flood defence project to stabilise and improve 1km of embankment
following a tidal surge in 2013
by Rob Culledge BEng (Hons)
piling platform with piles installed in the river - Courtesy
of JBA Bentley Ltd
the temporary pile installation - Courtesy of JBA Bentley
project to stabilise and improve 1km of flood defence embankment
was initiated by the 2013 tidal surge. This caused widespread
flooding to the east coast and raised water levels by over 1.5m
in the Dutch River. Post-event inspection highlighted that this
length of channel side was at high risk of failure and could
potentially flood up to 3,000 properties. This watercourse has a
high tidal range which has been constrained and aggravated by
engineering works over many centuries. The tidal surge disturbed
the dynamic equilibrium and triggered on-going erosion,
compounded by extremely high tides forecast in 2015.
A three-staged approach
was used to deliver the project in order to best manage the
flood risk and maximise efficiency. The project team worked
collaboratively at all stages to establish optimum solutions.
Works have been delivered ahead of programme and under budget
allowing reinvestment of funding. Key challenges to the project
were to ensure that a 1:100 year standard of flood protection
was maintained at all times, working in a tidal environment as
well as responding to a dynamic river channel. This had a
significant impact on construction methods and temporary works
Phase 1: Investigation
The ground investigation,
geotechnical analysis and feasibility study identified that the
existing defences were insufficient to cope with future flood
events and rapid draw down conditions.
Breach analysis identified the potential severity and magnitude
of a failure of the defence. Despite the height of the defence
being in place, there were significant defects which reduced the
standard of protection. Initial options were identified and used
for the business case and to procure the subsequent phases of
Drone survey - Embankment failure before works -
Courtesy of JBA Bentley Ltd
installation within the river
Courtesy of JBA Bentley Ltd
Phase 2: Temporary works
Urgent work was carried
out to stabilise the defence before forecast astronomical high
tides in September 2015.
The temporary works
solution was to install a 150m length of 13m long sheet piles as
advanced works, which could then be incorporated into the
long-term solution, affording time for the permanent design to
be developed. The site compound was formed at an early stage,
and the public access road diverted for the duration of the
The piling platform was
used to provide additional support in the most critical section,
and methods of pile installation were trialled to determine the
preferred process for the permanent works.
A procurement exercise
was carried out for Phase 3, resulting in the design and
construction of the permanent works being directly awarded to
JBA Bentley. This was due to excellent performance and
demonstrated efficiency savings.
Evaluation of design
options and commercial models took place during this time,
culminating in the identification of the baseline solution. This
comprised a piled foreshore and a stabilising layer of rock
armour from the crest of the embankment extending to the centre
of the river channel.
Typical section drawing through embankment - Courtesy
of JBA Bentley Ltd
Click to enlarge
Phase 3a: Permanent works
A value engineering
exercise was conducted to investigate options for reducing the
quantity of rock armour required. This resulted in modification
of the design to incorporate 11m piles at the toe of the defence,
removing the need to armour the channel bed.
The piles were also
redesigned to increase driveability in response to poor ground
conditions encountered during phase 2. The coordination,
collaboration and cooperation between designer, contractor and
end-user was key to the timely delivery of this engineering
Because of the dynamic
river environment and the need to minimise operational costs,
the design had to accommodate ongoing erosion. The required
section size, length and weight of pile required to restrain
these forces in cantilever alone was not practical, and was
resolved by designing stainless steel tendons connected to an
anchor restraint system.
An innovative design,
comprising a pair of adjacent 12m compression and tension
continuous flight auger (CFA) piles, was established as an
alternative to a raked compression pile and anchor tie
arrangement. The new flood defence was formed by integrating the
reinforced concrete pile cap and wailing beam. This generated
savings in steel volume and time, due to quicker installation.
The design was
complicated by the presence of a steel girder railway bridge
that bisects the site, meaning a piled solution was not possible
in this area. An alternative approach was agreed in conjunction
with Network Rail for placement of rock armour on this section.
Post installation of rock armour under the Network Rail
bridge - Courtesy of JBA Bentley Ltd
Phase 3b: Permanent works
During the construction
phase, higher than normal tides were experienced. Flood incident
duty officers interacted with the site team to give advance
warning such that the site could be monitored during these
times, and emergency response plans were put in place.
Following the challenges
faced during the installation of the temporary solution in
(phase 2), the supply chain was engaged to undertake trial pile
installation to determine the correct methodology for the
permanent solution. This ensured the project hit the ground
running to achieve the programme and minimise the impact on
Access to the west end of
site under the railway bridge involved passing directly in front
of a small row of houses. Early engagement was critical to agree
the use of part of their front gardens.
The pinch point was
hoarded to minimise the disruption, and temporary parking and
footpath arrangements formed to suit the residentsí needs. In
addition, each house had a structural survey in advance of the
site activity, and the works were planned to minimise vibration
during installation of the piles.
Before the works started,
the new flood defence wall was confirmed as permitted
development. However, the new wall was replacing an existing
brick wall to a concrete structure with the exposed ends of the
To gain early acceptance
from the local residents, an artistís illustration of the
finished scheme was shared.
Due to limiting
operational space, the permanent works necessitated using heavy
plant on the narrow receding foreshore. The key to successful
delivery was to ensure that the temporary works were sufficient.
To this extent 700t of temporary sheet piles and 25,000 tonnes
of stone were used to create a ramp over the defence, construct
the piling mat and access roads.
Disposal and import costs
were minimised through diverting material from a nearby site
that was destined for landfill disposal.
There was opportunity to
utilise a newly available lighter pile section with the
equivalent strength and make further cost and carbon reductions.
Before procuring the permanent piles, this section was used in
the early enabling works to prove the methodology and temporary
Interaction at this stage
between the design and construction team enabled the piling mat
to be modified in position, level and thickness. This generated
savings in time, material, transport and carbon emissions,
increasing the projectís sustainability credentials.
Completion of the works looking east - Courtesy of JBA
Works under the HV
overhead cables posed both safety and programme risks. The
installation method for this section was carefully planned with
specialist tier 2 contractors, and included using smaller plant
and a lower piling platform. This avoided the requirement for
outages, saving £24,000.
Health and safety
Health and safety
performance has been exemplary. Over 55,000 hours have been
worked on the project with over 480 positive safety and
environmental interventions, along with a positive behavioural
discussion reporting ethic. Throughout the project there has
been only one minor first aid injury.
The site manager
proactively engaged with local residents; noise, dust, vibration
and disruption from site traffic were major concerns. These were
fully addressed through sympathetic site management and regular
updates. Betterment for local people was provided by permanent
tarmac surfacing of the 900m access track between the main road
and site, installed to accommodate construction traffic.
The site was located near
the Humber Estuary which is internationally designated for
nature conservation. Environmental specialists were involved
throughout the development of the scheme to minimise negative
Works below mean low
water level were reduced through design and consultation with
the marine management organisation.
The site was near to a
breeding pair of protected marsh harriers and the works
progressed with close liaison with Royal Society for the
Protection of Birds and proactive on site monitoring. The local
MP paid particular interest in the scheme and was fully informed
on the progress of the scheme at regular intervals throughout
the works looking west - Courtesy of JBA Bentley Ltd
and publishers would like to thank Rob Culledge, Contracts
Manager, with JBA Bentley Ltd for providing the above
article for publication. The author thanks Angie McKinney,
Project Manager with the Environment Agency, for her input.