Quick sand
QUESTION: What are the conditions favourable for the formation of quick sand?
T.S. Chellammal Anni, Chennai
ANSWER 1: Quick sand occurs when a saturated wet sand mass of semi solid consistency tends to reach a liquid consistency. Quick sand occurs naturally where the water pressure below a saturated sand deposit is greater than the usual static water pressure similar to that of near artesian condition.
Quick sand contrary to popular belief is not a type of sand but is a condition of seepage flow of water occurring in a sand mass with an upward flow gradient that exceeds unity. Quick sand condition can occur in a mass of pebbles and coarse sand, as quickly as it will, in fine sand and coarse silty sand deposits.
However large volume of water will be required for such a condition to prevail in pebbles and coarse sand. Hence it is frequently observed only in fine sand and silty sand. In deserts, this condition may occur by the upward pressure caused by hot springs.
S. Boominathan
Professor of Soil Mechanics (Rtd)
Anna University
ANSWER 2: Quick sand is not a type of sand but a flow condition occurring within a cohesionless soil when its effective stress is reduced to zero due to upward flow of water. The effective stress in the soil is the difference between the total stress and the pore pressure inside the soil mass. The higher the effective stress, the more tightly the soil grains are held together, generally resulting in higher strength. A soil is said to be in a quick condition when the effective stress drops to zero. Quick sand occurs in nature when water is being forced upward under pressurised conditions. Quicksand is found where water and sand mix every day. A good place to find quick sand is in hilly country with abundant caves and underground springs.
In this case, the pressure of the escaping water exceeds the weight of the soil and the sand grains are forced apart. The result is that the soil has no capability to support a load. There is no `sucking' or downward force associated with quicksand. Consequently, it is literally impossible for a person to be sucked into quicksand and disappear. So, a person walking into quicksand would sink to about waist depth and then float.
R. Sathya Nathan
Institute of Remote Sensing, Anna University,
Chennai
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