underground construction pitfalls to avoid

5/19/2018 Underground Construction Pitfalls to Avoid

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IEAust Singapore 12 Jan 2012 1

Underground Construction

Pitfalls to Avoid

D Wen, BSc, PhDPE, PE(Geo), AC(Geo), CEng, MICE, MIEAust, CPEng


2/692IEAust Singapore 12 Jan 2012

Soil Investigation & Interpretation

Maintenance

Interfaces Base Stability

Gaps in Retaining Walls

Buildings on Mixed Foundations

Underground Construction - Pitfalls to Avoid



Interpreted Profile of Surface Wave Velocity

Interpreted Rock Profile




Maintenance


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IEAust Singapore 12 Jan 2012

Maintenance


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TBM Launching


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TBM Launching


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Braddell Road Proposed Depressed Road

TBM Launching


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F1

30m3 F1 sandsflowing into the

TBM launching

chamber through

the gap between

the TBM shield andthe shotcrete

support of the

launching

chamber.

Road closed foremergency repair.

CPT carried out to

confirm no cavities

under road.

TBM Launching


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15IEAust Singapore 12 Jan 2012

TBM Launching


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TBM Launching or Docking

Ground treatment for TBM launching /

docking: TBM Length + One Ring Length

Quality control of ground treatment


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Interface between Stable & Weak Soils


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600mm of

settlement

on CTE

due to

DTSS

tunnelling



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Weak ground / strong, stable ground

Face pressure governed by overlying

weak ground



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IDEALISED PROFILE IN GRANITE

Interface between Rock & Soil


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Flushing of the annular gap

Water ingress

Limit of EPB Technology

Coarse granular

material / rocks

Insufficient fines to form

plastic plug


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Good EPB Spoil


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Difficult to Turn a Mixture of Cut Rock and Granular Spoil into EPB Spoil

May Be Unable to Maintain Target Pressure

Cut Rock & Granular Spoil


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Mixed face of strong or very strong rock

with flowing or squeezing soils

Predominantly granular spoil

Need to have high face pressure, but spoil

does not readily form a plug in EPB

machines



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Cutterhead Intervention


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Over-excavation by Slurry TBM


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Feeding Pipe

Discharge Pipe

Flow meterDensity meter

Excavation Management System


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Normal Excavation Condition Over Excavation Condition

Excavated

Qty(Cum)

Jack Stroke (m)

Theoretical

Value

Theoretical

Value

Jack Stroke (m)

Excavate

dQty(Cum)

Excavation Management System


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Base Stability of Excavation

Hydraulic uplift

D D

Uk1

k2


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Example of hydraulic uplift failure reported by Davies (1984)

Hydraulic uplift


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Shaft Excavation


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B St bilit f E ti


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Soil stability in the gaps

marine clay squeezing through the gaps into

excavation, causing localized soil collapse

Large ground settlements at areas with

gaps in diaphragm walls

Water leaking through the gaps into the

excavation


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Ground level

Final excavation level

Underground services

Jet grouted piles to close

the gap in diaphragm wall

Untreated zones below the

service

Diaphragm wall


G i R i i W ll


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G i R t i i W ll


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SandGrouts Gravelcoarse medium fine

Cement

Sodium silicate

Grain size (mm) 2 0.5 0.25 0.074

Groutability Based on Grout

Type vs Soil Particle Size

(after Henn, 1996)


B ildi Mi d F d ti


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UNIFORM

SETTLEMENT

UNIFORM TILT

Building on Mixed Foundations

B ildi Mi d F d ti


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Uniform Deformation: Not Damaging




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BENDING SHEAR

Damaging Modes of Movement

HORIZONTAL

EXTENSION




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Adjacent buildings on different foundations: Old buildings on footings

butted against newer buildings on deep foundations.



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Settled by 107mmPiled

Building tilting away fromthe adjacent structure




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Annex structure

on footings

abutting buildings

on piles




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Main frame on piles, floor slab notsuspended



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FILL (6.5m THICK)

SOFT MARINE CLAY

(33m THICK)

SANDY OLD ALLUVIUM

SETTLEMENTBuildings on Mixed

Foundations




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Slabs are suspended; columns

not suspended



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Walkway - piled

Main load bearing

column - shallow

foundations




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Part of the building on piles and part on footings

Damaging at small differential settlements.




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Masjid Wak Tanjong


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Masjid Wak Tanjong



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Fill & Organic clay Weathered Jurong Formation

No 7 No 9




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Party Wallto No 9

Shirlaw, Wen, Algeo & Patterson-Kane (2003)




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Pocket of soft clay


Oth I ith G d S ttl t


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Other Issues with Ground Settlement

Oth I ith G d S ttl t


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Other Issues with Ground Settlement

Do not forget service connections to buildings!



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To identify buildings on mixed foundations

- A&A drawings

To identify locations of variation in

geology, in particular along terrace houses

A general walk around the site building

inspection




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Soil Investigation & Interpretation Maintenance

Interfaces

Base Stability


Buildings on Mixed Foundations



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THANK YOU

underground construction pitfalls to avoid

Documents

ieaust singapore

tbm length

weak ground strong

stable ground face pressure

docking ground treatment

idealised profile

retaining walls buildings

overlyingweak groundinterface