Dredging underneath and near the RDM ship-lift, Rotterdam
|Location||Rotterdam, the Netherlands|
|Client||Port of Rotterdam|
Beside out grab dredger Rival, we also use the self-propelled hopperbarges “Prinsenstad” and “Geroma” of our survey vessel “Alk”, which is no longer in use and been replaced by another survey vessel.
|Dredging volume||Approx. 8.500 m3|
About this project
In view of plans to recommission the ship lift on the RDM yard, the access channel to the ship lift needed to be dredged. This work involved dredging underneath the lifting platform, the open portion of the ship lift, and the areas in between and behind the guiding system at the mouth of the entrance.
This kind of dredging required a specialised dredging technique. Apart from the limited width of the ship lift entrance, height was the main limiting factor to be reckoned with. Depending on the tide, there was no or hardly any space for manoeuvring between the underside of the lift platform and the waterline. The dredging area was enclosed by quay structures and the lift platform. These objects were exposed to the risk of damage as a result of dredging operations. Due to the poor accessibility of the area underneath the lift platform and near the quay it was decided to commit divers to the dredging operation.
We carried out a diving inspection underneath the lift platform prior to commencing dredging. This inspection afforded us insight into the density of the consolidated silt as well as an accurate picture of the situation below the platform. Based on these findings, we came to the conclusion that a dredge pump fitted with a suction hose would generate sufficient power for dredging away the consolidated silt. This pump assembly was hung from pontoons.
In earlier projects we had worked successfully with DUC-Diving on Urk (Netherlands). This company also supported us in the subject specialised work. The divers were able to access all locations such as sheetpile casings and to simply dredge to the required level without damaging the quay.
The use of a silt pump would have generated a higher delivery rate, with a larger volume of dredging material to be transported and dumped. In the technique used here, the silt was not pumped directly into hopper barges but into a temporary underwater depot. This way the dredged material was allowed time to consolidate, which ultimately resulted in a lower delivery rate (approx. 30%). The properties of the silt were such that it settled fairly quickly (in less than two hours).
To prevent water turning turbid as well as dispersal of the seriously contaminated silt, we deployed a bubble screen near the existing turbidity curtain and the mouth of the discharge pipe was kept beneath the top of the existing turbidity curtain.
The consolidated material could be removed from the temporary underwater depot with the aid of a crane vessel and transported to the Slufter depot.
While dredging underneath the ship lift was quite a complex operation, carrying out soundings there was a challenge as well. We usually carry out soundings with the aid of multibeam echo sounding underneath a survey vessel. This method was also applied in the water in front of the ship lift. This way we were able to take soundings with an accuracy of 2 cm.
However, this sounding method could not be applied underneath the ship lift because of the very limited clearance height. Besides, as the measuring equipment underneath the ship lift did not have GPS connectivity, test values did not correspond accurately with the correct location. This was why another method was used under the ship lift: the manual sounding method.