innovative precast design solution to protect 260 properties
from catastrophic flood risk in the event of culvert failure
delivered within a 12-week programme
designed a precast solution that improved workforce safety
by reducing time spent working within confined spaces
Granary Culvert is a 60m long channel with a 6m span. It has a
2.5m headroom and forms a section of Worsley Brook. The 150
year-old brick culvert had suffered structural defects. These
included some full section cracking, ring delamination, heavy
deformation to the barrel, and rotation to the abutment.
Sections were in such poor condition that the Environment Agency
had declared “red zones” to prevent personnel access. The busy
B5211 Barton road runs adjacent to Granary Culvert. This key
local route links Worsley with the M60 Manchester Outer Ring
Road. Surrounding the culvert are offices and residential
properties meaning any collapse would have a catastrophic effect
on the 11,000-strong surrounding community. This includes 260
properties that would be at risk of flooding if a failure
Background to the
The repair to Granary
Culvert was part of a much larger Environment Agency Water and
Environment Management (WEM) framework scheme. This had been
awarded to VBA, a joint venture between VolkerStevin, Boskalis
Westminster, and Atkins. The larger scheme covered repair works
to Bridgewater and Granary culverts on Worsley Brook.
Due to its complexity,
VBA awarded the £895,000 Granary Culvert section of the project
to VolkerBrooks based on their specialist expertise. The
intricacy of works undertaken required a collaborative approach
from a fully integrated team. This delivery team included the
Environment Agency, VBA, VolkerBrooks and GHD.
VBA’s winning tender was
based on a concept to strengthen the culvert using a sectional
steel liner. They chose this solution to provide safe cover for
the workforce during installation. Due to the red zones, the “no
person-entry” was an important part of the contract.
The main challenge
centred on ensuring construction work could be carried out
safely inside the defective culvert. This was a designated
confined space and the work needed to be delivered within a
tight programme to minimise local disruption. The team opted for
an alternative precast concrete solution. This would provide the
same level of protection to personnel as the concept solution
but would be more achievable within the tight 12-week programme.
This was critical to achieve with further works planned
effecting the highway.
Areas had been declared ‘red zones’ in the culvert after
suffering full section cracking, ring delamination,
heavy deformation to the barrel, and rotation to the
team worked with the EA to gain approval for a pumping
regime to divert Worsley Brook around the culvert and
allow VolkerBrooks to work in dry conditions
Design and construction
Instead, the team
developed a solution using bespoke arched pre-cast concrete
units to line the culvert. This solution provided the required
strength without the need for restrictive core propping. The
units also acted as a shield to protect personnel during the
In total 36 (No.) bespoke
concrete sections were lifted and placed to line the defective
areas of the culvert and minimise the risk of flooding to nearby
GHD and VolkerBrooks
developed the culvert design with cognisance to the
manufacturing process (formwork striking forces), transportation
and handling (lifting anchors) and permanent location. The
culvert ran adjacent to heavily trafficked highways and was to
be designed to support permanent loadings from vehicles and
backfill materials. The collaborative approach was continual
throughout the project with all GHD designs independently
reviewed by VBA to ensure compliance with the contract and the
The design balanced
permanent and temporary works parameters to maintain the channel
area of 6.0m2. Due to the use of precast concrete the prescribed
roughness coefficient of 0.03mm was not achieved, however VBA
assessed the hydraulic capacity of the culvert and by making
changes to the bed slope could achieve the required capacity.
This ensured the solution delivered the specified hydraulic
High strength concrete
was specified for the bespoke culvert units; this allowed early
demoulding to speed up the manufacturing process. The fast track
construction method used precast technology to reduce the amount
of time required for confined space working.
GHD designed the short
section of temporary support inside the existing culvert to
ensure safe access for VolkerBrooks’ installation team. This
used modular frames that were within the manual handling limit.
The solution allowed manual erection and installation within a
confined space with limited room to manoeuvre.
The team also delivered a
permanent formwork solution during concreting and grouting
operations. The team connected the new units into the existing
culvert by forming an arched structure in brickwork at the
outlet to match the existing structure. At the inlet to the
culvert, two flume walls ensured that the hydraulic design
requirements were met.
The team also minimised imported materials by stabilising silt
from the river bed using cement and lime. They then reused this
as fill material in the void around the precast units. As a
result, only 20 tonnes of contaminated materials were taken from
site for further processing.
The project team adopted
a fully integrated approach to design. This included combining
permanent and temporary works designs, assuring buildability,
and incorporating construction sequencing.
The team worked closely
together, coordinating multiple disciplines to carry out
collective reviews. This involved considering all structural
load cases during transportation and handling.
By developing a precast
solution, the team minimised road closures. As a result, they
alleviated potential disruption for commuters along this busy
Workforce and public
The team incorporated
measures to protect public and workforce safety from design into
delivery. These measures included safe confined space working
through structural designs and traffic control measures during
VolkerBrooks put temporary traffic lights in place to
protect passing pedestrians and motorists while work was
Workforce safety: VBA’s
confined space-trained structural engineers inspected the
culvert to establish the temporary works design and confirm
exclusion zones. GHD then designed temporary propping for the
areas of the culvert where access was critical. This included
areas where the team would carry out further inspection and
subsequent installation work. The temporary propping designs
used modular frames to improve installation efficiency.
Throughout design and
delivery, the project team incorporated a safety in design
approach. This spanned from revising the client’s original
sectional liner concept solution to refining the precast design.
GHD reduced the weight of the precast units by changing the
precast concrete invert to pumped fibre reinforced concrete.
This reduced manual handling risk and avoided reinforcement
fixing in the confined space. VBA reviewed the GHD design to
ensure the works complied with the WEM framework health and
safety code of practice - Safety is Paramount.
Public safety: With the
scheme located in a busy residential area and along a key local
route to the busy M60 motorway, public protection was at the
forefront of delivery. VolkerBrooks ensured the safety of
passing pedestrians and motorists by putting temporary traffic
lights in place between 9:30am-3:30pm on Barton Road each day.
They also provided local businesses and residents with
alternative parking options to minimise disruption during the
VBA also undertook a
public safety risk assessment to ensure that the public was
protected within the Worsley Brook area as it is a Flood and
Coastal Risk Management site.
Precast unit installation
The use of precast
culvert units eliminated temporary shuttering and fixing rebar
to form the new lining. This solution was much more cost
effective and less time consuming compared to in situ reinforced
The units were lifted
into place by a crane working within a lane closure. Working
area constraints were dictated by the location and agreements
with the highway authority. This meant that each unit had to be
designed within a 4T crane lifting capacity. The installation
process involved lifting the units onto polytetrafluoroethylene
(PTFE) rails in front of the culvert. These Teflon rails reduced
frictional resistance and were used to pull the units into
position using Tirfor® machines. Once positioned, 400m3 of
free-flow concrete was pumped into the void between old and new
VolkerBrooks installed 36 (No.) precast units within a
lane closure on the busy adjacent road
The team developed a
bespoke environmental management plan for the project. This
highlighted the key risks to be managed including minimising
impacts on local wildlife and the watercourse itself. This
covered bats, fish, and culvert diversion approvals.
Pre-design survey work
had identified the presence of bats in the area. As part of the
solution, the team installed screens at the entrance and exit of
the culvert to ensure bats did not get trapped in the working
area. Bat boxes were installed in the culvert to meet Natural
England licence requirements.
Safe working relied on
the culvert being repaired in dry conditions. To facilitate
this, the team worked closely with VBA and the Environment
Agency to gain approval. This involved developing, designing,
and delivering an approved pumping regime to divert Worsley
Brook around the culvert. As a result, the new units could be
installed safely in dry conditions.
VBA’s design also
included gravel traps for installation on the riverbed with GHD
designing the connection detail. These gravel traps help
encourage fish breeding, restore the habitat, and by extension
improve the biodiversity and ecological profile of the area.
VBA, VolkerBrooks and
GHD’s collaborative approach to safety, efficiency, and value
drove successful delivery of the scheme. Having already improved
efficiency by using precast units, the 36 (No.) sections sat on
an in situ concrete slab reinforced with steel and polypropylene
fibres. This eliminated the need to fix traditional
reinforcement within the culvert, reducing health and safety
risks to the workforce and speeding up construction.
Despite the confined
space and safety challenges, the team completed the works within
a tight 12-week programme. This was critical to the project by
finishing in time to avoid further works planned affecting the
motorway and ensure the improvements were winter-ready for the
local community’s peace of mind.
The culvert repair was
completed within the shortest possible timescales with zero
accidents or incidents. This was testament to the team’s
stringent approach to safety, proactive discussions with local
authority highway engineers and ongoing collaboration with the
local business and residential community
and publishers would like to thank the Environment Agency,
VBA, and GHD for providing the above article and images for