Unexpected Ground Settlement in Tseung Kwan O in the Late 1990s

Investigation, Modelling and Analyses

Suraj De SilvaAECOM Asia Co. Ltd.

9 April 2013

Introduction (1)AECOMs (Former Maunsells) Role;

Was not in the SSDS Tunnel C Construction

Carrying out Reclamation in Town Centre South

The then TDD (now CEDD) requested Maunsell to investigate the unusual settlement behaviour of the reclamation

TKO Industrial Estates Corporation also requested Maunsellto investigate the settlements

Introduction (2)

The ground investigation and Findings The geological and hydro-geological model Analyses and Modelling

Settlement/Heave Groundwater Modelling

Findings

Aerial View of New Reclamation

in TKO Town Centre South in

1998

Surcharge Mounds Along Proposed Road Alignments

The Settlement of the Surcharge mounts were monitored

SURCHARGING AND SETTLEMENT MONITORINGSURCHARGING AND SETTLEMENT MONITORING

Dredged Areas

Surcharged Areas

SETTLEMENT CURVES FROM ZONE 3SETTLEMENT CURVES FROM ZONE 3

1300 mm settlement

Commencement of Unusual Settlement

Expected Settlement Curve

Unexpected Unusual Settlement

PROPOGATION OF UNUSUAL SETTLEMENTPROPOGATION OF UNUSUAL SETTLEMENT

Ground Investigation(1)Main Objectives

Establish the sub-surface ground water regime.

Establish the geological model

Identify the sub-surface layers contributing to settlement

Establish soil parameters to predict the future settlement and recharge behaviour of the ground

Ground Investigation(2) Pore Pressure Measurements

Vibrating wire piezometers in Marine mud and Alluvial clay/silt layers Standpipe piezometers in Alluvial sands, gravels, Completely decomposed

volcanic layer and in rock Standpipe observation wells in Fill

Settlement/Heave Measurements Magnetic Extensometers (Spider Magnet type) Surface settlement markers

In-situ tests in Boreholes SPTs, Vane Shear, Permeability, Pressure Packer tests in rock

Laboratory Tests Index tests, strength and 1D consolidation tests

At E

ach

Inst

rum

enta

tion

Clu

ster

Instrumentation Cluster

INSTRUMENTATION CLUSTER LOCATIONSINSTRUMENTATION CLUSTER LOCATIONS

PIEZOMETRIC ELEVATION FOR BEDROCK AT BH1 TO BH18

-80

-70

-60

-50

-40

-30

-20

-10

0

10

01-Jan-00 31-Jan-00 01-Mar-00 01-Apr-00 01-May-00 01-Jun-00 01-Jul-00 31-Jul-00 31-Aug-00 30-Sep-00 31-Oct-00

DATE

PIEZ

OM

ETRI

C E

LEVA

TIO

N (m

PD)

BH1B-VWPa BH2B-VWPa BH3B-A BH4B-A BH5B-A BH6B-A BH7BBH8B BH9B-A BH10B-A BH10B-B BH12B-A BH13B-A BH14A-ABH15B-A BH15B-B BH16B-A BH17A-A BH18A-A

PIEZOMETRIC ELEVATION IN ROCKPIEZOMETRIC ELEVATION IN ROCK

Normal Phreatic Surface

PIEZOMETRIC ELEVATION FOR CDV AT BH1 TO BH18

-60

-50

-40

-30

-20

-10

0

10

01-Jan-00 31-Jan-00 01-Mar-00 01-Apr-00 01-May-00 01-Jun-00 01-Jul-00 31-Jul-00 31-Aug-00 30-Sep-00 31-Oct-00

DATE

PIE

ZOM

ETR

IC E

LEV

ATI

ON

(mP

D)

BH1B-VWPb BH2B-VWPb BH3B-B BH4B-B BH5B-B BH6B-B BH9B-B

BH12B-B BH14A-B BH15C-VWPa BH16B-B BH17A-B BH18A-B BH18B

PIEZOMETRIC ELEVATION IN CDVPIEZOMETRIC ELEVATION IN CDV

Normal Phreatic Surface

PiezometricProfile Showing the Drawdown

PIEZOMETRIC PROFILESPIEZOMETRIC PROFILES

LAYOUT PLAN OF SUBLAYOUT PLAN OF SUB--SURFACE CROSSSURFACE CROSS--SECTIONSSECTIONS

F

F

D D

SECTION F SECTION F -- FF

Phreatic Surface in Rock

Phreatic Surface in CDV

Development of Drawdown Profile within the Confined Aquifer

Settlement Analyses

Objectives of Settlement Analysis

Estimation of current settlement due to drawdown effect

Prediction of future settlements with drawdown

Prediction of Settlement/ground heave with future re-charge

Settlement/Heave Estimation Methods

Settlement due to drawdown Using conventional Consolidation Theory Using Asaokas Method

Heave due to recharge Using coupled numerical continuum modelling

using FLAC

v = (- u),

Where v - vertical effective stress

u - pore water pressure

(- u) - decrease in pore pressure

v - change in vertical effective stress

The primary consolidation settlement or the settlements due to drawdown can then be estimated from below:

S = i

voivivoi

oi

ici

ehC

'' 'log

110

Where Cci - Compression Index of the ith layer

eoi - Initial void ratio of the ith layer

voi- Initial vertical effective stress at the mid-point of the

ith layervi

- Increase in effective stress at the mid-point of the ithlayer

hI - Thickness of ith layer

Estimation of Settlement from Consolidation Theory

Asaokas Curve fitting MethodAsaoka's method (1978) is based on the assumption that the primary consolidation settlements follow an exponential curve of the form;

2

2

2 48

Hc

nFDc

c vhWhen S0 = 0 where

If ch = 0 then ; 2

2

4 Hcc v Where

tLnc 1

Sett

lem

ent

S

Time t

Si+1

Si

S1

S(t) = S - (S - S0)(1- e-ct)

S(t) = S (1- e-ct)

Estimation of Settlement due to Drawdown using Asaoka Method

01-Sep-94 01-Sep-95 31-Aug-96 31-Aug-97 01-Sep-98 01-Sep-99 31-Aug-00 31-Aug-01 01-Sep-02 01-Sep-03 31-Aug-040

200

400

600

800

1000

1200

1400

Sett

lem

ent (

mm

)

Settlement due to Drawdown Estimated from Both Methods

Area Drillhole Settlement Marker

Total Monitored Settlement up to July

2000 (mm)

Estimated Background

Settlement(mm)

Estimated Drawdown

Settlement from AsaokaMethod

Theoretical Settlement Estimated

from PiezometricData (mm)

Difference between two

Estimates (mm)

Zone 1 BH16 A1 67 3 64 97 27

BH16 A7 81 10 71 91 20

Zone 2 BH32 PS19 671 413 258 278 20

Zone 3

BH1 PS21 1150 589 561 587 26

BH3 PS10 981 320 661 603 58

BH8 SM6 933 845 88 91 3

BH20 PS20 1067 330 737 754 17

Groundwater Modelling

Objectives of Groundwater Modelling

Transient Flow Groundwater Modelling (Coupled Models)

Establish drawdown profile and extent (taking account of recharge of rock from soft clay)

the time required to attain the observed drawdown

how long it would take for the groundwater levels to recover (recharge)

Steady State Groundwater Modelling

Establish the Steady State Flow conditions

Groundwater Models Developed

Coupled numerical modelling using FLAC 3.4.

Plane Strain 2km long Model of TKO North Plane Strain 1.6 km and 2.0 km long Models

of TKO South (TIE Area) Plane Strain small models at 3 borehole

locations to model local recharge behaviourinclude ground heave

Axisymmetric model of TKO Shaft and Stub tunnel

Steady State Seepage Modellingusing SEEP/W

Plane Strain 2km long Model of TKO North Plane Strain 1.6 km and 2.0 km long Model of

TKO South Axisymmetric model of TKO Shaft and Stub

tunnel

N

North of Tunnel C Geological Section Used in Groundwater Modelling

2 km

South of Tunnel C Geological Section Used in Groundwater Modelling (Plane Strain and

Axisymmetric Models)

2 kmSSDS Tunnel C

TKO Drop Shaft

DiaphragmWall

Parameters Adopted in Coupled FLAC Modelling

FLAC Coupled Model Results TKO North of Tunnel C (Town Centre)- Drawdown in Bedrock

FLAC Coupled Model Results (Plane Strain) South of Tunnel C Drawdown in BedrockT

otal

Hea

d in

Roc

k at

an

Ele

vatio

n of

-84

mPD

in (m

)

Drawdown in Rock from AxisymmetricCoupled Model drawdown in bedrock

Steady State Profile

Distance from Access Shaft (m)

Tot

al H

ead

in R

ock

at a

n E

leva

tion

of -8

4 m

PDin

(m)

Groundwater Inflow Rate into Shaft from Axisymmetric Model

Measured inflow = 1100 Litres/min

Time (Months)

Inflo

w R

ate

into

Sha

ft (L

itres

/min

)

Groundwater Recharge

Grouting and Sealing of Tunnel C

Period Activity

Nov 1999 Breakthrough

Nov 99 Feb 00

Temp Concrete Lining

Feb May 00 Concrete Invert

May Dec 00 Install twin steel tubes

Dec 00 Mar 01

Repair and grouting

Mar April 01 Contact grouting

925 925

4230 mm (Excavated)

1350mm Steel Tubes

Sectional View of Tunnel C

Pore Pressure Recovery with Time at Top of Alluvium during Recharge (From Plane Strain

Model)

% P

ore

Pres

sure

Rec

over

y (%

)

0

10

20

30

4

0

50

60

7

0 8

0

90

100

0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 96 102 108 114 120 Time in Months

80% recharge in 12 months

Evolution of recharge at the top of alluvium with Time(with rock mass permeability krock = 1 x 10-6 m/s)

PIEZOMETRIC ELEVATION FOR ALLUVIUM AT BH19 TO BH32(RECORD UNTIL 02 March 2002 )

-50.00

-40.00

-30.00

-20.00

-10.00

0.00

10.00

17-Jun-00 16-Sep-00 16-Dec-00 17-Mar-01 17-Jun-01 16-Sep-01 16-Dec-01 18-Mar-02 17-Jun-02

DATE

PIEZ

OM

ETRI

C EL

EVAT

ION

(mPD

)

BH19C1 (B) BH19C1 (C) BH20C1 (B) BH20C1 (C) BH21C1 (A) BH21C1 (B) BH22C1R (B)

BH22C1R (C) BH22C2R (C) BH23C1 (B) BH23C1 (C) BH23C2 (C) BH24C1 (B) BH24C1 (C)

BH25CI (C) BH25C2 (B) BH26C1 (B) BH26C1 (C) BH32C1 (C) BH32C2 (B) BH32C2 (C)

Rise in the Groundwater level

About 10 months

Pore Pressure Recovery with Time at Top of Bedrock during Recharge (From Plane Strain

Model)

% P

ore

Pres

sure

Rec

over

y (%

)

0

10

2

0

30

4

0

50

6

0

70

8

0

90

10

0

0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 96 102 108 114 120

Time in Months

80% recharge in 8 months

Evolution of recharge at the top of Bedrock with Time(with rock mass permeability krock = 1 x 10-6 m/s)

Piez

omet

ric

Ele

vatio

n in

m

PD

80 % in About 8 months

Recharge and Ground Heave Modelling with Small Coupled FLAC Models

CDV

Alluvium

Marine Mud

Reclamation Fill

Volcanic Tuff Bedrock

Initial Pressure Profile

46m Head

46m Head

t1t2

t3

Predicted Ground Settlement/HeaveLocation Predicted Movement from (heave is +ve) Time T

when Tunnel Inflow is reduced to nil(mm)

T+ 6 months

T+ 1year

T+ 2

years

T+ 3

years

T+ 4

years

T+ 6

years

1) Developed areas

BH15 Tong Tak Street West 11 7 0 -5 -10 -18

BH16 Tong Ming Street 1 -3 -13 -21 -28 -40

BH17 Road D1. North -East from Beverly Garden

-3 -6 -13 -19 -24 -31

2) Areas under development

BH3 Road D4, North West of Bauhinia Garden

73 73 72 68 65 59

BH4 Road D4, North East of Bauhinia Garden

11 9 0 -9 -17 -30

BH5 Road D4 West, next to box culvert

-2 -5 -12 -18 -23 -31

- 2 1 0

- 1 9 0

- 1 7 0

- 1 5 0

- 1 3 0

- 1 1 0

- 9 0

- 7 0

- 5 0

- 3 0

- 1 0

1 0

0 3 - F eb -0 0

1 3 - A p r-0 0

2 2 - Jun-0 0

3 1 - A ug -0 0

0 9 - N ov-0 0

1 8 - Jan-0 1

2 9 - M ar-0 1

0 7 - Jun-0 1

1 6 - A ug -0 1

2 5 - Oct -0 1

0 3 - Jan-0 2

1 4 - M ar-0 2

2 3 - M ay-0 2

Date

(-ve

Set

tlem

ent)

(+ve

Hea

ve)

(mm

)M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 1st Datum

Ground Heave = 20 -25 mm

Magnetic Extensometer Readings

Summary of Findings

1. Ground investigations revealed that the settlement was due to drawdown and the drawdown was caused by Tunnel C.

2. Settlement analyses showed that the settlement due to drawdown ranged from 90 -750 mm

3. The coupled FLAC modelling showed that measured drawdown can occur within the time frame.

4. FLAC models showed that recharge will occur relatively quickly within 8 to 12 months.

5. Modelling showed that up to about 75 mm ground heave can take place at some locations.

Thank You