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