1Danish Geotechnical Society Meeting

G t h i l M it i d th Ob ti l M th dGeotechnical Monitoring and the Observational MethodCopenhagen, 13-9-212

The Observational Method in Ground Engineering g g- Recent UK Experience

ByyDuncan Nicholson,

Di t ADirector, Arup

2Introduction

Background Peck 1969 Background – Peck 1969 Ciria Report 185 (1999) - Observational Method OM Principles

– Uncertaintyy– Design - Predefined and OM– Design Codesg– Rapid deterioration– Trigger Criteriagg

Technical and Commercial Risk Management Case Histories Case Histories

3History - Key datesy y

40 to 60’s - Terzaghi and Peck 1969 - Peck’s Rankine LectureEarly 90’s - Channel Tunnel, Limehouse LinkEarly 90 s Channel Tunnel, Limehouse Link

1994 G t h i S i i P i t 1994 - Geotechnique Symposium in Print 1995 - EC7 OM Clause 1996 - ICE and HSE NATM publications 1999 CIRIA OM Report No 185 1999 - CIRIA OM Report No 185 2001 - Managing Geotechnical Risk 2003 - Ciria C580 – Embedded Walls.

4Peck (1969) – Eight Ingredients( ) g g

a) Sufficent SI to establish general nature / ti f d itproperties of deposits.

b) Assess Most Probable and Most Unfavourablediticonditions.

c) Establish Design based on Most probable.d) Select Monitoring parameters and calculate

values.e) Calculate values for most unfavourable conditions.f) Select design Modification options.g) Monitor and evaluate actual conditions.h) Modify design to suit actual conditions.h) Modify design to suit actual conditions.

5Peck’s OM applicationsec s O app cat o s

Best way out OM applicationsy pp– Cleveland Ore Terminal - soft clays – stockpiles of iron ore– Cape Kennedy Causeway – Hydraulic fill

Ab Initio OM applications– Harris Bank – Chicago strut monitoring– Bay Transit Tunnels – Volume loss

PitfallsGeology could be worse buried chanels– Geology could be worse. – buried chanels

– Monitoring – Exclude progressive failure and brittleness.– Reporting and interpreting – Timely and robustp g p g y– Design must vary during construction – Contract issues

6

Ciria Report 185 (1999)Ciria Report 185 (1999)

7CIRIA Report 185, (1999)

The Observational Method in ground engineering:

principles and applicationsD P Nicholson, C-M Tse, and C Penny

Ch2 Definition and History Ch3 Concepts Ch3 Concepts Ch4 Technical Considerations Ch5 Management Considerations Ch5 Management Considerations Ch6 Contractual Framework Ch 7 9 Applications such as : Ch 7-9 Applications such as :-

Tunnelling, Excavations, Embankments, Environmental Geotechnics etcEnvironmental Geotechnics, etc..

8CIRIA (1999) - OM Definition

The Observational Method in ground engineering is a The Observational Method in ground engineering is a continuous, managed, integrated, process of design, construction control, monitoring and review which enables gpreviously defined modifications to be incorporated during or after construction as appropriate. All these aspects have to b d t bl b t Th bj ti i t hi tbe demonstrably robust. The objective is to achieve greater overall economy without compromising safety.

The Method can be adopted from the inception of a project or later if benefits are identified. However, the Method shouldlater if benefits are identified. However, the Method should not be used where there is insufficient time to implement fully and safely complete the planned modification or emergency plans.

9

The OM PProcess

for Ab initio approach

10

Concepts (Ch 3)Concepts (Ch 3) Uncertainty Design - Predefined and OM Design Codes Design Codes Rapid deterioration

T i C it i Trigger Criteria Implementation of Modifications Value Management

Talk based on Ab Initio applicationM t OM li t t k Most OM applies to temporary works

11Uncertainty in the GroundU ce ta ty t e G ou d

Geological Geological• eg- Complex geology and hydrogeology.

Parameter and modelling• eg- Undrained soil verses drained• eg- Undrained soil verses drained.

Ground treatment Ground treatment • eg - Grouting, dewatering.

(Construction Uncertainty)• eg – complex temporary workeg complex temporary work

12Geological Uncertainty - Weathered ChalkDriven piles - length variation

13Parameter Uncertainty – Undrained soil Two props “progressively modified” to one

14Ground Treatment

Uncertainty

Ground Water control

Philippines - dry dock

-Additional Well pointsfor drawdownfor drawdown

15Construction Uncertainty - CofferdamD d i d k ilt d fill tiDredging - dock silt and fill compaction

16

C t (Ch 3)Concepts (Ch 3)

UncertaintyDesign - Product and ProcessDesign Product and ProcessDesign CodesRapid deteriorationTrigger CriteriaTrigger Criteria Implementation of ModificationsValue Management

17The Design Productg

The Design Product is:- Drawings, Calcs, Spec etc.

HSE CDM Regs (1994)(Construction Design and Management)

D i i l d d i d i d t il• Design includes: drawings, design details, specifications and BOQ

• Address Buildability• Identify hazards

A i k Ri k i t• Assess risks – Risk register

• Design and construction links via:-Design and construction links via:• ‘Health and Safety Plan’• Planning Supervisor

Stronger design and construction links

18Predefined Design Process Permanent works

The OM Process Temporary works Permanent works

One set of parameters O d i / di ti

Temporary works Two sets of parameters T d i d di ti One design / predictions

Outline of construction method

Two designs and predictions Integrated design and

construction methodsmethod

Contractors temp design

construction methods

Methods relate to triggers Contractors temp design /method statement

Monitoring checks

Methods relate to triggers

Comprehensive and robustMonitoring checks predictions not exceeded

Comprehensive and robust monitoring system

Review and modify process Review and modify process– contingency plan– improvement plan

Emergency plan Emergency Plan

19Design Codes - Comparison with

Peck’s (1969) OM and Current Codes

Peck (1969) UK Codes– OM conditions/values – CIRIA 580, BS 8110, BS 8002

– Eurocode – EC7• Most Probable• Not used

• Not used• Mod conservative orNot used

M t

Mod conservative or Characteristic

• Most Unfavourable • Worst credible

20Eurocode EC7 Cl 2.7Eurocode EC7 Cl 2.7

(1989 and1995) Recognised prediction is difficult in Geotechnics –

OM d i thOM used in these cases.

– 1) Establish limits of behaviour.– 2) Acceptable probability actual behaviour within limits.– 3) Monitoring plan, response times and contingencies.– 4) Contingencies adopted if real outside acceptable

range.

21UK Design Codes - Soil Strength Parameters

rop

erty

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ive

ob

able

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en

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e

ble

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ead

ing

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Mo

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mat

eu

ctu

ral

avo

ura

b

erat

ely

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f R

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ract

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ost

Un

fa

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dNo

Ch

ar(u

sed

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Soil Strength Parameter Results1 i 21 i 201 i 1000 1 in 21 in 201 in 1000 (Eg Undrained strength, SPT etc)

22Ideal EC7 Predicted versus Measured PerformancePerformance

Predicted EC7

Predicted mostprobable value

GREEN

ding

s "Ideal" distribution ofmeasured deflections

Predicted EC7Characteristic

Value (SLS)AMBER

o. o

f rea

d

REDMost U f bl

No

5%

Unfavourable(ULS)

Deflection ( )

23

Trigger CriteriaTrigger Criteria

Traffic light conditions include:Traffic light conditions include:-

• Green = Safe site condition.

• Amber = Decision stage

• Red = Implement planned modifications

• EmergencyEmergency(Not normally part of OM. Required under CHSW Reg (1996). Relates to Ultimate Limit State.)

24Trigger Criteria Example : TunnellingEmergency plan

men

tg y p

implementation ULS( adjacent

buildings &project

Without modification

ce s

ettle

m

ED

Emergency Trigger structure )

val

ue

or S

urfa

c RE

SLS( adjacentbuildings )Discovery

Mon

itore

dve

rgen

ce

AM

BER

Red trigger

Amber trigger

With planned modification

buildings )Discovery

Mnn

el C

onv A

EN Behaviour

eg T

unG

REE

Decision Implementation

T d t i t t

Time

Trend rate – important

25

Multi stageMulti stage Construction

26OM Limitation - Rapid Deterioration‘...... the Method should not be used where there is insufficient time to implement fully and safely complete the planned modification or emergency plans.....’ (CIRIA, 1999)

D i i d dDeterioration rate depends on:

- Ground conditions (ductile/brittle soil behaviour)

Groundwater conditions (rainfall/burst water mains)- Groundwater conditions (rainfall/burst water mains)

- Temporary surchargesp y g

- Construction sequence and programme

27

Implementation of the OMImplementation of the OM

Construction Implementation Co st uct o p e e tat o

• Peck (1969)• Peck, (1969)• Starts with most probable ( MP )

conditionsconditions

• CIRIA Report 185, (1999)• Starts with characteristic ( Moderately

Conservation) conditions. • Progressive Modification to Most Probable

28Failure characteristics

Ductile behaviour– Significant displacement to mobilise peakcharacteristics Significant displacement to mobilise peak

strength => possible warnings

– e.g. Soft clays, Sands, Ash, Previous failures (residual strength mobilised)

Mobilised strength failures (residual strength mobilised)

– Cyclic loading – “creep”

– Also seasonal changes (wetting/drying)

OM applicable

Brittle behaviourDisplacement

P k – Rapid, run-away failures

– Low displacement to mobilise peak strength => Minimal warning

Mobilised strength

Peak

g g

– e.g. Stiff clays

– Also ‘high rainfall - slope flooding’Residual Also high rainfall slope flooding

OM not applicable?Displacement

29Technical & Commercial Risk ManagementRisk Management

OM Technical Balance

30Technical & Commercial Risk ManagementRisk Management

OM Commercial Project Risks

31Contractual (Ch 6)Contractual (Ch 6)

Types of Contracts.ypContractual Difficulties using OM.Contractual RisksContractual Risks.Strategies.

– Traditional Contracts• VE Clauses• Novation• Partnering

– Design and Build • Strong Interface Manager.

32

T l52

Tunnels(ch 7)(ch. 7)

Mainly NATMy Design and Planning

– JLE NATM tunnels Post ARUP Balfour Beatty

LUL JUBILEE LINE EXTENSION CONTRACT 102

JLE NATM tunnels Post Heathrow

Monitoring50

Monitoring Review process

ARUP Balfour Beatty

33Excavations

(Ch 8) Cuttings

– Slopes – Soil Nails– Contingencies -

dewatering Retaining walls

– Cantilever – Multipropped– Contingencies:-

Berms and soft proppingp pp g

34Other Applications (Ch 9)Other Applications (Ch 9)

1Without compensation grouting, excessive settlement leading to cracking.

Crack

Ground Treatment

Grouting

g

Acceptable level of Settlement

– Grouting– Dewatering– Deep compaction Settlement without

compensation Embankments

– Soft clays E i t l

compensation grouting

GROUND TREATMENTUNCERTAINTY -Settlement Controlusing Compensation Grout

Final grouting

Environmental– Containment– Clean up NATM

Initial grouting

p Structures

– Bridges and Dams

NATMTunnel

GEOLOGICAL UNCERTAINTY- tunnel face logged- variable groundPARAMETER UNCERTAINTY

con ergence monitoring– Tunnelling effectsg

scour holesconvergence monitoring lining pressure

Compensation Grouting

35Conclusions (Ch10)The CIRIA Report has achieved the following objectives:

• Established a definition of the OM Process.

• Clarified Uncertainty and Risk Management.

• Integrated OM into design codes and site control procedures.

• Provided technical, managerial and contractual guidelines for the team.

• Summarised applications/case histories.

36

Thank you for your AttentionThank you for your Attention.