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Evaluation of Ground Movements Caused by TBM Tunneling and Assessment of their Impacts

Raghu Bhargava

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Content

• Nature of Ground Movements• Method of Analyses:• Sources and Contributing Factors in TBM Tunneling• Potential Impacts on Buildings• Methods of Assessment of Impacts

Tunneling Induced Ground Movements

• 3D-Ground Movements (Attewell and Woodman, 1982)

• Transversal Trough(s); • Longitudinal Trough(s); • Start about 2.4i /1.5D ahead and,

reach maximum value about 3i / 2D behind tunnel face

• Ground movements consists of:- Vertical Deformation / Settlements- Horizontal Deformations • Induced effects comprise:- Differential Settlements (Angular

Distortions)- Horizontal Strains

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Semi-Empirical Methods

Transversal settlements are well characterized by Gaussian Distribution (Peck, Cording)

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Main Contributions to Semi-Empirical Method

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Modes of Ground Deformations above Tunnel

In Cohesionless Soils- Soil above roof does not yield

and moves down as a block- Localized yield around sides

propagates to surfaceIn Cohesive Soils- Continuous yield zone that

expands around tunnel. No Localized yielding

Resulting in larger max. settlement but narrower trough width in cohesionless soils ( K = 0.2 to 0.3) compared to cohesive soils (K= 0.4 for Stiff to 0.7 for Soft Clays)

• After Wong & Kaiser, 1987

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Approach of Cording and Hansmire (1972

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Tunnel Depth vs. Trough Width for different soil types

Subsurface Settlement Profiles (after Mair 1993)

• Trough width decreases with depth

• Smax increases with depth• Provides settlement profile

at base of foundations

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Ground Movements in TBM Tunneling

• Main Contributing Factors- stress change (relief) ahead and axial ground deformation

toward face of tunnel ~ σ’H + Pw > σT

- ground deformations due to overcutting – radial - ground deformations into tail void (annulus around TBM/Lining)- Deformations of Lining- Due to primary and secondary consolidation – increased if

excessive face pressure are applied • Main Controlling Factor- Face Support Pressure- Backfill Grouting: Workability, Early Setting, Strength, Stiffness

GROUND LOSS

• In soft ground tunnels, ground loss is key parameter

• It is physical gap around TBM shield and the lining + Lining Deformation (typically squatting)

• Minimize the gap by grouting through TBM tail, early setting, non-shrinkage higher stiffness grout mix.

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TBM Overcut & Annular Gap (Tail Void)

• Size - 5” to 10” all around TBM

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Annular Grouting Through TBM Tailskin

• Grouting concurrently with Excavation & TBM Advance• Grouting as close as practicable in the tail void

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Numerical Analyses

3D and 2D Numerical Modelling

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- 3D analyses shows stress distribution around tunnel face- Rotation of major and minor principal stress and resulting ground

arching is captured- Stress relaxation factors can be obtained from model to undertake

a number of 2D analyses along the tunnel

Numerical Methods of Ground Movement Estimation

3D Numerical Analyses allow consideration of:- Site specific geo-stratigraphy & groundwater- Foundations and surcharge loads above and adjacent- Soil and structure interaction- Modelling of construction method and Sequence ; TBM

Geometry & Annulus, Face Pressures, Ring Building- Ground relaxation and arching effects are implicit 2D-Axi Symmetric Analyses & 2D Plain Strain Analyses together can be used instead of 3D Analyses- Obtain longitudinal displacement profile and stresses from axi-

symmetric model- Obtain radial displacements from 2D model to obtain stress

relaxation factors

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2D Axisymmetric and Plain Strain Analyses

Analysis of Longitudinal & Radial Displacements

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2D Axisymmetric and Plain Strain Analyses

• LDP-AxiSymmetric Model • Radial Disp.- 2D Plain Strain

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2D Plain Strain Analyses -Example

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BUILDING DAMAGE ASSESSMENT

3-Stage Assessment • Stage 1- Based on total and differential settlements- Provides a corridor wide screening criteria• Stage 2- Based on strains induced in Building- Correlate strains to limiting strains and a extent of likely

damages to building and required repair and mitigation • Stage 3- Modelling of buildings and ground and their interaction effects- Only for buildings assessed slight or higher damage categories

in stage 2 assessment

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Stage 1- Building Damage Assessment

Based on maximum absolute differential settlement caused by settlement troughDoes not consider horizontal movement and strainsDoes not consider location of building within the settlement trough Damage Classification and Risk Assessment Based on (CIRIA RP30)Initial screening criteria to assess impacts and determine need for further assessment

Riskcategory

Maximum slope of building

Maximum settlement of building

(mm)

Description of risk

1 < 1/500 <10 Negligible: superficial damage unlikely

2 1/500 – 1/200 10 – 50 Slight: possible superficial damage which is unlikely to have structural significance

3 1/200 – 1/50 50 – 75 Moderate: expected superficial damage and possible structural damage to building, possible damage to relatively rigid pipelines

4 > 1/50 > 75 High: expected structural damage to buildings and rigid pipelines or possible damage to other pipelines

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Ground Settlement Contour Plan

• Contours of total settlement• Determine influence zone of settlements & affected buildings

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Stage 2 Assessment

• Building idealised as a deep beam with span L and height H

• Buildings assumed to follow the deformed ground at the foundation level

• The settlement trough subdivided into sagging and a hogging zones, delimited by the point of inflections

• Ground movement profiles are obtained from numerical or semi-empirical methods.

• Based on concept of limiting (critical) strain proposed by Burland & Wroth (1974) and modified by Boscardin & Cording (1989)

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L-hogL-sag

i

Sagging ZoneHogging Zone

BuildingH

∆ −

hog

∆ −

sag

Tunnel

Calculation of Strains in Building

• Deflection ratio in hogging and sagging spans (∆ / L)

• Maximum bending and diagonal strains (εbt and εdt)

• Horizontal Strains (εh ) OR

• Angular distortion in building (β)• Horizontal Strains (εh )

H, height of the building.E/G, = 2.6 Masonry and 2.5 for RC Bldg.L, Length of the considered building span.I, Sagging I = H3 / 12; Hogging I = H3 / 3t, Sagging t= H / 2; Hogging t = H

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⋅

⋅⋅⋅⋅

+⋅

∆

=

GE

HLtI

tL

Lb

23

12

ε

Calculation of Strains in Building

• Maximum bending and diagonal strains in building

• Angular Distortion in building• In Hogging Zone: due to the restraining

effect of the foundation, the neutral axis can be assumed at extreme fibre of the beam. Then, all strains due to bending are tensile.

• In Sagging Zone, it is reasonable to assume that the neutral axis is located in the middle of the beam. In this case, bending will generate both tensile and compressive strains.

• 6-damage categories identified based on limiting strains (Boscardin & Cording, 1989)

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⋅

⋅⋅⋅⋅

+⋅

∆

=

GE

HLtI

tL

Lb

23

12

ε

Ground Movement Profiles

Twin Tunnels

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• Single Tunnel

Transversal Deformations due to TunnellingCombined: 2 & 3Caclulatio 270Building n19 H.B.F. Building Piles: NoChainage:115PM / 117PMSoil type: UGU

-5.0

-4.0

-3.0

-2.0

-1.0

0.0

1.0

2.0

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

15

20

-50 -40 -30 -20 -10 0 10 20 30 40 50

Hor

izon

tal S

trai

ns (m

m/m

)

Dis

plac

emen

ts, v

ert.

& h

oriz

. (m

m)

Distance from Alignment Axis (m)

Settlements

Horiz. Displacements

Horizontal Strains

Transversal Deformations due to TunnellingCombined: 2 & 3Caclulatio 270Building n19 H.B.F. Building Piles: NoChainage:115PM / 117PMSoil type: UGU

-5.0

-4.0

-3.0

-2.0

-1.0

0.0

1.0

2.0

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

15

20

-50 -40 -30 -20 -10 0 10 20 30 40 50

Hor

izon

tal S

trai

ns (m

m/m

)

Dis

plac

emen

ts, v

ert.

& h

oriz

. (m

m)

Distance from Alignment Axis (m)

Settlements

Horiz. Displacements

Horizontal Strains

Assessment of Building Damage • Based on Angular Distortion

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• Based on Strains

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

Bui

ldin

g St

rain

s (1/

100)

BUILDING STRAINS AND CLASSIFICATION OF DAMAGE

ebtsz ebthz edtsz edthz

0.05/1000 > Negligible

0.15/1000 > Slight > 0.075/1000

0.075/1000 > Very Slight > 0.05/1000

0.3/1000 > Moderate > 0.15/1000

Severe > 0.3/1000

Building Damage ClassificationDegree and Description of Building Damage from Ground Settlements Caused by Tunnelling (After Burland et al, 1977 and Boscardin and Cording, 1989)Risk Category

Description of Degree of Damage

Description of Typical Damage and Likely Forms of Repair

Approx. crack width (mm)

Max Tensile Strain (%)

0 Negligible Hairline Cracks Less than 0.1mm Less than 0.05

1 Very Slight Fine cracks easily treated during normal redecoration. Damage generally restricted to internal wall finishes. Perhaps isolated slight fracture in buildings. Cracks in exterior brickwork visible upon close inspection.

0.1 to 1mm 0.05 to 0.075

2 Slight Cracks easily filled. Redecoration probably required. Recurrent cracks can be masked by suitable linings. Exterior cracks visible: some repainting may be required for weather-tightness. Doors and windows may stick slightly.

1.0 to 5mm 0.075 to 0.15

3 Moderate Cracks may require cutting and patching. Tuck pointing and possibly replacement of a small amount of exterior brickwork may be required. Doors and windows sticking. Services may be interrupted. Weather tightness often impaired.

5 to 15mm or a number of cracks greater than 3mm

0.15 to 0.3

4 Severe Extensive repair involving removal and replacement of sections of walls, especially over doors and windows required. Windows and door frames distorted. Floor slopes noticeably. Walls lean or bulge noticeably. Some loss of bearing in beams. Services disrupted.

15 to 25mm but also depends on number of cracks

Greater than 0.3

5 Very Severe Major repairs required involving partial or complete reconstruction. Beams lose bearing, walls lean badly and require shoring. Windows broken by distortion. Danger of instability.

Usually greater than 25mm but depends on number of cracks

Greater than 0.3

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Stage 2 vs Stage 1 Assessment

• Considers horizontal ground movement and strains• The settlement trough subdivided into sagging and a hogging

zones• Consider location of building within the settlement trough.• A building can be considered separately at each side of a point

of inflexions, an hence assessed separately in 2-zones. • Structural strains in the building are obtained and assessed

against limiting strains associated with a defined level of damage

• Quantifies structural impacts and can be used as an approach to assess risks and need for monitoring and mitigation measures for buildings,

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Stage 3 Assessment

• By Potts and Addenbrooke (1197), Mair & Taylor (2001)• Analyzes effect of stiffness of structures on ground settlements

by number of 2D numerical analyses.• Provides modification factors for deflection ratio and horizontal

ground strains: greenfield vs. presence of building

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THANKS FOR ATTENDING !

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