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Project Spotlight - Outfalls and Headwalls from Kaymac

The existing flap valve and headwall at Tuddenham Outfall, Suffolk UK, had fallen into disrepair and a new headwall structure and HDPE flap valve were required to prevent the River Lark flooding through the Tuddenham Stream. The Works were carried out by Kaymac on behalf of the Principal Contractor JN Bentley for the Client, Environment Agency.

Situated at the confluence of the Tuddenham Stream and the River Lark, lack of land access to this area meant transport plant and materials were transported to the site by a barge constructed from sectional steel pontoons; maintained during the works by a pair of 6m long spud legs.

The project called for the installation of 38 PU18 steel piles which would form the new headwall structure. The piles were to be driven using a EMV220 piling vibrator mounted onto a Komatsu PC138US-10 excavator.

Prior to the construction of the new headwall, the old steel piles were removed from the existing headwall using the EMV, the redundant flap door was then removed and new piles installed. A temporary cofferdam, formed in front of the outfall, was then dewatered using hydraulic submersible pumps to allow a section of the existing 1600mm corrugated steel pipe carrying the stream (once a blanking plate had been fitted to the inlet end) to be cut out and removed.

A steel back shutter was installed to stabilise the ground behind the outlet. The new steel headwall, with a spigot to fit the bore of the existing pipe, along with the flap valve frame was fully welded to the piles on both sides.

Stone was then laid onto geotextile in front of the outlet, and a 200mm thick reinforced concrete slab constructed on top of the stone. Once piles had been flame cut down to the required height, a steel capping beam was welded to the top. The ground behind the piles was then filled with materials won from reducing the bed level of the river down to the invert level of the stream. Once the door was fitted to the flap valve frame, divers cut the piles of the cofferdam to allow the river and stream levels to equalise.

Finally, a 400mm diameter Key-Klamp handrailing system was fitted to the top of the capping beam. Top soil was then imported onto the top of the backfilled areas to promote the reinstatement of vegetation along the rear side of the completed structure.

Wakisa Mphwanthe, Quantity Surveyor at principal contractor Mott JN Bentley, said, ‘The completed works look fantastic. we appreciate your level of workmanship in accommodating the various changes - from initial enquiry to project completion’.

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