Case study

1. Case Study on Site Investigation
Plan at Chek Lap Kok, Hong Kong
O. BOU GHANAM, C. TANISSA
DEPARTMENT OF CIVIL & ENVIRONMENTAL
ENGINEERING, UNIVERSITY OF ILLINOIS
URBANA-CHAMPAIGN

2. Project Description
 Construction of Hong Kong replacement airport by
levelling the island of Chek Lap Kok and Lam Chau
and Reclaiming almost 600 Ha of land
 Preliminary offshore site investigation (1973-1978)
revealed the presence of
 Up to 20 m of very soft marine mud forming the
seabed around the islands.
 Additional cohesive sediments, somewhat stronger
and less compressible, sporadically indicated at
greater depths.
 Anticipated Construction problems due to large
settlements/ differential settlements and large
mudwaves.
Wu et al. (2020).
O. BOU GHANAM & C. TANISSA 2/18
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3. Project: Geotechnical Risks and Constraints
.
Stability
• Embankment Stability
• Bearing Capacity
Failures
• Mud Waves
Settlements
• Excessive Total
Settlements
• Intolerable Differential
Settlements
Seawalls
• Stability of Seawalls
• Excessive deformation
Cost and Time
• Decide on most
efficient construction/
ground improvement
solution
O. BOU GHANAM & C. TANISSA 3/18
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4. Geotechnical Program
 Purpose of Geo-technical Program:
 Fully defining the problems;
 Assessing the viability of using vertical drains and careful filling techniques to allow successful
construction over the mud, eliminating the need to excavate and replace.
Two parts:
 Part I: Extensive site investigation to provide detailed data regarding the foundation soils at the site.
 Part II: The construction of a heavily instrumented large test fill into the sea from Chek Lap Kok to
evaluate feasibility of reclamations and staged construction works with various vertical drain
configurations
O. BOU GHANAM & C. TANISSA 4/18
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5. Part I: Site Investigation
 77 marine boreholes, drilled from floating barges using land drilling
equipment with good sampling procedures (drilling mud, casing, fixed
piston and shelby sampling)
 93 piezocone penetration soundings, 60° conical piezocone tip,
projected area of 15 cm2, sleeve area of 200 cm2, porous piezometer
element located midway between the tip and the base, rate 2cm/sec
 95 pore pressure dissipation tests;
 Extensive field vane testing; 65-mm x 130-mm field vane tip was used
in the soft marine mud, while a 55-mm x 110-mm vane tip was used in
the stiffer clays. performed at a rate of rotation of 6°/min to develop the
peak (undisturbed) strength, after which 10 time rotation and a second
shear test performed at 6°/min
 In-situ variable head permeability tests using Geonor Piezometers
 Marine geophysical studies
 Extensive laboratory testing program including both routine and
specialized tests
O. BOU GHANAM & C. TANISSA 5/18
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6. Subsurface Stratigraphy
 [0 -15m]: Upper Marine Clay: Very soft and
compressible Seabed Mud
[15 – 23m]: Upper Alluvium: Very stiff
desiccated and fissured clay with frequent
dense sand lenses [OCR 4-8]
[23 – 38m]: Lower Marine Clay: Medium stiff
light marine clay [OCR 2-3]
[38 – 48m]: Lower Alluvium: Very dense coarse
to fine sand, becomes very silty, with occasional
lenses of hard clay, frequently grades into
gravels and cobbles.
[48 – End]: Completely Decomposed Granite
(CDG) overlaying Granite Bedrock
O. BOU GHANAM & C. TANISSA 6/18
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7. Piezocone-Borehole Correlations
O. BOU GHANAM & C. TANISSA 7/18
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8. Upper Marine Clay

Lower Marine Clay
O. BOU GHANAM & C. TANISSA 8/18
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9. Properties of Upper/Lower Marine Clay
Upper Marine Clay Lower Marine Clay
Classification High Plastic Clay - CH Medium to High Plasticity CL/CH
Plasticity Index 45-65% 20-40%
Water Content MC > LL 30-60%
OCR 1.5 to 2.0 4.0 to 7.0
Normalized Shear Strength
from FVS: su/σ’vo
0.42 0.4 to 0.6
Cc 0.3 to 0.5 0.15 to 0.35
Cr 5 to 10% Cc 0.02 to 0.06
Cv 1.3 ± 0.5 m2/yr 2.0 ± 1 m2/yr
Ch 4 m2/yr
O. BOU GHANAM & C. TANISSA 9/18
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10. Part II: Test Fill - Objectives
 Objective:
 Development of in-situ information regarding the
consolidation characteristic of the seabed mud layer
Development of in-situ information regrading
embankment stability
Assessing the viability of staged reclamation and the
applicable rates of constructions
Assessing the feasibility of multiple vertical drain
configurations ( Sand Drains Vs at various diameters/
spacing) in speeding consolidation settlements
Test Fill Construction: (a) March, 1982, Seawall Coming
above Water; (b) April, 1982, Drain Installation in Main Test
Area, Haul Road on Left; (c) July, 1982, Completed Test Fill
O. BOU GHANAM & C. TANISSA 10/18
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11. Part II: Test Fill – Construction Details
 Located 200 m offshore, 100 m square of plan
 Built over a soil profile consisting of:
 4 m of sea water to reach the seabed
 7 m of fairly uniform mud,
 5-6 m of upper alluvial crust deposits,
 3-8 m of lower marine clay deposits,
 7-13 m of lower alluvial deposits,
 CDG residual Soils
 Four Quadrants:
 Control Area - No treatment
 500-mm diameter displacement sand drains at 3.0 m
triangular spacings
 Plastic band Alidrain at 1.5 and 3.0 meters triangular
spacing in the remaining twi quadrants
Plan of Test Fill
O. BOU GHANAM & C. TANISSA 11/18
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12. Part II: Test Fill – Instrumentation
Instrument Measured Parameters Installed
to June, 1982
Operational
Nov., 1982
Pneumatic piezometer
Porewater pressure
63 63
Hydraulic piezometer 13 13
Settlement plate Surface settlement and
groundwater level
58 57
Settlement profile pipe Surface settlement profiles 19 5
Pneumatic settlement gauge Subsurface settlement 10 ?
Inclinometer Subsurface lateral
movement
9 7
Subsurface settlement
anchors/Inclinometer
Subsurface settlement
and lateral movement
12 10
Surface movement
point
Surface settlement and
horizontal movement
54 54
O. BOU GHANAM & C. TANISSA 12/18
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13. Part II: Test Fill –Measurements & Constraints
 Measurements of :
◦ Consolidation and settlement of the
subsurface soils;
◦ lateral movement in the fill and
subsurface soils;
◦ pore pressure generation and
dissipation; and
◦ Embankment filling.
• Use of instrumentation
specifically designed to
accommodate large
strains.
Instrumentation
to accommodate
large
movements
• Duplication of instruments to allow for
loss of some installations.
• Duplication of the method of
measurement by using two or more
different instrument types
• Extensive protection measures,
especially
Anticipated large
damage rate due
to ongoing
construction
activities
O. BOU GHANAM & C. TANISSA 13/18
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14. Part II: Main Test Fill – Results and Interpretations
Settlement profiles (one-dimensional/ Uniform) observed via a pipe installed at the end of the
Construction Stage 4 across the 1.5-m Alidrain and 3-m sand drain quadrants
O. BOU GHANAM & C. TANISSA 14/18
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15. Part II: Main Test Fill – Results and Interpretations
Settlements of Upper Marine Clay in Main Test Area
 The inclusion of Alidrains accelerated the
consolidation process;
 Closer spacing of the Alidrains resulted in
faster settlement.
 The effectiveness of the sand drains, on the
other hand, appears to be limited
 Estimated Settlements appeared reasonable
when compared to measured values
O. BOU GHANAM & C. TANISSA 15/18
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16. Part II: Main Test Fill – Results and Interpretations
 Data from permanent Piezometers and
Piezo-profile tests
 Pore pressures were extrapolated from
Pore pressure isochrones and Coefficients of
Consolidations of soils were back-calculated:
Estimated Cv for Soft mud of 2.9 m2/ yr,
higher than the the value estimated from
testing program ( 1.3 m2/yr)
Effective Vertical paths were estimated for
each quadrant
Distribution of Excess Pore Pressures with Depth-
Main Test Area
O. BOU GHANAM & C. TANISSA 16/18
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17. Part II: Main Test Fill – Conclusions
 The test fill successfully demonstrated the viability of building the Chek Lap Kok Airport
reclamation over the soft seabed mud
The development of mudwaves could be avoided by controlled filling in layers
Vertical band drains could be used to accelerate consolidation of the mud
Displacement sand drains were not successful
Alidrains tested were found to be very effective despite the large vertical strains experienced
 Solution: Install Alidrain or an equivalent product at 1.5-m spacing and with a 12-month
consolidation period, with 25% Preload
O. BOU GHANAM & C. TANISSA 17/18
CEE - Geotech

18. Performance: Settlement
Measurements -Multi-temporal InSAR
(a) Estimated overall settlement velocity over the two decades. (b)-
(g) Time series of settlements of six selected points estimated from
MT-InSAR (blue triangles) and the settlement model (curves)
 Significant settlement since its operation in
1998 due mainly to consolidation of the
materials used in the land reclamation and
underlaying mud deposits;
 Settlement Measurements using Interferometric
Synthetic Aperture Radar (InSAR)
 The results reveal both spatial and temporal
variations of the land settlement, accumulating
to over 40 cm over two Decades
 The results are validated through cross-
validation between the datasets and with
leveling and GPS measurements on the Airport
platform.
O. BOU GHANAM & C. TANISSA 18/18
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