The 3rd Training courseTUNNELLING IN URBAN AREA

Prague, 4-5th May 2007

Settlements Induced by Settlements Induced by Tunneling in Soft GroundTunneling in Soft Ground

Dr. Eric LecaDr. Eric Leca((SYSTRA, FranceSYSTRA, France))

ITA - AITES WORLD TUNNEL CONGRESS 2007 PRAGUE

TRAINING MATERIAL PREPARED BY

5Conclusions Conclusions and and referencesreferences

Chapter Chapter 1 1

Chapter 2Chapter 2

IntroductionIntroduction

Chapter Chapter 33

Index

2

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4

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Introduction

Ground motion induced by tunneling and impact

on existing structures

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Chapter 1

Tunneling Induced Ground Movements

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Displacements of the Excavation Profiles

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Yielding Zone Around the Opening

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a. Limited extension b. Weaker grounds

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Tunnel Face Stability

a. Clayey Grounds

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Tunnel Face Stability

b. Frictional Grounds

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Tunnel Face Stability

• Clayey Grounds:– N=(σS+γH- σ T)/Su– N<5-6– U- shear strength: Su

Settlements Induced by Tunneling in Soft GroundSettlements Induced by Tunneling in Soft Ground

• Frictional Grounds:– σ S /σ c, γD/σ c, σ T /σ c

– σ c =2c’ cosφ’ /(1-sinφ’)– Cohesion: c’– Friction angle: φ’

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Propagation of Movements Towards the Surface

a. Deeper Tunnel

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Propagation of Movements Towards the Surface

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b. Shallow Tunnel

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Influence of Support Conditions

• Support stiffness

• Installation timeframe

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Surface Settlement Trough

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Chapter 2

Causes For Construction Induced Settlements

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Main Sources For Settlements(sequential construction)

• Face intake/instability• Impact of characteristics and installation

conditions of temporary support• Staging of the excavation works (cross-

section• Final liner installation and response

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Settlements Along a Shield Driven Tunnel

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Typical Contributions to Shield Tunneling Induced Settlements

• Face intake: 10-20%• Along the shield: 40-50%• Tail skin: 30-40%

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Sources For Additional Settlements

• Effect of groundwater– During construction

• drawdown of ground water table• Seepage towards the tunnel face

– In the long term (consolidation)• Effect of worksite conditions

– e.g. impact of vibrations of weaker grounds

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Chapter 3

Evaluation of Ground Movements

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Settlement Evaluation Methods

• Empirical and Semi-empirical– Analytical– Observational

• Numerical

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Settlement Trough Characterization

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3D Distribution

Cross-section

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Settlement Trough Characterization

s(x)=smax .exp(-x2/2i2)Vs=(2π)1/2.i.smax

Key parameters: smax, i

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Dampening Effect

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• Vl = Volume loss at the opening• Vs = Volume loss at ground level

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Ground Movements at Depth

• S(x,z)=smax (z).exp(-x2/2(Kz)2)• H(x,z)=S(x,z).x/z• Vs(z)=(2π)1/2.Kz.smax(z)• K = empirical coefficient

– (varies typically from 0.5 for stiff clay and sandy clay to 0.25 to softer sands and gravels)

(after O’Reilly & New, 1982)

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Chapter 4

Incidence of Ground Displacements on Existing Structures

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Typical Idealized Building Response to Ground Motion

(after Attewell et al, 1986)

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Induced Vertical MovementsρvA= settlement at AδvAB= differential settlement

b/w A-Bω = tiltβBC= relative rotation b/w B-CαC= angular deformation at CΔAD= relative deflection b/w A-

DΔAD/LAD= deflection rate

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Induced Horizontal Movements

εhAB=(ρhA- ρhB)/LAB

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Induced Horizontal Deformations

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Damage ClassificationDamage Type Damage Degree Damage Description Crack width (mm)

0 Negligible Micro-cracks <0.1

1 Very slight Architectural <1

2 Slight Archit. - to treat <5

3 Moderate Functional 5-15 or sev.>3

4 Severe Structural 15-25 (number)

5 Very severe Structural >25 (number)

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Relationship Between Critical Extension and Cracking

Damage Type

0 1 2 3 4&5

εcrit (%) ≤0.050 0.050< ≤0.075

0.075< ≤0.150

0.150< ≤0.300

0.300<

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(after Boscardin & Cording, 1989)

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Range of Serviceability Limit State For Standard Structures

Damage Type Average Slope of Settlement Trough

Under Structure (%)

Maximum Settlement of the Structure (mm)

1 < 2 < 10

2 2 < < 4 10 < < 20

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Design Methodology• Phase 1: Investigation of Existing Buildings• Phase 2: Information Summary• Phase 3: Selection of Damage Criteria• Phase 4: Modeling (after Burland, 1995; Mair et al, 1996)

– Preliminary Assessment– Second Stage Assessment– Detailed Evaluation

• Phase 5: Determination of Allowable Thresholds• Phase 6: Back-analysis & Calibration of Models

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Chapter 5

Settlement Control

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Preventive & Remedial Approaches1. Improvement of Overall Project Conditions

- Aim at larger depth of cover- Look for ground layers of good mechanical properties- Minimize excavated cross-section- Aim at straight alignments

2. Improvement of Ground Characteristics3. Structural Improvement of Building4. Improvement During Construction

a. Sequential/Conventionalb. Shield Tunneling

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Pre-support

Face Bolting

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Pre-support

Fore-poling

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Pre-support

Umbrella Vault

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Pre-support

Pre-vaultSettlements Induced by Tunneling in Soft GroundSettlements Induced by Tunneling in Soft Ground

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Crown Support

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Underpinning

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Chapter 6

Observation and Monitoring of Ground

Response

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Observation & Monitoring

• Inspection• Instrumentation

– Monitoring of existing structures– Ground measurements

• Monitoring Program

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Chapter 7

Contractual Aspects

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Contractual Aspects

• Usual contractual clauses• Position of the different players

– The Owner– The Engineer– The Contractor

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Conclusions and references

Further Developments and References

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Recommendations and Possible Ways of Improvement

• At Owner – and Engineer - Level– Organize preliminary inspections & studies– Commission a comprehensive program of studies– Set appropriate settlement limits– Provide available information at tender stage

• At Contractor’s Level– Provide assistance required during construction– Set accountabilities

• Possible contribution of Insurance Companies– Obtain greater technical clarity when defending cases– Require risk analyses prior to finalizing binding agreements– Exercise greater diligence when reviewing litigious situations

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References• AITES-ITA, Working Group 2: Leca, E. & New, B., “Settlements

induced by tunneling in Soft Ground”, Tunnelling and Underground Space Technology (TUST) 2007, 22, Nr. 2, 119-149.

• AFTES, Groupe de Travail n°16 : Leblais, Y., André, D., Dubois, P., Gigan, J.P., Guillaume, J., Leca, E., Pantet, A., Riondy, G., 1994, « Tassements liés au creusement d’ouvrages souterrains », Tunnels et Ouvrages Souterrains (TOS), 132.

• Additional references quoted in WG2 document (TUST, 22)

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