The most recent was on a hike to the Keet Seel pueblo ruins in the Navajo National Monument in northern Arizona. The ruins are reputed to be the best preserved anywhere, probably because the site can only be accessed after a strenuous eight mile hike, most of which is along a sandy stream bed. The guides warn all visitors of the quicksand, which most often forms on the downstream side of boulders, since the surface water flows around the boulder but the subsurface water is pulled upwards and suspends the sand. The quicksand forms at other spots as well, particularly near the edge of the stream on the inside arcs, and we learned to anticipate it after a while. For the most part, the quicksand is no more than calf or knee deep, and is really just an annoyance.

In regards to dry quicksand, I remember my father telling me about a death at his chemical plant that often gave me nightmares as a child. An operator was filling a boxcar full of a very fine chemical powder, which my father likened to microbeads used for fiberglass repair. The powder was blown in through a large flexible hose, somewhat like an air conditioning duct. The boxcar was mostly full when the operator somehow fell in and sank to the bottom as if he were falling through air. The man apparently suffocated quickly, and was quite dead when they were finally able to fish him out.

It states “A moderate amount of water works to increase the friction between sand particles. This is what allows you to build sand castles.”.

This is absolutely wrong. The effect that allows you to build sand castles is surface tension. As with most any soil/sand, a moderate amount of water acts as a lubricant allowing the particles to consolidate easier. This is the basis of the Proctor test, and describes the moisture/density relationship of a soil/sand with a parabolic curve indicating at what percent moisture maximum consolidation can be achieved.

Beaches are not quicksand because the water migrates up into the sand after it has already been deposited by liquid force and consolidated due to wave action and the weight of the sand. If you have a water pocket in the depositional sequence it could theoretically become quicksand, but I have never seen anything like this.

Although, a mobile dune could migrate into tidal pools or such and probably create quicksand. This is much more likely than beach/river sands becoming quicksand for reasons I will get to later.

Quicksand forms is stream/river environments because there is large amounts of sand being transported by the water. Depending on the sands mineral composition, it can cause it to have some buoyancy. Most river sand is sub-angular mature quartz. It is mature, because most of the softer minerals have been eroded into silts and clays (feldspars in particularly). So anyway, quartz sand that gets deposited into a water “hole” can exhibit some buoyancy, causing it to become a loosely consolidated sludge. I suppose the current could probably undercut a bank or something and form quicksand at a deeper level also.

Note: some buoyancy, very minimal

In my opinion I don’t think that a saturated sand within a few feet of the surface could turn into quicksand. Most alluvial sands have a dry density of about 115 pcf within a few feet of the surface. Maximum dry density of alluvial sands by the Standard Proctor Method is usually around 120pcf. That tells you several things, mostly that the sand is relatively consolidated to begin with, and the pores between the sand grains are most likely in a state of positive pore pressure, not allowing water to migrate into the sediment package.

In the case of water loss, the sands are going to remain compacted, but they are going to exhibit negative pore pressure, allowing water to migrate back into the package when it becomes available. The overburden of the existing sediment will keep the sand compacted though, even against the buoyancy of the sand grains.

Ok, about the sand dunes. Wind deposited (aeolian) sand is typically very mature, has well sorted grain sizes, and is rounded. Wind is a much more efficient erosional force than water, something on the order of about 100 times. It also is very good at sorting sand, because wind can only transport the smaller particles, leaving the gravels behind. It also tends to pick up some silt as it is transported and deposited. (silt is the same thing as sand, just smaller particle sizes)

I will state that aeolian sands are not entirely consistent everywhere. They can be beach or river sands that have been moved by wind, but in some instances they are actually glacial sediments (glaciofluvial outwash). Essentially, these blew off the moraines from the last ice age. This is what the sandhills in Nebraska are.

I would think that wind deposited sand would have a much greater tendency to form quicksand than alluvial sands because these sands are typically in place at about 85-90pcf dry density, with a max of about 115pcf dry density, they don’t interlock well because of their rounded shape, and they have up to about a 15% silt content.

Note: dry densities are relative as is 15% silt content. Density would increase as depth increased due to consolidation of the sediment from overburden. The 15% is kind of an arbitrary number. It is what I’m used to seeing around here. The UCS would classify it as a cohesive soil instead of a sand at around 30%. The silt is an important factor here though because ambient moisture will cause it to form a sort of weak matrix with the sand grains in the first 5 or so feet of sediment. Increased contact with moisture or any type of overburden will cause the matrix to collapse, but it makes for larger pore sizes in said upper layer.

So aeolian sands have relatively more pore space than alluvial sands due to their small amount of consolidation. This would allow for more water to infiltrate the package, along with increased buoyancy due to smaller particle sizes. The same could be said about a sand dune migrating into a tidal pool or small water pocket along a river. As the sand was deposited in the water, it would not consolidate under it’s own weight, allowing a very loose suspension to form, giving us quicksand.

This also implies some things about Peorian Loess that I had not previously though of, but thats for a different time and place.

Anyway, this was my first post. If I’m not supposed to be posting in this old of an entry, sorry. I really do enjoy this site, I have slowly been working my way through every article and I’m gonna have to pick up the book.

So why are they deadly?

The bay of Mt Saint Michel is a beautiful place and it is really tempting to go for a walk when the tide is low. If you careless you can find yourself trap in the quicksand and this is when troubles start, because if, for sure, you are not about to sank in the sand and if nobody helps you to get out , it will be difficult to extract yourself of it. Then the tide is back and, as the people around say, it is coming “a la vitesse d’un cheval au gallop” (at the speed of a galloping horse) you will just drown.
Anyway you can visit the bay with a guide by feet or by horse and it is, somehow, a mystic experience due to presence of the Mont itself.