Tunnel in Soft Soil
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Transcript of Tunnel in Soft Soil
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Geotechnical Surveys for Designing andConstruction
of Underground Works in Big Cities
Seminar on Tunnel Construction in Soft SoilCondition
Hanoi, VietnamFebruary 29, 2008
byHarvey W. Parker
Past PresidentInternational Tunnelling and Underground Space Association
(ITA)
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Geotechnology & Planning
Everyone in planning and design oftunnels must give geology seriousconsideration
Therefore, Use Geology & Geotechnology
as early as possibleConceptual stage
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Challenges of the Underground
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Strange & Unique Functions of the
Ground in Tunnels The ground and groundwater is the load
The ground is the medium that transfersthis load to the tunnel lining
The ground is the structural material that
actually carries most of the load Tunnel linings only have to carry part of totalload
We call it Arching
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Selected Challenges of theUnderground: Variability
Geology can be Subtle and IllusiveVariability is Often Abrupt
PropertiesMagnitude Varies Significantly
Time, Seasons, Sample Size, Rate of Loading
Properties of Geologic Materials have anenormous range of values
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Range of permeability
Greater than any other engineering parameter Roughly 10 -7 to 10 +3 cm/secFactor of 10,000,000,000
Units used are factors of 10
For Comparison: Strength
Much Smaller RangeSoft Clay to Concrete ~ 1,000Soft Clay to Steel/Rock ~ 100,000
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Geotechnical ExplorationsComparative Sampling Frequency
Concrete Structure Every 20 to 50 m 3
~ 0.1% See 100% off Truck
Geotechnical Exploration Borings: 6 to 8 cm Diameter
core @ 90 m spacing = ~ 0.0005%
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Geotech sees only 0.0005% of Tunnel Volume
(Thimble to ~50 Oil Drums)
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Selected Important GeotechnicalIssues
Geology Groundwater Groundwater
Groundwater Contaminated Soil & Groundwater
Obstructions, Gas, & Other Safety Issues Ground and Groundwater Properties
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Challenges of theUnderground
Geologic investigations are more difficult
Use Well-Qualified Engineering Geologistand Geotechnical Engineer with Tunnel
Experience
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Success in Spite of Vast Uncertainty
Exploration sees only miniscule volume Never see ahead of the face
Major TBM issue
Yet Fortunately - Owners, Designers &Contractors do a remarkable job
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Special Challenges in Big Cities
Very Few outcrops to actually see & touch soil Difficult to find safe location for borings Because of Utilities or other obstructions
May not get all borings on alignment or wherewanted Borings may not be equally spaced or off line
May encounter: Obstructions contaminated soil
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Geotechnical Database for Cities
Summarizes all available Geotechnical data All cities should create, maintain, & update such a database
Note the quality of the data entries; some are better than others Should Include:
Geotechnical Records Borings, Test Pits, Basement Excavations Existing Foundations including Pile Driving Records
Water, Oil, and Geothermal Well Logs Groundwater Information
Information on Contamination Archeological Information Man-made Subsurface Features
Obstructions such as Utilities, Old Sea Walls, Piers, Piles, WellCasings, Tunnels, etc
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Groundwater Information for Cities Document all information on groundwater
Water Levels and Pressure Trends Current (Include Artesian Conditions) Future (Consider Global Warming)
Groundwater Flow Directions For environmental purposes To evaluate flow of contaminated plumes To evaluate salt water intrusion, if applicable
Document Permeability of Ground Part of Geotechnical Database Identify strata or features of High or Low Permeability
Impermeable layers
Highly permeable zones Obstructions to water flow
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Asset Management Program Database of Existing Utilities and Other
Subsurface Features Documents Location and Condition
Very useful for managing potential replacementand upgrading costs during your project or infuture
Valuable Input to Overall Planning ofUnderground Space for your City
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Geotechnical SurveyExplorations
Methods & Principles
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NO LONGER SUFFICIENT TODESCRIBE STRATIGRAPHY &
GROUNDWATER TABLE
MUST PREDICT BEHAVIOR
MUST ESTABLISH A BASELINE
So "changed condition" (if encountered) can be administered fairly
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Intact vs. Mass PhysicalProperties
Intact properties are the properties of thesoil on a small (core sample size) scale
Mass physical properties govern behavior Includes the fundamental intact properties Modified by the effects of the ground at the
scale of the tunnel
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Determine Groundwater Location
& Mass Permeability of Soil
After Parker, 1996After Parker, 1996
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Tunnel Behavior is Complex Behavior of the Ground During Tunnelling
Determined by MANY factors including: Geology Engineering Properties of Materials Means and Methods
Excavation Method Ground Support & Lining Method Ground Modification Method, if any
Terzaghis Tunnelmans Ground Classification Firm, Ravelling, Squeezing, Running, Flowing, Swelling
Generally: Owner Owns the Ground Contractor Owns the effects of Means & Methods But these are always interdependent and never so clear
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Past PracticesNo accepted standard
# BoringsSpacingDepth
Geo.Investigation Cost1/2 to 3+ Percent of Construction Cost
Some up to 8++ % have been reported.Special Class over these guidelines
Nuclear Waste
Hazardous Waste
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USNC/TT Recommendations onSite Investigation
1984 Study of exploration practices in USA
U.S. National Committee on Tunneling Technology(USNC/TT)
Exploration = 3.0 percent of project cost
Boring Length = 1.5 linear meters of boreholeper route meter of tunnel alignment
For typical urban tunnel situation
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USNC/TT Recommendations onSite Investigation
Make Geotechnical Report a ContractDocument (Now Required by ITIG)
No DisclaimersEstablish a BaselineThis evolved into the Geotechnical Baseline Report(GBR) also called Geotechnical Conditions Report
Compile As-Built Report
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Recommendations for How Much to DoDetermine Project Magnitude
Major or Complex Project
Small or Ordinary Project
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Guidelines for Determining Project
MagnitudeMajor or Complex Tunnel Projects
Greater than 4 m in diameterLength greater than 300 m
Alternative Alignments ConsideredMultiple PhasesUnusual, complex, or unproven construction procedures
Difficult or adverse geologyConsiderable risk
Considerable sharing of risk
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Guidelines for Level of GeotechnicalEffort
All Tunnel Projects
Emphasize GeologyConduct at least two exploration phasesFund the initial phase well
confidently select the alignmentestimate of the likely construction methods, liningand cost
Pre-approve contingency fund Answer technical questions resulting frominitial boring program
Use Contingency only if needed
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Guidelines for Level of Geotechnical
Effort All Tunnel Projects
Get more information than needed for design
Get enough data to predict Ground Behavior Alternative construction methods
TBM or Traditional techniques
Get enough data to minimize uncertainty
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Guidelines for Level of Geotechnical
Effort All Tunnel Projects
Don't do any exploration unless itspecifically fills a genuine need
Often, reduction of uncertainty is a genuine need
Good Exploration reduces BID COST by:10 to 15 Times Exploration cost
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Guidelines for Level of GeotechnicalEffort
All Tunnel Projects
Conduct a Supplementary CostExploration Phase
After alignment is fixed & after designedTo Confirm the design concept
Get information contractor needs to estimateProductivity (rate of advance)Estimate the costs and to bid the job
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Guidelines for Level of GeotechnicalEffort
Large or Major Projects
Develop multi-phased program to fill actualneeds
Phasing increases quality and reduces cost
Use non-traditional techniques if theyreduce uncertainty
Geophysics
In Situ TestsPressuremeterspump tests, etc., as appropriateshafts, adits, pilot tunnels
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Guidelines for Level of Geotechnical
EffortLarge or Major Projects
For initial phases of design, budget,and fund
Boring length = About equal to route lengthCost = 1.5 to 2.25 percent of construction cost
Overall budget = 3.0 Percent of
ConstructionIncluding contingenciesImplementation of contingencies must be quick
and easy
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Selected Geotechnical Issues
Sonic Drilling
Geophysics/TomographyGPS and GIS
Video & Digital PhotographySystematic Risk Management ProgramComputer GraphicsMethane, H 2S, & Ground ContaminationMuck Disposal
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RISK & CHOICES ARE PLENTIFUL
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Geotechnical RiskManagement
Conduct Geotechnical SurveysSpecifically to Reduce Uncertainty of High-risk Issues
Identify Issues using Risk Management
TechniquesWorkshop sessions by technical staff
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ITA Guidelines for Risk ManagementTUST, 2004
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ITIG International Risk Code Requires Risk Registers and
RMP from project conceptionthrough construction
Re-Insurance Industry isdemanding compliance withCode
Revised following ITAintervention. Use 30 January2006 version
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Risk Worksessions Risk worksessions force the project team toconsider potential problems
Not complicated Process:1. Identify & characterize potential threats2. Develop risk reduction strategies3. Determine cost/benefits for these strategies4. Decide a prudent course of action.
Source: Reilly
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C Ro
R r
p
RMP: Risk IdentificationRMP: Risk Identification
Probability/Impact classification - exampleProbability/Impact classification - example
RISKRISKIntolerableSignificantSubstantialTolerable
Insignificant
17-2513-169-125-8
1-4
UnacceptableUnacceptableEarly attentionRegular attention
Monitor
Negligible Unlikely Likely Probable Very likely
I M P A C T
PROBABILITY
Very high
High
MediumLow
Very low
5
4
32
1
10
8
64
2
15
12
96
3
20
16
128
4
25
20
1510
5
R=P IR=P I RESPONSERESPONSE
In this case the impact prevails tothe likelihood of the event
In this case the impact prevails to
the likelihood of the event
The detailed process of riskidentification is project-specific
The detailed process of riskidentification is project-specific
Source: Grasso
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Methane, Hydrogen Sulfide &Ground Contamination
More Prevalent in Tunnels & Mines thanPreviously Thought
Must Investigate for these HazardsImportant Decision Regarding SafetyMeasures and Cost
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Muck Disposal- Major Issue(Often Neglected)
Muck is generated 24 hours/day Potential Public Disturbance
Muck disposal sites are often far away
Cost and Disturbance Issue Muck is sometimes contaminated
Can go only to certain very expensive disposalsites
EPB and Slurry Muck needs special treatment
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Geotechnical Aspects ofContracting Practices
No Disclaimers for ground conditions
Include a Fair Changed Condition Clause
Share Risk and Share Fairly
Adapt ITIG Requirements to your Country
Prepare Geotechnical Baseline Report (GBR)GBR also called Ground Reference ConditionsCross-checked with plans & specificationsMake GBR part of contract documents
Provide for Disputes Review Board (DRB)
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References AFTES, 1994, The Choice of geotechnical Parameters and
Tests Useful to the Design, Dimensioning and Construction ofUnderground Structures, Association Francaise de Travauxen Souterrain, Paris, France
Guglielmetti, Vittorio; Grasso, Piergiorgio; Mahtab, Ashraf; &Xu, Shulin; Editors, 2008, Mechanized Tunnelling in Urban
Areas , Taylor & Francis Group, London, 507 pp.
Parker, Harvey W. 1996, Geotechnical Investigations,Chapter 4 of Tunnel Engineering Handbook , 2nd Edition,edited by Kuesel & King, Chapman & Hall, New York
USNC/TT (1984), Geotechnical Site Investigations forUnderground Projects , U.S. National Committee on TunnelingTechnology, National Research Council, Washington, D.C.
Geotechnical Surveys:
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Geotechnical Surveys:
ConclusionsGeology Dominates Every Major Decision
Allow Geo-Issues their Proper Role in PlanningChallenges and Potential Benefits of theUnderground are EnormousExploration programs see only a very small % ofvolume to be tunnelled. (0.0005%)
The Ground is the Load and it also Provides
Support for Most of the Load
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Geotechnical Surveys:Conclusions
Incorporate Formal Systematic Risk Studies
As soon as project identifiedUse risk management to direct geotechnical studies
Follow Guidelines: Adapt to ITIG Code
Start Geotechnical Work in Conceptual Stage &
Continue Geotechnical Observations During Construction Prepare As-Built Report
While in Service
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Geotechnical Surveys:Conclusions
Many Challenges Exist in Big Cities
Difficult to find safe boring locations Few outcrops Many Obstructions; Archeological Issues Contaminated Soil & Groundwater
Recommended Practices
Geotechnical Database Groundwater Trends
Asset Management Program
Formal Planning of Underground Space
h l
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Geotechnical Surveys:Conclusions
Determine Mass Physical Properties at Full Scale
Not just intact properties Behavior is Complex & Hard to Identify Cause:
Owner owns ground
Contractor owns means & methods Prepare a Geotechnical Baseline Report (GBR)
(also known as Geotechnical Reference
Conditions Report) Distinguish between Fact and Interpretation
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Geotechnical Surveys:Conclusions
Budget 3% for Geotechnical Investigation
Don't do any exploration unless it specifically fills
a genuine needHowever, often, reduction of uncertainty is a genuine need
Good Exploration reduces BID COST by:10 to 15 Times the Cost of Exploration
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Overall Conclusion
Geology & Geotechnology areEssential to Planning, Design &Construction
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Thank YouHarvey W. Parker
Past PresidentInternational Tunnelling and Underground Space Associaton
Source: Apollo 17