The existing flap valve and headwall at Tuddenham Outfall, Near Barton Mills, Suffolk, had fallen into disprepair 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 on behalf of Principal Contractor JN Bentley for the Client The Environment Agency.


The outfall was situated at the confluence of the Tuddenham Stream and the River Lark. Due to lack of access into this area by land, a barge constructed from sectional steel pontoons was used to transport plant and materials down to the site. The barge was maintained in position 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, then the redundant flap door was removed. The new piles were then installed. The design allowed the formation of a temporary cofferdam in fornt of the outfall, which was then dewatered using hydraulic submersible pumps to allow a section of the existing 1600mm corrugated steel pipe carrying the stream to be cut out and removed removed.

 

A blanking plate was fitted to the inlet end of the pipe to allow dewatering to take place.Once the pipe had been cut back to the required length, a steel back shutter was installed to stabilise the ground behind the outlet, and the new steel headwall was fitted, along with the flap valve frame.

 

The headwall was designed with a spigot which fitted into the bore of the existing pipe, and once in position, it was fully welded to the piles on both sides.

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


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