1Asia

Managing Geotechnical RiskManaging Geotechnical RiskAre we learning from the failures Are we learning from the failures

““The Use of Instrumentation and Monitoring to Verify The Use of Instrumentation and Monitoring to Verify Design and Control Construction of Deep Design and Control Construction of Deep

ExcavationsExcavations (modified)” (modified)”

Andy PicklesAndy Picklesof of

GCG (Asia) Ltd.GCG (Asia) Ltd.

2Asia

Overview of PresentationOverview of Presentation

Presentation relates to deep excavation for major Presentation relates to deep excavation for major infrastructure projectsinfrastructure projects

• Risk in design and role of instrumentationRisk in design and role of instrumentation

• Typical scope and cost of instrumentationTypical scope and cost of instrumentation

• Obtaining added value (reducing risk) from instrumentationObtaining added value (reducing risk) from instrumentation

• Interpretation of instrumentationInterpretation of instrumentation

3Asia

Risk in Design of Deep ExcavationsRisk in Design of Deep Excavations

Instrumentation is associated with the management of Instrumentation is associated with the management of risk (i.e. uncertainties). These include:risk (i.e. uncertainties). These include:

• Soil stratigraphySoil stratigraphy

• Geotechnical design parametersGeotechnical design parameters

• Construction related factorsConstruction related factors

4Asia

Uncertainty of Soil StratigraphyUncertainty of Soil StratigraphyVariation of Continuity of Sand LayerVariation of Continuity of Sand Layer

Discontinuous Layer Continuous Layer

5Asia

Uncertainty in Design Parameters - 1Deep Excavation in Clay

-36

-32

-28

-24

-20

-16

-12

-8

-4

0

4

0 20 40 60 80 100 120 140 160

Undrained shear strength, cu (kPa)

Re

du

ced

leve

l (m

)_

Strength_1

Strength_2

FILL

Soft CLAY

Stiff CLAY

Clayey SAND

Hard CLAY

7540 Final exc

-36

-32

-28

-24

-20

-16

-12

-8

-4

0

4

0.0

00

0.0

40

0.0

80

0.1

20

0.1

60

0.2

00

0.2

40

Wall horizontal disp. (m)

Re

du

ced

leve

l (m

) _

Strength_1

Strength_2

Bend Mom 2800 kNm

5000 kNm

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Uncertainties in Design Parameters – 2Uncertainties in Design Parameters – 2Variation of Rate of SofteningVariation of Rate of Softening

Low Permeability High Permeability

7Asia

Construction Related Uncertainties -1Construction Related Uncertainties -1

Performance of Grout Cut-off

CLAY

8Asia

Construction Related Uncertainties - 2Construction Related Uncertainties - 2

• Ground settlement on wall installationGround settlement on wall installation

• Effectiveness and extent of dewateringEffectiveness and extent of dewatering

• Properties of jet grout layersProperties of jet grout layers

• Effectiveness of ground treatmentEffectiveness of ground treatment

• Effects of previous construction workEffects of previous construction work

• Interaction with adjacent structuresInteraction with adjacent structures

9Asia

Instrumentation to deal with these risksInstrumentation to deal with these risks

• All design approachesAll design approachesDesign parameters from previous experienceDesign parameters from previous experienceDetermined from Determined from instrumentation resultsinstrumentation results

• Routine design approachRoutine design approachOffset risk by using conservative assumptionsOffset risk by using conservative assumptionsVerify design using Verify design using instrumentation resultsinstrumentation results (AAA) (AAA)

• Observational approach (progressive modification)Observational approach (progressive modification)Manage risk through engineering inputManage risk through engineering inputLess conservative assumptionsLess conservative assumptionsContinuous improvement of design based on Continuous improvement of design based on instrumentation resultsinstrumentation resultsAs knowledge improves design improves and risk remains constant.As knowledge improves design improves and risk remains constant.

10Asia

Trigger Levels in Routine DesignTrigger Levels in Routine Design

• Trigger levels are used as part of the AAA systemTrigger levels are used as part of the AAA system

• Important to distinguish between trigger levels set using the design Important to distinguish between trigger levels set using the design predictions, on the basis of additional calculation and based on predictions, on the basis of additional calculation and based on engineering judgementengineering judgement

• Design predictions are unreliable and AAA value requires a design review, Design predictions are unreliable and AAA value requires a design review, if appropriate AAA values can be updatedif appropriate AAA values can be updated

• Additional calculation is based on actual serviceability and AAA values Additional calculation is based on actual serviceability and AAA values should not be exceededshould not be exceeded

• Engineering judgement are often guesses or best practice and more Engineering judgement are often guesses or best practice and more detailed analysis is then required and AAA values can be reviseddetailed analysis is then required and AAA values can be revised

11Asia

Managing Risk and Obtaining ValueManaging Risk and Obtaining Value

• If the major role of instrumentation is to manage risk how If the major role of instrumentation is to manage risk how can we make it more effective?can we make it more effective?

• Alternatively, how can we get more value out of Alternatively, how can we get more value out of instrumentation?instrumentation?

12Asia

The Role of InstrumentationThe Role of Instrumentation

• Routine design and constructionRoutine design and constructionTo verify an already conservative designTo verify an already conservative design

• Observational approachObservational approachUse data to continually improve designUse data to continually improve design

Reduce construction costsReduce construction costs

Save construction timeSave construction time

• Long term perspectiveLong term perspectiveFull interpretation published with all background Full interpretation published with all background information allows future designs to be improvedinformation allows future designs to be improved

Benefits accrued by major clientsBenefits accrued by major clients

0%

10%

INCREASING

VALUE

13Asia

Typical Scope of Standard InstrumentationTypical Scope of Standard Instrumentation

Standard instrument Standard instrument typetype

1 km cut and cover 1 km cut and cover with station in with station in

SingaporeSingapore

1 km cut and cover 1 km cut and cover with station in with station in

Hong KongHong Kong

Inclinometers in wallsInclinometers in walls 120120 5050

Strut load monitoringStrut load monitoring 400400 200200

Piezometers Piezometers

(VW and standpipe)(VW and standpipe)

5050 120120

Settlement pointsSettlement points 800800 500500

ExtensometersExtensometers 1010 1515

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Scope of Data CollectionScope of Data Collection

• Typically 1000 to 2000 separate instrumentsTypically 1000 to 2000 separate instruments

• Readings required daily during excavation and strut Readings required daily during excavation and strut removal stages and weekly at other timesremoval stages and weekly at other times

• Typically of the order of 200 readings per instrumentTypically of the order of 200 readings per instrument

• 400,000 separate sets of readings400,000 separate sets of readings

• 1,000,000 or more data points1,000,000 or more data points

15Asia

Typical cost of instrumentationTypical cost of instrumentation

Cost includesCost includes

• InstrumentationInstrumentation

• Monitoring teams (instrumentation and survey)Monitoring teams (instrumentation and survey)

• Engineering support to coordinate workEngineering support to coordinate work

• Development and maintenance of databaseDevelopment and maintenance of database

• Daily, weekly and monthly reporting with associated meeting costsDaily, weekly and monthly reporting with associated meeting costs

Cost of instrumentation and Cost of instrumentation and monitoringmonitoring

1.5% to 2% of contract value1.5% to 2% of contract value

Management cost (engineering Management cost (engineering support, meetings etc.)support, meetings etc.)

0.5% to 1% of contract value0.5% to 1% of contract value

Total costTotal cost 2% to 3% of contract value2% to 3% of contract value

16Asia

Cost of Instrumentation Routine DesignCost of Instrumentation Routine Design

• Total cost for routine instrumentation of a deep excavation contract is S$ Total cost for routine instrumentation of a deep excavation contract is S$ 5 to 10 million5 to 10 million

• Primary use is to verify the design of the excavation worksPrimary use is to verify the design of the excavation works

• Cost is of similar order to the original design costs (i.e. pay as much to Cost is of similar order to the original design costs (i.e. pay as much to verify as to design)verify as to design)

• Instrumentation is often seen as an onerous contract requirementInstrumentation is often seen as an onerous contract requirement

• No added value obtained from instrumentationNo added value obtained from instrumentation

• Designer is often not closely involved with constructionDesigner is often not closely involved with construction

17Asia

How to Add Value to InstrumentationHow to Add Value to Instrumentation

• Adding value is reducing risk and reducing costAdding value is reducing risk and reducing cost

• Involve designer in the construction workInvolve designer in the construction work

• Prepare good quality instrumentation interpretative reports Prepare good quality instrumentation interpretative reports (comprehensive rather than instrument specific)(comprehensive rather than instrument specific)

• Carry out back analyses to determine actual performance parametersCarry out back analyses to determine actual performance parameters

• Encourage value engineering through observational approachEncourage value engineering through observational approach

• Publish results and improve standards and codes of practicePublish results and improve standards and codes of practice

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Cost of Adding Value to InstrumentationCost of Adding Value to Instrumentation

• Additional cost for proper interpretation of results and Additional cost for proper interpretation of results and associated report is S$ 0.1 millionassociated report is S$ 0.1 million

• Additional cost for back analysis in order to obtain useful Additional cost for back analysis in order to obtain useful design data for future work is S$ 0.1 to 0.2 milliondesign data for future work is S$ 0.1 to 0.2 million

• Large savings in Contract costs and times are achievable Large savings in Contract costs and times are achievable by making better use of monitoring databy making better use of monitoring data

• Additional costs can usually be offset by better control of Additional costs can usually be offset by better control of day to day monitoring (i.e. cut down frequency)day to day monitoring (i.e. cut down frequency)

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Examples of Excessive MonitoringExamples of Excessive MonitoringManual Settlement ReadingsManual Settlement Readings

-15

-5

-120

0

Jan 00 Nov 01

20Asia

Examples of Excessive MonitoringExamples of Excessive MonitoringManual Piezometer ReadingsManual Piezometer Readings3

2

1

0

Gro

un

dwa

ter

Le

vel m

PD

18 Months with 500 Measurements

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Examples of Excessive MonitoringExamples of Excessive MonitoringDatalog Piezometer ReadingsDatalog Piezometer Readings

2

1

0

Gro

un

dwa

ter

Le

vel m

PD

1 Day Piezometer

Tide Gauge

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Examples of Excessive MonitoringExamples of Excessive MonitoringDatalogged Piezometer ReadingsDatalogged Piezometer Readings

2.5

-15

-5

Jan 00 Nov 01

-1.5

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Monitoring Frequency, Value and RiskMonitoring Frequency, Value and Risk

• My experience approximately 50% of monitoring is unnecessary. My experience approximately 50% of monitoring is unnecessary.

• Obtained because nobody is reviewing the data.Obtained because nobody is reviewing the data.

• Extra value can be obtained without additional cost by better Extra value can be obtained without additional cost by better control of instrumentationcontrol of instrumentation

• Better value is associated with reduced risk Better value is associated with reduced risk

• Additional value can be obtained through Additional value can be obtained through Use of Observational Approach (Progressive Modification)Use of Observational Approach (Progressive Modification)Adoption of updated design codes (e.g. CIRIA C580)Adoption of updated design codes (e.g. CIRIA C580)Feeding back information to future contractsFeeding back information to future contracts

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Proposal for Increasing Value of InstrumentationProposal for Increasing Value of Instrumentation

• Use of Observational Method can achieve substantial cost savings Use of Observational Method can achieve substantial cost savings and more importantly time savings (e.g. Powderham, Nicolson)and more importantly time savings (e.g. Powderham, Nicolson)

• Adoption of CIRIA C580 which makes more use of instrumentation Adoption of CIRIA C580 which makes more use of instrumentation and can reduce cost of Dwalls by approximately 5% (Sze and Chan and can reduce cost of Dwalls by approximately 5% (Sze and Chan 2005)2005)

• Scope for reducing factor of safety or load factor by 10%Scope for reducing factor of safety or load factor by 10%

• Paper to ICDE 2006 proposes adoption of C580 with improved Paper to ICDE 2006 proposes adoption of C580 with improved instrumentation as an interim measure. Better use of instrumentation as an interim measure. Better use of instrumentation, involvement of designer and more widespread instrumentation, involvement of designer and more widespread adoption of Observational Approach.adoption of Observational Approach.

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Comment on Interpretation of InstrumentationComment on Interpretation of InstrumentationThe 4 Basic StepsThe 4 Basic Steps

1.1. Baseline readings and background trends must be Baseline readings and background trends must be established (daily rainfall, seasonal rainfall, tide, established (daily rainfall, seasonal rainfall, tide, temperature etc.)temperature etc.)

2.2. Construction records are critical to data interpretationConstruction records are critical to data interpretation

3.3. Instrumentation results must be correlated against Instrumentation results must be correlated against baseline trends and construction historybaseline trends and construction history

4.4. Analysis and presentation of data must consider cause Analysis and presentation of data must consider cause and effectand effect

26Asia

Establish Baseline TrendEstablish Baseline Trend

“X”

“Y”

Movement ofViaduct Structure

40 m

Alert = 10 mmAction = 15 mmAlarm = 20 mm

MaximumAllowableWater DrawdownIs 1m

27Asia

Establish Baseline Trend – 1Establish Baseline Trend – 1Movement of Bypass Pier HeadMovement of Bypass Pier Head

50

-30

20

0

Jun 00 Oct 01

X-dir

Y-dir

July 00

Feb 01

40 mm

18 mm

28Asia

Establish Baseline Trend – 1Establish Baseline Trend – 1Movement of BypassMovement of Bypass

50

-30

20

0

Jun 00 Oct 01

X-dir

Y-dir

July 00 July 01

40 mm

18 mm

29Asia

Establish Baseline Trend – 2Establish Baseline Trend – 2Variation of Groundwater LevelVariation of Groundwater Level

0

1

2

3

Gro

un

dwa

ter

Le

vel m

PD

Jan 00 July 00 Jan 01 July 01

30Asia

Construction Records – Tabulated DataConstruction Records – Tabulated Data

Pile No. Cap Mark InclinationDrilling

Start Date

Drilling Completion

Date

Ground Level

Casing Top Level (for

R.H.)

Length of Casing

(mPD) (mPD) (m)

                   

MP33A-1

MP33A

1 : 4 14-Jul-05 14-Jul-05 6.800 7.060 21.66

MP33A-2 1 : 4 26-Jul-05 26-Jul-05 6.800 7.119 21.60

MP33A-3 1 : 4 03-Aug-05 04-Aug-05 6.800 7.030 21.54

MP33A-4 0 : 1 22-Aug-05 23-Aug-05 6.800 7.000 20.13

MP33A-5 1 : 4 05-Sep-05 06-Sep-05 6.800 7.037 20.45

MP33A-6 1 : 4 18-Jul-05 19-Jul-05 6.800 6.942 20.53

MP33A-7 1 : 4 09-Aug-05 17-Aug-05 6.800 6.892 20.15

MP33A-8 1 : 4 27-Jul-05 27-Jul-05 6.800 7.104 20.97

MP33A-9 1 : 4 02-Aug-05 03-Aug-05 6.800 7.151 20.94

MP33A-10 1 : 4 16-Jul-05 16-Jul-05 6.800 6.989 20.94

MP33A-11 0 : 1 05-Sep-05 05-Sep-05 6.800 7.051 19.9

MP33A-12 1 : 4 15-Jul-05 15-Jul-05 6.800 7.028 20.32

MP33A-13 1 : 4 08-Aug-05 09-Aug-05 6.800 7.015 20.21

MP33A-14 1 : 4 29-Aug-05 30-Aug-05 6.800 6.884 19.78

31Asia

Construction Records - EnvironmentalConstruction Records - Environmental

32Asia

Construction Records – Progress SummariesConstruction Records – Progress Summaries

33Asia

Construction Records - PhotographsConstruction Records - Photographs

3 March 2002

34Asia

Correlate Response to Construction ActivitiesCorrelate Response to Construction ActivitiesCause and Effect PlotsCause and Effect Plots

Excavation level (mPD)

Settlement (mm)

Inclinometer (mm)

Piezometer (mPD)

-15

-5

-120

0

50

+2.5

-1.5

Jan 00 Nov 01

35Asia

Correlate Response to Construction ActivitiesCorrelate Response to Construction Activitiesand Baseline Trendsand Baseline Trends

-2

-1

0

+1

+2

Jan 00 July 00 Jan 01 July 01

Tidal Response

Start Dewatering

Complete Dwall

Stop Dewatering

Rainfall Control

Gro

un

dwa

ter

Le

vel m

PD

36Asia

SummarySummary

• The primary purpose of instrumentation is to manage riskThe primary purpose of instrumentation is to manage risk

• Risk arises due to uncertainties in ground engineeringRisk arises due to uncertainties in ground engineering

• Designer is aware of uncertainties so should be involved in construction Designer is aware of uncertainties so should be involved in construction (i.e. continuity) but is often excluded(i.e. continuity) but is often excluded

• Much instrumentation is wasted, not enough engineering inputMuch instrumentation is wasted, not enough engineering input

• Reduced costs and reduced risk (i.e. added value) can be obtained by Reduced costs and reduced risk (i.e. added value) can be obtained by better use of instrumentationbetter use of instrumentation

• Requires good quality interpretation of data and reporting to manage the Requires good quality interpretation of data and reporting to manage the risksrisks

37Asia

End of PresentationEnd of Presentation

Thank youThank you