seabed: from cannons and cannonballs to anchors and ship parts. On top of all that, large numbers of German sea mines were placed in the Solent strait during the Second World War and approximately 40,000 bombs were dropped. The entire area had to be inspected and cleared to prevent risks and delays in the dredging work. We were working with colleagues from Boskalis Hirdes, which specializes in the detection and clearance of UXO.” ADVANCED SURVEYS The inspection of the port began in November 2015. About 800 objects were detected using traditional survey techniques such as side-scan sonar and magnetometer survey. A crane vessel then brought the objects to the surface. “We, however, also unintentionally retrieved a German sea mine with 700 kilograms of explosives,” said Van den Bosch. “It was made of aluminum and so it escaped detection by our equipment. Sea mines can still explode and we therefore switched to an advanced survey technique known as Sub-Tem that is also used to detect non- ferrous metals. This technology was developed for use on land, but we adapted it for use underwater. It worked perfectly. More than 12,000 different objects – weighing in total more than 1,100 tons – were removed from the seabed.” RISKS “Given the risks, our NINA safety program played a leading role on this project,” Van den Bosch continued. “Portsmouth is a busy port with a lot of leisure shipping. To keep the public at bay, the Smit Stour was put on patrol. We worked with a team of certified UXO divers who didn’t start to work until we were certain they were not exposed to any danger.” A light-sensitive underwater camera was fitted to the grab of a pontoon’s crane to obtain a clear picture of the objects on the seabed. A pump was also fitted to the grab that was used like a vacuum cleaner to carefully remove the sand from around obstructions, which could be a torpedo, bomb or sea mine. That process was monitored with the camera. “The UXO divers came into action only if we couldn’t make a clear assessment of the object on the seabed with the camera,” Van den Bosch added. “If the divers believed that the object was indeed UXO, then it was removed and disposed of by the Royal Navy’s own UXO divers. We also found all sorts of other objects that could disrupt the dredging work, such as anchors and big bundles of twisted steel cables.” LAYER BY LAYER The dredging work was done layer by layer. “The Sub-Tem surveys allowed us to inspect the seabed to a depth of two meters,” Van den Bosch explained. “We repeatedly conducted intensive investigations of the area to make sure we didn’t miss anything. Then we removed the objects we found and dredged out that layer of soil before investigating the next layer. It was a time-consuming approach but the number of objects found dropped as we removed more material and then things started to speed up.” More than 80 Boskalis colleagues were at work in Portsmouth, which is now ready for the arrival of the aircraft carriers in 2017.
Dredging work by the trailing suction hopper dredger Shoalway in the port of Portsmouth.
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