Since the completion of the Poole Bay Beach Renourishment Programme of Works in January 2006 it had become necessary to extend the surface water outfall at Branksome Chine.
The existing outfall was 50m long, constructed from a steel pipeline encased in concrete and supported on both sides by interlocking steel sheet piles. Although the pipeline was previously able to reach the sea, following the replenishment of beach sediment it is now found almost 30m short. As a result, the outfall is found completely buried by the beach and unable to function efficiently. The flows through the outfall originate from the Branksome Chine stream and the surrounding surface water drainage system. Flow volume increases rapidly following periods of intense rainfall and the outfall needs to be able to cope with these periods of high demand. This situation also poses a potential health and safety issue to beach users. A prompt response by Jenkins Marine was effected to extend the outfall and solve the problem.
The civil engineering works were quickly undertaken by Jenkins Marine, a testimony to our strong supply chain relationships in co-ordinating the delivery of the specially produced pipeline sections within a tight time frame. All works were planned around tidal cycles, demanding fast and efficient procedures. The outfall was successfully extended by a further 48m, permitting effective operation. The contract was completed below budget price and within the time anticipated.
Jenkins Marine is based in Poole, and thanks to it’s fleet of specialised workboats and barges all materials were able to be delivered by sea, causing minimal disruption to beach users. Health and safety of the public was given top priority throughout the duration of the works.
As a direct result of our performance Jenkins Marine has subsequently been invited to submit budget proposals to extend a number of similar outfalls along this stretch of coastline.
Foreness point, Margate – Specialist dredging works
June/July 2006
Jenkins Marine have recently been involved in underwater construction works for the waste water outfall at Foreness Point, Margate, as part of a £80m wastewater treatment scheme to serve Margate and Broadstairs. The scheme involves engineering work at the existing coastal wastewater treatment works at Foreness Point and North Foreland in Broadstairs to enable wastewater to be transferred to a new inland Treatment Works, before being piped out to sea. The existing outfall pipes are being replaced by a single 1800mm diameter, 600m long pipe tunnelled through the underlying chalk reef.
Jenkins Marine were involved with the marine works to excavate a 30m long by 8m wide reception pit in the sea bed to enable the recovery of the boring machine. At 8m below the existing seabed level it will be backfilled with gravel. When the boring machine reaches the end of its tunnel drive it will enter the reception pit into the gravel. The gravel will be pumped out and the TBM recovered. The works were completed by fitting a pipe diffuser to manage the periodic discharge of wastewater.
The works involved utilising specialist equipment supplied by Jenkins Marine, and operation within a high degree of precision. The services of WM Plant Hire were used to supply a 45 tonne long reach excavator with a reach of 22m, fitted with Prolec underwater dig profile equipment. The computerised cab display provides an image of the machine boom, dipper arm and bucket position, co-ordinated to GPS data. Mounted on the dredge pontoon ‘Doreen Dorward’, the excavator was positioned using survey equipment from the shore and the dig parameters set to the Prolec system. A diamond tipped milling machine was attached to the excavator to profile the edges of the excavation and penetrate any hard areas. A hydraulic ramming plate was attached to compact the deposited gravels.
Excavated material was loaded on to the 300m3 split hopper barge ‘Needles’, able to instantaneously deposit spoil material. ‘Needles’ is propelled by a powerful V12 Caterpillar engine, and when loaded is capable of speeds up to 8 knots en-route to the spoil ground, and 10 knots when light, enabling a swift return to the dredge area.
With a tidal range of around 4m and a depth of 16m at high water, strong tidal currents and an exposed headland location over a rock ledge, this was a challenging working environment. During of the excavation and backfill operations, a team of divers were used to monitor the progress and check the accuracy of the underwater construction.
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