installation of a 2km pipeline in Oxford Eastern Bypass to
replace a 24” burst trunk main
by Kikuo Matsushima CEng MCIWEM MICE
jacking pit - Courtesy of Optimise
a burst on the existing 24” trunk main running along the Oxford
Eastern Bypass in September 2014 causing severe flooding, Thames
Water made the decision to replace the high risk section of the
main to prevent future pipe failures. The Optimise Joint Venture
delivered the £6m project, installing 2km of replacement trunk
main along the busy dual carriageway, whilst minimising
disruption to local traffic and effectively engaging with third
parties and the local community.
On 23 September 2014, a
burst occurred on the existing 24” cast iron trunk main running
along the Oxford Eastern Bypass, which caused severe flooding to
properties in the adjoining residential area. The main also had
previously burst along the same stretch of the road in 2007. In
order to reduce the risk of the pipe bursting again and the
potential major impact of any further events, Thames Water made
the decision to replace the section where the main is closest to
residential and commercial properties.
In January 2015, Optimise
was contracted to replace a 2km section of the trunk main that
runs between Magdalen Wood to the north and Garsington
roundabout to the south. Optimise is a joint venture made up of
Barhale, Clancy Docwra, Murphy and MWH (now part of Stantec) to
deliver TWUL’s Asset Management Plan 5 (AMP5) capital works
The site is located along
the A4142 Oxford Eastern Bypass Road in Cowley, approximately
3km east of the centre of the City of Oxford. The road is a busy
dual carriageway, which forms the south eastern section of the
Oxford ring road. One section of the road where work was carried
out is bounded by MINI Plant Oxford, which is the central
assembly facility for the MINI range of cars.
Along the pipeline route, there are 2 subways crossing under the
road (one for pedestrians and one for cyclists), which needed to
be negotiated by the new main. On the northern section of the
pipeline route, the Brasenose Wood and Shotover Hill Site of
Special Scientific Interest (SSSI) is located adjacent to the
eastern side of the road and Open Magdalen ancient woodland
adjacent to the western side of the road.
construction along Oxford Eastern
and insertion of PE liner into sleeve
from tunnel jacking pit
Courtesy of Optimise
The route of the pipeline
is underlain by various strata and the ground at tunnelling
depths was predominantly a firm sand with seams of limestone and
sandstone running through it, which posed challenges to the
tunnelling works carried out under this project.
Pipeline route and
With regards to the
pipeline rehabilitation and replacement options, sliplining and
on-line replacement options were immediately discounted as the
existing 24” trunk main was vital for the water supply to the
City of Oxford and surrounding areas and it could not be
isolated for the works.
From the beginning of the
project design, the Optimise design and construction teams
collaboratively investigated the route of the new replacement
trunk main and installation methods, taking into account the
Minimise disruption to
traffic along the Eastern Bypass and connecting roads.
Avoid working on and
adjacent to the existing fragile 24” trunk main.
Route the pipeline away
from the area affected by the previous main bursts.
Minimise disruptions to
vehicles entering and exiting the MINI Oxford plant.
Route the pipeline away
from existing high risk underground services.
Avoid working within
environmentally sensitive areas.
The selected pipeline
route and installation methods for the pipeline sections is
shown in the figure below. The new main was generally routed on
the opposite side of the road from the existing main. In terms
of the pipe installation methods, open-trenching was planned in
the carriageway and foot/cycle paths where possible, and the
trenchless method was adopted where the new main crosses the
roads and pedestrian and cycle subways.
trenching - Courtesy of Optimise
Site investigations and
As a part of the
engineering investigation, a total of 16 boreholes were sunk
along the proposed pipeline route, mostly along the tunnelling
sections, in order to ascertain the nature of the underlying
strata and assess the feasibility of the tunnelling work. A soil
electrical resistivity survey was also carried out to determine
the level of protection required against ground corrosivity for
new metallic pipes.
In terms of the pipe
material selection, the use of 600mm ID ductile iron pipes was
selected for open-cut sections for ease of installation, due to
the limited working space. For the trenchless sections, 630mm OD
SDR 17 PE pipes were chosen to be installed within 1200mm ID PCC
jacking pipes. Hydraulic modelling work carried out during the
design stage confirmed that the selected pipes would have no
effect on the transfer capacity of the trunk main and pressures
in the supply zones.
In addition, in order to
reduce the risk of future pipe failures and the consequent
impact on the surrounding area, the installation of a Syrinix
TrunkMinder leak and burst monitoring system was requested by
The proposal of
installing a section of the new main in the road next to the
MINI plant raised concerns for the BMW Group which operates the
Mini Oxford plant. They were concerned that a possible future
pipe leak or burst would affect the operation of their plant and
business and requested the project team carry out detailed
assessment on future pipe failure risks. Subsequently, a
cross-functional team of engineers within Optimise, including
civil engineers, a pipeline expert and a mechanical engineer,
carried out a Failure Modes and Effects Analysis (FMEA), in
order to identify possible failure modes of the new main and to
assess the probability and consequence of each failure mode. The
quantitatively presented results indicated that the choice of
pipe materials, the pipeline design and the proposed
construction methods would significantly reduce the risk of
future pipe failure, compared to the risks associated with older
mains. The assessment results were shared with the BMW Group who
accepted the installation of the main on their side of the road,
and the construction work could proceed.
Site plan - Courtesy of Optimise
Click to enlarge
Due to the reactive and
emergency nature of the project, a fast track design was carried
out to enable the pipeline construction to commence from June
2015. The outline of the project scope included:
Installation of DN600 DI
pipes along the open-cut sections with line valves and other
pipeline ancillaries (1560m in length).
Installation of 630OD
SDR17 PE liner within 1200mm PCC jacking pipes (470m in length),
along with construction of 9 (No.) tunnelling launch and
reception shafts (depths up to 9m).
Connections to the
existing distribution and trunk mains at 6 locations along the
Installation of Syrinix
Trunk Minder units at four locations, along with 1 (No.)
ultrasonic flow meter chamber.
Environment and ecology
In the design stage, the
design team carried out environmental surveys, including ecology
surveys and a contaminated land assessment. The northern end of
the existing 24” main ran through Brasenose Wood and Shotover
Hill SSSI, however the new pipeline avoided the SSSI. This
provided a permanent positive impact on the SSSI, as the need
for any future maintenance works in the SSSI was removed. Some
works were required to decommission the existing main within the
SSSI - the impact of these works were assessed and assent was
obtained from Natural England.
The construction work
started in June 2015 from the northern end of the new pipeline,
which mainly consisted of open-trench sections. Following the
completion of the open-cut pipe installation, the tunnelling
work began. For the tunnelling work, 6 (No.) tunnelling sections
were completed, using a 1200mm diameter full faced TBM, although
the TBM needed to be replaced with an open face shield to allow
manual excavation along 2 (No.) tunnel sections where hard rock
impeded the use of the TBM.
The construction team
carried out the works whilst closely coordinating with the
Oxfordshire County Council Highways Team and MINI plant to
ensure that disruptions to the traffic along Eastern Bypass Road
and deliveries to MINI plant were minimised. The Council
Highways Team permitted a reduction of the speed limit to 30mph
on the relevant stretch of the Eastern Bypass and closures of
sections of the foot and cycle paths. These helped to create
safe working conditions for the work force.
Throughout the project
period, Thames Water set up a pipeline monitoring programme for
the existing 24” main and carried out leak detection surveys
along the previous burst pipeline section at regular intervals
until the new water main was commissioned, to ensure that
flooded properties didn’t experience flooding again during the
The construction of the
new pipeline was successfully completed and the new main was
commissioned in February 2017, along with the new trunk main
monitoring system. Following this, the old 24” main was
decommissioned, which removed the risk of pipe failure and
flooding to the local area.
from local school with Tunnel Boring Machine - Courtesy
school pupil and his mum inside the tunnel
machine with TBM Driver
Courtesy of Optimise
Engagement with the local
community was a key element of this project. A community drop-in
session was organised by Thames Water and Optimise prior to
commencement of the construction work, to inform residents of
the local community of the proposed work and to discuss any
project related issues. Regular letters, texts and a project
webpage were set up to help keep local residents and users of
the footpath and cyclepaths informed. A local school also took
part in a competition organised by Optimise, to name the tunnel
boring machines and design artwork to decorate them with to
encourage young people to think about STEM (Science, Technology,
Engineer and Mathematics).
and publishers would like to thank Kikuo Matsushima, Lead
Design Engineer with MWH (Now part of Stantec), for
providing the above article for publication. The author
thanks the following for their input: Jonathan Wickens
(Thames Water), Roger Thatcher (Clancy Docwra), Mark Tierney
(Optimise) and Rachel Groves (Murphy).