A huge extension to Denmark's Aarhus Harbour has marked the largest-ever single marine project for Danish contractor Per Aarsleff. Its two year, DKK300M ($36M) job is part of a new DKK650M container terminal which will be opened in the spring of 2001.
Won on competitive tender in several separate contracts, work involves building a main outer protective breakwater and a separate access embankment. An enclosed basin was then backfilled with sea dredged sand to form the 25ha terminal area.
Per Aarsleff mobilised a huge fleet of specialist marine plant for the contract, including suction dredgers, barges, tug boats, jack-up platforms and sand pumps, together with piling rigs, excavators and wheeled loaders.
Work started on the 2km long main sand core, rock armoured, L shaped outer breakwater in October 1998. The core of the huge embankment, about 140m wide at the toe, was formed using a computer controlled suction dredger equipped with a global positioning system. Sand excavated from Aarhus Bay was dumped along the line of the breakwater to an initial height of about 6m above the sea bed.
Working in 12m to 14m deep water, the outer slope was graded at 1:10 and the inner at 1:5. The suction dredger continued its journey back and forth to Aarhus Bay to form the remaining 5m to 6m tall pinnacle. But this time, the sand and water mixture was pumped onto a steel ramp mounted on the deck of a flat-topped barge. The sand slid down the chute dropping into the core to form a natural 1:3 slope on the outer face. The inner face was graded to match the shallower 1:5 slope leaving the top of the sand core about 2m below sea level.
With about 40m of the core complete, Per Aarsleff began the the second stage of the operation, covering the top and part of the slopes with an initial protective capping layer of sea-dredged gravel and pebbles, 10mm to 200mm diameter. Also dredged from Aarhus Bay, the material was brought in on flat-topped barges. These were moored to a row of temporary tubular steel piles installed at 30m centres along the line of the embankment. The material was offloaded by a Cat 350 hydraulic excavator onto a feeder conveyor and strategically placed to from a layer varying from 600mm at the toe to about 3m at the crest.
A second 1m thick protective layer of 80mm to 500mm quarried stone, also brought in on barges, was then placed over the pebbles to a height at the crest of 2m above sea level. Placement used the same methods, running about 40m behind the initial capping.
The final, outer rock armour followed, another 40m behind.
Thousands of granite blocks, weighing between 1.5t to 6t, were brought in on a fleet of barges from a quarry in Sweden.
Per Aarsleff used a pair of wheeled loaders - a Cat 980 and a Komatsu WA 600 - to pick up the rocks and drop them in reach of a Cat 350 hydraulic excavator to form a 2m thick final capping layer, producing a crest width of 3m at a height of 4m above sea level. The staggered layering process minimised exposure and potential scouring of the core by rough seas.
Operating round the clock, seven days a week, Per Aarsleff worked initially 1.6km in a north easterly direction before turning 90degrees north west for the remaining 400m, completing the outer breakwater in just six months.
The container terminal's 600m long north breakwater, a parallel and staggered continuation of the outer breakwater's 400m north westerly return, was built in a similar sequence, but without protection on the inner slope, which eventually formed part of the infill area. A sheet piled pier head was built on the end of the breakwater together with a roll-on/roll-off ferry ramp.
The southern end of the outer breakwater was joined to an inner breakwater. This was protected only by a layer of pebbles and a final layer of 140mm to 400mm stones over part of the outer face of the sea-dredged sand core. This inner breakwater was continued back to form one face of the main 20m wide access embankment, to widen the access route from the existing bulk terminal. The other face of the access embankment was protected with the layers of pebbles and stones, snaking around to join up with the western tip of a 530m long steel sheet piled quay wall. The other end of the quay was joined to the northern end of the outer breakwater to form an enclosed basin.
Per Aarsleff's team installed the 21m long Hoesch 3600 steel sheet piles for the quay wall with one of its in-house designed Hitachi 125 crane-based selferecting piling rigs, working from a jack-up platform. The rig drove the sheet piles over water to toe about 4.5m into the underlying stiff clay.
Standing on the dredged sand infill, a second piling rig, based on a Sennebogen 655 crane with PVE vibrator, drove a second line of 6m long lighter section Hoesch 2500K sheet piles, parallel to and 34m away from the main quay wall. The two rows of piles were anchored together with tie bars and connected 3m below sea level by divers. A second, deeper row of 25m long anchors were also installed from the quay wall at 9.5m below sea level and connected to concrete blocks placed in the sand fill.
With the sheet piled quay wall finished and the basin within the inner breakwater closed, the company continued pumping in the remainder of the project's 5M. m 3of sea-dredged sand at night and weekends to fill the enclosure to 1.7m above sea level.
Per Aarsleff then transferred its piling operation to the compacted fill inside the basin, adjacent to the sheet piled quay wall.
A larger Hitachi 180 crane based leader piling rigs, with a 9t Junttan hydraulic drop hammer, installed a line of 400mm square precast concrete piles, 2.3m away from and parallel to the sheet piled wall.
The piles, made at Per Aarsleff's Centrum Paele factory and ranging in length from 22m to 31m, were supplied in two pieces, each with a maximum length of 18m, and connected on site. A parallel row of concrete piles was then added at 31m distance - just within the area confined by the second row of sheet piles.
The two lines of concrete piles are directly under the centre line of the rail tracks of the quayside container handling crane. A 3.5m wide, 2m deep concrete capping beam connects the sheet piled quay wall and the first row of piles. The second row of concrete piles was capped with a 2m deep, 1m wide cast insitu beam.
The container crane rails were fixed to the capping beams and berthing fenders were then attached to the 530m long quay wall.
The final task was finishing off the main access road and laying railway tracks on a 2.4m wide 200mm thick concrete plinth along the access breakwater to connect the new terminal area with the existing port.
Per Aarsleff has now completed its contracts and handed over the 25ha site ready for the erection of the main container handling crane and terminal buildings, and the laying of block paving over the entire area. This work is due to be completed this spring in time for the opening of the Aarhus Harbour extension by Denmark's Queen Margrethe on 1 April.