Careful dredging for historic site
This summer saw the completion of the capital dredging works on the historic Millbay Inner Basin at Plymouth in the UK. Dredging is not often thought of being ‘precision’ work, but this latest project ‘raised a number of challenges’ said Dan Jenkins of Jenkins Marine.
Firstly, the works are located within the Millbay inner basin, which was originally constructed by Brunel in the in the 1850’s for the Great Western Railway Company, and has listed granite and limestone quay walls, which are undergoing repairs and must be given due care in order not to damaged them.
For many decades silt had been allowed to collect in the Inner Basin. The English Cities Fund (ECF) developed detailed plans to bring this back into use and form it into the centrepiece of a newly regenerated area, but simply dredging the basin was not possible due to the environmentally sensitive nature of the area and the need to prevent re-suspended material from drifting into the surrounding waterways.
Further, due to the restrictive size of the basin the licence stipulated that a backhoe dredger must be used for the works.
This saw Jenkins Marine use state of the art computer guided dredging equipment, including a Prolec PCX-3D RTK GPS dredge system, accurate to 10mm, and a new Case CX350 long reach excavator with a 6.5m boom and 6.5m dipper arm which was able to dredge the 37,000 cubic metres of material required.
To add to the complications, there was found to be trace contamination within some of the sediments to be removed, which has been addressed by excavating in 1m deep layers.
Therefore two thirds of the material was disposed of at sea at the sensitive Rame Head dump ground using Jenkins Marine’s 300m3 self-propelled split hopper barges Nab and Needles, while about one third of the material, the layer which had trace contamination, was brought to shore and then unloaded by excavator ready for dewatering and remediation by main contractors Morrison Construction.
The project took around three months to complete. However, the backhoe dredger Doreen Dorward is still on site helping demolish part of the quay in order to recover blocks for rebuilding other sections of the listed wall.
‘It was by no means straightforward,’ says Mr Jenkins, ‘but we are very pleased with the result and it shows we are capable of taking on these kinds of sensitive projects.’
