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 may 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 most common size for sandbags is 14 inches by 26 inches. These dimensions and weight of sand a bag this size can hold allows for the construction of an interlocked wall like brickwork. They are not too heavy to lift and move into place. 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.
Big bags are much larger than traditional sandbags. Moving a bag of this size typically involves a forklift truck. Thailand utilized big bags filled with sand to erect temporary walls to protect against the 2011 Thailand floods.
Other uses 
Sandbags have been used since at least the late 18th century. They have traditionally been filled manually using shovels. Since the 1990s, machine filling has become more common, which allows the work to be done more quickly and efficiently.
Sandbags are also used in weight training, as a cheap and effective resistance training tool. There are several different types of sandbags are used in the world of strength and conditioning. 
Carried within vehicles to provide improved traction during inclement weather. Typically stored above the drive wheels where the increased weight improves traction. If ever stuck, sand can be removed and placed directly onto the slippery surface thereby providing greatly improved traction.
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.
Sandbags are often used to temporarily stabilize soil from erosion, such as oceanfront structures whose foundations have been undermined from heavy waves.
An often unknown use of sandbags includes panel restoration and re-creation. The sandbag being used behind the panel as it is beaten by a hollowing hammer to create initial angles and curves.
In addition, sandbags are often used when shooting a long gun, specifically a rifle, from a rest, as it provides support for the weapon, allowing for less movement during shooting.
In slang 
The word can also refer to a simple 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.
See also 
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- Hesco bastion
- Metalith, brand name/manufacturer of an alternative flood control technology
- War Office. Military Training Pamphlet No 30, Part V: Protective Works. 1941. p. 39 specifies 20 inches (510 mm) of sand in bags as being proof against armour piercing bullets and bomb fragments.
- Hunnicut, R.P. "Sherman: A History of the American Medium Tank" ISBN 0-89141-080-5
||This article needs additional citations for verification. (June 2008)|
- A Directory of UK local council sandbag policies.
- California Department of Water Resources and the California Conservation Corps - How to fill and place sandbags.
- US Army Corps of Engineers Sandbagging pamphlet (PDF)
- US Army Corps of Engineers - How to use Sandbags (PDF) (HTML)