticity clays are
subject to much greater
levels of permanent
deformation under
repeated loading
compared to low plasticity
clays (Li (Association of
American Railroads) &
Selig (University of
Massachusetts),
‘Cumulative Plastic
Deformation for Fine
Grained Soils’)
10
RSSB1498-T679 The
effects of railway traffic
loading on embankment
stability
4 November 2015
Effects of axle load and plasticity
Stage 1
Deviatoric Stress
 Mohrs Circles - major (σ1)
and minor (σ3) principal
stresses
 Deviatoric stress, σd
σd = σ1 - σ3
σ1
σ3
11
RSSB1498-T679 The
effects of railway traffic
loading on embankment
stability
4 November 2015
Stage 1
Development of Analytical Model for Potential Impact Assessment
Prediction of Cumulative Plastic Strain within the Service Load Range for a
given number of applied stress cycles adopting power law model of Li &
Selig (1996):
σ
εp = a( σd )m Nb
s
where
εp is the plastic strain in the soil,
σd is the applied deviatoric stress,
σs related to the threshold strength of the soil,
a, m and b are constants to be determined, and
N is the number of stress cycles applied.
12
RSSB1498-T679 The
effects of railway traffic
loading on embankment
stability
4 November 2015
Analytical Model - Derivation of Constants
13
RSSB1498-T679 The
effects of railway traffic
loading on embankment
stability
4 November 2015
Potential Impact Assessment Philosophy
 In common with practice for structural design and assessment, the limit
state principle is adopted for geotechnical structures such as
embankments
 The two limit states of relevance are
- Ultimate Limit State: verification of strength and stability
- Serviceability Limit State: verification of acceptable performance
under service loading including deformation and ride quality
14
RSSB1498-T679 The
effects of railway traffic
loading on embankment
stability
4 November 2015
Stage 1
Assessment Methodology
SLS check
satisfied. No
maintenance
for traffic
loading
effects
required
Detailed
assessment
required
Start
Determine
- threshold
stress, σT
- proposed
change in
deviatoric
stress, σd
ULS
check
YES
σd > σT
YES
NO
SLS
check
Use
reload
load
model
scenario
Determine
plastic
deformation,
δp
δp< δallowable
NO
Embankment potential impact assessment for a typical train: Impact of increase in railway traffic
loading
15
RSSB1498-T679 The
effects of railway traffic
loading on embankment
stability
4 November 2015
Maintenance
impact
Ultimate Limit State Check
Comparison of the predicted change in the deviatoric stress, Δσd, within the
clay under a given train axle load against an allowable (or ‘threshold’) stress,
σT, for the clay.
If Δσd < σT then the ULS check is satisfied.
If Δσd > σT then failure of the embankment is possible.
16
RSSB1498-T679 The
effects of railway traffic
loading on embankment
stability
4 November 2015
Threshold Stress Level
 For fine grained materials such as clays,
under repeated loading permanent
deformation increases with the logarithm of
the number of cycles of applied load, the rate
of accumulation of plastic strain increasing as
the stress level increases
 Eventually a ‘threshold’ deviator stress level
(qthreshold), at which the rate of accumulation
of permanent deformation increases
exponentially, is reached
17
RSSB1498-T679 The
effects of railway traffic
loading on embankment
stability
4 November 2015
Serviceability Limit State Check
Comparison of the predicted deformation, δp, which will
accumulate under repeated axle loading against an
allowable limit, δa
If δp < δa then the SLS check is satisfied; no maintenance
for traffic loading effects required
If δp > δa then traffic loading effects contribute to
maintenance requirements
Effectiveness of drainage provision is also a key factor
18
RSSB1498-T679 The
effects of railway traffic
loading on embankment
stability
4 November 2015
Allowable Deformation (δa )
Allowable deformation over defined length of track, δa, comprises
deformation due to:
• Ballast degradation
• Seasonal effects – shrink/swell
• Plastic deformation due to rail traffic loading
Defined length of track
Design track level
19
RSSB1498-T679 The
effects of railway traffic
loading on embankment
stability
4 November 2015
Utilisation of Assessment Methodology
 Ultimate Limit State – checks for potential embankment
failure will provide information for NR Earthworks and
Track Maintenance Engineers
 Serviceability Limit State – checks for permanent
deformation of the track will provide information for NR
Earthworks and Track Maintenance Engineers
20
RSSB1498-T679 The
effects of railway traffic
loading on embankment
stability
4 November 2015
Potential Implementation within Network Rail
 Potential impact assessment model; a tool for
–Route clearance
–Maintenance assessments/budgeting
–Track quality assessments
–Business planning (i.e. Control Period 6)
 NR engagement with Stakeholders
–Office of the Rail and Road Regulator – budget
determination
–Train Operating Companies (TOCs)/Freight Operating
Companies (FOCs)
21
RSSB1498-T679 The
effects of railway traffic
loading on embankment
stability
4 November 2015
Summary
 RSSB1498-T679 completed May 2014
 Potential Impact Assessment Methodology delivered to NR
 Assessment Methodology Testing and Validation
– NR RAM (GEO)s briefed and model made available
 Wider Project Dissemination
– PWI presentation
– Stephenson Conference paper/presentation
– ECSMGE – Edinburgh 2015
– Abstract accepted, 11th World Congress on Railway Research, Milan
– Journal paper (in preparation)
22
RSSB1498-T679 The
effects of railway traffic
loading on embankment
stability
4 November 2015
Next Steps
 Implementation of improved embankment impact assessment
methodology within NR for assessment of maintenance
requirements
 CP6 Policy – Development of New Asset Metric to quantify
capability of embankments to accept increases in axle
load/overall loading
 Future development of assessment methodology (e.g. site
instrumentation and monitoring)
23
RSSB1498-T679 The
effects of railway traffic
loading on embankment
stability
4 November 2015
Thank you