Giant foundations for the Venice floodgates

Spring 2011
Seabed
titans
Giant foundations for
the Venice floodgates
Plus:
Tottenham Court
Road diaphragms
Wind farming
powers up
Emap Inform, Emap Limited
First Floor, Greater London House,
Hampstead Road,
London NW1 7EJ
Tel: Direct dial extensions on
(020) 7728 exchange:
EDITORIAL
Fax: 020 7728 4666
Consulting editor
Adrian Greeman 4550
[email protected]
Production
Andy Bolton
Lucien Howlett
Graphics
Anthea Carter 4548
DISPLAY ADVERTISING
Fax: 020 7728 4666
DISPLAY ACCOUNT EXECUTIVE
Olivia Horne 4529
[email protected]
DISPLAY PRODUCTION MANAGER
Paddy Orchard 4111
MEDIA REPRESENTATIVES
Italy/Switzerland: Fabio Potesta,
Mediapoint & Communications, Corte
Lambruschini, Corso Buenos Aires,
8 v Piano, Interno 7, 16129 Genova.
Tel: 0039 010 570 4948.
Fax: 0039 010 553 0088.
Email: [email protected]
www.mediapointsrl.it
Germany/Austria: Randolf Krings,
EuroMediaConnect, Dambachtal 10,
D-65193 Wiesbaden, Germany.
Tel: 00 49 611 532 4416
Fax: 00 49 611 532 4519
Email: [email protected]
www.euromediaconnect.com
USA and Canada: Olivia Horne,
as above.
EUROPEAN FOUNDATIONS
is published four times a year by
Part of Emap Limited
Printed by William Gibbons, Sussex.
ISSN Number: 0017-4653
www.foundationworld.org.uk
The European Federation of Foundation
Contractors aims to promote the common
interests of members of the federation, to
improve standards of workmanship and
to maintain high standards of technical
competence, safety and innovation.
It is also a primary objective of the EFFC
to express the point of view of members to
the European Commission, national
governments, consulting and civil
engineers, architects, contractors,
professional institutions, public authorities
and other interested parties in member
countries or abroad.
For more information contact
the EFFC secretariat at:
Forum Court, 83 Copers Cope Road
Beckenham, Kent BR3 1NR, UK
tel: +44 20 8663 0948
fax: +44 20 8663 0949
european foundations spring
2011
No 49
Spring 2011
Tunnel is preferred
Fehmarnbelt option
Danish politicians are supporting
a e5.1bn immersed tube tunnel
option for the Fehmarnbelt crossing,
as proposed by Femern, the stateowned company in charge of planning the link.
One of Europe’s largest-ever construction projects, the future Fehmarnbelt link is set to comprise a
combined motorway and rail tunnel
immersed under the Fehmarnbelt
between Rødbyhavn in Denmark
and Puttgarden in Germany.
The 18km tunnel, which is scheduled to open in 2020, will cut travelling times between the European
continent and Scandinavia by allowing high-speed train services to operate between northern Germany and
Scandinavia’s main cities.
It should create a new and
important
northern
European
growth area.
After comparison between a subsea tunnel designed by consultants
Ramboll, Tec and Arup against
Cowi and Obermeyer cable-stayed
bridge solutions, it was concluded
that the tunnel option will not only
have advantages during the construction phase, but also during the
operational phase.
As well as technical, environmental and safety advantages, the impact
on the environment and marine traffic are considered to be lower for the
tunnel solution. Construction costs
for the tunnel are also lower.
When complete, the Fehmarnbelt link will be the third huge
infrastructure project to have taken
place in southern Scandinavia over
the past three decades.
In 1997 and 1998, the Storebælt
Bridge opened to rail and road traffic respectively, linking the Danish
islands of Funen and Zealand. In
2000, the Øresund Bridge between
Denmark’s capital, Copenhagen,
and Malmö, Sweden’s third largest
city, opened to traffic. Since their
commissioning, the two bridges
have contributed significantly to
economic growth in their respective
areas.
“We welcome the political support for our recommendation that
the future link be designed as an
immersed tunnel,” said Femern
CEO Leo Larsen. “The decision
means that Femern A/S has reached
an important milestone in the planning of the fixed link.
“As our conceptual design projects are based on an extremely
thorough, technical foundation,
we can now focus on ensuring that
the authorities approve the project,
including from an environmental
perspective.”
Over the coming year, Femern
will prepare an environmental
impact statement to be considered
by the authorities in Denmark
and Germany in accordance with
national regulations.
Femern expects to submit an
application to the German authorities during the first six months of
2012. A construction bill will then
be submitted to the Danish parliament, Folketinget, in 2013. This
more or less coincides with the timetable for the German authorities’
approval of the project.
“The aim is to build and operate one of Europe’s safest and most
modern tunnels for both trains and
cars, which will bring northern
Europe and Scandinavia even closer
together,” said Larsen.
“As a result we’ll create opportunities for increased growth and prosperity for more than 10M residents
in the Fehmarnbelt region.”
The construction of one of
Europe’s biggest infrastructure projects is expected to commence in
2014. The link is scheduled to open
to traffic in 2020.
4
Worldwatch
Crossrail’s Woolwich station
box funding deal imminent
6
Worldwatch
Trapped TBM halts work on
s135M UK storm water tunnel
8
Thinking inside the box
Work continues in tight
conditions for the new
Tottenham Court Road station
10
Housing the Venice gates
Flood gates will protect
the Italian city, but huge
foundations must be built first
20
Pass the port
A proposal for a new offshore
shipping terminal near Venice is
under consideration
22
In at the deep end
A 576MW wind farm to be built
off the Welsh coast highlights
the size of the challenge facing
the UK’s wind farm foundation
Cover photograph:
Rolls Royce winches at the Venice flood barrier
3
Crossrail’s Woolwich station
box funding deal is imminent
By Alexandra Wynne
Final sign-off on the funding of the
Woolwich station box for London’s
Crossrail project was still pending
as EF went to press, despite hopes
agreement had finally been reached.
The future of the station has
been uncertain for over a year, since
developer Berkeley Homes was hit
by the economic downturn.
Berkeley had promised to
put €218M towards station box
construction and planned to develop
the space around it.
Berkeley is understood to have
spent €6M on the station design
but its finances suffered badly in
the recession and it has since been
attempting to renegotiate the extent
of its funding commitments with
Crossrail.
Crossrail chief executive Rob
Holden told the London Assembly’s
Transport Committee in early
February that Crossrail sponsors
had been working “very hard”
to agree the financial details of
Berkeley Homes’ commitment. He
had expected that papers would be
signed over the deal on 8 February.
He had declared himself “as
confident as anybody can be that
we can bring it to a successful
conclusion (shortly)”.
The plan involves building a
291m long by 23.5m wide and 17m
deep box to give capability for a
station at Woolwich on the route as
well as creating 2,500 homes.
Agreement about who will
finance the fit-out of the box and
build the station at ground level
must also be reached.
Long-time
backer
of
the
station, MP for Greenwich &
Woolwich Nick Raynsford said
he was anxious for news and has
challenged
transport
secretary
Philip Hammond for an update.
Delays to the funding deal had
caused Crossrail to accommodate
changes in the schedule, said
Holden. But he said that the
government review of the scheme
and subsequent plan to postpone
the opening of the route meant that
the opening schedule of the “limbs”
surrounding the central Crossrail
section had still to be confirmed.
The central section is scheduled
to open in the third quarter of
2018 – a year later than originally
planned. This means it is likely that
the remainder of the route will be
opened in 2019.
Crossrail chairman Terry Morgan
added that investigations into the
potential for a station at Kensal
Green in west London were still
ongoing and that a decision would
be made in the next few months.
The
Royal
Borough
of
Kensington & Chelsea is lobbying
for the station to be built.
Network Rail is leading this work
and is looking at the implications
of building the station against
criteria set by London mayor Boris
Johnson, said Morgan.
These are that the station must
be delivered at no additional
cost, that it has no effect on the
timetable and that work causes no
construction delays.
Outgoing Crossrail boss wants engineer replacement
By Alexandra Wynne
The new Crossrail boss should have
an engineering background rather
than one in project management,
says outgoing chief executive Rob
Holden.
Holden
told
the
London
Assembly Transport Committee in
February that “skillsets [of the chief
executive] will change and evolve”
4
as the e17bn mega-rail scheme shifts
through different phases.
He said the job would be
different as it moved into the heavy
construction stage.
“The skillset needs to be less
managerial and more engineering,”
he said before singling out Crossrail
finance director David Allen and
programme director Andy Mitchell
– last week picked as the engineers’
favourite to take over from Holden –
as potential beneficiaries from such
a boss.
“People like Andy Mitchell and
David Allen, the two executive
directors I’m leaving behind, will
benefit from somebody different –
somebody who has those skillsets to
help them in their career ambitions
and to help them in delivering
Crossrail.
“My engineering colleagues will
benefit from a person who can talk
more engineering talk,” he added.
Mitchell was clear favourite
after EF sister magazine New
Civil Engineer canvassed engineers
following Holden’s surprise decision
to quit last month.
europeanfoundationsspring2011
Unique machine to take
apart Hong Kong tunnel
By Andrew Bolton
A French machinery manufacturer
is building a revolutionary tunnel
dismantling machine, which will
help contractors in Hong Kong
splice an existing metro to a new
extension.
Bessac, part of Bachy Soletanche,
is making the machine, which will
be trialled in a purpose-built test
tunnel in France.
A joint venture between contractors Dragages, Maeda and Bachy
Soletanche has the contract to connect the new tunnels with existing
ones on Hong Kong’s Island Line.
The machine will remove bored
tunnel lining segments along a
120m section of overrun tunnel
,which serves as siding at the western end of the existing Island Line,
beyond its terminus station.
It will work in compressed air,
which will support the mixed
ground around the tunnel while a
sprayed concrete lining is applied,
replacing the segments.
The machine will work a few
metres at a time, backfilling the tunnel with low-strength concrete to
give added support.
Dismantling the lining will allow
a tunnel boring machine working at
the eastern end of the new exten-
sion, known as the West Island
Line, to punch through into the
existing tunnel.
The joint venture is boring a first
770m section of the new tunnel
through soft ground with the TBM,
which will finish in the dismantled
joint section.
This TBM section will connect to
the rest of the €1.53bn West Island
Line being built with drill and blast
bores in hills to the west. The project
is due for completion in 2014.
The one-off machine is under
assembly now and will go through
proving trials near Toulouse before
delivery to Hong Kong.
Machines Czech in for micropiles
A e3M slope stabilisation
project begun in February
last year is continuing
at the giant Bilina opencast brown-coal mine in
the rainswept and hilly
Bohemia region of the
Czech Republic, near the
German border.
Micropiles and 400
anchors are being installed
by Soletanche Czech
europeanfoundationsspring2011
Republica to prevent slips
in old waste material which
has been deposited over
the years on the edge of
the deep pit. The mainly
sandy clays and claystones
rest on harder schists
beneath and tend to slide,
threatening the huge
excavators which continue
working below, says project
manger Jan Libus.
Soletanche is using
two Comacchio MC1200
machines to drill 160mm
diameter holes between
15m and 40m deep, half
in the waste and half into
the bedrock. Concrete
blocks made by another
contractor are held by the
anchors.
Mine owner SD –
rekultivace is the client.
Austrians
pass cash
for tunnel
The Austrian government has
committed funds for the next phase
of the 55km long Brenner Base
Tunnel (BBT). It will be the second
of the great Alpine deep high-speed
rail connections to be built.
A tranche of e1.3bn from the
Ministry of Transport in Vienna will
match a similar sum by the country’s Italian partners in the project.
“Along with subvention funds from
Brussels, that will bring funding to
just over e3bn,” said a spokesman
for BBT, the Austrian company set
up by the Austrian state railway
company to manage and commission the project.
The project is part of the Berlin to
Palermo high-speed corridor, which
is top of the list of Trans European
Network (TEN) projects designated
by the European Union.
Work can now continue until
2015 to complete all the preparations for main bores, including a 6m
diameter exploratory and service
tunnel, side adit access for the main
drives, and spoil disposal facilities.
“We will also be able start the main
drives at the north and south portals,” said the spokesman.
The funding also makes it almost
certain that the tunnel will eventually be built, after almost a decade
of ground investigation, design, and
exploratory tunnels.
The BBT spokesman said the
biggest contracts would be carried
out from three intermediate points
along the 55km length of the tunnel. Funds for the main drives will
be needed after 2015.
Like the Gotthard, the Brenner
will be a low base tunnel with gradients held as flat as possible by
passing deep under the high peaks
of the Alps. Maximum cover for the
bores will be more than 1,700m, less
than on the Gotthard but still posing
major problems.
Unlike Gotthard the tunnel will
have three bores, two main rail tunnels of around 8m diameter and a
central service tunnel of 6m diameter. The additional bore serves for
drainage and as an exploratory tunnel to investigate the rock. A 3km
long section of a 6km initial bore
through potentially difficult rock has
been driven from Innsbruck, and a
10.5km section from Aica on the Italian side was completed in November.
5
Trapped TBM halts work on
£114M UK storm water tunnel
PRESTON TUNNELS
Storm water tunnel (unaffected by stuck TBM)
Storm water main
A5085
oa d
Blackpool R
A583
Preston New Rd
Preston waste water
treatment
le
Ribb
works
River
Pumping
Station
Direction of drive
Location of stuck tunnel boring machine
then filling them with a coolant
such as brine.
The JV is also considering dewatering the area.
“We could reduce the water pressure on the tunnel, but we would
need to use something else to make
[the tunnel] safe, such as a compres-
Li v
Contractors grappling with a
trapped tunnel boring machine
(TBM) in Lancashire say that jetgrouting, ground freezing or dewatering are the three most likely
methods to be used to free it.
The KMI Water joint venture
(JV) of Kier, J Murphy and Interserve had halted work on part of a
£114M United Utilities tunnelling
project in Preston after silt breached
the tunnel lining of the 3.5km long,
2.85m diameter storm water tunnel underneath Preston and Penwortham in Lancashire.
The TBM is stuck 25m below
ground in alluvial clay deposits.
As a result, work was stopped on
24 November and the Lovatt earth
pressure balance shield machine has
not progressed since.
KMI Water said it was looking
at several methods to stabilise the
ground and allow the TBM to be
recovered. “We’re working up different options to get back into the
tunnel and repair [the tunnel lining],” said KMI Water contracts
manager Andy Parker.
“We could drill a hole from the
surface down to the tunnel and
inject grout around the TBM and
the tunnel lining [to alleviate pressure around the TBM]”.
KMI Water is also looking
at freezing the ground around
the TBM.
This would be done by inserting pipes around the TBM and
Ro
ool
e rp
ad
9
A5
sive air lock,” added Parker.
KMI is still unsure what caused
the silt breach.
The lining of the 2.85m internal
diameter tunnel is made up of 1m
long precast concrete rings. Each
ring consists of six sections which
are bolted together and then sealed
with a rubber gasket.
Tunnelling for the project was
going to plan until engineers noticed
silt below the ferryplate – a sledge
pulled along behind the tunnel boring machine containing the electrical panels, grouting equipment and
a conveyor belt to take away the
spoil.
“We couldn’t get to it, or see
where it was coming from,” said
Parker, adding that the ingress was
a fine silt that could “get through
any gap”.
A stack wall made of timber and
sandbags was built to contain the
ingress whilst the extent of the problem was assessed.
When it was evident that silt was
still entering the tunnel, engineers
decided to flood the tunnel to make
it stable.
A decision on the recovery method will have to be sanctioned by the
Environment Agency due to the
tunnel’s close proximity to the River
Ribble.
The Agency will seek assurances
that jet grouting or ground freezing
would not lead to pollutants entering the river.
Health & Safety Executive
approval will also be needed.
Parker said he could not put a
timescale on the recovery attempt
but insisted the project team was
“working flat out”. The remainder
of the project in Preston city centre
remains unaffected.
Foundations reconstruction for Kent dockyard building
A listed pumphouse in the historic
dockyard in Chatham, Kent is to
have its foundations rebuilt.
The South East Development
Agency has commissioned Campbell Reith as consultant for the delicate work .
Campbell Reith will design and
replace two of the pile foundations,
which have worn away over time.
The historic Pump House Five
is a Grade II listed building dating
from 1879.
It has planning permission for
use as restaurant and leisure facilities.
Original design of the brick build6
The listed building is being converted into a leisure development
ing and its foundations still exist
from 1871 drawings signed by a
member of the Institution of Civil
Engineers, supervising engineer
EA Bernays, MICE. Investigations
reveal them to be “remarkably”
accurate.
They showed that some of the
structure was supported with timber
piles, which were drilled into deep
fill and alluvium into the underlying
gravel.
Land raising around the building
early and dewatering of the dockyards in the 1990s for the Medway
tunnel are blamed for significant differential settlement of foundation
elements.
An initial assessment shows two
damaged piers need support with
new underpinning, said the consultant. Monitoring indicates that the
remainder of the building is stable.
The pump house was originally
built to expand the dockyard in
preparation for the construction of
then new iron clad warships.
This development will go alongside the Dockyard Museum.
EuropEanFoundationsspring2011
GE Awards attracts
record number of entries
A largescale cofferdam for a
Crossrail station construction in
a water filled dock at London’s
Canary Wharf, dramatic plunge columns for top down construction on
the London Bridge Shard skyscraper and use of large scale, deep, steel
pile techniques to remove a huge
buried granite block seawall in very
soft ground in Singapore are among
a record number of entries in this
year’s Ground Engineering awards.
The entries from respectively
Canary Wharf Contractors, with
consultant Arup and specialist contractor Expanded Piling, Balfour
Beatty Ground Engineering and
Gammon Construction have each
been shortlisted for the technical
excellence category.
They join grouting work on the
new Shanghai Yangtse river tunnel in China by the Shanghai Tunnel Engineering Company together
with the Shanghai Yangtse River
Tunnel & Bridge Construction &
Development Company; data man-
agement and software integration
for bored tunnel design on Crossrail
by a joint Arup-Atkins consultancy
team; bi-directional pile testing
using the Osteberg method for large
piles on the Farringdon station redevelopment on London’s Thameslink
by Atkins; and development of a
large scale asset management and
recording system for earth embankment structures on London Underground, also by Atkins.
Seven other categories make up
the awards, including a new one of
product innovation in the geotechnical sector. Shortlisted entries for
all of them were published on 10
February and the winners will be
selected over the next six weeks by
the panel of five judges drawn from
the geotechnical community.
The eight categories are for international project of the year, UK
geotechnical or geo-environmental
project over £1M, and separately
under £1M, sustainability, health
and safety, innovation, technical
excellence, and finally young geotechnical or geo-environmental
engineer of the year.
The winners of the eight prizes
for this fourth year of the event,
which is sponsored by Bam Ritchies and Bachy Soletanche, will be
announced on 11 May at a presentation lunch Park Lane’s Grosvenor
Hotel in London UK.
n Full details of the shortlisted
entries are at www.geawards.com
St Helena gets
rockfall protection
Major rockfall protection works
will wind up in March on the
remote southern Atlantic island of
St Helena after construction began
last August. Appropriately for the
famous last internment of Napoleon Bonaparte, the project has been
carried out by French contractor
CAN France.
The project extends emergency
protection carried out in 2009 by the
same contractor, after a 300t rockfall
damaged parts of the island capital
Jamestown and forced evacuation
of a primary school. Local workers trained at that time have largely
been used for the work.
The volcanic island remains
vulnerable to falls because of its
geology of heavy basalt lava flows
interbedded with easily eroded ash
and pyroclastic deposits, which
make the island’s steep valley sides
unstable. Much of its 4,000 resident
population plus water and power
infrastructure is at risk in the narrow
EuropEanFoundationsspring2011
valley of the capital.
The latest works involve installation of proprietary GBE rockfall
barriers from Swiss manufacturer
Geobrugg along the steep slopes
above the town.
Just over 2.5km of 500kJ capacity
has gone in, 1.3km of 1,000kJ and
210m of 2,000kJ and 900m of extra
strong 3,000kJ.
As with all supplies to the island
this was delivered via a 10 day journey from Capetown in South Afria
aboard the poster ship St Helena, the
only means of access.
The barriers are formed with
minipile anchored plates supporting
a steel post tied back at the top with
cables to rock anchors further up the
slope.
Design of the works for the client, the UK’s Department for International Development acting for the
St Helena Government, is by consultant WA Fairhurst of Glasgow,
which also supervised.
7
transport
The small site is hemmed
in by offices and
residential buildings
thinking insidE thE box
Minimising ground movements while building a 44m deep diaphragm wall box
at London’s Tottenham Court Road station is challenging enough. But joint
venture Bauer Keller must also install 48.5m deep piles with plunge columns
to millimetre accuracy. Ruby Kitching reports.
E
ngineers can reel off the problems of working in congested
city centre construction sites
with relative ease: very little storage
and loading area for supplies; noise
restrictions due to nearby residents
and businesses; and limitations on
ground movements to protect existing underground structures and
infrastructure. So for subcontractor
Bauer Keller joint venture, these
constraints came as no surprise
when it arrived on site at central
London’s Tottenham Court Road
station last year.
But the degree of accuracy
required to build some elements of
8
the £12M Goslett Yard basement
still came as quite a shock.
Tottenham Court Road station
is situated at the junction of the
famous Oxford Street and Charing
Cross Road and under the towering
gaze of the 34-storey Centre Point
building. Here, the road above and
the Tube tunnels below experience
some of the heaviest traffic in central London. Since the station currently serves two Underground lines
A new line foR london
Crossrail is a new cross-London rail line that will run 118km from
Maidenhead and Heathrow in the west, through new 21km twin-bore
tunnels under central London to Shenfield and Abbey Wood in the east.
With a total budget of £15.9bn, the new line will have 37 stations, with
eight brand new central London stations and 28 upgrades of others,
11 of these major reconstructions. When Crossrail opens in 2018 it will
increase London’s rail-based transport network capacity by 10%.
– the Central and the Northern – it
is a popular interchange as well as
one of the main stations serving
the West End office, shopping and
entertainment areas. Built in 1900,
the station is now used by 150,000
passengers a day and by 2018, this
will rise to more than 200,000 with
the new Crossrail station opening
(see box). Anyone who has elbowed
their way through the crowds down
the greasy steps of Tottenham Court
Road station will be able to testify
that the century old station is well
past its use-by date.
It’s not surprising, then, that a
£1bn station upgrade is currently
europeanfoundationsspring2011
PLAN
2.32m diameter piles
with plunge columns
1m thick, 44m deep
reinforced concrete
diaphragm wall
A
A
Precast concrete stop
ends with water bar
0
m
10
SECTION A-A
Level (+0)
Ground level (0m)
Made ground
Level (-1)
Level (-2)
River Terrace
Deposits
44m
48.5m
Level (-3)
Level (-4)
London Clay
Lambeth Group 1
Lambeth Group 2
Plunge columns
Thanet Sand
Chalk
Plunge columns will support buildings on top of the station
The diaphragm wall grab takes 3.2m long, 1m wide bites as it digs
under way to improve the situation.
This overall scheme involves creating a new station six times bigger
than the existing one and with twice
the original capacity. The project
also involves reducing congestion above ground for pedestrians,
cyclists, bus passengers and other
road users. It will also have much
improved passenger access by escalator and lift.
Client London Underground is
undertaking the project on behalf of
Crossrail. Main contractor is Vinci
Bam Nuttall.
Bauer Keller’s contract is part of
this upgrade project and involves
building a 44m deep diaphragm
wall box to create a four-storey basement which will eventually house
four escalators and one lift. These
will take passengers from the new,
larger street level ticket hall to the
Northern and Central Underground
lines and the new Crossrail line. A
third of the perimeter diaphragm
wall has been completed and work
is about to begin (after Easter) on
11 large diameter piles, each with
plunge columns.
“We’ve got a substantial excava-
Cartwright. But in central London
where there are Tube tunnels and
shallow foundations nearby, it was
decided that it would be prudent to
build the walls using the minimum,
single bite length.
Each panel is excavated to its
final depth 44m below ground level,
requiring six to seven hours of
straight digging. The steel reinforcement cages are nearly at their limit
for bar size and spacing with 50mm
diameter bars used for the main
reinforcement. They are detailed to
accommodate slab connections and
utilities ducts.
Each cage is fabricated in one
complete length on the factory floor
in south Wales by supplier Express
europeanfoundationsspring2011
tion – a perimeter of 150m – and
to make sure we’re as good to our
neighbours as can be, we’ve had to
look very closely about how to build
it,” says Bauer Keller JV project
director Alex Cartwright.
The diaphragm wall is being built
in shorter lengths, or panels, than
would usually be expected. This is
because shorter panels disturb the
ground less than longer ones and
hence induce much smaller ground
movements. A crane-mounted grab
excavates the ground in single bites
3.2m long and 1m wide. Walls of
the excavation are supported by
bentonite. “In an open field, you
could build panels about 7m long by
taking bites next to each other,” says
“We’ve got a substantial
excavation and to make sure
we’re as good to our neighbours
as can be, we’ve had to look very
closely about how to build it”
Alex Cartwright, Bauer Keller
Reinforcements. The cage is then
dismantled and brought to site only
the night before it is needed since
there is no space to store them on
site. After one panel has been excavated 44m below ground level and
the reinforcement lowered, it is then
filled with concrete and the bentonite displaced.
The site operates 24 hours a day,
Monday to Saturday. Not only does
night-time work have to be carried
out quietly and with careful co-ordination with the local council, noise
restrictions also apply by day. The
basement will be constructed using
top-down techniques to ensure
noise levels are kept to a minimum.
Top down construction involves
creating a perimeter wall in the
ground, and installing piles with columns on top (“plunge columns”) to
create the basement structure. With
the basement columns in place, a
ground floor slab and further superstructure can be erected and basement soil dug out in tandem.
At Goslett Yard, when the diaphragm wall is complete, 11 piles
of 2.32m diameter and 48.5m deep
will be built inside. A plunge
9
transport
column will be carefully embedded into each pile just before the
concrete hardens. The plunge column steels are bespoke sections with
web and flange depths of 700mm.
Thicknesses vary.
“The plunge columns are 33m
long – so are really quite big – and
serve two roles: as the box is excavated, they work in tension and
reduce the buoyancy of the box and
secondly, they serve as the supports
for the future development above
ground level,” explains Cartwright,
who most recently oversaw plunge
column installation on London’s
Shard of Glass skyscraper. On the
Shard, top-down construction was
adopted so that the above ground
structure could progress swiftly,
unhindered by complicated basement construction.
But at Tottenham Court Road,
the scheme is solely concerned with
underground construction and purpose of the top down technique is to
keep noise levels low and, more crucially, to keep ground movements to
a minimum, reducing the effect on
nearby Tube tunnels or foundations.
The site is currently gearing up for
the start of piling and installation of
the colossal plunge columns.
“They are the challenge for this
project,” says Cartwright rather
gleefully. “They are some of the
heaviest you’ll ever see.”
The columns have been fabricated
by steel contractor William Hare.
“They [William Hare] stood out as
one of the few who could get these
columns built really straight,” Cartwright explains. The columns are
32m long and have to be placed to
a 1 in 400 accuracy to ensure they
work efficiently. “One of the difficulties was that the 1 in 400 tolerance could be used up during fabrication.” The column arrives on
site in three pieces: a 12m and 20m
length which make up the permanent column and a 6m length “follower” piece which is recovered.
Simply connecting the lengths of
column on site within these tolerances will be a challenge, considers
Cartwright, “so we will be doing a
lot of practicing before we do our
first column”, he admits.
Pile construction begins with the
“This is certainly a
raising of the bar in
our industry. We’ve
got just 2.5mm in a
metre to play with.”
Alex cartwright,
Bauer keller
10
Fixing steel for one of the plunge columns
installation of 10m of 2.4m diameter temporary steel casing to seal
the loose and wet soils in the upper
formations. A BG40 excavator then
works to 48.5m below ground level
under bentonite. The pile cage is
then lowered in and concrete is
tremmied from the bottom of the
pile to the required cut-off level,
displacing the bentonite. The bentonite is then drawn out from the
remaining top section of excavation,
(where the plunge column will sit)
while the concrete cures below.
“The challenge then will be to
pick up the plunge column and
place it to fairly tight tolerances
into the pile before it hardens,” says
Cartwright.
The column will be embedded
into the top 11m of the pile which is
made up of 32/40 strength concrete,
supplied by Hanson. The concrete
has been designed to take longer to
cure so that there is more time to
position the column.
A bespoke plunging frame has
been designed and loaded up with
laser guides and instrumentation
to help position the column correctly. This frame has spider-like legs at
the bottom which spread out onto,
and push against, the inner face of
the excavation, clamping the frame
in place.
A square opening runs along the
length of the frame to guide the
plunge column. Lasers are fired
up from the bottom of the frame
to the top to check that the square
openings are all in line. The frame
is then effectively steered until it is
plumb. Positional tolerances require
the top of the column to be within
10mm of its intended plan position and 10mm of its correct level.
Cartwright adds: “This is certainly
a raising of the bar in our industry.
We’ve got just 2.5mm in a metre to
play with.”
The
data
collected
by
instrumentation on the frame will
give Bauer Keller the confidence
that the column is plumb. Designer
Atkins assumed that this verticality
could be achieved on site, and has
designed the column to work in
compression to an even greater
degree than normal. This means
that the column section size is as
slim as possible.
The basement is to be excavated
and ready for fit out and connection
with other London Underground
and Crossrail infrastructure in 2013.
The wider Tottenham Court Road
upgrade project will be complete in
2016.
concrete keeps the noise down
Diaphragm wall panels are usually
separated during construction by a thin
steel stop-end which runs the full depth
of the wall. The main job of this stop-end
is to incorporate a water bar which
allows water to pass out of the wall.
The stop-end is usually a piece of
sheetpile which is broken out using a
crane-mounted chiselling tool after a
panel section has cured. The water bar
remains embedded on one side of the
panel to sit hard up against the adjacent
panel about to be built.
Breaking out the steel was considered
too noisy for the Tottenham Court
Road site. Instead, a much thicker,
unreinforced precast concrete stopend is being used which will be cast
permanently into the wall. Since the
permanent precast steel stop-end is
much thicker than the removable steel
one, there is a greater length of wall
which will be unreinforced. “While this
is less efficient than the steel stopend option, it is the quieter solution,”
Cartwright says.
Night-time site activity is kept to a
minimum because there are residents
nearby. However, London Underground
stakeholder manager Adam Healy
says that currently, the average nighttime site noise levels are currently
below the ambient levels from nightclubs
in the area.
europeanfoundationsspring2011
POrts
HOusinG tHe
venice Gates
A large reclaim work site has
been formed at Malamocco
channel. It will be removed later
Few foundations are as gigantic
as the base units for the Venice
flood gates. Adrian Greeman
visits the site.
G
reat fleas have lesser fleas upon
their back to bite ’em”, so the
poem says. And great foundations have lesser foundations also,
to support them. For the great
Venice flood prevention scheme,
the foundations even have further
temporary foundations during their
construction.
The main founding blocks of the
project are huge concrete caissons
that will be inserted flush with
the seabed in the three channels
connecting Venice Lagoon with the
sea outside. The three channels are
the Lido channel to the north, the
central Malamocco channel and the
Chioggia channel to the south.
Sitting within cavities in these
giant cellular concrete blocks will be
the steel flap gates that will rise from
the bottom during storms and tidal
surges to protect the canal city and
its unique architecture, history and
irreplaceable artistic heritage.
The gate housing blocks are
enormous, the largest rising to a
height of 11.5m with a 60m length
and 48m width – the size of eight
tennis courts. Even the smallest
are 36m wide and the same length
and height.
The differing widths will
accommodate different gate sizes,
according to the depth of the
various channels. Four lines of gates
are needed in total, one each for
the two southern inlets and two in
the northern Lido inlet, the closest
entrance to the cluster of islands
making up the old city.
“The Lido is the widest
entrance and was too big for
the gates to work effectively,”
explains Monica Ambrosini, an
information officer at Consorzio
Venezia Nuova, the client for the
€4.6bn project. Tests showed the
gates would only be effective in a
channel up to 400m wide.
The designer, consultant Technital
from Verona, early on decided to
divide the Lido channel in two by
12
the formation of an artificial island,
now fully dredged and with building
construction already under way.
Conveniently, the island could also
be used as the location for all the
control and monitoring equipment
and for the air compressors and
other machinery needed for the
gates once they are in operation.
Lido’s Treporti channel, passing
north of the new island, is the
shallowest with a 6m depth of
water. Small private vessels, light
craft and yachts will use it.
It will have 21 gates in a row.
Each caisson will seat three of the
20m-wide gates, which means seven
will be needed. At either end there
will be two higher and narrower
caissons to form the abutments and
give access to the underwater units.
The other Lido channel, San
Nicolò, has another seven caissons
plus two abutments. It is slightly
narrower and only 20 gates are
required, so one of the caissons
will be smaller, holding just two
gates. But the water is deeper here at
11.5m, to allow the big tourist cruise
ships through to the city, and so the
gates are longer and the units must
be longer too.
In the far south the lagoon’s
Chioggia channel will need a
12m depth to allow fishing vessels
through to their nearby port of
Chioggia. Here, 18 gates are needed,
housed in six caissons with again
two additional abutment units.
Largest of all the works are for
the central Malamocco entrance.
This is the most important for
shipping with a deep water channel
continuing right across the lagoon
to the modern industrial port of
Marghera. It will have 19 gates in
seven caissons with another two
abutment units.
All these huge units are now
in construction. For the smaller
Treporto channel in the Lido inlet,
contractor Mantovani is taking
advantage of a small 500m long
harbour space built as part of the
new channel. The harbour is a storm
refuge and also a waiting space for
a small side lock which will allow
boats to pass through even during
gate closures. A second harbour is
on the inside.
“By strengthening the walls
and sealing the entrance with a
cofferdam the harbour makes
a useful workspace for caisson
construction,” says Ambrosini.
“At Malamocco, there was no land suitable for
a giant work yard. It had to be large, because
the caissons need a year of construction work”
Enrico Pellegrini, GLF
The 9m deep dock will be flooded
eventually and the units floated out.
A similar temporary drydock
arrangement prevails at Chioggia
where a larger lock and harbour
refuge will allow the fishing crews to
pass. Contractor Condotte d’Acqua
is working on these.
By the big channel which runs
through the Malamocco inlet, and
for Lido’s cruise ship channel, there
is no harbour space. The cruise
ships do not get a lock either as they
can easily reschedule their routes.
The big oil tankers need no refuge,
though they do require a lock, the
largest, as they cannot reschedule
during storm closures.
For contractor Grandi Lavori
Fincosit (GLF), which is carrying
out the work in the Malamocco and
southern Lido channels “another
solution was required”, says GLF
site manager Enrico Pellegrini. That
was to build the caissons on land
and move them, on completion,
to the sea, where they will be
lowered into the water using a giant
Synchrolift ship lift.
“The equipment for that is
europeanfoundationsspring2011
Lagoon
Venice
Marghera
Port
Lido
lagoon
inlet
1. Refuge haven and navigation lock
2. Abutment
3. Treporto Gates
4. New island and abutments
5. St Nicolo Gates
6. Abutment
7. New breakwater
Sea
Lido lagoon inlet
1. Abutment
2. Row of gates
3. Abutment
4. Navigation lock
5. New breakwater
6. Temporary worksite
Lagoon
Malamocco
lagoon inlet
Sea
6
Malamocco lagoon inlet
1. Refuge haven with craft harbours
2. Abutment
3. Row of gates
4. Abutment
5. New breakwater
Chioggia
Chioggia
lagoon inlet
Sea
Lagoon
Chioggia
fishing
port
Chioggia lagoon inlet
The Venice lagoon with three existing entrances and the new configurations with gates
literally Rolls-Royce standard,”
says Pellegrini. It has been
designed and assembled by the
US Syncrolift company, a part of
Rolls-Royce’s marine equipment
division since the 1980s.
“But at Malamocco, there was
no land suitable for a giant work
yard,” says Pellegrini. “It had to
be large, because the caissons need
a year of construction work. With
some 18 to build and install by 2013
they all needed to run concurrently
in a single space.” GLF’s existing
facilities in southern Italy, where it
makes marine structures for delivery
all around Europe, were too small.
Land had to be made.
Conveniently there was a space
close to the channel. Just here a
new breakwater has been built out
to sea as part of the scheme to slow
the fastest tides. It could be joined
to land with a sheetpile quay wall
extension to protect a shoreside
space which could be filled.
“Sand had to be dredged anyway
for the shiplock approaches,” says
Pellegrini. Work began in 2005 for
three years to make a 13,000m2
europeanfoundationsspring2011
Venice is threatened by flooding
several times a year
13
POrts
Breakwaters, which define the
existing channel, have been
extended and a ship lock added
temporary platform “with great
care and tight organisation”, jetting
sand onto the beach and close shore.
The problem was to avoid
settlement as much as possible under
the maximum 24,000t weight of
each unit “and particularly any
differential
settlement”
says
Pellegrini. A key issue was the sloping
shoreline profile to a maximum
filled depth of 7m. Juggling the fill
sequences was critical.
“We also drilled 21 large
diameter dewatering wells,” adds
Pellegrini. These tapped into two
water tables in the mostly clay layers
underneath, which are separated by
a 2m thick band of very hard clay
at around 10m depth. “We dropped
one of these by 2.5m and one by
nearly 9m and this forced some
150mm of settlement.”
On top of the dredged sand,
1.5m of gravel was laid in 300mm
layers, each compacted to highway
standards, he says “around 98%
or 99%”. On that went a further
500mm layer of leanmix concrete.
Pellegrini would have preferred more
robust asphalt but environmental
restrictions prevailed.
On top of that each caisson gets
its own temporary concrete slab
during construction, and the whole
area is being constantly monitored
for settlement, just in case, as the
caissons are built.
But before that could begin
last year there was plenty more
to do, says Pellegrini. Each of the
four channels has to be prepared
and shaped, partly to receive the
caissons and partly for tailoring the
overall hydraulic regime.
“First
work
actually
was
strengthening and defending the
lagoon islands,” he says. The sea
edge of the lagoon is little more
14
Foundation mattresses are laid by unwinding from huge floating rollers
than sand spits stretching its 50km
length and only a few hundred
metres wide, narrowing at one point
to around just 50m. It was here
the sea broke through in 1966’s
devastating floods.
New groynes and other sea
defences have been made. New
additional
breakwaters
and
reinforcements to the existing ones,
shelter the sea entrances.
Then each channel had to be
dredged to a square profile, from
the natural V-shape. At Malamocco
dredging will fill in the 16m deep
centre, while cutting back the
edges to 14m. Grab barges fitted
with sonar did this work. “And it
is amazing how precise you can be
with the 3D images you get from
modern multi-beam underwater
survey equipment,” says Pellegrini.
To protect the new sea floor,
giant mattresses were made from
aggregate sandwiched in geofabric
and steel mesh tied across with
steel bar. These are like the seafloor
protection used on the Osterschelde
barrier on the North Sea coast in
the Netherlands. The 200m long
blankets, each 10m across, were
made up on land and then rolled
Site manager Enrico Pellegrini
europeanfoundationsspring2011
ports
Flood gate operation
Lagoon
Sea
Air pumped in
Expulsion of water
A piling barge was built for
the 1,800 trench bed piles
onto large floating rollers. A
specially designed and built vessel
could hold the rollers in Y-shaped
arms and then gradually unroll the
mattresses across the channel width.
A heavy weighted section formed
the start point.
But this was easier said than done
says Pellegrini. Accuracy in placing
the rolls was vital, particularly as
they had to overlap by about a
metre. “That can be achieved with
modern GPS positioning equipment
controlling the balance between six
anchor cables” he says.
But a major problem for all the
channel works is the tides. The
water from the lagoon wants to pour
through the three narrow entrances
and back twice a day, explains
Pellegrini, “which can mean
currents up to five knots, which is
almost impossible”.
For the mattresses, only dead
calm would suffice, which limited
the work window to just over half
an hour on the turn of the tide,
just long enough for one mattress.
“But you only get one chance,” says
Pellegrini.
Above the mattresses a 2m thick
layer of rockfill has been placed by
grab barges, the material coming
from limestone quarries in Croatia
across the Adriatic.
This shaped layer extends 200m
either side of the gates which have
to sit flush in a trench in the middle.
Forming and excavating this is the
most crucial work.
The first step was to make a heavy
sheet pile wall either side of the
16
future trench. This is being driven
by a pontoon-mounted hydraulic
pile hammer, driving 30m into the
dense clays. Accuracy is vital to
form the trench wall and positioning
crucial – even using GPS and four
anchor cables it can sometimes take
half a day to set up, says Pellegrini.
The tide race means the pontoons
have been fitted with extendible legs
to fix their position once it is done.
Work is complicated by the need
to keep the channel open for the big
ships. Closure of one side is possible
with constant liaison with the port
authority, but switching from side to
side has been needed.
The inside of the sheet pile
walls was next dredged 15m down
with grab barges and then comes
a second stage of piling. This time
concrete piles have to go in, to form
a grid pattern across the bed of the
trench, some 1,800 in all.
Another purpose-made and fairly
large barge has been used for this
work which just completed before
Christmas. It carries two trackmounted pile masts with hydraulic
hammers, one each side. They
could move up and down to cover
the grid each time the barge was
repositioned.
Driving the piles 30m down
requires pile extenders. The 500m
diameter circular precast piles
arrived from a local supplier and
were delivered by a small barge and
lifted out by crane to slide into the
pile rig.
Another 2m thick layer of stone
goes on to the trench floor and
Lagoon
Sea
2m thick rubble mattress
Hinge
Water filled cells
Gate housing
Gate
Plant tunnels
Access tunnels
Concrete
filled cells
Seabed consolidation piles
Aggregate
mattress
Aggregate layer
1.5m rubble
Grouted
gravel infill
Cross section of the caisson trench. Profiled flat seabed, prepared
by laying huge mattresses (below), extends 200m either side
europeanfoundationsspring2011
ports
The huge caissons
require a carefully
programmed sequence
of concreting (right)
Special mobile jacks will lift the caissons when complete
The reinforcement required for the caissons is exceptionally dense
18
1
4
2
5
3
6
this time must be compacted
carefully to form a flat base. The
contractor has made a giant plate
compactor for this, 3m square with
vibrator units and kentledge on top.
It will be suspended by barge cables.
Some of the trench piling
continues at the edges, but attention
is now on the casting yard where the
huge caissons began construction
last year. Each is built on a concrete
slab above which is a grid of 2.3m
high pillars, keeping a cavity
underneath to be used later to move
the 24,000t units.
Casting is complicated as each
unit a highly complex structure in
itself with multiple cells and walls
to form, all linked together with
piping and ducts. These will allow
water to be pumped to and fro in the
float out stage, explains Pellegrini,
to help balance the buoyancy of the
irregularly shaped elements.
Once immersed the units will
also be filled with water to weight
them down, and with concrete in
the cells around air-filled tunnels,
which are needed for inspection and
maintenance.
Reinforcement for the big box-like
units is extensive, with a density of
up to 350kg/m3 and bars of 22mm
to 26mm thickness everywhere. For
air/water boundary walls, highly
expensive stainless steel bar is used.
So congested is the steel in the cell
walls, which are often only 250mm
thick and a maximum of 500mm,
that ordinary vibrator compaction is
ruled out. Instead, a self-compacting
concrete mix has been designed.
“It is a difficult mix with tight
tolerances,” says Pellegrini, and
mainly for that reason the contractor
has set up his own batching plant
on site even though there is a local
ready mix firm nearby. “That also
makes delivery very quick,” he adds.
An extensive programme of
pretesting and pour trials was
carried out, as well as trying out the
packed-in reinforcement assembly.
Trials included pressure testing a
variety of waterstop options for
joints. In the end a stainless steel
strip with bitumen was selected.
“The sequencing of pours and
timing to compensate for shrinkage
is critical,” says Pellegrini. The yard
has to follow a stringent routine.
“One unit would be difficult, but
doing all of them on a production
line basis makes programming
especially complicated,” he adds.
About one third of the units have
been completed so far.
Once finished rails will be laid
underneath each unit on which will
run small hydraulic jack trolleys
each capable of lifting 350t. The
trolleys are specially designed
versions of Norwegian TTS mobile
jacks used in shipbuilding and oilrig
work. About 80 units will be linked
by computer to lift the caisson and
move it to the Syncrolift.
This too is based on shipyard
design with a 24-strong rank of 500t
winch units each side and a platform
of beams between them. These are
twice as wide as a normal ship lift
and 5m deep to resist the deflections.
Load testing was under way when
EF was on site, but it will be a while
before the lift is used for a complete
caisson. That has to happen before
2013 when the gates will be ready
for installation.
But that is a way off yet and
Pellegrini says even the precise
method for the immersion stage has
not yet been worked out. He has
enough on his plate for the moment.
groundengineeringspring2011
Marine
The total 1.5km of quays
will be for container ships
Oil will be offloaded
and piped to shore
pass the port
A new offshore shipping terminal is being planned outside Venice to increase
the city’s port capacity. Richard Johnstone looks at the plans.
A
plan to build a port in the north
of the Adriatic about 14km east
of the port of Venice is being
examined by the Italian government
and the Venice Port Authority.
The proposed facility is a deep-sea
container terminal with a maximum
capacity for 3M container units a
year. Plans for the €1.3bn project
also include a crude oil terminal
with annual throughput of 7Mt.
The 80ha site was originally
planned as an oil terminal in 1984,
according to Halcrow’s maritime
project director Payam Foroudi,
who has undertaken concept design
work for the project on behalf of the
Venice Port Authority.
Foroudi says the idea was rejected
to due to difficulties in extending the
existing inland port in Venice without increasing the risk of flooding in
the historic city.
The proposals that Halcrow
has developed are now with the
port authority which, according
to Foroudi, is now examining the
logistics of the scheme, including
how to transport cargoes between
20
the new port and Venice.
Halcrow suggests a fleet of barges
could transfer containers, but Foroudi says that even if this is agreed,
“five or six years of design and
assessment” will be needed before
work can begin. This work will
include detailed design of the 3.5km
protective breakwater planned for
the site, and the 1.5 km of quays for
deep-sea containerships.
Foroudi says with the available
information the structure would
undergo a large amount of settlement due to the fine sediments on
the seabed 20m underwater. These
will be compacted by the breakwater
and engineering solutions will have
to be proposed to deal with this.
Two options, which were outlined in Halcrow’s report for the
breakwater – a rubble mound or
caisson design – and one of four
types of design for the quay wall,
will address this.
More information on conditions
will become available through the
Venice Tide Barrier project that is
constructing 78 oscillating inflat-
able flood gates that will guard the
city against rising tides by 2014 (see
pages 12-18).
Marco Redaelli, a geotechnical
engineer with Halcrow who has also
worked on the project, confirms that
“the information we had in terms
of ground conditions was quite
limited”.
However, he adds: “We are aware
that the main geotechnical issues
will be settlement and consolidation
of the ground where we are constructing.”
The final solution would be based
on a “number of factors”. Regarding the breakwater, Redaelli says:
“From a geotechnical point of view,
the more flexibility in the structure,
the better.”
Quay construction could involve
building concrete caissons, a sheet
piled “combi-wall”, a piled concrete
deck or blockwork concrete solution.
Blockwork concrete has advantages as “this technique is quite simple,
it’s not complicated, and it is quite
robust”. However, he argues that
they are “not great” at being adapt-
ed, and this would have an impact as
one of the criteria of the design was
future expansion of the port.
The caisson design had advantages because it could be constructed
off site and then transported and
sunk into place.
The project’s logistical issues
include the necessary engineering
to pump crude oil directly from the
offshore terminal to inland refineries by a pipeline.
Halcrow proposes that electrical
power to the port be supplied by submarine cables, with water and fuel
for the terminal provided by barges.
The port itself is to form part of
the new Northern Adriatic multiport
system, which aims to play a greater
role in handling more Europe-Asia
and Europe-Mediterranean cargo
flows over the next 10 years.
The North Adriatic Ports Association envisages that development
projects worth €3.43bn in the
region are needed to provide Europe
with a consolidated port base in the
Mediterranean, to attract shipping
coming from the Suez Canal.
european foundations spring
2011
Renewable eneRgy
in at the deep end
A 576MW wind farm to be built off the Welsh
coast highlights the size of the challenge
facing the UK’s wind farm foundation
designers. John McKenna reports.
l
ike a swan, an offshore wind turbine glides gracefully above the
water, offering little clue as to
the strenuous activity taking place
beneath the surface.
Foundation design for offshore
wind farms is the great unseen element of the UK’s green energy
revolution and in many ways it is
the most challenging part of each
project’s construction.
The UK’s offshore wind plans are
the most ambitious in Europe, if not
the world, and consequently pose
the greatest challenge to foundation
designers.
The UK is already the world
sector leader with 1.34GW of offshore generating capacity installed,
according to figures published by
the European Wind Energy Association (EWEA) in January. Most
of the existing operational capacity
comes from the Crown Estate’s first
round of leasing for offshore wind
sites, which restricted projects to 30
turbines, with wind farm generating
capacities ranging from 60-90MW.
Its second round of leasing will
22
total 7.2GW, with project generating capacities ranging from 300MW
to 1.2GW. The vast majority of
round two schemes are either under
construction or in the advanced
stages of design. On top of this will
come a 32GW round three, comprising mega offshore development
zones ranging between 600MW and
9GW in generating capacity.
Allowing for the big turbines in
round three a rough average is 4MW
capacity for each turbine. This means
the UK stands to have more than
10,000 wind turbines installed off its
shores over the next 15-20 years.
With variable ground conditions and sea levels requiring each
turbine’s foundation to be designed
individually, this represents a
sizeable challenge for geotechnical
engineers.
“The sheer size of the piles that
need to be installed on most of
the offshore foundations is mind
blowing,” says Ramboll director
of geotechnical engineering Mohsen
Vaziri. “They can be up to 6m in
diameter, with wall thicknesses up
to 100mm and each one around
500m apart.”
The 576MW Gwynt y Môr wind
farm off the north Wales coast
is the project for which the team
is designing foundations, and it
offers a fascinating insight into the
“We are designing a cantilever, which is
really a simple design. But because you
have so many variations, what is very simple
soon becomes very complicated”
Mike Hallett, Ramboll
challenge facing the UK’s offshore
wind ambitions.
At a cost of e2bn, Gwynt y
Môr (Welsh for “wind in the sea”)
is being delivered by a project
company in which Germany’s RWE
Innogy is the main shareholder with
a 60% stake. The other shareholders
are Germany’s Stadtwerke Munich
(Munich Municipal Utility – 30%)
and Siemens (10%).
The total development area is
124km2, although the area within
which the turbines will be built
will be no more than 79km2. It sits
13km off the north Wales coast at
the nearest point to shore, 16km
from Llandudno, and 18km from
the Wirral.
The project comprises 160,
3.6MW wind turbines, and is being
built in two stages.
Driving
of
the
monopile
foundations for the first 92 turbines
will begin in November, in water
depths of between 13.3m and 21.3m
with respect to lowest astronomical
tide and a tidal range of 10m. The
construction of the monopiles
europeanfoundationsspring2011
Renewable eneRgy
“The sheer size of
the piles that need
to be installed is
mind blowing”
Mohsen Vaziri,
Ramboll
is expected to continue through
to late 2013, with the second stage
comprising foundations for the
remaining 68 turbines in deeper
waters. These are in the region
of 30m with respect to lowest
astronomical tide and, again, a 10m
tidal range.
Each of the 160 foundations
consists of two parts: a steel
monopile of between 50m and 70m
in length and between 4.7m and
6m in diameter plus a 25m steel
transition piece that is fixed on top
of the monopiles, connecting them
with the turbine tower. The pile
might extend between 13m and 30m
through water, and then a further
5.5m above the water. The whole
above-water height, including the
transition piece and turbine tower,
is 85m.
A e240M (£200M) contract for
the manufacturer of the foundations
and transition points was awarded
in January to a consortium of
Danish fabricator Bladt Industries
and
German
manufacturer
Erndtebruker Eisenwerk (EEW)
Special Pipe Construction.
However, before they can begin
manufacturing the monopiles and
transition pieces, Ramboll must
conclude its design work, which
requires designing each foundation
individually.
“What we are doing essentially
is designing a cantilever, which
is really a simple design. But
because you have so many
variations, what is very simple soon
becomes very complicated,” says
Ramboll offshore wind associate
Mike Hallett. “Each of the 160
foundations will be different.”
As a rule of thumb, wind turbines
need to be spaced apart by seven
times the diameter of the turbines’
rotor blade. On Gwynt y Môr, the
rotor blades have a diameter of
107m, so with turbines spaced so far
apart it is understandable that the
ground conditions can change for
each foundation.
“The ground is glacial, but no
point in the ocean is the same,”
says Ramboll geotechnical design
engineer Laurence Cross.
“There’s about 15-20m of sand
and clay, laterally and vertically
variable, underlain by mudstone. We
24
Britain is becoming the
largest offshore wind
energy user in Europe
generally embed the monopiles in
the mudstone by 20m, although the
mudstone level itself might range 5m
to 35m below mudline,” adds Cross.
The varying ground conditions
and the various loads the monopile
has to deal with mean that on each
monopile there is a different result
each time when calculating the pile
carrying capacity.
The loads can be grouped broadly
into three categories: axial loads
from the mass of the foundation,
tower, nacelle, and rotor nacelle
assembly; lateral loads from the
wind and waves; and torsion as
the wind turbine tries to twist
the foundation. In the harsh and
variable marine environment, it is
not surprising that the design of
each monopile is a job in itself.
“There’s a ridiculous number
of load cases, somewhere between
2,000-3,000,” says Hallett.
“You are looking at issues like
different wind directions, wave
lengths, is the turbine operating,
scour or no scour? Each one
produces a different load case.”
But if the ground conditions are
so variable, then why not simply
design each monopile to the greatest
load case. Surely this replication
would be more efficient in terms of
both time and cost?
Not so, says Hallett: “You have to
be location specific to optimise the
amount of steel required. It does
cost the client more money in
Gwynt y Môr
Consented December 2008
Rhyl Flats
Generating since 2009
Liverpool
North Hoyle
Generating since 2003
Rhyl
Conwy
0 km 10
Bangor
WALES
Chester
Above: the location of various schemes now under way off Wales.
Below: the monopile supports needed for the turbines are huge
europeanfoundationsspring2011
Renewable eneRgy
26
Turbine
tower
20m
mudstone
Pile
foundation
15-20m
sand and clay
Transition piece (25m)
Wind generators must be
carefully spaced. Right:
monopile cross section
50-70m
design time, but the returns are
vastly exponential.”
More important, as the projects
become larger all the design
parameters, such as water depths, soil
conditions and waves, become more
varied, making it simply impossible
to design a single foundation.
“If we design for the worse case
the deeper positions would probably
be okay,” says Hallett. “However,
as the water depth reduced the
foundations would likely be too
stiff and so would be outside of the
frequency limits the wind turbine
generator can operate in.”
Cross adds that optimising the
design of the monopiles so that each
one is as small as it can be helps
in the installation process. “The
smaller your piles, the easier they
are to drive,” he says.
He also says that one of the
biggest challenges in designing these
monopiles is the lack of case history
to inform his work. “A lot of the
standards and industry papers are
generally for much smaller diameter
piles and this job becomes all about
relating those principles to larger
piles,” he says.
“Also, thinking about this project
in particular, there aren’t many 6m
diameter monopiles that have been
installed to that depth in mudstone.
Other offshore wind farms are
typically embedded in London Clay,
chalks etc, so there really isn’t the
case history there to back up what
you are doing.”
This point is proven by last
year’s revelation that many of
Europe’s windfarms with monopile
foundations were at risk of fatigue
cracking due to the settlement of the
transition piece caused by the failure
of its grouting.
A group of major European energy
firms commissioned Norwegian
standards body Det Norske Veritas
(DNV) to investigate the problem,
and in January it concluded that
the grouting failures were due to the
axial capacity of cylindrical grouted
transition pieces being lower than
that previously assumed.
The effect of large diameters, the
lack of control of tolerances that
contribute to the axial capacity, and
the abrasive wear of the grout due to
the sliding of contact surfaces when
subjected to large bending moments
from wind and waves, all reduced
the axial capacity to a far greater
extent than the limited case history
of offshore wind was able to predict.
“The original guidance didn’t
allow for the scale,” says Hallett.
“The tolerances aren’t there. This
is a very harsh environment with
lots of loading and unloading.”
The DNV report recommends
that conical-shaped transition pieces
be used on new projects. According
4.7-6m
(100mm thick)
to this design, the monopile and
transition piece are fabricated with
a small cone angle in the grouted
section.
If the bonds between the steel and
grout are broken during in-service
life, some slight settlement of
the transition piece will occur.
This settlement will introduce
compressive
contact
stresses
between the steel and grout which,
together with some friction, will
provide sufficient resistance against
further settlement.
At the same time as making
its recommendation on conicalshaped transition pieces, DNV also
announced a further investigation
into the viability of cylindrical
grouted connection with shear keys
as an alternative solution.
Shear keys are circumferential
weld beads on the outside of the
monopile and the inside of the
transition piece in the grouted
section. The shear keys’ purpose
is to increase the sliding resistance
between the grout and steel so that
no settlement occurs.
According
to
DNV,
the
existing design standards for such
connections are based on limited test
data for alternating dynamic loading.
The consultancy says that before
this solution can be recommended,
design practices for shear keys
should be developed and properly
incorporated in design standards.
As a result, Hallett says the
transition piece design for Gwynt
Y Môr is yet to be agreed. But he
denied that the problems with the
grouting connections meant that
monopile foundations should be
abandoned altogether.
On a number of projects,
“Other offshore wind farms are typically
embedded in London Clay, chalks etc, so
there really isn’t the case history there to
back up what you are doing.”
Laurence Cross, Ramboll
including another RWE scheme in
Germany, steel jacket foundations
– reminiscent of North Sea oil rig
structures – are being used instead
of monopiles. These four-legged
structures rest on smaller piles and
act as a base for the turbine towers
to sit in.
“Jacket structures are certainly
a valid solution,” says Hallett.
“However, they currently tend to be
more expensive than steel monopiles
for the depths we are looking at.”
He adds that they may become
more viable in round three in deeper
water, and that projects are very near
the depths where monopiles fare less
favourably against jacket structures.
Cross agrees: “With deeper water
comes bigger piles, and monopiles
may become too big for barges at
the depths some of the round three
projects are at. Jackets may become
more appropriate.”
With wind turbine installation
due to start in 2013 and the project
due be fully operational in 2014,
it may well be that Gwynt y Môr
is one of the last great monopiled
offshore wind farms.
Regardless of whether it is or not,
there is still a fantastic foundations
challenge ahead as Britain seeks to
once again rule the waves.
europeanfoundationsspring2011