unexpected ground settlement
DESCRIPTION
Unexpected Ground SettlementTRANSCRIPT
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Unexpected Ground Settlement in Tseung Kwan O in the Late 1990s
Investigation, Modelling and Analyses
Suraj De SilvaAECOM Asia Co. Ltd.
9 April 2013
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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
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Introduction (2)
The ground investigation and Findings The geological and hydro-geological model Analyses and Modelling
Settlement/Heave Groundwater Modelling
Findings
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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
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SURCHARGING AND SETTLEMENT MONITORINGSURCHARGING AND SETTLEMENT MONITORING
Dredged Areas
Surcharged Areas
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SETTLEMENT CURVES FROM ZONE 3SETTLEMENT CURVES FROM ZONE 3
1300 mm settlement
Commencement of Unusual Settlement
Expected Settlement Curve
Unexpected Unusual Settlement
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PROPOGATION OF UNUSUAL SETTLEMENTPROPOGATION OF UNUSUAL SETTLEMENT
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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
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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
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INSTRUMENTATION CLUSTER LOCATIONSINSTRUMENTATION CLUSTER LOCATIONS
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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
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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
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PiezometricProfile Showing the Drawdown
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PIEZOMETRIC PROFILESPIEZOMETRIC PROFILES
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LAYOUT PLAN OF SUBLAYOUT PLAN OF SUB--SURFACE CROSSSURFACE CROSS--SECTIONSSECTIONS
F
F
D D
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SECTION F SECTION F -- FF
Phreatic Surface in Rock
Phreatic Surface in CDV
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Development of Drawdown Profile within the Confined Aquifer
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Settlement Analyses
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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
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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
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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
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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)
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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
)
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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
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Groundwater Modelling
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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
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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
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South of Tunnel C Geological Section Used in Groundwater Modelling (Plane Strain and
Axisymmetric Models)
2 kmSSDS Tunnel C
TKO Drop Shaft
DiaphragmWall
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Parameters Adopted in Coupled FLAC Modelling
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FLAC Coupled Model Results TKO North of Tunnel C (Town Centre)- Drawdown in Bedrock
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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
)
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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)
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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
)
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Groundwater Recharge
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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
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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)
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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
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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)
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Piez
omet
ric
Ele
vatio
n in
m
PD
80 % in About 8 months
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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
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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
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- 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
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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.
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Thank You