open the bag to form a collar opening. The second team member places the tip of a pointed shovel with sand into the opened sandbag. A square shovel is not recommended as the blade of the shovel will not fit into the sandbag when filling. The third team member will transport and stockpile the filled sandbags. Properly placed sandbags will be set lengthwise and parallel to the water flow with the folded or open end of the sandbag facing upstream. All loose debris should be removed from the placement surface and the lowest areas are the first spots to be filled in with sandbags. Each bag must be set consecutively with the tightly packed bottom slightly overlapping the previously placed sandbag. Subsequent layers of bags should be offset by 1/2 the length of a sandbag to eliminate voids and improve the wall seal. Each placed bag should be tamped and flattened to improve the seal. The two primary methods for stacking sandbags to build flood control structures are the (1) Single Stack placement, and; (2) Pyramid Placement Method. Fortification 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 tiny air gaps makes sandbags an efficient dissipator of explosive blast. The most common sizes for sandbags are to . 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. Some of the World War I memorial trenches were rebuilt with concrete sandbags after the First World War—although criticized as looking unnatural, they have lasted well. During World War II in Great Britain, some aircraft revetments and pillboxes