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Sitting on the rock of the bay

Construction of a new container terminal in a Danish port involved a wide variety of geotechnics, all carried out by one contractor. Rodney Byles reports.

A recently completed container terminal on the east coast of Denmark is Danish contractor Per Aarsleff 's biggest marine project to date.

The firm's DKK300M (£26M) contract at Aarhus Harbour involved building a main outer breakwater and a separate access embankment. The embankment links to an inner breakwater and quay wall to form an enclosed basin, which was backfilled with sea dredged sand to form the 25ha terminal area.

Work began on the 2km long L-shape outer breakwater in October 1998.The sand core of the huge rock armoured embankment, about 140m wide at the toe, was formed using a computer controlled suction dredger equipped with a global positioning system. The dredger excavated sand from Aarhus Bay and dumped it along the line of the breakwater to an initial height of about 6m above the sea bed.

Working in 12-14m deep water, the seaward slope was graded at 1:10 and the inner slope at 1:5. The suction dredger then formed the centre of the 5m to 6m tall ridge.

A sand and water mixture was placed on a steel ramp mounted on a flat-topped barge and slid down a chute on to the core to form a natural 1:3 slope on the seaward face.

The inner face was again graded to match the shallower 1:5 slope, leaving the top of the sand core about 2m below sea level.

After completing about 40m of the core, Per Aarsleff covered its top and part of the slopes with an initial protective layer of sea-dredged gravel and pebbles of 10-200mm diameter. Dredged from Aarhus Bay, this material was brought on barges that were moored to a row of temporary tubular steel piles installed at 30m centres along the line of the embankment. The gravel and pebbles were placed over the core in a layer 600mm thick at the toe and about 3m at the crest.

A 1m thick layer of 80mm to 500mm quarried stone was then placed over the pebbles to a height at the crest of 2m above sea level. Placement of this layer lagged about 40m behind the initial capping layer. The final, 2m thick outer rock armour protection of granite blocks followed on a further 40m behind, forming the finished 3m wide crest 4m above sea level. The staggered layering process was adopted to minimise exposure and potential scouring of the core by rough seas.

Per Aarsleff worked initially in a north easterly direction for 1.6km before turning 90degrees north west for the remaining 400m, completing the outer breakwater in only six months.

The new 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 way, but did not have protection on the inner slope because it 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, protected only by a layer of pebbles and a final layer of 140-400mm stones over part of the outer face of the sand core. This inner breakwater was continued to form one face of the main 20m wide access embankment, which was extended to widen the route from the existing bulk terminal. The other face of the access embankment was similarly protected with the layers of pebbles and stones and was extended, 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 installed 21m long Hoesch 3600 steel sheet piles to form the quay wall with an in-house designed Hitachi 125 crane-based self-erecting piling rig working from a jack-up platform. Piles were driven to about 4.5m into the underlying stiff clay.A second piling rig, based on a Sennebogen 655 crane with PVE vibrator, standing on some of the dredged sand infill, drove a second line of 6m long lighter section Hoesch 2500K sheet piles into the sand fill, 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 was also installed from the quay wall at 9.5m below sea level and connected to concrete blocks placed in the sand fill.

Once the quay wall and the inner breakwater were finished to form the sealed basin, 5M. m 3of dredged sand was pumped in, filling it to 1.7m above sea level. The firm then installed a line of 400mm square precast concrete piles through the compacted fill,2.3m away from and parallel to the quay wall. These were between 22m and 31m long, each in two sections up to 18m long. A second parallel row of piles was then installed 31m away, just inside the area confined by the second row of sheet piles.

The two rows of concrete piles are located 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 piles was capped with a 2m deep, 1m wide cast insitu beam. The container crane rails were fixed to the capping beams. Per Aarsleff completed its contract by 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 reclaimed terminal area with the existing port. The new facility is due to open in April.

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