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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, shielding glass windows in war zones, ballast, and in other applications requiring mobile fortification. 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. They can degrade prematurely in the sun and elements once deployed. They can also become contaminated by flood waters making them difficult to deal with after flood waters recede. ==Usage== 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.〔War Office. Military Training Pamphlet No 30, Part V: Protective Works. 1941. p. 39 specifies of sand in bags as proof against armour piercing bullets and bomb fragments.〕 The most common sizes for sandbags are to .〔Kim Williamson. (How Sandbags Work ). Answers.com.〕〔Kelly Hart. (Type of Earthbags to Use ). EarthbagBuilding.com.〕 These dimensions, and the weight of sand a bag this size can hold, allow for the construction of an interlocking wall like brickwork. Individual filled bags 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 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 military and for flood prevention are made of circular woven polypropylene. Sandbags have been used since at least the late 18th century. For example, British loyalists used sandbag and log fortifications in the 1781 Siege of Ninety-Six during the American Revolutionary War. Nathanael Greene was familiar enough with the fortification technique to equip his troops with hooks to pull down the sandbag and log walls when they stormed the Star Redoubt in Ninety Six, South Carolina. Sandbags have traditionally been filled manually using shovels. Since the 1990s, machine filling has become more common, allowing the work to be done more quickly and efficiently. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「sandbag」の詳細全文を読む スポンサード リンク
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