APPLICATION OF THE OBSERVATIONAL METHOD FOR RAILWAY EARTHWORK STABILISATION IN THE UK

Sangmin Lee, Senior Geotechnical Engineer

INTRODUCTION

• Fundamentals of the Observational Method

• Application of the Observational Method for Railway Earthwork

Stabilisation in the UK: South of Manningtree Railway Embankment

- Project Background

- Base Design

- Contingency Design

- Monitoring Plan & Trigger Levels

2

FUNDAMENTALS OF THE OBSERVATIONAL METHOD

3

Brief History

Quoted from Soil Mechanics in Engineering Practice, Terzaghi, 1945:

In the past, only two methods have been used for coping with the inevitable

uncertainties:

either to adopt an excessive factor of safety, or else to make assumptions in

accordance with general, average experience…..The first method is wasteful; the

second is dangerous.

Soil mechanics, as we understand it today, provides a third method which could be

called the experimental method.

FUNDAMENTALS OF THE OBSERVATIONAL METHOD

4

FUNDAMENTALS OF THE OBSERVATIONAL METHOD

Brief History

Peck’s Rankine Lecture (1969):

•An integrated process for predicting, monitoring, reviewing and modifying

designs without compromising safety

•To successfully implement OM, it was necessary to have two designs

compared with the traditional single design approach in Geotechnics:

a design starting with the most probable (best estimate) condition

and varying the design and/or construction to the planned most

unfavourable condition, should observed behaviour be worst than

those predicted based on best estimate conditions.

Powderham’s Approach – Progressive modification

5

FUNDAMENTALS OF THE OBSERVATIONAL METHOD

Guidances and Standards available in the UK

• CIRIA (Construction Industry Research and Information Association in the

UK): 1999. CIRIA Report 185 “The Observational Method in Ground

Engineering: principles and applications”

• Network Rail: 2009. Guidance Note (NR/GN/CIV/801) “The application of

the Observational Approach to the design of remedial works to

earthworks”

• Eurocode: EN 1997-1: Section 2: Basis of geotechnical design

6

APPLICATION OF THE OBSERVATIONAL METHOD FOR RAILWAY EARTHWORK STABILISATION IN THE UK

• Project Background

• Base Design

• Contingency Design

• Monitoring Plan & Trigger Levels

7

PROJECT BACKGROUND

Location

8

PROJECT BACKGROUND

Topography

• Embankment Length = 600

metres

• Embankment Height = 4.5

metres to 8 metres

• Side slopes = 31 and 42 from

the horizontal (1 in 1.10 to 1.67)

• Embankment running on the

valley side of the River Stour

9

PROJECT BACKGROUND

Stratigraphy

10

PROJECT BACKGROUND

Signs of unstable embankment slope

11

PROJECT BACKGROUND

Signs of unstable embankment slope

12

PROJECT BACKGROUND

Project History

Preliminary Design:

Gabion Toe Wall +

Regrade of Slope with

overall FoS > 1.3

Issues with Network Rail Boundaries.

Re-grading slope angle had to be

reduced. Therefore, FoS < 1.3

Developing alternatives: Sheet Pile Wall, Soil Nailing,

Observational Method, etc

Value Engineering: NR decided to

go for the Observational Method

with FoS > 1.15

13

WHAT TO DO WITH THE OM?

• Base Design

• Contingency Design

• Monitoring Plan

- Intervention Plan, Trigger Levels, Monitoring Frequency

14

DESIGN REQUIREMENTS

• Design in accordance with Network Rail Guidance Note (NR/GN/CIV/801)

and CIRIA Report 185

• Overall Factor of Safety and Soil Conditions

• Settlement limit for remediated embankments

Base Design Contingency Design

Overall Factor of Safety

FoS > 1.15 FoS > 1.3

Conditions Most Probable Most Unfavourable

Time after re-opening the track to traffic

Maximum permitted cumulative settlement (track)

4 weeks 15 mm

6 months 25 mm

12 months 30 mm

15

SOIL PARAMETERS

Base Design Contingency Design

16

BASE DESIGN

• Most probable conditions

• Overall Factor of Safety > 1.15

• Gabion Toe Wall + Regrade of the existing steep embankment

17

CONTINGENCY DESIGN

• Most Unfavourable Condition

• Overall Factor of Safety > 1.3

18

MONITORING PLAN

• Groundwater & SMD : Piezometers, standpipes and a local rainfall gauge

• Shear movement within the embankment: inclinometers and gabion wall

survey points

• Settlement under the track: track monitoring and CCQ data

19

MONITORING PLAN

Colour Coded Quality Data (CCQ)

• Quality of a track, that is dependent on vertical and horizontal positions and tilts between the

two tracks and also along the track length

20

MONITORING PLAN

Intervention level (trigger value): a pre-determined point at which a decision is

made about a change to the design, construction programme or detail(s) of the site

works.

• Action level (RED value) -

The point at which a planned

modification is implemented

• Warning level (AMBER

value) - The point at which

an intervention plan is

implemented.

Traffic light system for an incremental excavation process (CIRIA 185, 1999)

21

TRIGGER LEVELS

Soil Moisture Deficit

Intervention

Levels SMD Actions

Green Dry/Normal

( 50mm)

Continue to monitor monthly unless other instruments/monitoring equipment

have reached intervention levels

Amber Wet (>0mm,

<50mm)

Review if other instrumentation trigger levels are exceeded

Yes: increase piezometer monitoring frequency twice weekly and check

the piezometer data with the trigger level

No : continue to monitor monthly

Red Saturated

( 0mm)

Increase piezometer monitoring frequency twice weekly

Increase gabion wall survey points/inclinometer monitoring frequency to every

2 weeks until SMD falls below saturation

Review piezometer/gabion survey point/inclinometer data

Senior Earthwork Management Engineer to assess/review the site with Territory

Geotechnical Engineer

22

TRIGGER LEVELS

Piezometer

0.75

0.8

0.85

0.9

0.95

1

1.05

1.1

1.15

1.2

2 3 4 5 6 7 8 9 10

Fac

tor

of

sa

fety

Ground water level (mAOD)

Piezometer trigger levels (mAOD) at Mid-slope

23

TRIGGER LEVELS

Piezometer

Intervention Levels

Piezometer

/standpipe

(h, mAOD)

Actions

Green zone various for each

instrument

Continue to monitor monthly unless other instruments/monitoring

equipment have reached intervention levels

Amber zone various for each

instrument

Continue to monitor twice weekly until SMD falls below saturation

and/or the intervention level goes down to Green

Consider to increase frequency of gabion wall survey points and

inclinometers

Red zone various for each

instrument

Continue to monitor twice weekly

Increase monitoring frequency of gabion wall survey points and

inclinometers to at least every 2 weeks

Review inclinometer and gabion wall data

Senior Earthwork Management Engineer to assess/review the site

with Territory Geotechnical Engineer

24

TRIGGER LEVELS

Gabion Wall Survey Point

• Measure tilt or strain of gabion wall due to move or fail by sliding and bulging.

• Survey point interval: 5 metres

0

5

10

15

20

25

30

35

40

45

0 20 40 60 80 100 120

Tra

ck M

ovem

en

t (m

m)

Gabion Wall Movement (mm)

Intervention level for gabion wall survey points

25

TRIGGER LEVELS

Gabion Wall Survey Point

26

TRIGGER LEVELS

Gabion Wall Survey Point

Intervention

Levels

Gabion wall

displacement

(d, mm)

Actions

Green zone d <40 Continue to monitor monthly unless other instruments/monitoring equipment

have reached intervention levels

Amber zone 40 d <65

Review CCQ data, inclinometer data alongside gabion data

Increase monitoring frequency of gabion wall survey points to every 2 weeks

Network Rail representative to undertake site visit

Senior Earthwork Management Engineer to assess/review the site with Territory

Geotechnical Engineer

Decision on further action (if any) to be documented on file

Red zone 65 d <80

Increase monitoring frequency of both gabions and inclinometer to at least every

2 weeks until movement slows or is agreed with Territory Geotechnical Engineer

Senior Earthwork Management Engineer to discuss site with Track Section

Manager

Network Rail personnel to undertake site visit ASAP

Advise mainframe contractor of site mobilisation possibility

Give strong consideration to undertaking soil nail installation in problematic areas

Decision on further action (if any) to be documented on file

27

TRIGGER LEVELS

Inclinometer

Intervention

level

Maximum

horizontal

displacement

(dh, mm)

Actions

Green Zone dh < 5 Continue to monitor monthly unless other instruments/monitoring equipment

have reached intervention levels

Amber Zone 5 dh < 10

Review CCQ data, inclinometer data alongside gabion data

Increase monitoring frequency of inclinometers to every 2 weeks

Network Rail representative to undertake site visit

Senior Earthwork Management Engineer to review with Territory Geotechnical

Engineer & Decision on further action (if any) to be documented on file

Red Zone 10 dh < 15

Increase monitoring frequency of both gabions and inclinometer to at least

every 2 weeks until movement slows or is agreed with Territory Geotechnical

Engineer

Senior Earthwork Management Engineer to discuss site with Track Section

Manager

Network Rail personnel to undertake site visit ASAP

Advise mainframe contractor of site mobilisation possibility

Give strong consideration to undertaking soil nail installation in problematic

areas & Decision on further action (if any) to be documented on file

28

POST CONSTRUCTION MONITORING PROCESS MAP

Inclinometer

Post Construction Monitoring Process / scheme for 2 years – to

be reviewed every year

Gabion Wall Track Piezometers / Standpipes SMD (Soil Moisture Deficit)

Monitoring undertaken every month (12No spaced 20m apart altering in toe and midslope).

Monitoring of Gabion wall undertaken every month. Absolute total position (XYZ) to be recorded

Monitoring to be undertaken and reviewed by NR during construction by a track engineer only in accordance with NR/SP/CIV/071 and NR/BS/LI/045 (issue 2). Information (CCQ data, etc) to be made available for the DE, SEME etc.

Monitor Piezometers monthly. 8 Piezometers monitored with a data logger and 4 monitored manually (Intervention levels to be obtained from slope stability analysis at location of Piezometer).

Soil Moisture Deficit to be remotely monitored on a weekly basis using site rainfall data.

Green dh<5

Amber

5 dh<10

Red

10 dh<15

Green d<40

Amber

40 d<65

Red

65 d<80

Green Amber Red Green Amber Red Green <50

Amber >0, <50

Red ≤0

Green

Continue to monitor at existing frequency unless other instruments/ monitoring equipment have reached amber or red intervention levels

Consider the Following:

What does the monitoring data of the instruments show?

Is there an anomalous reading?

Is a site visit needed by DE, CM and NRR?

Do we need to assess / interpret all data (including site visit data)?

Is a decision meeting needed / N.R to review?

Does the SEME need to review the above with TGE?

What actions need to be taken? All considerations need to be documented on file

Amber

Red

Consider the following:

What does the monitoring data of the instruments show?

Is there an anomalous reading?

Is a Site visit needed by DE, CM, NRR?

Does the SEME need to asses / review the site with Track Section Manager?

Does there need to be a Decision meeting / N.R to Review?

What actions need to be taken? All considerations need to be documented on file

UNDERTAKING REACTIVE SOIL NAILLING IF:

TME has problems with top/alignment levels over the effected area attributed to the embankment, or

Features of earthwork instability apparent (i.e. drop in cess), or

Movement continues by a further 10mm for Inclinometers and 50mm for Gabion wall survey points beyond the red threshold level after the initial triggering reading, or

It is considered necessary by TGE

What to do at each intervention level (Green, Amber, and Red) Key DE – Design Engineer CM – Construction Manager NRR – Network Rail Representative SEME – Senior Earthwork Management Engineer TGE – Territory Geotechnical Engineer TME – Track maintenance Engineer dh – Maximum horizontal displacement d – Displacement

Actions:

Do Nothing

Amend Trigger Levels

Increase monitoring frequency

Proceed to Red Trigger level All actions need to be documented on file

Actions:

Do Nothing

Amend Trigger Levels

Increase monitoring frequency

Advise mainframe contractor of site mobilisation possibility

Activate soil Nailing All actions need to be documented on file

29

CONCLUSIONS

• 1st time used in Network Rail East Anglia Territory

• Network Rail saved £1million

• Trigger values stay in Green, and Intervention Plan/ Planned Modification

not implemented up to date.

• Base Design: Overall FoS > 1.15 with Most Probable conditions

• Contingency design: Overall FoS > 1.3 with Most Unfavourable Conditions

• Monitoring System: a weather station including a rainfall gauge,

piezometers, inclinometers, gabion wall survey points, and CCQ Data

30

THANK YOU

31