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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
3
4
1
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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
Expanded Concrete Segment VaultSettlements Induced by Tunneling in Soft GroundSettlements Induced by Tunneling in Soft Ground
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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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