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Build Sandbag Dikes the Right Way

A sandbag dike could fail if it isn’t built correctly.

“Planning, organization and identifying someone who will supervise the dike-construction process are keys to building effective flood protection,” says Ken Hellevang, North Dakota State University Extension Service flood expert.

Woven polypropylene is the most common material for sandbags. Bags usually are about 14 inches wide and 24 to 26 inches long. Other sizes are available, but the bags are easier to handle if they weigh no more than 35 or 40 pounds with filling material in them.

Sand is the easiest material for filling and shaping sandbags. Silt and clay in bags will form a good dike, but working with those materials is more difficult.

Here are some sandbagging tips:

  • Fill sandbags about one-half full.
  • Do not tie bags closed. Untied bags form into a dike better.
  • Lift bags using your legs to limit the strain on your back.
  • Build the dike at least 1 foot higher than the projected crest to allow for fluctuations in the water level.
  • When selecting the dike’s location, take advantage of natural land features that keep the dike as short and low as possible. Avoid obstructions that would weaken the dike.
  • Do not build the dike against a building wall because of the force the dike may place on the building. Leave at least 8 feet to maneuver between the dike and buildings.
  • Remove ice and snow from the ground before starting to build a dike or the dike will leak when the snow and ice melt.
  • Since friction holds a dike from sliding, create a good bond between the ground and the dike. Remove anything slippery from the dike site. Do not put plastic sheeting under the bags since it will increase the potential for the dike to slide.
  • If the dike is to be more than about 3 feet high, dig a bonding trench where the dike will be placed. The trench should be at least 4 to 6 inches deep and 18 to 24 inches wide.
  • Use a sump or skimmer pump to remove water that leaks through the dike. Use ground fault circuit interrupters on circuits or extension cords to reduce the risk of electrocution.

The U.S. Army Corps of Engineers recommends building a dike three times as wide at the base as it will be tall. For example, a 4-foot-high dike should be 12 feet wide at the base. You would need about six sandbags per foot of dike length for a 1-foot-tall dike, 21 sandbags per foot for a 2-foot-tall dike, 45 sandbags per foot for a 3-foot-tall dike and 78 sandbags per foot for a 4-foot-tall dike.

At a minimum, dikes should be twice as wide at the base as they are tall. In this case, you would need six sandbags per foot of dike length for a 1-foot-tall dike, 17 for a 2-foot dike, 30 for a 3-foot dike, 57 for a 4-foot dike and 93 for a 5-foot dike.

You’ll need about a cubic yard of sand to fill 100 30-pound sandbags or 75 40-pound bags.

Filling sandbags usually is a two-person job. One person holds the bag on the ground slightly in front of his or her spread feet and the second shovels the sand into the bag. The bag holder should use gloves to protect his or her hands. Using safety goggles also might be a good idea, especially during dry and windy days.

Here’s how to build a dike:

  • Place the first layer of bags lengthwise (parallel to the water flow), overlapping the bags so the filled  portion of one bag lies on the unfilled portion of the next, with the untied open end facing downstream.
  • Offset adjacent rows or layers by one-half bag length to eliminate continuous joints (it’s similar to laying bricks).
  • Compact and shape each bag by walking on it. Continue to walk on the bags as you place succeeding layers to eliminate voids and form a tight seal. Five feet of water can exert about 310 pounds of pressure per square foot at the base of a dike.
  • Seal the completed dike with a sheet of plastic to improve water tightness. Spread a layer of soil or sand 1 inch deep and about 1 foot wide along the bottom of the dike on the water side. Lay polyethylene plastic sheeting over the loose soil or sand so the bottom extends 1 foot beyond the bottom edge of the dike. Place a row of sandbags on the bottom edge of the plastic to form a watertight seal along the water side. The upper edge of the plastic should extend over the top of the dike. Use sandbags to hold down the top edge of the plastic.

Lay the plastic sheeting very loosely. The water pressure will make the plastic conform easily to the sandbag surface. If the plastic is stretched too tightly, the water could puncture it. Also, avoid puncturing the plastic with sharp objects or by walking on it.

Use poly sheeting that’s at least 6 mils thick. It generally is available in 100-foot rolls from construction supply firms, lumberyards and farm stores.

For more information about dike building, visit www.ag.ndsu.edu/publications/disasters/sandbagging-for-flood-protection or view these videos: www.ag.ndsu.edu/flood/sandbagging-safety-tips and www.ag.ndsu.edu/flood/home/building-a-sandbag-dike-video.

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