Sandbag

A sandbag is a bag or sack made of hessian (burlap), polypropylene or other sturdy materials that is filled with sand or soil and used for such purposes as flood control, military fortification in trenches and bunkers, shielding glass windows in war zones, ballast, and in other applications requiring mobile fortification, such as adding improvised additional protection to armoured vehicles or tanks.

The advantages are that the bags and sand are inexpensive. When empty, the bags are compact and lightweight for easy storage and transportation. They can be brought to a site empty and filled with local sand or soil. Disadvantages are that filling bags is labor-intensive. Without proper training, sandbag walls can be constructed improperly causing them to fail at a lower height than expected, when used in flood-control purposes. They can degrade prematurely in the sun and elements once deployed. They can also become contaminated by sewage in flood waters making them difficult to deal with after flood waters recede. In a military context, improvised up-armouring of tanks or armoured personnel carriers with sandbags is not effective against cannons (though it may offer protection against some small arms).

Sandbags may be used during emergencies when rivers threaten to overflow their banks, or when 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 effective in preventing flooding because water will eventually seep through the bags and finer materials, like clay, may leak out through the seams. 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 and 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 sizes for sandbags are 14 by 26 inches (36 by 66 cm) to 17 by 32 inches (43 by 81 cm). These dimensions, and the weight of sand a bag this size can hold, allow for the construction of an interlocking wall like brickwork.

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