Deposition Of Dredged Material At Reclamation Areas In Ancient Chinese And Modern Times

Hills of Jingshan Park Beijing
Hills of Jingshan Park Beijing

As promised, I still have several stories for you and this is another one. As you may remember, we’ve visited China for attending the WODCON in Shanghai1 and afterwards travelled to Beijing for sightseeing. A must see destination in Beijing is the Forbidden City. The epicentre of ancient Chinese power, the seat of the emperor. Once the exclusive domain of the supreme ruler, now a tourist attraction for the general public. The Forbidden City was mainly build in the Yongle era of the Ming dynasty2 between 1407 and 1420. It comprises numerous courtyards and halls and temples. All the buildings are surrounded by thick walls and a moat.

Moat around the Forbidden City
Moat around the Forbidden City

This moat is an impressive 6 meters deep and 52 meters wide. That is a big moat. But remember it is long: 3.5km around3. So, it is an impressive moat. Now consider this moat is dug in the fifteenth century. It has been dug by hand! Imagine, thousands of labourers digging, carrying and removing the soil from the moat. That is quite an operation.

To put this in perspective. The moat has a volume of 6x52x3,532m=1,101,984m³. Yes, that is over a million cubic meters. Even for a modern dredging project it is a serious volume. And digging a hole at one place is the first step. Where do you dispose it? At a dredging project, there is a reclamation area. As this was dry land, there was no reclamation area. So, what do you do with such a volume? If you pile it up, you can store a volume of V=1/3 pi r² h in a cone. Assume a slope of one third of the height to the radius, the height of the pile can be calculated and will be around 49 meter. And that is exactly what the ancient engineers did: they created the hill of Jingshan Park4. With its five peaks, it is not exactly a cone, but the estimated height was quite close!

Height marker at the top of the hill in Jingshan Park
Height marker at the top of the hill in Jingshan Park

The engineers had probably carefully planned how they constructed this hill and planned the delivery of the material accordingly. Nowadays, with the much higher production rates and shorter project delivery times, it is highly inadvisable to build a reclamation area with this height. There are several reasons why not to do it like that. First, it would take time to drain the pore water away from the core of the hill. Loading more on top quickly would make it very instable. Sometimes with disastrous results5. Another is when you create high banks, it will be easier for shear planes to form and collapse the structure that way. Lastly, a lower reclamation area will also have a larger surface area and more choice to select multiple locations to evenly distribute the material in volume and composition. A well designed reclamation area requires good knowledge of the deposited material and a skillful team that operates the equipment to manage the deposition.

Explanations of issues with depositing sand at reclamation areas
Explanations of issues with depositing sand at reclamation areas

Based on the exposed rocks sometimes seen on the sides of the Jingshang Park hill, the core is probably consisting of bigger rocks as a kind of backbone. But not every rock found in the moat ended up in the hill throughout the area. Several decorative rocks can be found that have a typical size that could just be handled by manual labour. Just another tribute to the perseverance of those classic engineers.

Decorative stone in Bei Hai Park west of Jingshan Park
Decorative stone in Bei Hai Park west of Jingshan Park

References

  1. WODCON XXII, EADA
  2. Yongle Emperor, Wikipedia
  3. Forbidden City, Wikipedia
  4. Jingshan Park, Wikipedia
  5. Aberfan disaster, Wikipedia

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

Paying Tribute to the Hard Life of Peat Dredgers

Sculpture ‘Verveners’ by John Rijnen
Sculpture ‘Verveners’ by John Rijnen

It’s been a while since my last update. Don’t worry. All is well. The delay was due to my change in habitat. Moving can be draining attention and creativity, but can also create opportunities for exploring the new neighbourhood. And that is exactly what I will share as this first post. One of the first dredging related discoveries was a sculpture on a roundabout nearby. Probably most passers-by will not notice it or at most will be puzzled by the strange flat figures. To us, as dredging community, we immediately recognise the typical posture of manual dredging. Intrigued by the unexpected discovery of dredging art at this location, I went on an investigation on the background of this statue.

Sign at sculpture ‘Verveners’
Sign at sculpture ‘Verveners’

The sign at the sculpture did reveal some details on the name, date and artist of this monument1. Obviously, the topic of this statue was a tribute to the people who have been working as peat dredgers and contributing to the wealth of the town. It is also a reference to the origin of the current geography of the municipality: lots of lakes and ponds that are a recreational area for the population and a resting place for nature, that were unbelievably all excavated manually over ages.

The most characteristic feature of the sculpture are long poles2 in combination with the posture for back breaking manual labour. The long pole has a hoop at the lower end with some kind of fishing net attached. The hoop is used to scrape sediment (in this case peat) from the bottom and collected in the pouch. Mud and peat are sticky enough to stay in the net for vertical transport, but the flexibility and openness enables easy discharge. For peat dredging the material was scooped up on shore and dried3. For maintenance dredging, the material was brought on board and discharged at a convenient disposal location4. The Dutch dredging industry cherishes this and remembers it with all kinds of small statues found in many offices.

Paper weights about manual dredging with a ‘baggerbeugel’ (Credit: Hendrik Jan de Kluiver)
Paper weights about manual dredging with a ‘baggerbeugel’ (Credit: Hendrik Jan de Kluiver)

Manual dredging shaped much of Holland’s geography, either the canals and lakes or the reclamation of marsh into pasture land. It has been identified as a typical Dutch tradition. But is it?? Preparing this post, I’ve discovered it is not. Several references say that we did not invent the ‘baggerbeugel’2. Before we started to use it, it has been applied already for ages in East Anglia5, which had a very similar landscape. Already from the tenth century, peat has been excavated from the Fenlands and later on from the 14th century, they started manual dredging already. Only later on in the 16th century Holland started to take over this practice. And in good Dutch tradition, we refined the trade and even exported consultancy on these projects back to England!

Stills from an instructional video on peat dredging (Credit: modified from Open Beelden Project)
Stills from an instructional video on peat dredging (Credit: modified from Open Beelden Project)

Peat dredging is a laborious procedure and later on, this industry was mechanised. There is a nice video explaining about peat dredging in the Netherlands. Especially the hydraulic dredging starts at 08:10 into the video. All processes in a regular dredging project are identifiable: a) dredging, b) discharge, c) bulldozering, d) compacting, e) drying and f) further transport of the merchandise.

Viewing a video is one way to experience manual dredging. In the dredging garden of the National Dredging Museum6, there is an exhibit to experience this trade first hand.

Visitors experiencing manual dredging in the National Dredging Museum
Visitors experiencing manual dredging in the National Dredging Museum

References

  1. De oude tol, Rotondekunst
  2. Baggerbeugel, Wikipedia
  3. Vervening, Wikipedia
  4. Barges, IADC
  5. A Tale of Norfolk Peat Cutting, Norfolk Tales, Myths & Beyond
  6. Bagger Praktijk Tuin, Nationaal Baggermuseum

See also