the observational method in ground engggineering - … · 8 ciria (1999) - om definition the...
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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
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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
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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.
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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.
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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
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6
Ciria Report 185 (1999)Ciria Report 185 (1999)
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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..
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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.
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9
The OM PProcess
for Ab initio approach
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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
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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
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12Geological Uncertainty - Weathered ChalkDriven piles - length variation
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13Parameter Uncertainty – Undrained soil Two props “progressively modified” to one
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14Ground Treatment
Uncertainty
Ground Water control
Philippines - dry dock
-Additional Well pointsfor drawdownfor drawdown
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15Construction Uncertainty - CofferdamD d i d k ilt d fill tiDredging - dock silt and fill compaction
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16
C t (Ch 3)Concepts (Ch 3)
UncertaintyDesign - Product and ProcessDesign Product and ProcessDesign CodesRapid deteriorationTrigger CriteriaTrigger Criteria Implementation of ModificationsValue Management
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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
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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
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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
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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.
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21UK Design Codes - Soil Strength Parameters
rop
erty
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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)
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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 ( )
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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.)
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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
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25
Multi stageMulti stage Construction
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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
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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
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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
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29Technical & Commercial Risk ManagementRisk Management
OM Technical Balance
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30Technical & Commercial Risk ManagementRisk Management
OM Commercial Project Risks
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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.
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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
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33Excavations
(Ch 8) Cuttings
– Slopes – Soil Nails– Contingencies -
dewatering Retaining walls
– Cantilever – Multipropped– Contingencies:-
Berms and soft proppingp pp g
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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
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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.
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36
Thank you for your AttentionThank you for your Attention.