The International Association of Dredging Companies (IADC) recently released the article named ‘Environmental dredging of a chromium contaminated fjord’.
The article , written by authors from DEME, gives a detailed look into the efforts made to complete the environmental dredging works in Valdemarsviken fjord in Sweden.
The sediments in this fjord were contaminated with chromium from the discharge of untreated wastewater from tannery operations some decades ago. Thus, the remediation of the fjord was very important for the water quality in the Baltic Sea.
The contaminated sediments in the inner part of the fjord were dredged, dewatered, solidified and placed as engineered fill on a nearshore area.
It was observed that the dredging operations on one side, and the backfill operation on the other side, would affect the stability conditions along the shores. As such, stabilisation of quays, shores and the backfill area by means of lime-cement columns was required.
The dredging of contaminated sediments covered about 200,000m³ spread over a 350,000m² large area. During the detailed design phase, prior to the start of the works, the site conditions and sediments were monitored and investigated thoroughly.
Lab scale tests were performed to verify that the chosen process for the dredging, dewatering and solidification met the set requirements with regards to the strength of the backfilled stabilised sediments specified by the client.
The lab work also formed the basis of the process design for the on-site solidification plant. The dredging operations met the stringent environmental demands towards turbidity and the generation of spill.
The sediments were dredged by using hydraulic excavators equipped with a level-cut clamshell bucket or a visor bucket. The sediments were then pumped to a continuous mixing unit for solidification with a cementitious binder.
Any water released from the activities was collected and treated by a specially designed on-site water treatment plant.