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

Don’t rock the boat, don’t tip the boat over

DOP Dredge ‘Roanoke’, Long Island, USA

We were quietly enjoying our dinner on a relaxed evening in our vacation. Suddenly, we were rudely disturbed by rumble and clatter from across the valley. For our eyes developed a rock slide. Just as sudden as it started it was already over. Perplexed, we were too slow to capture the event and put it on social media. Afterwards, I took some pictures of the rubble. As you can see, it was not even a proper rock slide, more the collapse of a retaining wall.

Retaining wall collapse, Sóller, Mallorca, Spain, June 21, 2018

Come to think about it, it was not the first collapse I witnessed. Back in 2006, I was visiting our DOP dredge at Roanoke on Long Island, NY in the USA. I had to do some measurements and general inspection. I was below decks connecting the data recorder to the drive system and had to check something with the dredge master. Just when I climbed on deck, he yelled at me to hold on. Immediately a torrent of water and sand was flung horizontally over the dredge. Some stones cracked a window in the control cabin. Within seconds a tsunami lifted the dredge for about a meter and we kept rocking until the reflecting waves in the pit eventually subsided.

DOP Dredge Roanoke with pit bank in the foreground, before it collapsed. Older bank collapses in the background.

That was one big bank collapse to me. A bank collapse is a known, although undesirable phenomenon in dredging1. It is a result of dredging methods, relying on the development of an active bank to produce a heavy slurry, that is sucked up. However, the sediment does not consist of a uniform block of sand. Usually, the sediment is deposited in different layers, each with their own geo technical properties. These result in varying propagation velocities of the active banks. When a ‘faster’ sand is under a ‘slower’ sand, the upper layer is not supported anymore and collapses. As the bank slumps down, it displaces an enormous volume of water and this often causes a tidal wave of its own. At Roanoke, the effects were aggravated by the fact, that the upper bank ran all the way to above water level.

Progression of an active bank and bank collapse

As this bank collapse can be expected when dredging with active banks and different sand layers, dredging companies are very keen on predicting these nasty consequences. Not only for the safe working condition of the crew, but also to prevent material damage and eventually for a stable and reliable delivered profile. Exactly this is what is being investigated by dr. Askarinejad in the Laboratory of Geo-Engineering at the Technical University Delft2. He has a beautiful rig, where exactly those conditions can be simulated and measured. With a neat trick he tips the whole test facility to form an instable bank. This makes the bank collapse on demand3.

Static liquefaction tank TU Delft (Credit: dr. A. Askarinejad)

Basically, this is exactly what we can demonstrate with the ‘breaching exhibit ‘ in our dredging experience4. Of course you are welcome to come over. For those who are not in the circumstance to visit us, you can also visit the National Dredging Museum as they now have a breaching exhibit of their own5.

Handover of our old breaching exhibit to the National Dredging Museum

References

  1. Breaching Process OE 4626, van Rhee, TU Delft
  2. Amin Askarinejad, TU Delft
  3. Statische liquefactietank , Delft Integraal
  4. Loose sand, how hard can it be?
  5. Baggermuseum krijgt model van Damen Dredging, Binnenvaartkrant

See also

Loose Sand, How Hard Can it Be?

Breaching exhibit at the Damen Dredging Experience

Did you ever tried to build a sand castle? Probably yes. Felt frustrated it always collapsed unexpectedly? At least I did when I was a child. But it took me an academic study to know why. Lucky you. You just have to read this blog post and experience a moment of enlightenment. So, this is good moment to stand up and get some coffee. You will enjoy reading it more and remember my lecture every moment you take a sip.

The second exhibit in the Damen Dredging Experience is an installation, which we call: ‘the Bank’. Usually there is some mechanical or hydraulic action, that will cause the sediment, to become unstable. In this exhibit, we can turn the little wheel at the lower right corner. The first thing you will notice is that at the higher end of the soil surface, the grains will slightly move and start to tumble down along the slope. Where the activation of the particles start at the slope is called the active bank.

Breaching the bank and density flow

The effect we would like to demonstrate, is that different soil types, do have different behaviour in this process. There are three different soil types, from course to fine, from the front to the back. The finer material at the back seams to stay the longest at rest. This is due to a phenomenon that we call ‘dilatancy’1. If a stack of grains is sheared, they have to hobble over the tops of the layer below.

Under pressure due to dilatency on a shear plane

When the grains do hopscotch over each other, they require more space to do so. Effectively the pores increase in volume and the total sediment expands. The extra space cannot be accommodated for by expanding water, it has to be replenished. The extra water has to come from the outside. But the grains themselves are in the way and form resistance to the incoming water. The resistance causes a differential pressure under the ambient pressure, commonly known as ‘vacuum’. And grains under vacuum tend to cling together and form chunks. This happens mostly, when the pores are small, or when the grains are small. Exactly what you can see in the exhibit.

Once the sediment is loosened from the active bank, it rolls down the slope, it behaves like a dense fluid, driven by gravity2. When the slope becomes less, or the running fluid encounters resistance, the sediment will settle again at the so called ‘passive bank’.

Outflow of density current and sedimentation

Here the reverse process happens, the water has more trouble getting out of the suspended flow and run longer. The passive slope will be flatter at finer grains than in more coarse material.

Both processes can be identified in e.g. the DOP3. It is usually suspended on a wire and lowered onto the seabed. Powerful jets excavate a small pit where the suction head takes up the suspended material. The walls of the pit become unstable as an active bank. The loose material flows into the pit. This turns into a continuous process and the active bank, runs away from the suction pit.

Breaching and density flow in a DOP process

Now, it is immediately evident, why DOP pumps have this characteristic suction pipe. It fits snugly in the pit and has the least resistance for the incoming density flow. Another benefit of the suction tube, is that if the bank collapses on the DOP, the suction pipe can be extracted without too much trouble. Extracting a pump from under a collapsed bank imposes the same trouble as creating a passive bank: suction due to dilatancy.

So, your sand castle collapses when water enters the pores. A demonstration of grains becoming as strong as a concrete block by under pressure is a well-known household phenomenon: vacuum packed coffee. Now, you will think of this, whenever you open a new pack of coffee.

Vacuum packed coffee is stable due to under pressure in the pores

References

  1. Dilatancy, Wikipedia
  2. Density current, Wikipedia
  3. DOP pumps, Damen

See also