Gahir, J.S.

Underground Space – the 4 th Dimension of Metropolises – Barták, Hrdina, Romancov & Zlámal (eds)
© 2007 Taylor & Francis Group, London, ISBN 978-0-415-40807-3
Design and construction of a cofferdam for a sub-sea tunnel on The Palm
Jumeirah, Dubai
J.S. Gahir
Kellogg Brown & Root, Leatherhead, UK
A. Mochizuki
Taisei Corporation, Tokyo, Japan
N.O. Othman
Parsons PTG, Washington, USA
M.A. Qamzi
Nakheel, Dubai, UAE
ABSTRACT: The paper presents design, construction and performance of a sheet piled cofferdam required to
construct a 1.4 km long sub-sea vehicular tunnel in dry conditions. An overview of the challenges faced whilst
piling and how these were overcome are presented. Measures taken to maintain the cofferdam during tunnel
construction are included.
1 INTRODUCTION
The Palm Jumeirah in Dubai, United Arab Emirates,
involves the reclamation of land to form the shape of
a palm tree comprising of a crescent, 16 Fronds, Spine
and Trunk. Reclamation extends some 5.5 km into the
Arabian Gulf and the artificial island is protected at
the seaward side by a rock armoured semi-closed elliptical
Crescent (Figure 1). The resulting development
increases the Dubai shoreline by 75 km and when completed
will provide for a community of about 100,000
people.
To cross a body of water some 500 m wide a sub-sea
vehicular tunnel connects the Spine to the 11.5 km long
Crescent. The tunnel cross-section comprises 3 cells,
the outer cells carrying 3 lanes of traffic in each direction
while the central cell is reserved for services and
emergency evacuation.
The construction methodology adopted was cut and
cover in dry conditions which required a 2.4 km long
sheet piled cofferdam as temporary works, with the
sheet piles acting as cut off to reduce seepage into the
cofferdam to manageable levels. Offshore and onshore
operations were undertaken to install 30 m long steel
sheet piles into the crest of temporary dykes and into
the recently reclaimed islands where the tunnel portals
are located. The construction sequence was to dredge,
form dykes, install sheet piles and drain the cofferdam.
After draining, the dykes provide construction
Figure 1.
Overall view of The Palm Jumeirah.
access to the Crescent, a significant advantage over
an immersed tube approach. The cofferdam constitutes
some 30% of the total project cost of the tunnel
works.
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Figure 2.
Tunnel alignments.
Figure 3.
Typical cross section and ground conditions.
2 PROJECT DESCRIPTION
The total length of the tunnel structure is 1398 m and
comprises of a 241 m long Spine portal approach,
969 m of reinforced concrete box section and 188 m
of Crescent portal approach.
The horizontal and vertical alignments were dictated
by a maximum grade of 6% for a 60 kph design
speed, in compliance with Dubai Municipality Geometric
Design Manual for Dubai Roads (1999), and the
need to ensure that the tunnel was sufficiently deep to
maintain a 125 m wide arc-shaped navigation channel
with a minimum draft of 10 m, above the 1.5 m thick
layer of armour protection on the roof slab. The vertical
alignment and the horizontal alignment, which is a
reverse curve with a central straight section, are shown
in Figure 2.
Mean sea level is +1.0 mDMD (Dubai Municipality
Datum, which is 0.09 m below Admiralty Chart
Datum) with a tidal range of 1.2 m between MHHW
and MLLW. The reclaimed islands are at +3 m and
+4 mDMD at the Spine and Crescent, respectively.
Seabed elevations before The Palm reclamation were
in the range −11 m to −12 mDMD. Pre-construction
surveys indicated that the minimum seabed elevation
along the tunnel alignment and the cofferdam
region was −15 m and −19 mDMD respectively, in
areas where materials had been won for breakwater
construction.
A Design and Build contract was awarded to Taisei
Corporation in 2004 for the construction of the
vehicular tunnel.
3 GEOLOGY AND GROUND CONDITIONS
The local geology in the vicinity of the project site
has been shaped by the current hot arid climate and
by fluctuations in sea level and is described by Gahir
et al. (2006).
Ground conditions were determined by sinking 13
offshore boreholes to a depth of 30 m into the seabed
and 3 boreholes up to 30 m depth on the onshore
reclamation. The minimum elevation explored is
−45 mDMD. The offshore boreholes were drilled
along the alignment of the dykes and are offset up to
230 m from the tunnel centre line. All boreholes were
grouted after completion.
The boreholes revealed that under a 12 m thick
recent fill a 1 m to 2 m thick band of weak caprock
is present which in turn is underlain by 2 m to 4 m
thick layer of medium dense sand. The sand overlies
an 18 m to 22 m thick sequence of very weak to weak
interbedded calcarenite and sandstone. A conglomerate
layer whose upper horizon is some −35 mDMD is
present across the site (Figure 3). The UCS of rock
encountered is in the range 1 to 3.5 MPa. In isolated
boreholes a thin layer of soft silt was recorded
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under the fill, and is suspected to be the original
seabed.
In-situ permeability of the rock units, measured
using standard Lugeon tests and variable head permeability
tests, indicated k values ranging from
4×10 −6 to 5×10 −4 m/sec with lower values at depth.
4 DESIGN CONSIDERATIONS
The cofferdam is required for some 17 months and at
peak periods a workforce of about 1000 was expected
within its confines and thus safety was of paramount
consideration in the design process.
The cofferdam outline was dictated by the following
considerations (a) the dykes have a 12 m wide crest at
−2 mDMD; an elevation adopted after a soil-structure
interaction analysis was undertaken for different pile
sections and various crest elevations, to provide a
balance between the fill available and the structural
performance of the piles (b) the sheet piles were
planned 2 m from the seaward edge so that a 10 m
wide corridor was available for a 2 lane temporary
road inside the cofferdam after draining (c) the tunnel
trench at its deepest is −26 mDMD and the trench
slopes, located in interbedded sandstone and calcarenite,
were planned at 1V:0.5 H (d) a 15 m minimum
clearance was maintained from the top of the trench
excavation to the toe of the dyke thus avoiding surcharging
and undermining the stability of the trench
slope and (e) a dyke slope of 1V:4 H from crest to
−10 mDMD and 1V:7 H thereafter, was adopted from
slope stability considerations based on recent experience
gained in the construction of The Palm. A 2.4 km
long cofferdam outline was thus identified having a
plan area of some 300,000 m 2 and a maximum width
of about 400 m. This outline involved 1550 m of offshore
piles and 850 m of onshore piles. Local filling
adjacent to the recently formed islands was undertaken
to maximize land based operations.
Both marine and land driven sheet piles are up to
30 m long and the top of the piles were planned at
+3.3 mDMD for minimal overtopping under storm
surge and wave conditions, and the toe at −26 mDMD
based on seepage analyses which required 1 m minimum
penetration into the lower less permeable sandstone
which underlies the calcarenite. Bending stresses
and deflections complying with Technical Standard
and Commentary for Port and Harbour Facilities
(2001) indicated that pile sections SX27 (section modulus,
z = 2700 cm 3 /m) for the offshore piles, and PU25
(z = 2500 cm 3 /m) for onshore piles were appropriate.
The estimated maximum deflections at the top
of the marine piles, after draining, was calculated to
be about 200 mm and the relative deflection between
+3 m and −2 mDMD i.e. the cantilevered section, to
be about 150 mm. These estimates are based on a
sea level of +2.45 mDMD, to take account of storm
surge, and a horizontal modulus of subgrade reaction
of 10,000 KN/m 2 /m based on an empirical relationship
to an SPT-N value of 5.
Seepage analysis using 2D FLOW with an average
k value determined for each substrata indicated that
some 820 m 3 /hr could be expected under the cofferdam
once steady state conditions had established.
French drains at the base of the tunnel trench, with
pits and submersible pumps of sufficient capacity, and
lines of well point systems were designed to collect the
inflow (Figure 4). The collected inflow was directed
to two locations, one either side of the tunnel trench,
before being pumped back to the sea over each dyke
with due care taken not to erode the outer slope.
5 CONSTRUCTION METHODOLOGY
5.1 Dyke construction
Cutter suction dredgers were used to excavate the tunnel
trench, one for shallow and one for deep dredging
below −15 mDMD, with operations commencing at
the end of the alignment i.e. at the newly formed
islands and progressing towards the centre where the
trench is deepest.The dredgers lowered the seabed elevation
by some 2 m in a single step until the desired elevation
along the tunnel alignment was achieved within
0.5 m of the invert elevation, with the final grade being
achieved using earthworks equipment in dry conditions.
Seabed elevations were constantly monitored
by surveying equipment on the dredger and from a
survey boat.
The dredged material was sucked into the dredger
holds and placed at the dyke locations using a spread
pontoon positioned by GPS and controlled by anchors
and winches. Both dykes were built up concurrently
and care was required to achieve the 1V:4 H
slope by considerably reducing the volume discharged
through the pipe. Total volume for the two dykes
was 920,000 m 3 of which 660,000 m 3 was obtained
from the tunnel trench excavation and the balance
was obtained from a borrow source nearby, with a
small quantity purchased from a supplier. A larger
volume than had been estimated at the tender stage
was necessary due to the flattening of the dyke slopes
from 1V:4 H to 1V:7 H below −10 mDMD as already
described in Section 4. Bathymetric surveys were carried
out after filling to either confirm that the desired
dyke profile had been achieved or identify local areas
requiring additional fill.
5.2 Sheet pile procurement and preparation
Procurement of the 12,500T of sheet piles was a lead
item due to a shortage on the world market. Some
9500T of new SX27 sections in 15 m lengths were
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MAX. DSP=130mm
MAX. DD=4.9mm
WLD -15.9DMD
CRESCENT
NW
SW
SE
MAX. DSP
MAX. DD
WLD
Figure 4.
Dewatering regime and cofferdam monitoring points.
procured from Japan and had a2to3month delivery
time. Used PU25 sections of some 10 m length were
purchased locally and taken to an onsite yard at the top
of the Spine where 20 m lengths were assembled. To
determine the sheet pile drivability a trial was undertaken
near the Spine portal close to a borehole location
using PU series. The piles were spliced and driven to
depths of 33 m using vibrohammer ICE815C (1250 kN
max centrifugal force) without difficulty.
The newly purchased SX27 sections were taken to
Hamriya Port, about 35 km east of the project site,
where they were chamfered and welded to form 30 m
lengths. Hyperseal DPS-1000 was applied to the pile
clutch. This material can expand up to 10 times its
original volume in fresh water but its performance offshore
is curtailed due to the salinity of sea water and
it expands to half the figure. The piles were paired to
form 1.2 m wide panels, so that each prepared offshore
panel comprised of 4 sheet piles, before being barged
to site.
5.3 Onshore sheet piling
Sheet pile sections, 20 m long and 0.6 m wide, were
pitched into a guide frame capable of accommodating
16 sheet piles and driven one by one with a 1.5 m stick
up. Longer sheet piles could not be accommodated
due to limitation of the crane boom. Ten metre long
pile sections were hooked through the guide frame and
spliced to the driven piles. Driving recommenced using
vibrohammer ICE815C and progress was slow due to
difficult driving. Some hard nodules present within the
calcarenite and the wear and tear on the used piles were
felt to contribute to difficult driving. Additionally two
splices in the desired pile length did not always provide
a plumb vertical section and this led to an increase in
the frictional force in the pile clutch.
At some locations especially on the Crescent piles
were terminated 5m short of the required design penetration
due to refusal with the driving system. A
reassessment of the seepage inflow into the cofferdam
was undertaken using k of calcarenite 2×10 −5 m/s
which indicated that the anticipated inflow in this
area increased 9 times locally with the nett effect that
the overall infow into the cofferdam after draining
increased from 820 m 3 /hr to 1600 m 3 /hr. To cope with
the additional inflow contingency measures were put
in place and comprised of (a) drive secondary short
length sheet piles in local areas at the top of the tunnel
trench excavation (b) increase the number of wells and
(c) increase pump capacity.
5.4 Offshore sheet piling
Offshore piling was undertaken from 2 barges, each
with a 150T crane, and the prepared panels 30 m long
and 1.2 m wide, were driven through a guide frame
using a double clutch vibrohammer ICE1412 (2300 kN
max centrifugal force). The contractor reported difficult
driving conditions which required greater effort
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in getting the necessary penetration than had been
anticipated initially. On average 2 to 3 panels were
being achieved in a 12 hour shift. When the second
barge, operating a week later on the opposite dyke,
also experienced similar difficulties it was recognized
that the problem was not local.
Piling records indicated that the penetration rate
decreased significantly below −17 mDMD in the second
calcarenite layer. A review of the unconfined
compressive strength results indicated weak rock and
that although the presence of hard nodules may explain
some of the driving difficulties, something fundamental
had been overlooked. Pairing the piles doubled the
area at the cutting edge and also doubled the friction
from the fill and underlying rock, and this resistance
could not be overcome by simply doubling the hammer
capacity required for a single pile section.The decision
to pair the piles was taken after discussion with a local
piling contractor who had successfully driven paired
piles but not to the depth required for this project. An
offshore pile driving trial may have revealed the problem
earlier, and more work in this area would have
been useful.
To overcome the difficulties in the short term predriving
was undertaken with a panel connected to a
water jetting system to −25 mDMD and withdrawn,
and the final 1 m was driven with the permanent pile.
Following the initial success with water jetting, a decision
was taken to weld four 25 mm diameter steel pipes
in the corners of all the sheet piles along its total
length and connected to 2 mm diameter jetting nozzles
located near the toe of the sheet pile. The pipes were
hooked to a high pressure pump capable of delivering
250 l/min at 40 bar pressure. Simultaneous water jetting
and driving with the same hammer overcame the
driving difficulties. To make up for lost time a third
barge was mobilized and a night shift introduced. In
86 working days 1290 paired sheet piles were successfully
driven to the desired toe elevation. Towards the
end a production rate of 5 to 6 pairs was being achieved
in a single shift.
5.5 Cofferdam closure
Seven onshore and 2 offshore closures were necessary
to complete the perimeter. Sheet pile sections driven
from the opposite ends were overlapped so that an open
box was formed. A uPVC tube was inserted from the
top of the piles to key into the top of the dyke and
grouted, thus sealing the open box. After setting, a
borehole was drilled through the grout, the dyke and
into seabed extending to the same elevation as the toe
of the sheet piles. A rapid setting chemical grout, colloidal
silica with sodium chloride, was then injected
at 0.5 m centres working from the bottom up. The colloidal
silica when set has a very low permeability, thus
effectively sealing the closure point.
The offshore closure was undertaken during a Neap
tide within a 4 hour window. Due to an increased flow
velocity some scouring of the dyke, up to 2 m deep over
an 8 m long section, was observed by divers deployed
periodically during the course of the works. The dyke
was repaired after closure.
5.6 Draining
Some 4,350,000 m 3 of water inside the cofferdam
(excluding seepage) was pumped out in stages to
reduce the water level from +1mto−23 mDMD. This
was undertaken using 9 high capacity pumps each fitted
with a flow meter, of which 7 were operational at
one time, connected via valves and manifolds to the
discharge pipes and over the northern cofferdam wall
and to sea. The pumps were located on two floating
pontoons placed centrally at the deepest section of the
trench excavation. Discharge was completed in 45 days
and the rate of water level drop was maintained at less
than 1 m/day. Some leaks in the sheet piles joints were
sealed using underwater welding and sealants.
6 IN-SERVICE PERFORMANCE AND
MONITORING
6.1 Summary of instrumentation installed
To provide information on sheet pile deflections, the
deformation of the dykes, and water levels in the cofferdam
6 total station targets with a mirror attachment,
8 inclinometers, 16 observation wells with ground
water loggers, and settlement monitoring stations were
installed at strategic locations and depths. Base readings
were established on top of the sheet piles before
draining and the instruments were installed shortly
thereafter. Temporary bench marks were installed at
50 m intervals on the shoulder of the dyke for monitoring
settlements. Trigger and action values were
identified for each item along with an action plan
should the values be exceeded, and to provide an early
warning system in the event that unanticipated conditions
arise.Typical results are included on Figure 4 and
an aerial view of the cofferdam is shown on Figure 5.
6.2 Strengthening works
A maximum total deflection of some 1000 mm and
a relative deflection of 285 mm was recorded along
a 200 m long section at the end of draining, which
were in excess of the action values set. The measured
deflections, which incorporates pile rotation, is
in an area where the original dredged channel was
the deepest and the dyke height maximum. CPT testing
indicated the presence of very loose material with
entrapped silty clay for which a horizontal modulus
of subgrade reaction of 2000 KN/m 2 /m was appropriate.
A best fit analysis using a 2D FEM analysis with
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ITEM
DESIGN / PROCUREMENT
COFFERDAM CONSTRUCTION
TUNNEL CONSTRUCTION
COFFERDAM REMOVAL
TRAFFIC OPERATION
2004 2005 2006
2007
Figure 6.
Overall schedule.
8 HEALTH, SAFETY AND ENVIRONMENT
Figure 5. Aerial view, December 2005.
the surveyed dyke geometry and the lower modulus
value was undertaken to simulate the actual condition,
and indicated that the stresses in the steel piles were
within tolerances. As a result, the trigger values for
sheet pile deflection were revised up to 1250 mm at
the top of the sheet pile as an absolute deflection and
335 mm between +3.0 m and −2.0 mDMD as relative
deflection to reflect yield stress situation. Vibrotreatment
works on 2.5 m grid spacing and to a depth of
15 m were undertaken in this portion of the dyke fill
to densify the material and the crest level was raised
by 2 m to 0 mDMD and the dyke widened locally to
limit further deflections.
6.3 Maintenance
In addition to visual inspections the instruments are
monitored daily. Once steady state conditions had
established there was no further deformation of the
dyke or additional deflections of the sheet piles, other
than minor cyclic fluctuations due to tidal variations.
The measured discharge averages 1650 m 3 /hr and
compares very favourably with the calculated inflow
estimate of 1600 m 3 /hr indicated by the seepage
analysis.
Corrosion of the sheet piles due to seawater
aggressivity was observed along the perimeter. Ultrasonic
testing revealed that the rate of corrosion was
0.8 mm/yr at the upper half, where tidal fluctuations
occur, and 0.3 mm/yr at the lower end of the cantilever
section. A couple of moist areas were observed on the
dyke slope and standpipes were installed in these areas
which recorded water level at a depth of 3.5 m below
the dike slope. The moist spots are related to the capillary
rise in the area and the capillary fringe is consistent
with the results reported by Fookes et al. (1983) for
sandy silts.
7 PROGRAMME
The overall project schedule is shown on Figure 6.
A plan was prepared to evacuate the cofferdam in
an emergency and drills undertaken at periodic intervals
demonstrated that the work force could be safely
evacuated in 8 minutes. Discharge water quality was
visually monitored on a daily basis with water quality
samples analysed weekly.
Whilst draining the cofferdam local fishermen were
engaged to catch fish trapped within its confines using
wire net traps of different sizes, up to 2 m in diameter.
The trapped fish were removed and placed in
oxygenated cool boxes before being returned to sea.
Almost 1900 fish comprising 35 species were safely
returned to open waters.
9 CONCLUSION
The successful completion and draining of the cofferdam
has resulted in a safe environment enabling cut
and cover tunnel construction to proceed. At the time
of writing in September 2006, 85% of the tunnel structure
is complete and the tunnel will be operational in
December. Difficulties encountered whilst pile driving
were successfully overcome with additional effort and
within programme constraints, without compromising
safety.
ACKNOWLEDGEMENTS
The authors are grateful to Nakheel for their kind
permission to publish this paper.
REFERENCES
Dubai Municipality Geometric Design Manual for Dubai
Roads. 1999 Edition.
Gahir J.S., Radwanski R. and Hafez T.A. 2006, “Design and
Construction Challenges of Sub-sea Directional Drilled
Crossings on the Palm Jumeriah, Dubai. ITA congress,
Korea.
Technical Standard and Commentaries for Port and Harbour
Facilities in Japan (The Overseas Coastal Area
Development Institute of Japan). 2001 edition.
Fookes P.G., French W. J. and Rice M. M. 1983. The influence
of ground and groundwater geochemistry on construction
in the Middle East. Quarterly Journal of Engineering
Geology, London, 18, 101–128.
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