When civil engineering contractor Kier was constructing a fish pass as part of major environmental works on the River Severn, we supplied a bespoke groundworks support package. During the course of the works, this package had to respond to the design changes imposed by the local geology, as well as the load conditions and sequencing of the works.
metre long excavation
levels of bracing frames
Super Bracing Struts used
Work on the Lincomb Fish Pass near Stourport-on-Severn began in March this year, with Kier Professional Services having undertaken the design of the 75-metre-long reinforced concrete structure. The finished structure will enable twaite shad, a type of herring, to once again migrate freely upstream to spawn. This is one of four fish passes on the River Severn being installed as part of Unlocking the Severn - an ambitious river conservation project led by Canal & River Trust in partnership with Severn Rivers Trust, the Environment Agency and Natural England. Keir is the lead delivery partner for these fish pass construction works.
Taking the form of a series of ascending pools that allow the fish to swim from a lower level downstream to a higher level above the man-made weir, the pass also features a control structure at the upstream exit to facilitate future maintenance of the channel.
Our temporary works design was created using a cofferdam to enable the safe propping of sheet pile walls, while the ground was excavated by a long reach excavator, supported by a mini-digger.
Then, as a layer of siltstone bedrock within the zone of works prevented the piles being sunk to the original depth intended, our groundworks solution was revised as work progressed, to enable a secondary lower level to be installed.
The equipment supplied to Kier included Super Shaftbrace frames with runs of walers extending up to the full 75 metre length of the excavation, along with 21 Super Bracing Struts, as well as the accompanying hydraulic pumps and ancillary equipment.
Super Bracing Strut is a high load capacity strut designed for use in conjunction with Super Shaftbrace and Supershaft Plus systems, and typically used as a flying shore. Also, on the fish pass project, to allow sections of the bracing to be removed during the programme, our workshops produced special sacrificial sections for the walers, which enabled them to be welded against the vertical sheet piles, and then cut off again without damage.
Richard Leigh is a Project Engineer with Kier Infrastructure, currently seconded to the ‘Unlocking the Severn’ team and filling the role of Temporary Works Coordinator. He commented: “Having put out enquiries for the type of temporary works we believed necessary for the project, we carried on developing our methodology. In the case of the Lincomb Fish Pass, a solution was required to resist ground pressure before the base slab was installed to achieve the required strength. Mabey Hire came back to us with the best solution. Although we originally sought quotes for the design, supply and install of the bracing, one of our own specialist teams came available at the right time, so it was therefore decided to employ our own resource for the installation.
“Unfortunately, the sheet piles only penetrated a short way into the siltstone, when we would have wanted them to sink another metre to create a secure cantilever effect, meaning that the propping arrangement had to be changed at short notice. This was where Mabey Hire reacted so well in revising the solution to tackle a problem presenting itself on site.
“Mabey Hire was very quick to respond to this and worked collaboratively with the project team to amend the prop design, to ensure safety and minimise any project delays. Originally it was going to involve a single line of props at a higher level, but it was amended to introduce additional shoring at a lower level.”
When the Lincomb Fish Pass is completed as part of the Unlocking the Severn project in spring of 2021, the twaite shad - as well as species like salmon and eels - will be able to navigate this stretch of the Severn unhindered for the first time in 170 years, also benefitting the wider ecology.
“A solution was required at Lincomb Fish Pass to resist ground pressure before the base slab was installed to achieve the required strength. Mabey Hire came back to us with the best solution. They were very quick to respond and worked collaboratively with the project team to ensure safety and minimise any project delays.”
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