Who hasn’t seen people disappearing in quicksand in a movie scene? Do you think it’s true, do you think it can happen in reality? Well, from a geotechnical point of view, there is at least a scientific explanation that helps us understand why, when and how quicksand occurs. But surely, we don’t need to tell you that not everything we see in the movies is true.
First of all, it is important to clarify that the phenomenon of quicksand occurs in places where large quantities of water are found combined with fine sands, clays or silts. This combination forms a fluid with a very particular behaviour, in other words. physical characteristics that we call “non-Newtonian”. What does this mean? It means that the material resulting from the combination of water and fine sands, clays or silts is soft if a gentle force is applied to it, but very hard if the force is fast and dynamic.
In practice, this means that we could run across a quicksand, with fast and hard treads so that it does not have time to react. However, if we tread slowly, the water separates from the sand, the ground rearranges itself and this makes it easier for us to sink. Despite what the movies would have us believe, we would not end up in the centre of the earth. In reality, the density of the human body would cause us to sink to just above the waist.
A similar phenomenon, which can ‘liquefy’ the ground, is sub-pressure. This occurs when water flows upwards from a level below the water table. This occurs, for example, in the construction of buildings with excavations deep enough to reach below the water table. If lateral watertight screens are used and the floor is removed, the water, confined between the walls, will look for a way out. Its easiest path will be to rise from the bottom of the excavation to the excavated area, where the pressure of the terrain has disappeared.
In sandy soils, this upward flow can create an area that behaves similarly to quicksand. However, there is no reason for concern. This phenomenon is easily prevented by the construction of a water collection well to avoid any movie-worthy incidents.
By Jorge Laguna, head of the structures section of Amusement Logic’s architecture department.
