Quicksand is a mixture of sand and water, or sand and air, that looks solid, but becomes unstable when disturbed by any additional stress. In normal sand, grains are packed tightly together to form a rigid mass, with about 25 to 30 percent of the space (voids) between the grains filled with air or water. Because many sand grains are elongate rather than spherical, loose packing of the grains can produce sand in which voids make up 30 to 70 percent of the mass. This arrangement is similar to a house of cards in that the space between the cards is significantly greater than the space occupied by the cards. The sand collapses, or becomes 'quick,' when additional force from loading, vibration or the upward migration of water overcomes the friction holding the grains together .. المصدر :
https://www.scientificamerican.com/article/what-is-quicksand/