CEDA DMC Works On A Guidance Paper For Soil Investigation

CEDA Dredging Management Commission WG on Soil Investigation (Credit: CEDA)
CEDA Dredging Management Commission WG on Soil Investigation (Credit: CEDA)

Did you ever start a project and it turned out that the conditions were different than expected? Welcome to the dredging industry. One of the most underestimated preparations for a dredging project is the soil investigation. As this investigation is of the utmost importance for the dredging community, the DMC is preparing a guidance paper on this topic1, which we discussed last meeting (February 7, 2020, IMDC, Antwerp).

Working for a dredging equipment manufacturer, I am not much involved in the actual soil investigation. However, often our clients base their purchase of a specific type of equipment on the soil investigation and as such we are often presented with the reports on soil investigation. Based on these reports, we calculate the possible production for various types and advise the client for a dredge that will meet their requirements on the maximum production. most of the time we provide a good advise and the client is happy.

Off course there have been occasions where the performance was not as expected. Often because the report on the soil investigation was inadequate. Either the report did not contain all the details, or the investigation itself was lousy. Either way, rubbish in, is rubbish out. Just as an example, let me tell you what can go wrong, when the information is not representing the real circumstances.

One of our products are the so called ‘DOP Dredges’2. They are based around the versatile DOP pump. Basically, it a DOP suspended on an A-frame on a pontoon with a powerpack. The DOP can be lowered into the sediment and create a typical suction dredge pit. The production is more based on the rate that water can enter the bank face and the velocity that the banks recede. Our client provided us a Particle Distribution Diagram of the available sediment3. It was a nice narrow graded sand, but there was a considerable fines tail on the lower end. This was being dealt by the washing and screening installation. According to the client was this the sand characteristic from the whole pit. And what could be better? If you excavate all the material, you really know what is there, right?

Difference between expected soil conditions (left) and real situation (right)
Difference between expected soil conditions (left) and real situation (right)

Well no. As it happened, there were cohesive silt layers between the narrow graded sand layers. When dredging, they sucked at the bottom of the pit. Any silt layers gradually broke of and disintegrated by the eroding density flow. As the pit was created over a long period, the falling chunks of silt just slid down the slope, without causing any harm.

Enter: the new DOP dredge. It started in a new corner of the pit and initially had some trouble penetrating the silt layer. Eventually it managed to get through and started excavating a cavity below the silt layer. These broke of, burying the DOP. Without any possibility to recover the DOP, it turned into a very expensive anchor.

Risk of getting your DOP trapped in a cavity under the cohesive silt layers and the solution
Risk of getting your DOP trapped in a cavity under the cohesive silt layers and the solution

If the presence of these cohesive silt layers would have been known, we would have adapted the suction pipe for a deeper penetration. That prevents the DOP becoming covered and facilitates easier extraction. This story proves two things: 1. A proper soil investigation can prevent costly accidents and budget runovers. 2. A DOP can be modified to most requirements, when the circumstances are known.

Meanwhile, the DMC is preparing its guidance document to assist you in preventing problems like this. Follow CEDA for updates4.

Standard suction tube (left) and long suction tube (right)
Standard suction tube (left) and long suction tube (right)

References

  1. Dredging Management Commission, CEDA
  2. DOP Dredger, Damen
  3. A Sample of Soil Samples, Discover Dredging
  4. News, CEDA

See also

The Real Benefit Of Becoming An Individual Member Of CEDA

Young CEDA pitch talks at the CEDA Dredging Days 2017 (Credit: CEDA)
Young CEDA pitch talks at the CEDA Dredging Days 2017 (Credit: CEDA)

Do you remember the beginning of my website two years ago? The first public posts on this website were about the CEDA Dredging Days 20171. It has been a long time since then. Back then, I couldn’t have guessed what direction this website would take. By now, this is my sixtieth post! Some memorable moment in itself. Still, I would like to do what I intended to do: share knowledge about dredging. And a preview on the next CEDA Dredging Days in Rotterdam2 is a fitting commemoration of all the other posts in between.

Off course, the renowned CEDA Dredging Days are a platform to meet people and exchange knowledge. Especially all the presentations. That everybody has been sweating on writing the manuscript. Reviewed by the scrutiny of the Technical Paper and Program committee3. From personal experience, I can tell you, there are a lot of interesting papers. You’ve had already a teaser on the presentation of Camille Kapela4, about the dredging project in Monaco5.

Camille Kapela at the CEDA Dredging Days 2017 (Credit: CEDA)
Camille Kapela at the CEDA Dredging Days 2017 (Credit: CEDA)

Another I would recommend is the presentation of my colleague Frank de Hoogh about the new MAD gravel hopper dredge6. It is not just another boat, it is a change of concept. And there are so many interesting innovations on this dredge, that it merits its own sneak preview.

Innovative Marine Aggregate Dredge for gravel dredging
Innovative Marine Aggregate Dredge for gravel dredging

But there are more interesting perspectives on the event. In contrast to other commercial conferences on dredging, this is like a gathering of a community. It is a place for all the commissions and working groups within CEDA to present their efforts and show the reports and results of their work. There is a presentation of the Working Group on contract selection7, an update from of the Working Group on soil investigation and an interactive session themed on the recently published book: ‘Dredging for Sustainable Infrastructure’8.

The interactive session at the previous CEDA Dredging Days (Credit: CEDA)
The interactive session at the previous CEDA Dredging Days (Credit: CEDA)

Above all, it is a social event. All the knowledge presented would be worthless, when there is no community that absorbs the produced information. Would discuss about the content, the feasibility and possible applications. And these discussions generate innovative ideas on the new insights from the presentations and meetings.

Sometimes, I am asked: ‘What good is it to be a CEDA member myself? I can get the benefits of the presentations, the reduced price and the drinks, through the membership of my company. Why would I throw out my personal money?’ Well, it is not about all that. It really is about people and building a community. Having a (small) personal investment brings you in the state of mind, that you belong to this wonderful community with its long history in shaping the world. And that is the real benefit of being an individual CEDA member9. With this personal contribution you can take part of shaping the future of the world.

Thursday evening is the big get together of the dredging community; meet you there! (Credit: CEDA)
Thursday evening is the big get together of the dredging community; meet you there! (Credit: CEDA)

References

  1. Countdown to the CEDA Dredging Days 2017, Discover Dredging
  2. CEDA Dredging Days 2019, CEDA
  3. Committees, CEDA
  4. Dredging in Monaco: challenges and solutions, CEDA
  5. CEDA DMC Visits the Anse du Portier Project in Monaco
  6. Next generation marine aggregate dredger as platform for innovation and basis for fleet renewal, CEDA
  7. Effective contract selection: CEDA’s guide to optimised contracting methods, CEDA
  8. Interactive session, CEDA
  9. Why join CEDA?, CEDA

See also

CEDA DMC Visits the Anse du Portier Project in Monaco

CEDA Dredging Management Commission at a site visit in Monaco. (Credit: CEDA)
CEDA Dredging Management Commission at a site visit in Monaco. (Credit: CEDA)

In the past intermezzo, a lot of blog ideas past my mind. In due time, I will share some of them with you. Others already arrive by themselves naturally. e.g. Lately we’ve had another CEDA Dredging Management Commission meeting1. In preparation for the upcoming CEDA Dredging Days2, we discussed some publications that will be presented there3. Next to the meeting, we also did a site visit to a prestigious project. The Monaco extension project ‘Anse du Portier’ certainly demanded some serious management skills for the dredging works.

Overview of the Anse du Portier project
Overview of the Anse du Portier project

The extension project had already received a lot of attention in the press and in the dredging community. It certainly is a remarkable project, where a lot of disciplines are coming together. I would like to refer you from the excellent video on the Anse du Portier project itself4:

Extension en mer de Monaco – Techniques de construction (Credit: Anse du Portier Project)
Or, if you can hold your breath, to the presentation of Camille Kapella at the CEDA Dredging Days5, where she will elaborate on all the difficult challenges in the project. At the moment of our visit, the last caisson had just been placed in the construction6.

Last caisson in the constructed sea wall at Monaco
Last caisson in the constructed sea wall at Monaco

Caissons are a demanding construction in terms of dredging. Of course, there are examples, where location and placement were not so important, but usually the requirements are much stricter and the conditions much harsher. Caissons have to be placed next to each other in the first place. And joining them all together might end up in a big deviation as errors propagate through each misplacement. This has been recognised already for a long time. Even one of the first tunnels built of sunken caissons, the Maastunnel7, had specifications that are still in use today. So, how did they do this? There is a nice historic video available from the old Polygoon Journaal.

Building of the Maas Tunnel (Credit Polygoon Journaal)
In the case of the Maastunnel, they employed wires driving huge dials and sight line beacons. Under perfect conditions, enough time and an enormous amount of manpower, the objectives can be achieved. Nowadays, this approach would be too costly or can’t be used as the local circumstances prevent them. Waves, tides, difficult location or other factors are the edges of the envelope for modern caisson placement and all were present here in Monaco. Specifically, the challenges at the Anse du Portier site were the steep bedrock, the open coastline vulnerable to waves and environmental concerns. For each, of the challenges, appropriate solutions were chosen to manage the project.
The construction of the caissons to withstand the wave action during the lifetime is remarkable. The top of the caissons are equipped with so called patented Jarlan chambers8. This is a design concept known in the offshore construction to temper the wave action. Waves enter the construction through slots in the walls and enter a chamber with more columns for further dissipation of the wave energy. A similar approach is already discovered by nature itself: coastal mangrove forests.

Last caisson at the Anse du Portier with Jarlan chamber slots covered for transport
Last caisson at the Anse du Portier with Jarlan chamber slots covered for transport

References

  1. Dredging Management Commission discusses papers on contract-type selection and soil investigations, CEDA
  2. CEDA Dredging Days 2019, CEDA
  3. Effective contract selection: CEDA’s guide to optimised contracting methods, CEDA
  4. Anse Du Portier, Youtube
  5. Dredging in Monaco: challenges and solutions, CEDA
  6. Monaco Land Extension Project Reaches Milestone, Caissons Belt Completed, DredgingToday
  7. Maastunnel, Wikipedia
  8. Jarlan Chamber, Espacenet

See also