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

What Dutch Dikes are Really Made of

Placing protecting basalt blocks on a dike by a stone setter (Credit: van den Herik-Sliedrecht)

The Netherlands coastline is, to a great extent, defined by unnatural sharp lines: dikes. Even when you zoom into a more human scale, the dikes are decorated by geometric patterns: hexagons. Just like the dikes are not a natural component in the environment, the hexagons are an invasive specie: basalt blocks imported from more rocky countries, as Germany, England and Norway. They are carefully placed by hand to protect these structure from waves and tides. This armoured lining is usually the final stage of a dredging project: protecting what just has been constructed.

My personal experience with these blocks, is that we used to play with them, when we were just little boys. We sailed to the newly constructed Pampushaven, where there was a stockpile of these basalt blocks leftover from the reclamation of the Flevopolder. We puzzled them together like a real dike, or built forts of them. Even after forty years, the stock pile is still there!

Playground stock pile of basalt blocks for dike and shore protection (Credit: Google street view)

What I remember was, that they are extremely heavy. And their perfect prismatic geometry intrigued me intensely. I thought they were made that way. By now they feel less heavy and I have learned that they are some kind of volcanic rock called ‘basalt’. But it puzzled me how molten basalt can solidify into these perfect jigsaw pieces. Until last week, I’ve read an article in the Guardian1 about a geological feature called ‘Giants Causeway’.

Recent article about the origin of the basalt blocks at the Giant’s Causeway (Credit: Guardian News & Media Ltd)

As it happens, I also visited this Giant’s Causeway. And it really feels like standing on a dike at home. But this ‘dike’ was laid down by mother nature. Molten lava flowed over chalk beds. As it was clamped between other layers during cooling, the internal stresses caused the cracks to be distributed evenly in a nice geometric pattern. So, Finn MacCool2 wasn’t involved after all!

The internal structure of dikes, dams, jetties and groynes is a bit different and purposefully designed for the intended application. Usually there is a body of sand, that is designed to take the load of the tide and waves. Next an internal lining with appropriate permeability. Either watertight for keeping the water out, or open structure for draining the wave run-up. Height, width and slopes depending on the requirements.

Example of an internal structure of typical dike

Evaluating all these requirement choices in a well-balanced design is an art by itself. Don’t cut corners, it’s all about safety of the people living behind the dike. It is best left to specialised companies that are familiar with the design and construction of these civil works. In the Netherlands, the trade was often handed over from father to son and whole families became intertwined in these specialist dredging companies.

So, now we know the origin of the stones for the dikes and how they are used. But the real resource used to protect our dikes and the land behind it are: all those unnamed men that have literally put their back into placing those stones. We owe them our land!

‘The stone setter’ by Ineke van Dijk placed in 1982, on the occasion of the 50th anniversary of the Afsluitdijk (Credit: Wikipedia)

References

  1. Scientists solve mystery of how Giant’s Causeway was formed
  2. Finn MacCool

See also