ensuring continued water supply for Birmingham through three new
bypass tunnelling sections connected into the existing Elan
combining the tunnel drive pits and connection cofferdams,
the team made significant savings while simplifying
construction and reducing the delivery programme - Courtesy
sections will bypass key locations of the aqueduct to
improve resilience of water supply to Birmingham using this
100-year old asset - Courtesy of GHD
height of the industrial revolution, Birmingham’s population was
booming. Between 1800 and 1900, the city had grown
substantially. An innovative solution was needed to provide its
residents and industries with safe drinking water. James
Mansergh had identified the Elan Valley as a source for water.
With Birmingham located so far in land, the city needed a viable
way of transporting the water. Enter the Elan Valley Aqueduct.
The original aqueduct was built between 1893 and 1904. Fast
forward over 100 years and the aqueduct still provides drinking
water from the Elan Valley to Severn Trent Water’s customers in
Birmingham. The aqueduct transports high quality water entirely
Background to the
Having supplied water for
over 100 years, the aqueduct was beginning to tire in three key
locations and Severn Trent identified the need to protect the
structure. Severn Trent explored the possibility of internal
repairs but the time available between aqueduct draining and
restoring supply made this unfeasible. Having ruled out internal
repairs, Severn Trent identified that the best option would be
to replace three sections offline and use the planned shutdowns
to drain the aqueduct and connect into the existing pipeline.
Construction Alliance (BNMA) was appointed to deliver these new
sections across three locations in Bleddfa, Nantmel, and
Design and construction
The new aqueduct bypass
sections are segmentally-lined circular tunnels constructed
using an earth pressure balance tunnel boring machine (TBM).
Bleddfa: The new Bleddfa
tunnel section passes below the Welsh hills through the
Siltstone bedrock. This was excavated using a Herrenknecht TBM.
GHD began work on the project in August 2015 developing Severn
Trent’s feasibility designs. Working closely with BNMA, they
have simplified the tunnelling operation and removed a shaft.
► Nantmel: This bypass
section passes below ground. The solution for this section of
the scheme has been simplified. Instead of constructing a
cofferdam here, BNMA will use open cut techniques to remove
construction of a cofferdam and drive pit from the programme.
► Frydd Wood: The section
in the town of Knighton replaces the Frydd Wood conduit. The new
tunnel runs under the Scheduled Ancient Monument of Offa’s Dyke.
The project is located adjacent to a Site of Special Scientific
Interest (SSSI) meaning it is crucial that environmental impacts
are minimised. This was particularly important with the tunnel
drive shaft located just metres from a SSSI area and the
reception pit located on the hill side above the town.
BNMA’s upgrade of the 113 year-old Elan Valley Aqueduct
consists of tunnels constructed using an Herrenknecht
earth pressure balance tunnel boring machine - Courtesy
Innovative design and
The project involved a
number of innovative approaches in design and construction.
These solutions reduced cost, time and environmental impacts.
Reinforced soil wall: The
original solution was envisaged to use battered slopes. Instead,
the team introduced a near vertical reinforced soil wall on the
steep slopes of the downstream connection. It is the biggest of
its kind built in the UK in 2016.
TBM drive pit designs:
The new sections needed to connect into the existing aqueduct
during shutdowns and have flows switched to the bypass. The
shutdown duration for connecting was limited to five days
including a day to drain down the aqueduct and one day to
recharge. In addition, the team were limited by highway access
constraints, steep side slopes, limited working space, and a
minimal area for stockpiling excavated material within the site
The original solution for
these connections involved constructing TBM drive shafts to
build the new sections.
A solution to the
connection pit and TBM drive pit cofferdam was developed, which
involved a secant piled wall around the connection point between
the new tunnel and the existing EVA. This methodology removed
the need for an additional launch shaft and protected the
aqueduct at all times. It also allowed quick reinstatement of
flows and reduced shutdown requirements.
Finally, the design
minimised work within this period to ensure delivery to
programme and allowed for adaptation on site to suit the
geometry of the existing tunnel.
constructed the transition section to remain in this
condition until the tunnelling was completed from the
same portal pit
team installed precast box sections to form the
transition section after tunnelling completed
design meant the flows could be turned in to the new
bypass section without shutting the aqueduct down
Building the connection
and TBM drive pits
BNMA completed a bulk dig
above the crown of the aqueduct. They then built the piled wall
around the portal, maintaining safe clearance from the aqueduct.
This involved installing the sheet piles via dig and push
methods to protect the live pipeline.
To monitor any impacts of
ongoing work on the aqueduct, BNMA also installed real-time
monitoring systems using Shape Accel Arrays and vibration
monitors to monitor any impacts.
They inspected the tunnel
and conduit to assess the existing integrity of the structure
along the full length of the site compound. The aqueduct was
then protected with a concrete wrap.
Forming connections and
BNMA then constructed the
new connection sections. This involved constructing in situ
concrete walls and a base for the connection section. They also
installed double stoplogs ready to keep flows along the existing
During the shutdown, BNMA
removed the concrete wrap and cut into the existing pipeline by
removing the roof and wall. After connecting the transitional
section, the team then made good the surfaces and poured the
A key part of this
shutdown involved preparatory activities for the remaining
works. GHD carried out a 3D laser scan of the area for
fabrication of the deflector wall. Meanwhile, BNMA installed
stainless steel dowels for the deflector wall, placed double
stoplogs, and installed a temporary roof structure. Finally, STW
reinstated flows through the aqueduct.
BNMA then installed
precast concrete box sections to form the transition section.
These box sections were pre-benched to form the channel to the
required 3D transition cross-section.
Diverting flows into the
The next step focused on
diverting flows into the new tunnel. BNMA developed an
innovative solution that meant the flows could be diverted into
the new tunnel while the aqueduct was in service.
This involved removing
the temporary roof, placing precast concrete diversion blocks
into the live flow using guide rails and removing the stoplogs.
BNMA then placed the precast concrete roof complete with
permanent access portal.
designed the largest reinforced soil wall constructed in
2016 - Courtesy of CADSITE Services
The team has helped
secure significant time and cost savings through a range of
innovative designs at each project site. By combining the TBM
drive pits and the connection cofferdams, the design removed two
deep shafts from the scope. The reinforced soil wall design
helped the project achieve a cut-to-fill earthworks balance. At
Frydd Wood, the team removed a deep shaft by changing the
philosophy of hydraulic design.
On all three sites, the
project introduced a combination of cast, in situ concrete and
benched precast concrete units to remove the need for large
pre-fabricated lightweight steel sections. The project solution
included a staged excavation and exposure of the existing
structure. This removed the need for internal propping by
installing a concrete surround. This also kept the aqueduct
operational during excavation.
The method used across
all the sites allows flows to be turned while the aqueduct is
live. This has minimised the work during shutdown periods,
protecting the project programme and continuity of supply for
Severn Trent’s customers.
and publishers would like to thank Severn Trent Water,
Barhale/North Midland Construction Alliance (BNMA) and GHD
for providing the above article for publication.