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Pure shores

Diving and underwater engineering - A new underwater pipeline in Margate is cleaning up some of Britain's bestloved beaches, reports Jon Young.

Wastewater in Kent will soon be treated to a much higher standard thanks to the construction of a new £80M treatment works and a vital long sea outfall some 12km away.

Jenkins Marine is building the new sea outfall at Foreness Point in Margate. It will carry treated effluent out to sea after treatment at the new works at Weatherlees.

With a tidal range of about 4m and a depth of 16m at high water, strong tidal currents and an exposed headland location over a rock ledge, this is a challenging working environment.

'The pipe is being tunnelled through the underlying chalk reef by a tunnel boring machine (TBM) from the water treatment works to the outfall exit, ' says Brenden McGettrick, partner at equipment supplier and long reach excavator specialist WM Plant Hire.

Here Jenkins is excavating a 30m long by 8m wide reception pit in the seabed. This will serve a dual purpose. 'At 8m below the existing seabed level it will be backfilled with gravel material after the initial excavation of chalk. When the TBM 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, ' says McGettrick.

The pit will be completed by fitting a 'diffuser' to manage the periodic discharge of wastewater from the treatment works.

This operation involves some specialist equipment operating with a high degree of precision.

WM provided a 45t Caterpillar machine with a 22m reach. A Prolec underwater dig profile system was fitted to enable the operator to see underwater.

A computerised cab display provides an image of the machine boom, dipper arm and bucket position, co-ordinated to GPS data.

Mounted on Jenkins Marine's 30m x 9m self-propelled dredge pontoon Doreen Dorward, the excavator is positioned using survey equipment from the shore and the dig parameters set to the Prolec system.

The excavator is Caterpillar designed and manufactured for heavy excavation work and is able to dig through the chalk material.

When the digging bucket is not being used, a diamond-tipped milling machine is attached to the long-reach excavator to profile the edges of the excavation and penetrate any hard areas.

Excavated material is then loaded onto Jenkins Marine's 300m 3 split hopper barge Needles, which has a split bottom for instantaneous deposition of the spoil.

'Needles is propelled by a powerful V12 Caterpillar engine, and when loaded is capable of speeds of up to eight knots enroute to the spoil ground, and 10 knots when light, enabling a swift return to the dredge area, ' says McGettrick.

When complete, the new 1.8m diameter, 600m long outfall will replace two existing, shorter outfalls. These dispose of waste that has received limited treatment at plants at Foreness Point and North Foreland in Broadstairs.

They are being decommissioned as part of efforts to meet the standards of the more stringent EU Urban Waste Water Treatment Directive.

The directive requires that all wastewater from population centres with more than 15,000 inhabitants receive secondary treatment before discharging into water courses (NCE 21 April 2005).

The world's first fully inflatable rigid inflatable boat (RIB) was launched earlier this year by its creator De Batts RRIB Associates. The 5.3m rubber rigid inflatable boat was developed as an inflatable that could easily be carried by two men and which could be transported in a utility vehicle but had a considerable load carrying capacity. Originally developed for military use, the boat can carry a payload of up to 900kg but weighs just 110kg itself. 'It's good in surf and capable of long transits, ' claim the makers. Much thought went into ensuring that the craft fits the space constraints associated with a boat of this type while still offering good handling.The RIB is made from hard wearing Hypalon and German Neoprene Dropthread material. Its design incorporates a rigid inflated RIB type hull and replaceable inflatable floor with a wooden transom. Hull design has some redundancy in the event of damage.

The company claims it is considerably lighter than other conventional RIBs of similar size and due to its RIB characteristics it can outperform other inflatables of the same size while using the same size engines.

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