Sandbag 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 . Usage . 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 fortification s , 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 . Sandbags made using big bags . 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 are also used for disposable ballast in gas balloon s , and as counterweight s for theatre sets . Sandbags have been used since at least the late 18th century . They have traditionally been filled manually using shovel s . Since the 1990s , machine filling has become more common , which allows the work to be done more quickly and efficiently . During World War II , sandbags were also used as extemporized " soft armor " on American tanks , to help defeat German anti - tank rounds , but were ineffective . Sandbags can also be 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 . Sandbags are also used by off - road enthusiasts instead of sand plates or sand ladders to assist the vehicle to get traction and momentum after being stuck in soft sand . The same sandbags can be used to bridge deep holes or ditches . Apart from being very light and taking very little space ( when empty ) , the sandbags are a much cheaper option than any of the other options ( sand plates , sand ladders , multipurpose bags , etc ) . 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 by heavy waves . Sandbags are also used in earthbag construction to make inexpensive , environmentally sustainable homes . 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 .