Hey! I know this one!

Quicksand is sand with a high enough water content that separates the sand grains enough to allow shear modulus^{1} to become quite low. (That means the force necessary to slide the sand grains against each other becomes very low.) The sand and water mixture is thixotropic. it looks like a solid but acts like a liquid when stress is applied. The amount of water in the sand determines how much like a liquid the sand-water mixture will act.

A beach near the water is thixotropic. If you stand with your feet planted in one spot near the surf zone and move your body around the change in the center of gravity of your body's mass will apply shear force to the seemingly solid sand and your feet will sink a small amount into the sand. Quicksand exhibits the same property but to a much more pronounced extent. Weight alone applied to the surface of the sand is enough to turn an apparent solid into a liquid with a certain viscosity that is dependent on the water content. It becomes a non-Newtonian fluid^{2}.

So, the more you struggle while standing, the deeper you sink. The way out of this trap is to reduce the unit stress per area. In plain English, get horizontal. The more horizontal you become, the easier it is to move as if you were swimming and slowly get to solid ground. in fact, the faster you sink the easier it is to extract yourself **IF** you don't loose your head and just flail around.

The clay content will also effect the viscosity of the quicksand but the math becomes much more complicated. The formulas below assume a sand without clay content and of a small range of size of sand grains.

^{1} Shear modulus is defined as **S**. It is the tangential force applied between solid particles per unit of area divided by the angle of the tangential forces or **S**=(F_{tanΦ}/A)/ Φ.

^{2} A non-Newtonian fluid a fluid where the viscosity changes proportional to the applied strain or η= *d*τ/*dD*