A sandbag (floodbag) is a sack made of hessian/burlap, polypropylene or other materials that is filled with sand or soil and used for such purposes as flood control, military fortification, shielding glass windows in war zones and ballast.

Advantages are that burlap and sand are inexpensive, and that the bags can be brought in empty and filled with local sand or soil.

Sandbags may be used during emergencies when rivers threaten to overflood, or a levee or dike is damaged. They may also be used in non-emergency situations (or after an emergency) as a foundation for new levees, or other water-control structures. Sandbags are not always an effective measure in the event of flooding because water will eventually seep through the bags and finer materials, like clay, may leak out through the seam. After usage, dry sandbags can be stored for future use. Wet bags need to be disposed in a landfill as they may be contaminated by chemicals and fecal matter.

The military uses sandbags for field fortifications, or as a temporary measure to protect civilian structures. Because burlap and sand are inexpensive, large protective barriers can be erected cheaply. The friction created by moving soil or sand grains and multiple tiny air gaps makes sandbags an efficient dissipator of explosive blast. The dimensions and weight of sandbags used in fortification are carefully calculated so that the bags can be interlocked like brickwork and are not too heavy to lift and move around. They may be laid in excavated defences as revetment, or as free-standing walls above ground where excavations are impractical. As plain burlap sandbags deteriorate fairly quickly, sandbag structures that are meant to remain in place for a long time may be painted with a portland cement slurry to reduce the effects of rot and abrasion. Cotton ducking sandbags last considerably longer than burlap and are hence preferable for long-term use. However, the vast majority of sandbags used by modern militaries and for flood prevention are made of circular woven polypropylene. The easy availability to military personnel, size and construction of the bags has also led to the use of sandbags as makeshift hoods for prisoners of war.

See also: http://en.wikipedia.org/wiki/Sandbag

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