A sandbag (floodbag) is a sack made of 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.
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.
Sandbags have been used since at least the late 1700s. They have traditionally been filled manually using spades. Since the 1990s, machine-filling has become more common, which allows the work to be done more quickly and efficiently.
"Sandbag" can also refer to a crude weapon consisting of a small bag filled with sand for use as a cudgel typically by criminals, or to the act of striking a person on the head with such a weapon. This usage is obsolescent in normal speech, appearing mainly in legal codes. However the verb form is extended metaphorically in several slang expressions.
In games and various kinds of adversarial settings, the term sandbagging refers to the practice of purposely placing oneself in a weaker position so as to give the deceptive impression that one is less skilled than one truly is.