Singapore - Trenchless International

October 2010 - Trenchless International INdUstRy INNOvAtIONs
jack in the box
by James Thomson, Chairman, Jacked Structures SA
James Thomson, owner and principal of Jason Consultants Group until 2003, recently formed
Jacked Structures SA. Here Mr Thomson details the different concepts of installation.
The JackING Of large structures,
such as road underpasses, pedestrian
subways and culverts uses trenchless
jacking methods.
I developed and pioneered these techniques
in the 1960s and 1970s when
working with a major international contractor.
the initial projects were for pedestrian
subways using large diameter pipes to
provide the structure. the next development
was to jack box sections for culverts
and pedestrian subways. With this experience
we increased the dimensions to
accommodate larger structures. Figure
1 illustrates a typical situation where a
pedestrian subway is driven under a rail
embankment to allow separation of traffic
and pedestrians.
Figure 1 A Pedestrian subway under rail
embankment.
the most basic approach, which is still
widely used, is box jacking where the
structure is constructed on a launch pad
adjacent to where it is to be installed and
is jacked into the ground with excavation
taking place in the shield.
Dealing with a face excavation which
can be typically 15 m wide and 8 m
high you need to compartmentalise the
shield, which is fixed to the front of the
structure – very much like Brunel’s shield
for the very first tunnel under the thames
back in 1823. You also need to provide
thousands of tonnes of jacking capability
to push such large sections, which in
turn needs the provision of suitable reaction.
the frictional loads on a rectangular
structure are much greater than experienced
when installing circular sections
so there is a need to introduce anti-drag
systems to reduce this.
Many installations have been undertaken
under roads, rail tracks and
runways. Figure 2 shows one of the
first projects undertaken in the early
1970s where two underpasses were
driven below a major road in North
London to provide access to Brent Cross
shopping centre. Perhaps the most high
profile installation was the three large
underpasses jacked under rail tracks in
Boston, Us as part of the Big Dig project.
these boxes were approximately 24 m
wide, by 11 m high and the longest drive
was 109 m. Figure 3 gives some idea
of their size and the large number of
hydraulic rams that were used. Because
of the ground conditions – which were
mainly soft marine deposits with old piles
and buried quay walls – the ground was
stabilised by freezing.
Figure 2 Brent Cross jacked Underpass
under A41.
Figure 3 Jacked Boxes Boston Big Dig.
Box jacking, where the track is temporarily
supported and structures are
installed directly below the rail tracks, is
widely used in Europe.
there are some inherent limitations in
box jacking and in recognition alternatives
approaches have been developed.
If provision is made with the rail authority,
it is possible to obtain brief possession
to undertake work. With this in mind I
developed the modular approach.
Using a series of precast jacked boxes,
abutments and central piers can be
constructed without disrupting rail operations.
As can be seen, a whole series of
operations can be undertaken from the
boxes such as cross strutting and finally
stressing the boxes together and infilling
with concrete. During a short track
possession the bridge deck can be slid
in or bridge beams placed to complete
the structure. A large number of such
jobs have been undertaken in the UK.
A typical installation is the Wandsworth
under-bridge shown at figure 4. this
concept minimises the exposed face at
any time and requires much less launch
area as only relatively small jacking pits
are needed.
Figure 4 Wandsworth Under-Bridge
London.
More recently a new patented development
is the jacked deck which can
be combined with the modular system to
provide a complete structure without disrupting
rail services. the modular system is
used but the top box is modified internally
to allow a slide track to be installed. these
top boxes are also designed with a removable
section which exposes the track as the
deck is jacked into place. the jacked deck,
which can be formed from a complete cast
on site structure or formed from precast
beams, is fitted with a compartmentalised
shield where excavation takes place. this
same concept can also be used with a
jacked circular tube or box providing an
access gallery which allows piling to be
undertaken to provide abutment walls and
the installation of the slide track.
this approach is currently being used
to install a 20 m wide by 126 m long traffic
underpass under rail tracks in UK.
Another patented approach is to install
The relentless growth of
cities is creating a demand
to install structures below
existing facilities and
buildings. The potential
applications for creating
large underground spaces
by jacked methods without
disruption are unlimited.
initially jacked foundation boxes with
removable lids and install in these boxes a
slide track. In this case, precast arch sections
are jacked in along the track with lids
being removed within the shield as excavation
takes place.
the Jacked Arch has a number of advantages
such as the reduction of jacking loads
and face stability is improved. the use of
precast arch sections also minimises the
launch area. this approach enables larger
spans and longer length to be installed.
Metro station structures are quite possible
and full support to the excavation as work
advances would be provided thereby eliminating
the risk of collapse.
there are other techniques used such as
the provision of advanced support structures
where a canopy is installed as an
initial support for the final structure. typical
approaches are shown in the graphic
at figure 5. the steel tubes that form the
canopy can be installed by pipe ramming,
microtunnelling or auger boring.
Figure 5 Advanced support structures.
Although the majority of Jacked structure
installations have been in Europe, projects
have been undertaken in Us, India,
Canada, south Africa, Indonesia, taiwan,
Korea, Japan and Australia.
the relentless growth of cities is creating
a demand to install structures below existing
facilities and buildings. the potential
applications for creating large underground
spaces by jacked methods without disruption
are unlimited. the separation of road
and pedestrian traffic from rail traffic is
essential to avoid the all too frequent accidents
that occur at rail crossings. Valuable
and scarce development land cut off by
rail tracks and highways can be accessed.
New underground space such as parking
garages and storage can be created.
One interesting possibility that was put
to me in Australia was the possibility of
installing wine storage caverns. I am hoping
this will go ahead and I might get the
opportunity to check out the quality of
wine stored.
Biography
James Thomson has been involved in
Trenchless Technology over many years and
in developing and designing the installation of
large structures since the late 1960s. He was
the owner and principal of Jason Consultants
Group until 2003, a company specialising in
Trenchless Technology including a number
of jacked structure project. His experience
in the field has led him to develop some
concepts that have been patented. Along with
experienced colleagues who have worked with
him over the years he recently formed Jacked
Structures SA. A consultancy specialising in
developing concepts, detailed engineering,
installation design and contract management
for these projects. Work is undertaken for
owners, consultants and contractors.
Further information is available on the Jacked Structures SA web site
www.jackedstructures.com
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INdUstRy INNOvAtIONs
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