Fill the sandbag one-quarter to one-half full with sand. It should weigh about 40 lbs.
Fold the empty part of the bag over. Be sure that when you place the bag, the folded-over side is facing down, so that no water will seep in. If one bag leaks, the whole pile will be ruined.
Sandbags work best for filling in low areas of 2 feet or less.
Wait to place the sandbags until the rainy season is near, since the bags can deteriorate over an extended period of repeatedly getting wet and then drying.
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 size for sandbags is 14 by 26 inches (36 by 66 cm). 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 militaries and for flood prevention are made of circular woven polypropylene. The bags' easy availability, size and construction has also led to their use as makeshift hoods for prisoners of war.
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.