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Herring on the side of caution

Contractors are having to cope with rough seas, shipwrecks and migrating fish during construction of massive breakwater extensions at a Lithuanian port.

Every spring and autumn shoals of herring make their way through the narrow entrance to Klaipeda harbour on Lithuania's Baltic coast. Their goal is a huge 100km long lagoon behind the port.

In repairing and extending damaged breakwaters at the entrance to the harbour, Danish geotechnical contractor Per Aarsleff found that these migrations, combined with rough Baltic weather, had a significant impact on the construction programme.

'From mid-April to mid-May and from midSeptember to mid-October, we have to suspend dredging and cannot make any noise at night while the herring migrate into the lagoon, ' explains Per Aarsleff senior project manager Hans Jorgen Kallehauge.

Working for client Klaipeda State Seaport Authority, Per Aarsleff 's DKK140M (£11. 7M) contract involves restoring the toes and sides of the two parallel mass concrete breakwaters and extending them with rock armoured stone embankments founded on huge geotextile carpets.

The southern breakwater is being extended by 330m to the toe of its new pier head and the northern extension, at 250m long, will butt on to the existing breakwater on a dog leg, narrowing the channel to 150m.

In a separate £2. 9M contract, RN Dredging is excavating 1Mm 3of material from the 2. 4km long entrance channel to a depth of 14. 5m below sea level.

The breakwater extensions and the narrowing and deepening of the entrance channel will reduce swell into the harbour and improve mooring conditions at the port's oil terminal. The combination of river currents meeting up to 2m sea swell and onshore winds of up to 30m/s can make mooring very dangerous.

About 510,000t of graded granite will be used on the project, designed by Dutch consultant Frederic R Harris. Site supervision is by BCEOM of France.

Temporary works began in mid-February 2001 with the construction of a temporary quay in the shelter of the harbour. This is being used to store a small 30,000t buffer stock of stone. Construction proper, starting with the southern breakwater, began in March.

Both rock armour embankments are founded on huge geotextile carpets. For the southern embankment, a 75m wide and 150m long carpet was laid on the seabed along the line of the new embankment and a 100m square section was laid for the pier head.

Divers used special equipment designed and built by Per Aarsleff to unroll the geotextile on the seabed. Geotextile was delivered to site in 5m wide rolls, which were unrolled and stitched together to produce 24m wide sheets. These were rolled onto a drum supported and driven at each end by a pair of truck axles complete with wheels and tyres. As the wheels rotated, the drum also turned and gathered up the 150m long geotextile sheet.

The hollow steel drum was floated out to location during calm sea and slowly flooded in a controlled sinking operation to the seabed.

Divers then pulled off about 8m of geotextile and anchored it to the seabed with an initial 200mm thick layer of 0. 1kg to 300kg graded granite, the material also used for the embankment core.

A tug then slowly pulled the drum along the seabed to unroll the remainder of the geotextile.

This was covered with a 200mm layer of core material, placed by a pontoon-mounted long reach excavator equipped with a depth control global positioning system (DGPS). Compressed air was then blown into the hollow drum, displacing the water and raising it to the surface for towing back to the quay. The operation was repeated in a series of overlapping strips.

A line of steel tubular piles were driven through the stone-covered geotextile carpet and into the seabed at 20m centres, offset by 7m from the centre line towards the harbour side of the embankment. These act as moorings for the barges and ships offloading and placing stone directly into the embankment.

Per Aarsleff covered the geotextile with a 1m thick layer of stone before building up the central granite core to about 12m high at the crest and 40m wide at the toe. Most of the stone was placed directly from the barges and ships using their own onboard excavators. The stone was allowed to selfprofile, before final grading on a 1:2 slope using the DGPS controlled long reach excavator working from a pontoon.

As the staggered core and filter placing progressed, Per Aarsleff followed on with the main rock armour. This comprised 4-7t blocks on the harbour side of the embankment and 7-10t blocks on the seaward side. The armour stone was placed up to sea level and the entire breakwater capped with 10-13t granite blocks leaving a 5. 5m wide crest 3. 5m above sea level. The core for the semi-circular pier head was covered with a layer of 1. 5-4t granite blocks, followed by an outer protective ring of 4-7t rock armouring. The middle will be filled with 370 mass concrete 25t tetrapods.

'The weather and sea conditions are quite tough here and can change very rapidly, ' says Kallehauge. 'Due to a water depth of 12m we need to place a minimum 50,000t of core material before we are able to start placing the heavier filter layer.

Leaving the lighter core material exposed to the very changeable conditions can pose a major risk for us and is a big challenge to get the quarry to produce the correct material at the right time and get it to site in the right order for placing directly into the embankment.

Work on the southern embankment was due to finish at the end of last month. Work on the shorter northern breakwater extension will then begin, with repair of both breakwaters carried out simultaneously. Per Aarsleff has already broken up three shipwrecks at the end of the northern breakwater to make way for the extension.

Despite the delays caused by the herring migrations and bad weather, Kallehauge says the project is expected to finish by August 2002, almost 10 months ahead of schedule.

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