tunnel in soft soil

8/9/2019 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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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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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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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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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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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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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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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