An underwater world full of hidden surprises
Underwater dock structures are hidden from sight, but modern methods are now making their surprising details visible to everyone, not just divers.
The Port of Helsinki has about 5.6 km of different dock structures in the downtown area. Although construction methods have changed over the decades, the trends of past eras can still be seen.
“When a quay is repaired, the existing structures are sometimes left in place and the new ones are erected in front of or above them,” says Tero Sievänen, Port of Helsinki planning engineer.
The oldest quays were built on top of wooden boxes filled with sand and rock. For example, the oldest sections of the South Harbour Quay are still supported by boxes made of logs. Most of these are already hidden under modern surfaced areas, but some wooden boxes are still in use in support structures at the water’s edge.
“Water protects wood from rotting, so even older wooden structures stand the test of time,” says Sievänen.
In the past, wood has been used not only to make boxes but also in bottom-driven piling. The Helsinki coastline is mostly clay. This means that, in order to build stable quay structures, you will either have to dredge the soft soil from the seabed or use piling that extends down to the bedrock. After dredging, blasted stone will be laid on the seabed as a foundation for the actual support structures.
“These days, wooden boxes have mostly been replaced by L-shaped concrete retaining walls. Reinforced concrete has also replaced wood for piling,” says Sievänen.
Katajanokka’s own Atlantis
There is another interesting exception to be found at the port in addition to wooden boxes, L-shaped concrete retaining walls and piles made of various materials. Katajanokka has its own underwater vaulted structure reminiscent of the mythical city of Atlantis. The structure revealed by the modelling images is very surprising indeed.
“No one usually gets to see this except divers,” says Sievänen, laughing.
This one-hundred-metre-long structure brings to mind underwater catacombs. You could imagine it being Helsinki’s oldest quay, but it was in fact built after the Second World War. This area was completed in 1955, almost twenty years after the quay in the South Harbour that was supported by wooden boxes.
The Port has no idea why this decorative construction method was chosen.
“It’s possible that piling could not, for some reason, be installed,” says Sievänen.
Discoveries on the seabed
Technology has helped us to study underwater structures. The depth of quay areas was originally investigated using plumb lines, single-beam echo sounding, and diving. Divers have also provided more accurate information about the condition of quay structures, but it takes a lot of time to carry out diving surveys.
Multi-channel scanning can quickly provide information about the structural condition of quays and the land masses that have accumulated in the harbour basin. A variety of different imaging techniques have been available for the past couple of decades.
“In the past, the only technique we had at our disposal was to map seabed topography using depth sounding, but advancements are being made all the time. For the past five to ten years, we’ve also been able to use multichannel scanning and laser scanning, which enables us to study both underwater and terrestrial structures simultaneously.”
Although technology is developing all the time and images are becoming increasingly accurate, divers are still required.
“The images don’t show whether a pile is rusty or how thick its iron surface layer is. Divers can check these things and also make any necessary repairs.”
The scans can sometimes reveal some surprises as well:
“We find car tyres on the seabed every year, as they are also used as collision protection along the edges of quays. But we once found an entire car on the seabed as well,” says Sievänen, laughing.